Sports Medicine

https://doi.org/10.1007/s40279-019-01222-6

REVIEW ARTICLE

Optimising the ‘Mid‑Stage’ Training and Testing Process After ACL

Reconstruction
Matthew Buckthorpe1,2 · Francesco Della Villa1

© Springer Nature Switzerland AG 2019

Abstract
Outcomes following anterior cruciate ligament (ACL) reconstruction need improving, with poor return-to-sport rates and high
risk of secondary re-injury. There is a need to improve rehabilitation strategies after ACL reconstruction, if we can support
enhanced patient outcomes. This paper discusses how to optimise the mid-stage rehabilitation process after ACL reconstruc-
tion. Mid-stage is a difficult and vitally important stage of the functional recovery process and provides the foundation on
which to commence late-stage rehabilitation training. Often many aspects of mid-stage rehabilitation (e.g. knee extensors
isolated muscle strength) are not actually restored prior to return-to-sport. In addition, if we are to allow time for optimal
late-stage rehabilitation and return-to-sport training, we need to optimise the mid-stage rehabilitation approach and complete
it in a timely manner. This paper forms a key part of a strategy to optimise the ACL rehabilitation approach and considers
factors more specific to mid-stage rehabilitation characterised in 3 areas: (1) muscle strength: muscle and joint specific, in
particular at the knee level, with the knee extensors and flexors and distally with the triceps surae and proximally with the
lumbo-pelvic-hip complex, as well as closed kinetic chain strength; (2) altered basic motor patterning (movement quality)
and (3) fitness re-conditioning. In addition, the paper provides recommendations on how to implement these into practice,
discussing training planning and programming and suggests specific screening to monitor work and when the athlete is able
to progress to the next stage (e.g. late-stage rehabilitation criteria).

1 Introduction return to competitive sport after ACLR [4]. Of those who

do return-to-sport (RTS), 15% can expect a secondary ACL
Despite being perhaps the most discussed rehabilitation injury, with nearly 1 in 3 (around 30%) young recreational
topic, there is by no means a consensus on the best way and elite athletes experiencing a re-injury, usually within the
to rehabilitate a patient after anterior cruciate ligament first 2 years after RTS [5–9].
reconstruction (ACLR). It is well established that out- Recently, meeting RTS criteria prior to RTS has been
comes after ACLR are not perfect. Although, on one side, shown to reduce the risk of re-injury by 75–84% [5, 10].
patient-reported outcomes are often good in the short to However, within the Kyritsis et al. [10] paper, 12 of the 26
medium term after ACLR [1], a large proportion (35–45%) players with a second ACL injury actually met the RTS
of competitive athletes do not return to competitive sport criteria, while 28 of the 132 players with no second ACL
[2, 3]. Even 1 in 5 (18%) professional/elite athletes does not injury did not pass RTS criteria. This highlights a lack of
specificity and sensitivity for RTS criteria to identify high
Electronic supplementary material The online version of this risk athletes. Recently, we focused on optimisation of late-
article (https​://doi.org/10.1007/s4027​9-019-01222​-6) contains stage rehabilitation and RTS after ACLR and proposed addi-
supplementary material, which is available to authorized users. tional criteria, more stringent and reflective of factors which
may be linked with re-injury risk and athletic performance
* Matthew Buckthorpe
M.Buckthorpe@isokinetic.com [11]. However, given the fact that only 26% of patients meet
current criteria at 6 months after ACLR [12], making the
1
Isokinetic Medical Group, FIFA Medical Centre criteria harder is unlikely to solve the problem. Recent sug-
of Excellence, Education and Research Department, gestions have been to delay RTS until at least 9 months to
Bologna, Italy
reduce the risk of secondary injury [5]. This is because the
2
Isokinetic Medical Group, FIFA Medical Centre first 6–12 months after RTS is the period of the highest risk
of Excellence, 11 Harley Street, London WG1 9PF, UK

Vol.:(0123456789)
 M. Buckthorpe, F. Della Villa

addressed late-stage rehabilitation and RTS training and

Key Points testing after ACLR [11], to transition towards the optimi-
sation of the whole pathway after ACLR. In particular, we
Outcomes after ACL reconstruction are sub-optimal and discuss important factors specific to the mid-stage rehabil-
to improve outcomes we need to optimise our rehabilita- itation and provide recommendations on how to optimise
tion processes and practices this work, as well as outline screening tests and specific
Many factors which could be characterised as more criteria to achieve to effectively prepare for entry into the
‘mid-stage rehabilitation markers’ are typically not ‘late-stage rehabilitation and RTS programme’ [11]. It is
restored or not done so in a timely fashion after ACL hoped this will support practitioners working directly with
reconstruction, which may compromise entry into a late- patients after ACLR.
stage rehabilitation programme
The primary goals of mid-stage rehabilitation should be 2 The Functional Recovery Process
to address deficits associated with neuromuscular func-
tion, including muscle/joint specific strength imbalance It is important to have a well-structured functional recovery
resolution and closed kinetic chain strength, as well as process in place, and clear understanding of where mid-
basic motor re-patterning. In addition, for athletes a key stage rehabilitation fits within the overall approach. There
element of the programme should contain fitness re-con- is no gold standard ACL rehabilitation approach, but hav-
ditioning to avoid detraining and support an optimised ing criterion-based rehabilitation through stages or phases
physical fitness by the time of return-to-sport is regarded as best practice [24]. The functional recovery
Appropriate planning and programming is required to process can be broadly separated into pre-operative, early-,
be able to incorporate all training aspects effectively. mid- and late-stage rehabilitation and RTS training (Fig. 1).
The programme should be periodised incorporating load Pre-operative rehabilitation aims to prepare the athlete
management strategies. for surgery, normalising knee function through minimising
knee joint effusion, gaining full quadriceps activation and
normal gait [25]. Research shows that prehabilitation (5–6-
[8, 13] and there was a 52% reduction in knee re-injury risk week programme focusing on restoration of muscle strength,
for each month RTS was delayed up to 9 months in a single quadriceps hypertrophy and hop performance) results in
study [5]. However, even at 9 months one study showed that superior knee function post ACLR [26–28]. The early stage
only 11% of their ACLR patients actually met RTS criteria is focused on resolving pain and swelling, recovering suf-
[14]. One approach would be to delay RTS even further, with ficient knee joint range of motion, recovery of activities of
a suggestion to 2 years [15]. Delaying RTS of course allows daily living including the ability to walk without crutches,
more time to achieve the necessary functionality; however, and minimisation of muscle atrophy [25]. Late-stage reha-
this is only effective if this time is filled with high-quality bilitation focuses on optimising neuromuscular and move-
rehabilitation. It would appear more logical to optimise our ment performance and RTS training, defined as a continuum
rehabilitation strategies after ACLR. of sport-specific on-field rehabilitation, return to training,
Recent approaches have been done to optimise pre- return to play and finally return to performance [22]. Mid-
operative rehabilitation [16, 17], late-stage rehabilitation stage rehabilitation of course fits between early- and late-
[11, 18–20] and update the RTS frameworks [11, 21, 22]. stage rehabilitation and is the focus of this paper. For an
However, it appears that factors more associated with mid- optimal mid-stage rehabilitation, it is important to have clear
stage rehabilitation are typically not resolved after ACLR. goals and priorities, but also a clear understanding of when
In particular, there appears difficulty to be in restoring knee an athlete is ready to both start (see Table 1 for criteria to
extensor muscle strength in a timely fashion [14, 23]. With- commence mid-stage rehabilitation) and finish mid-stage
out high quality early and mid-stage rehabilitation, patients rehabilitation.
often do not overcome major aspects of dysfunction which
limits knee function and ability to transition through late-
stage rehabilitation and RTS training optimally. So, getting 3 Important ‘Mid‑stage’ Rehabilitation
mid-stage rehabilitation right is essential to optimising Considerations
patient outcomes after ACLR.
There is a lack of published recommendations on The main considerations for mid-stage rehabilitation can
‘how to optimise’ mid-stage rehabilitation. Therefore, we be grouped into three categories: (1) muscle strength, (2)
wrote this paper to accompany our previous work which movement quality and (3) fitness re-conditioning (Fig. 2).
Optimising the Mid-Stage of ACL Reconstruction

Of course, the stage also considers knee factors such as In the quest to recover knee extensor strength post-ACLR,
joint range of motion, effusion/swelling control, pain man- it is essential to utilise effective programme planning, as well
agement and joint stability, as well as considering the psy- as additional modalities to in part overcome the effects of
chology of the athlete (e.g. motivation, apprehension, etc.). AMI and the load compromised joint to support the restora-
Knee factors are though the predominant focus of earlier tion of muscle size, voluntary activation and strength. One
stage rehabilitation, and like psychology are then running such modality is blood flow restrictive training, which can
themes throughout the functional recovery process after support more optimised muscle hypertrophy and strength
ACLR. gains during the mid-stage, as this allows resistance training
in those load compromised athletes at lower loads [40, 42,
3.1 Muscle Strength 43]. It is also recommended to treat AMI and use a range of
modalities to facilitate increased neuromuscular activation.
3.1.1 Knee Extensor Strength Firstly, following the criteria to enter mid-stage rehabilita-
tion is essential, to avoid loading a painful and irritable un-
The key priority of mid-stage rehabilitation is the restoration prepared knee. In addition, managing pain and swelling is
of knee extensor muscle strength in a timely fashion. Injury important, as they will negatively affect joint proprioception
and surgery result in large deficits of quadriceps muscle vol- [44, 45], as well as result in neuromuscular inhibition via
ume, neural activation and strength [29]. Residual deficits the AMI process [46–49]. Thirdly, incorporating a range of
in knee extensor strength after ACLR are associated with techniques early during the mid-stage to target AMI within
poor biomechanics [30], reduced knee function [31, 32] and the session and support increased neuromuscular activation
increased knee osteoarthritis risk [33], as well as elevated during resistance training is recommended. Ice and trans-
risk of knee re-injury [5]. Knee extensor strength by the end cutaneous electrical stimulation have been shown to tempo-
of mid-stage rehabilitation should be within 20% of the con- rarily reduce the AMI effects of swelling [50, 51]. Finally,
tralateral limb, which provides the muscular strength foun- the utilisation of neuromuscular electrical stimulation can
dation on which to commence late-stage rehabilitation [11, support the recovery of knee extensor strength after ACLR
34]. Deficits greater than 20% are associated with reduced [52], and allows for the direct activation of the motor axon,
knee function and movement compensations during high and could allow for the direct recruitment of the inhibited
load activities (e.g. jumping and hopping] [30]. motoneurons. Muscle activation by means of neuromuscular
Extensive research indicates that most patients are unable electrical stimulation also allows for the recruitment of a
to sufficiently restore quadriceps strength after ACLR [29, greater proportion of type II muscle fibres when compared
35–37], with more than half of patients experiencing a defi- with voluntary contractions of a similar intensity [53–55]
cit greater than 10% versus the contralateral uninjured limb and, consequently, supporting more balanced recovery of
at the time of RTS [14, 29, 37–39]. Failure to achieve less muscle hypertrophy across all muscle fibres (e.g. avoiding
than a 20% difference versus the contralateral limb is com- preferential type I muscle hypertrophy in the presence of
mon at 6-month post-ACLR [29]. high-threshold motor unit inhibition).
Difficulties in restoring knee extensor strength appear due We also recommend the inclusion of more ‘isolated’ open
to arthrogenic muscle inhibition (AMI) present after injury and closed chain knee extensor strengthening techniques
and surgery, which remains and limits adaptations in knee (e.g. knee extension/leg press) as opposed to use of ‘func-
extensor muscle volume and strength [29, 40]. AMI typically tional strengthening’ (e.g. squatting, deadlifting, step-ups,
limits the ability to achieve desired neuromuscular activa- lunges) during most of the mid-stage, but particularly in the
tion and intensity levels for an optimal stimulus for strength earlier periods of the mid-stage [40]. Functional strength is
training adaptations, and is often present bilaterally follow- the ability to produce force in situations in which muscles
ing unilateral ACLR and, in some cases, can be equivalent are commonly used, whilst isolated strength tasks (i.e. knee
to the injured limb [41]. extension on the isokinetic machine) minimise the require-
ments for neural control to develop the muscle’s ‘capacity’

Fig. 1  The functional recovery process involving a criterion-based progression of five stages of pre-operative, early-, middle- and late-stage
rehabilitation and return to sport training (RTS)
Table 1  Recommended criteria for progression from the early- to mid-stage of ACL rehabilitation programme
Outcome measure Test Goal Reason for meeting criteria

Pain Numeric rating scale of pain 0–2 (knee specific). Tolerance to higher Pain along with swelling has a profound effect on joint
pain in non-specific area may be accept- proprioception [44, 45] as well as result in neuromuscu-
able (e.g. due to scar tissue) lar inhibition via the AMI process and resultant muscle
atrophy and weakness [46–49]
Effusion/swelling Stroke test [163] Zero to trace effusion Changes in knee joint effusion are frequently associated
Zero: No wave produced on downstroke with irritation of intra-articular structures and articular
Trace: Small wave on medial side with downstroke disorders in clinically active knees [167]. Swelling can
1 + : Large bulge on medial side with downstroke result in AMI, cause pain and prevent optimal range of
2 + : Effusion spontaneously returns to medial side after motion. It is also typically a sign of joint overload and a
upstroke joint reaction to loading. If the knee is swollen, it will not
3 + : So much fluid that it is not possible to move the effu- respond to higher loading and will also prevent optimal
sion out of the medial aspect of the knee recruitment of the knee extensor muscles, limiting the
ability to progress resistance training
Passive knee extension Prone hang test [168] Straight knee (0°) Restoring joint range of motion is a vital aspect of the
Subjects lie prone on a treatment bed with the lower rehabilitation process. Even small losses of knee extension
legs off the end allowing full passive knee extension. (3–5%) appear to adversely affect subjective and objective
The heel height difference is measured (approximately outcome markers later in the rehabilitation phase [34]
1 cm = 1°)
Passive knee flexion Supine or prone with long arm goniometer [169] At least 120° of knee flexion [25] Restoration of joint mobility is critical for the recovery of
normal or optimal gait biomechanics and proprioception.
Normal or optimal gait biomechanics cannot occur without
normal or optimal accessory (spin, glide) and physiologi-
cal (extension, flexion) joint motion [170]
Quadriceps recruitment Full quadriceps activation [25] Ability to sufficiently recruit the quadriceps Quadriceps inhibition can prevent recovery of quadriceps
(no quadriceps sag on single leg raise muscle strength and the safe and expedient progression of
through 10 repetitions) [25] rehabilitation [50, 171]. Persistent quadriceps lag on single
leg raise has been shown to indicate an inability to actively
fully extend the knee. If this is not achieved by week 5
post ACL reconstructive surgery, this would be considered
a predisposing factor for significant quadriceps weakness
at 6-months post-operation [164]
Walking gait Visual assessment of walking gait Sufficiently normalised gait without aid [25] Abnormal gait patterns have been associated with joint
weakness [172], low patient satisfaction with outcome
after surgery [173] decreased functional performance
[174] and post-operative complications including knee
osteoarthritis [175]. Abnormal gait patterns often become
further exacerbated as the athlete returns to running [161].
Thus, re-establishing normal gait early, as well as safely
after ACLR is a key priority

Each outcome measure, the specific test and goal as well as the justification for these criteria are presented
AMI arthrogenic muscle inhibition
M. Buckthorpe, F. Della Villa
Optimising the Mid-Stage of ACL Reconstruction

Fig. 2  A representation of the three important priorities of mid-stage rehabilitation training including muscle strengthening, movement training
and fitness re-conditioning and the sub-sections

to produce force [56] and do not mimic the way in which However, whilst there is a large quadriceps weakness and
the muscles function [57]. This is because when a patient AMI, it is recommended to utilise to a much greater extent
has large knee extensor strength deficits, they will adopt isolated strengthening than functional strengthening. More
movement patterns in which they ‘cheat’ and utilise the hip isolated open and closed kinetic chain exercises will reduce
extensors instead of the knee extensors [58, 59]. Even when the task degrees of freedom and support the minimisation of
achieving the optimal kinematics (e.g. correcting the com- cheat strategies, enabling more targeted work of the muscle
pensatory movement pattern of greater hip to knee flexion), group. This should also be accompanied as discussed with
there is still typically inhibition of the quadriceps, resulting additional modalities (e.g. blood flow restrictive training,
in lower neuromuscular recruitment, which may result in neuromuscular electrical stimulation). The balance of iso-
insufficient stimulus for adaptation [58]. This may explain lated to functional strengthening will then gradually change
the large residual deficits in knee extensors strength reported over time during the stage and reverse during the late-stage
in the literature after ACLR [14, 29, 36–38], even with elite of rehabilitation (i.e. a stronger use of functional to isolated
level athletes [39]. This is not to say functional strengthen- strengthening during the late-stage rehabilitation process)
ing is not important during the mid-stage, as it is essen- [11].
tial to develop both the isolated capacity of the muscle to Although there are doubts about the safety of open kinetic
produce force, as well as the intermuscular coordination to chain exercises, this is arguably unsupported by substan-
express this capacity during functional tasks [57, 60–62]. tial published evidence [63]. It is important though to avoid
 M. Buckthorpe, F. Della Villa

undue stress on the healing ACL graft through performing maximum (RM) would be complemented with 3–4 sets of
strengthening exercises at specific knee angles (e.g. 45°–90° 3–5RM for the uninjured side. Recent evidence suggests that
open kinetic chain knee extension, [34, 64]) and carefully high-intensity eccentric training of the contralateral limb
incorporating open kinetic chain strength exercise in general may be more effective than concentric training, in terms of
(and certain closed kinetic chain exercises). the cross-education benefit [69].
One major issue in the mid-stage is that many clinicians It is also recommended when measuring knee extensor
often ignore the contralateral uninjured limb, training only strength as part of the functional recovery process, or prior
the injured side. It is important to recognise that neuro- to RTS to consider both the relative (e.g. LSI) and absolute
muscular function deficits following ACLR are typically strength of the injured limb. We recommend, based on both
bilateral, in which the contralateral limb is weaker than evidence [11, 14, 70] and clinical experience, an LSI of 80%
its pre-operative values [23]. For example, only 29% of knee extensor strength (compared to a ‘preserved’ contralat-
patients achieved a limb symmetry index (LSI) greater eral limb) and > 2 Nm kg−1 peak torque on the isokinetic
than 90%, when the reconstructed limb was compared to machine (90° s−1) (80% of 2.5 Nm kg−1) be achieved prior
pre-surgery strength values at 6-month post-ACLR (note, to progressing to the late-stage rehabilitation and RTS pro-
pre-surgery, not pre-injury), whilst 57% were able to gramme [11].
restore the injured limb’s strength to within 10% of the
uninjured limb (i.e. the conventional LSI) [23]. The con- 3.1.2 Knee Flexor Strength
tralateral limb often serves as a ‘control limb’ on which
targets for the injured limb are based, both for progression Perhaps the second most important aspect of mid-stage
through the functional recovery approach (e.g. LSI > 80% rehabilitation is the recovery of knee flexor strength. Most
for progression to late-stage rehabilitation [11]), as well studies [71–75], report hamstring strength deficits which
as for RTS (> 90–100% LSI depending upon the sport). can persist for many years after surgery, with deficits of
Avoiding training the uninjured limb will likely result in between 0 and 20% at time of RTS [71–75]. Kyritsis et al.
an atrophy and strength loss, resulting in an earlier res- [10] showed a 10.6-fold greater risk of ACL re-injury after
toration of the LSI during the mid-stage (i.e. easier to ACLR for every 10% decrease in knee flexor to extensor
achieve 80% LSI, as the target is effectively lower) and strength ratio of the injured limb in professional football
result in the patient being under-prepared to tolerate the players.
higher loading demands of the subsequent programme (i.e. Hamstring strength recovery is particularly relevant
a sufficient LSI, but low levels of absolute strength). Fur- in those who have ACLR with hamstring graft, as they
thermore, RTS without sufficient training of the uninjured appear to show selective muscle atrophy, with the semiten-
limb will result in over-estimated knee function of the dinosus of the surgically repaired limb being significantly
injured limb, and under-preparedness of both limbs, likely smaller (10–28%) [76–79]. Semitendinosus atrophy from
increasing the risk of re-injury for both sides. The high graft usage is not accompanied by compensatory graci-
rates of ACL injury on the contralateral side after RTS lis hypertrophy [72], which may show up to a 30% defi-
following ACLR [65] are a particular concern. cit in muscle volume [77, 79]. This is often accompanied
Our advice is to include strength training for both limbs by reduced knee internal rotator strength [80]. It should
as part of the ACL functional recovery process. There is be considered that semitendinosus tendon regeneration
some evidence that training the contralateral limb can also after ACLR may take approximately 18 months [81] and
result in strength gains for the injured limb, via the cross- may not occur at all in 10–50% of patients [72, 78, 80].
education phenomenon [66–68]. The strength training Rehabilitation during this time and for individuals with no
approach to the contralateral limb should aim to preserve, tendon regeneration would presumably not load the sem-
not enhance, the strength and muscle size. Developing mus- itendinosus significantly and evidence indicates selective
cle size and strength of the contralateral limb beyond its neuromuscular inhibition of semitendinosus during high
pre-injury values will result in greater difficulties normalis- force contractions [82], which likely results in insufficient
ing LSI. However, maintaining strength on the uninjured stimulus following resistance training and persistent mus-
side, recovering the desired LSI (strength of the injured limb cle size and strength deficits. This selective inhibition may
versus the uninjured limb) as quickly as possible (mid- and require the semimembranosus to compensate to maintain
late-stage), prior to then adopting a conditioning approach optimal transverse plane control of the knee. After ACLR
to both limbs (e.g. RTS training) is recommended. Dur- with hamstring graft, there is often compensatory biceps
ing the mid-stage, this would involve performing the same femoris hypertrophy [83], which results in a reduced inter-
exercise on the uninjured as the injured limb, but doing so nal to external knee rotation strength ratio [83] and likely
at higher intensities and much lower volumes. For exam- contributes to the increased external tibial rotation seen dur-
ple, 6 sets of leg press for the injured side at 12 repetition ing running in ACLR patients [84]. Thus, we recommend
Optimising the Mid-Stage of ACL Reconstruction

that the addition of hamstring exercises which elicit more medius and maximus) may be a risk factor for ACL injury
selective medial hamstring muscle activation (e.g. Nor- [98]. The gluteus maximus is thought to become ‘inhibited’
dic hamstring curl [85], hamstring exercises performed (defined as reduced activation or delayed onset) after lower
with tibial internal rotations [86]) be incorporated to tar- limb injury [104, 105] and is an important muscle alongside
get potential residual deficit in medial hamstring muscle other gluteal muscles (gluteus medius and gluteus minimus)
size and strength. A holistic approach to hamstring muscle in preventing dynamic knee valgus during high load closed
strengthening [56, 87, 88], incorporating both knee and hip chain tasks [106, 107]. In addition, weakness of the glu-
dominant exercises is also recommended for all patients. teal muscles can contribute to altered movement patterns
In those with hamstring graft, a periodised resistance pro- which increase knee and ACL loading and are thought to be
gramme similar to the knee extensors should be adopted, important risk factors for ACL injury. Weakness of gluteal
and knee flexor strengthening delayed for 6–8 weeks after muscles would be expected to lead to a more upright and
surgery to allow healing of the harvested graft [64, 89, 90]. laterally positioned trunk during high load movements, to
Hamstring strengthening should commence with isometric position the centre of mass closer to the hip and thus reduce
knee flexor exercises as well as low to moderate intensity the requirements on the gluteal musculature. This would lead
hip extension exercises. Those without hamstring graft can to the centre of mass position further away from the knee
be less cautious, respecting the load capacity of the knee as and thus higher loads on the knee in the sagittal plane, as
a whole. Higher intensity pain-free hamstring strengthen- well as lateral shifting of centre mass, achieving a resultant
ing should be able to be commenced towards the end of vector line lateral to the knee joint, causing a knee abduc-
the mid-stage, with at least an 80% LSI by the end of the tion moment [101]. Thus, we recommend a strong focus on
mid-stage. After this, a stronger focus on high intensity, addressing dysfunction of the gluteal muscles during mid-
eccentric, high speed, longer muscle length and functional stage rehabilitation, as well as considering the trunk, pelvic
(e.g. higher speed running) strengthening should occur. and hip musculature in general (e.g. hip adductor strength,
trunk muscle recruitment and endurance).
3.1.3 Adjacent Joint Strength
3.1.4 Closed Kinetic Chain Strength
As well as the muscles about the knee, it is important that an
aspect of mid-stage rehabilitation be focused both distally As well as muscle/joint specific strength (e.g. knee exten-
and proximally to the knee joint. Deficits in plantar flexor sors/flexors), it is also important to have good closed kinetic
strength and muscle strength about the hip and lumbo-pelvic chain strength. The ability to perform functional tasks
region can occur and impact neuromuscular performance involves the ability of the neuromuscular system to develop
and movement quality. force (e.g. strength) [108], during certain movements. New-
Firstly, a key muscle group to consider restoring after tonian mechanics implies that movement acceleration is a
ACLR is the triceps surae. The soleus and gastrocnemius function of force applied/body mass (force = mass × accel-
are important contributors to muscle force generation dur- eration) and as such relative strength is theoretically impor-
ing running, particularly at speeds less than 7 m s−1 [91]. tant for optimal movement performance. In addition, there
In addition, the ankle joint eccentrically accepts around is good association between closed kinetic chain strength
40–50% of the impact forces when landing [92]. The soleus (e.g. isometric or dynamic squat strength) and athletic per-
muscle in particular acts as an agonist to the ACL, prevent- formance during sporting tasks such as jumping and sprint
ing anterior tibial translation, through restricting the shin running and change of direction ability [109–111]. In the
moving anteriorly in relation the knee, by providing restraint context of mid-stage rehabilitation, it is important to restore
to tibial advancement at the ankle [93, 94]. Schlumberger the necessary closed kinetic chain strength to support transi-
[95] reported an average of 8% deficit in calf muscle strength tion to more demanding functional tasks undertaken during
6 months following ACLR, which may be higher when con- the mid-stage (e.g. single leg squats, bilateral landing tasks)
sidering the contralateral strength deficits which are nor- and ultimately to late-stage rehabilitation [11]. For example,
mally present after injury [23]. bilateral landing, treadmill-based running and single limb
Additionally, a key aspect of ACL rehabilitation should plyometric tasks typically involve ground reaction forces
be to address strength deficits of the lumbo-pelvic-hip mus- of 1–1.5 [112], 2–3 [113] and 2–6 [112, 114, 115] times
culature. Weakness of certain muscles in the region has body mass, respectively. Therefore, developing and under-
been retrospectively or prospectively associated with lower standing the lower limbs ability to produce and accept force
limb and/or ACL injuries [96–103]. A systematic review by can provide the necessary foundation and understanding on
Petersen et al. [100] revealed deficits in hip muscle strength when they may be ready to perform these potentially danger-
after ACLR. In particular, weakness or reduced activa- ous exercises after ACLR. Inability to produce or eccentri-
tion of the hip abductors and external rotators (e.g. gluteus cally dissipate these forces via the neuromuscular system
 M. Buckthorpe, F. Della Villa

(i.e. insufficient functional eccentric muscle strength of the and joint proprioception. Given the residual deficits in move-
lower limb) would result in movement compensations [30] ment quality at the time of RTS, it would appear current
and/or overreliance/acceptance of the passive restraints such rehabilitation programmes do not effectively target aberrant
as ligament, joint complexes and fascial system, potentially movement patterns sufficiently [129].
resulting in either chronic overload [116, 117] and/or acute Altered movement quality is thought to be due to multi-
injuries such as graft failure due to ligament fatigue [118, ple factors including muscle imbalances/weakness (e.g. knee
119]. Having optimal task progressions according to both extensor weakness, [30]), altered posture (e.g. anterior tilted
task complexity and loading parameters is recommended as pelvis, [130, 131]), arthrokinetic dysfunction (e.g. reduced
too is developing sufficient closed kinetic chain strength to dorsi-flexor range of motion, [132]), altered reciprocal mus-
tolerate these tasks. It is recommended to be able to tolerate cle inhibition and synergistic dominance [133, 134] and
comfortably 1.5 times body mass single limb (e.g. leg press altered proprioception [135, 136]. Consequently, incorporat-
or single leg isometric squat, peak torque or predicted 1RM) ing corrective exercises to address these factors is important
before progression to late-stage rehabilitation. Increased to help establish more optimal movement quality. This would
focus on closed chain exercises to develop closed chain include a programme of re-activating and/or strengthening
strength as well as knee extensors strength is needed again weak/inhibited muscles as well as muscle release techniques
through a periodised resistance training programme within and flexibility training of over-active/tight muscles.
mid-stage rehabilitation. Patients may also move sub-optimally due to limiting
understanding of the tasks (low knowledge of movement)
3.2 Movement Quality or altered coordination during the task. So, practicing func-
tional tasks (with optimal coaching strategies) and re-learn-
Multiple studies have identified altered movement quality ing optimal technique are needed to reorganise skills fol-
(e.g. the ability to control the limb, maintaining balance and lowing a period of neuromuscular/corrective training [62].
optimal kinematics during movement) in the involved limb Motor learning is defined as the process of an individual’s
of both male and female ACLR patients when compared to ability to acquire motor skills with a relatively permanent
both their control limb and to uninjured controls during an change in performance as a function of practice or experi-
array of functional exercises [120–123]. Disruption to the ence [137]. Therefore, to gain expertise and induce a motor
native ACL leads to mechanical instability of the knee and learning adaptation, a skill must be practiced repeatedly. We
can alter neuromuscular control due to disrupted mechano- advise to incorporate optimal task progression based on both
receptors within the ligament [124] and altered somatosen- complexity and load for foundation motor pattern retraining.
sory input and joint proprioception. The resultant decrease This should include a progression from bilateral to single leg
in joint position sense and kinaesthesia, along with nocicep- tasks, with progression based on sufficient technique profi-
tor activity associated with pain and swelling, may poten- ciency and strength development (Fig. 3).
tially impair movement quality [125]. It appears that an ACL It is important prior to adding load that the optimal tech-
injury results in altered movement quality bilaterally, when nique is achieved, and that specific underlying dysfunc-
compared to pre-injury movement quality [120]. Altered tion has been addressed to ensure more optimal movement
movement quality has been associated with increased risk quality and adaptation from the programme. Utilisation of
for ipsilateral or contralateral secondary injury and the coaching techniques and use of bio-feedback through video
development of early onset of osteoarthritis of the knee joint analysis to maximise cognitive understanding are highly rec-
[126–128]. Paterno et al. [128] linked altered movement ommended. Furthermore, incorporating strategies to maxim-
quality prospectively with secondary ACL risk. The authors ise motor learning is encouraged including (1) adopting an
identified four predictive factors for secondary ACL risk in external focus of attention; (2) implicit learning; (3) differen-
a group of young female athletes, including increased knee tial learning and (4) self-controlled learning and contextual
valgus, asymmetry in internal knee extensor moment, signal interference [138].
leg postural stability and opposite hip rotation moment as
significant predictors of re-injury risk. In addition, Paterno 3.3 Physical Fitness Re‑conditioning
et al. [129] demonstrated that those with deficits in postural
control during functional type tasks (assessed using a single Successful RTS requires not only resolving impairments
leg balance type task) were at heightened risk of secondary at the knee, but also restoring neuromuscular function,
injury following ACLR. Thus, establishing symmetrical and sports-specific movement quality and sport-specific readi-
optimal movement quality in basic motor patterning tasks is ness (fitness, technical training and load readiness) [22,
important to ensure the right movement foundation on which 139, 140]. To achieve this, we need to think about ‘return
to retrain more demanding movement tasks. In addition, it to performance’ throughout the functional recovery process
is also important to restore/develop single control/balance [21, 22]. The rehabilitation and RTS process after ACLR
Optimising the Mid-Stage of ACL Reconstruction

are long (typically 6–9 months) [13] and offer an opportu- etc.), low or no load CV training (e.g. deep water running in
nity to develop an athlete’s physical fitness to higher levels swimming pool, stationary bike, cross-trainer, alter-g, etc.)
than before the injury, as long as it is appropriately planned. and additional corrective strengthening (e.g. extra ‘lumbo-
Additionally, there is also an opportunity to address physi- pelvic-hip strength session’ focusing on areas of weakness).
ological impairments which may have been limiting sport
performance (e.g. muscle morphology/upper body strength),
and increasing their susceptibility to injury or just previously 4 Recommendations for Implementation
unresolved aspects of function from previous injuries (e.g.
chronic groin pain). Although late-stage rehabilitation and In designing the mid-stage approach, it is important to focus
RTS training largely involve physical fitness re-conditioning on the goals/priorities and allocate training time according
[11], it is important that an athlete commences late-stage to these different training goals. The exact work and allo-
rehabilitation with a sufficient physical fitness profile to allow cated time on each training goal and in each environment
for intense training to achieve this higher level of athletic depend on the individual, their goals and actual time com-
performance. As such, an aspect of mid-stage rehabilitation mitment. In addition, certain factors should be considered
should be in avoiding detraining by incorporating ‘physical when rehabilitating patients with different graft types [34],
fitness reconditioning’. Evidence suggests that football play- as well as concomitant injuries such as meniscal injury,
ers’ cardiovascular (CV) fitness is lower at 6 months after chondral defects or multi-ligament injuries [34]. Mid-stage
ACLR than pre-injury values [141], indicating a need to bet- rehabilitation involves a big transition in terms of functional
ter optimise the training of this important physical fitness capacity of the injured athlete, in which they are transitioned
parameter. This could in part be due to insufficient intensities from the point of being able to walk normally at the start of
in late-stage rehabilitation, but may also indicate an insuf- the stage to being required to be able to run with sufficiently
ficient stimulus to preserve aerobic fitness during the earlier normalised running mechanics towards the end of the stage.
stages. Key fitness re-conditioning goals will depend on the It is essential to achieve optimal loading (defined as the
athlete’s sport, profile and previous injury history. But for load applied to structures that maximises physiological
most athletes incorporating specific sessions of re-condition- adaptation [142]), during the functional recovery process,
ing that include a focus on body composition, upper body which will be specific to the injured athlete and will change
morphology and strength, and importantly CV conditioning, reflecting their functional capacity. In particular, the stimulus
are recommended. This typically involves either incorpora- should target the specific training goal optimally (e.g. mus-
tion of this work into their existing rehabilitation training or cle endurance, hypertrophy, strength, power), whilst avoiding
separate re-conditioning sessions, involving predominantly excessive loads which may adversely stress the healing joint/
non-load-bearing upper body strength work (e.g. bench press, ligament [64] and/or patellofemoral joint. The knee is often
seated shoulder press, bench or seated rows, dips, pull-ups, load compromised throughout the mid-stage of rehabilitation

Fig. 3  A outline of our single leg squat task progression, beginning with bilateral unloaded squat, goblet squat, split squat, lunge, step up and
finally single leg squat. Progression is based on movement quality and ability to handle additional loading during the task
 M. Buckthorpe, F. Della Villa

and thus, cannot tolerate excessively high forces (e.g. very of the athlete to be performed. The first half of activity
high-intensity resistance training, 85–90%+, 5RM). Exces- recognises the load compromised athlete often with severe
sive forces on a compromised knee joint would result in over- quadriceps weakness and dysfunction, unable to perform
load and pain and swelling and could potentially stretch or the majority of functional tasks on land (Table 2). During
loosen the ACL graft [64, 143]. Thus, the functional recovery this stage, we recommend the use of lower intensities of
programme requires careful planning and well-designed pro- resistance (e.g. 12–20RM), and a stronger focus on non-
gressions of increasing challenge. weight-bearing exercise (including lower limb and lumbo-
Current evidence concerning training theory in unin- pelvic-hip exercises) and machine-based strengthening tasks
jured individuals reports that strength adaptations are (e.g. leg press, knee extension). We recommend that this
achieved across a range of training intensities (40–95% is accompanied by additional modalities to overcome the
maximal intensity) [144]. It is typically recommended that negative effects of AMI, allowing for higher neuromuscular
40–60% neuromuscular activation is needed at a mini- activation (e.g. neuromuscular electrical stimulation), and/or
mum for a strengthening effect [145], although there is a supporting the accumulation of metabolic stimuli (e.g. blood
dose–response relationship with greater gains in strength flow restrictive training). Although this lacks task specificity
from exercise which elicit higher neuromuscular activation and transference to functional exercises/movement [156], as
values [146–148]. The American College of Sports Medicine described, it can often target a specific muscle group better
recommends loads of 60–70% 1RM for the development of in isolation. We still recommend the use of functional exer-
muscle strength and 70–85% for hypertrophy [149]. Tradi- cise during this time, but mostly with the goal of teaching
tionally, it was believed that very high loads were necessary optimal technique as well as improving intermuscular coor-
to bring about activation of all type II motor units based on dination [60, 61], as opposed to for enhancing muscle size
the Henneman size principle [150] and achieve full and com- and strength. During this first block of training, the athletes
plete muscle hypertrophy (targeted at all motor units). How- will be unable to perform the majority of functional tasks
ever, it is suggested that more low-load training also recruits on land. As such, where possible we recommend utilising
fast-twitch muscle fibres and can achieve muscle hypertrophy the hydrotherapy/swimming pool to practice functional
and strength gains, provided that the working set is continued movement tasks such as lunging, squatting, as well as deep
close to volitional fatigue [151]. This would result in activa- water running (as well as the other benefits including active
tion and fatigue of the respective motor units, with the addi- joint range of motion, gait and CV conditioning with deep
tion of progressively larger motor units as the set progresses. water running). In water, the buoyancy force controls the
As such, although not optimal for muscle hypertrophy, low downward (landing) movement of the body, thus generating
load to fatigue can be used as a strategy for hypertrophy higher upward (concentric) and lower downward (eccen-
during the earlier stages of ACL rehabilitation, when the tric) forces. At the appropriate depths, there is also around
knee is load compromised. The efficacy of low to moder- a 45–60% reduction in body weight, allowing the earlier
ate load strengthening exercises (≤ 70% maximal intensity) practice of functional tasks at lower loads. This would allow
for developing maximal eccentric strength and rate of force for a learning effect which can then be more applied on land,
development is, however, questionable, thus requiring higher when the knee joint can tolerate higher loads (see supple-
load training for full optimisation of neuromuscular func- mentary video for examples of the exercises which can be
tion [152–155]. As such, there is a need to incorporate a performed in the pool as part of rehabilitation after ACLR).
periodised resistance training programme with increasing During the second block of training, we encourage a pro-
task difficulty. During the mid-stage, we recommend ini- gression to slightly higher intensities of strength training,
tially beginning with low- to moderate-intensity resistance typically 8–12 RM or 70–80% maximum for the injured
(e.g. 12–20 RM), focused predominantly on muscle work limb [40]. Furthermore, including progressive land-based
capacity and hypertrophy [40], targeting metabolic stimuli movement retraining and functional strengthening is now
for adaptation, as opposed to high mechanical stimulus or encouraged, as well as a combination of non-weight-bearing
muscle damage, when the joint is highly load compromised and weight-bearing lumbo-pelvic-hip exercises. The pool
[40]. There should then be a progressive increase in intensity can again be used where available to allow for the inclusion
and volume to support more optimised muscle hypertrophy of higher load movement activities such as single limb land-
and strength recovery, respecting the knee joint load toler- ing, as well as plyometric tasks. Plyometric type activity in
ance. Very high intensities are often contraindicated in this water versus land appears to result in reduced joint inflam-
stage, and more suited for late-stage rehabilitation, when the mation and perceived pain [157, 158], but similar gains in
knee can tolerate higher loads [11]. concentric power development [114, 115]. This is typically
In light of the above, we recommend splitting the mid- though done only in those undertaking a pro-athlete pro-
stage of rehabilitation in two separate halves/blocks of train- gramme in our clinics. Functional task progressions on land
ing, to allow specific work relative to the functional status
Table 2  The important priorities of mid-stage rehabilitation, the specific training approach to target the priority and the approach specific to the half of the stage
Priorities Training approach/goal 1st half of mid-stage 2nd half of mid-stage

Muscle strengthening
Knee extensor strengthening Resistance training to restore muscle endur- Low to moderate load OKC (initially at Moderate load strengthening (8–12 RM) in
ance, work capacity, hypertrophy and restricted range, 50°–90°) and CKC OKC and CKC (isolated [e.g. leg press] and
strength (within 20% of the contralateral (predominantly isolated e.g. leg press) functional [e.g. squat, lunge]) focused on
limb) of the quadriceps muscles strengthening exercises (12–20 RM) focus- muscle hypertrophy (and strength) through
ing predominantly on muscle endurance metabolic, mechanical stimuli and muscle
and hypertrophy through metabolic stimuli. damage mechanisms. Supplementary use of
Supplementary use of NMES and BFRT NMES and BFRT
Hamstring strength/activation Restore hamstring strength (within 20% Depending upon graft type—introduce hip Progress to isokinetic, isometric and concentric
contralateral side or pre-injury values), reac- dominant exercises, incorporate isometric strengthening. Knee and hip dominant exer-
tivate and strength the medial hamstrings, knee dominant exercises at lower loads cises at higher exercise intensities (8–12 RM)
Optimising the Mid-Stage of ACL Reconstruction

restore muscle hypertrophy and knee control Incorporate rotational control and strengthen-
ing drills
Closed kinetic chain strength Develop the strength of the kinetic chain to Introduce closed kinetic chain strengthening CKC strengthening to develop leg press
facilitate the ability to undertake functional at low to moderate loads, leg press isometric strength (8–12 RM)
exercises and more high load knee control in and dynamically (12–15 RM) Use of functional training for both motor re-
late-stage rehab patterning and closed kinetic chain strength
development
Calf muscle strengthening Restore plantar flexor strength to facilitate Bed-based/seated plantar flexor strengthening Weight-bearing calf raises employing dynamic
optimal control and strength to support the and bilateral standing calf raises actions at 8–12 RM. Example, 4 sets of 10
knee and for propulsion during running and single leg calf raises, with control eccentric
eccentric control in deceleration lowering and optimal foot and ankle control
Gluteal muscles strengthening Reactive and strengthening the gluteal mus- Non-weight-bearing muscle re-activation and Mix of WB and NWB exercises including sin-
cles. The gluteus maximus in particular is endurance exercises (e.g. clam, BL bridge/ gle leg bridges, loaded hip thrusts (12 RM),
prone to inhibition after injury hip thrust, side laying abduction) standing clam with band, lateral band walks,
standing hip extension drills, as well as closed
chain triple extension drills (e.g. RDL, good
morning, lunge)
NWB/isolated ‘lumbo-pelvic strength endur- Reactive and integrate the local core muscle Low load local stabiliser re-activation and Neuromuscular control in stance and during
ance’ which may have become inhibited neuromuscular control functional movements (e.g. pelvic and trunk
control in load-bearing exercises)
Adductor muscle strengthening Restore strength of the adductors to facilitateIsometrics/squeezes to reactive and strength- Progress and include dynamic adductor
late-stage rehabilitation ening the adductors beginning with the strengthening (ECC, CON and ISO)
knee bent avoiding long lever exercises in
hamstring graft patients
Hip flexor muscle strengthening Restore hip flexor muscle balance and strength Bed-based isolated superior hip flexor Weight-bearing and seated and loaded hip
strengthening and leg lifts with optimal flexor strengthening with machines, manual
knee control and use of bands/isoinertial training
Movement quality
Muscle release techniques Manual therapy to target over-active and Stronger focus on manual therapy with bed- Shift towards preparatory foam rolling prior to
tight muscles as part of holistic approach bed treatment, targeting RF, TFL, psoas, land-based movement re-education sessions;
to movement re-education, performed prior hip external rotators, lateral hamstring, VL, massage on recovery days with professional
to stretching and neuromuscular activation/ triceps surae athlete (pro-athlete schedule)
training
Table 2  (continued)
Priorities Training approach/goal 1st half of mid-stage 2nd half of mid-stage

Muscle flexibility Correct muscle length issues to ensure joint Use of hydrotherapy Dynamic flexibility in the pool
range of motion and muscle inflexibility do Strong focus on flexibility for joint range of Push to achieve optimal mobility/flexibility at
not compromise coordination and motor motion lumbo-pelvic region, knee and ankle (moni-
patterning Resolve muscle length asymmetries and toring hip range of motion and dorsi-flexion
muscle tension R.O.M)
Ensure optimal muscle tension of hip flexors
Balance and proprioception Restore basic static and dynamic balance Bilateral and unilateral static and balance Unilateral dynamic balance
(instance) to provide foundation for more Unilateral landing control in the pool (> 1 m
complex movement tasks depth)
Movement retraining Correct underlying deficits and retrain optimal Largely use of the pool for motor pattern Landing drills in the pool
basic motor patterning in tasks such as retraining, DWR and unilateral movement Unilateral movement based progressions on
squat, lunge, hip hinge, step up and lunge. progressions (split squat, lunging squatting) land
Progress to running gait re-education on Gait re-education and bilateral squats on land Bilateral loaded drills (goblet, front squat at low
treadmill (limited motor training on land) to moderate loads)
Treadmill based running gait re-education
Physical fitness re-conditioning
Aerobic conditioning in non- or low-load Maintain aerobic fitness of the athlete to Aerobic continuous moderate intensity on Continuous and interval cardiovascular condi-
situations ensure a foundation of fitness on which to the bike and cross-trainer, introduction to tioning using cross-trainers, bike, and pool
commence late-stage rehabilitation moderate intensity DWR based DWR
Introduction to alter-g running for conditioning
Upper body strength training Incorporate UB strength training to develop Implement where possible on recovery days, Include 2–4 times per week depending upon
athleticism and provide a good stimulus for lower priority, non-weight-bearing upper sport and goals generally as separate sessions.
recovery between lower limb rehabilitation body strengthening (e.g. bench press, seated Varying stimulus depending on day (meta-
sessions shoulder press, lat-pull down, etc.) bolic focus on LB recovery day), include
standing lifts (e.g. shoulder press)

OKC open kinetic chain, CKC closed kinetic chain, RM repetition maximum, NMES neuromuscular electrical stimulation, BFRT blood flow restrictive training, BL bilateral, WB weight-bearing,
NMW non-weight-bearing, RDL Romanian deadlift, ECC eccentric, CON concentric, ISO isometric, RF rectus femoris, TFL tensor fasciae latae, VL vastus lateralis, R.O.M range of motion,
DWR deep water running, UB upper body, LB lower body, lat latissimus dorsi
M. Buckthorpe, F. Della Villa
Optimising the Mid-Stage of ACL Reconstruction

should include introduction to bilateral landing/jumping, as levels of physical fitness, which are easier to maintain and
well as treadmill-based running re-education. restore after injury and can normally be incorporated into
One important consideration is when and how to initiate their rehabilitation sessions in the gym. However, elite
treadmill-based running. Importantly, running is a high load athletes have a much higher physical fitness profile, which
task and requires substantial strength and neuromuscular needs to be maintained and requires a greater investment
control. Each step in running represents around 2–3 times in re-conditioning to preserve [160]. They often have too
body mass [113]. Effective implementation of running can many objectives to allow for effective planning in a single
serve as a useful training stimulus for developing strength session design and require a greater volume of training with
and neuromuscular control, but it is important a patient is more carefully planned activity. As such, we typically plan
adequately prepared to commence running. In deciding a professional programme across 10 sessions, split across
when a patient is ready to return to running, most studies use different environments depending on the stage (Tables 3, 4).
time-based criteria, with the median time being 12 weeks To support optimal progress through the programme,
[159]. The ability to perform specific tasks though like run- it is also important to objectively track that progress. It is
ning is not related to healing times, but more specifically to recommended to include a series of regular monitoring as
function. Fewer than 1/5 studies reported clinical, strength well as screening tests to track the progress of the athlete.
or performance-based criteria for return to running, even This should be based around the objectives of the stage and
though best evidence recommends performance-based crite- assess knee status and response to training (e.g. pain, swell-
ria combined with time-based criteria to commence running ing, IKDC subjective form, etc.), strength, movement qual-
activities following ACLR [159]. Most other criteria used ity and physical fitness. Optimal rehabilitation progressions
beyond time are similar to the entry criteria for mid-stage have to be in line with the biological healing and ability
rehabilitation (e.g. pain < 2, > 120° or full knee flexion, zero of the joint to withstand the loading demands. To achieve
or trace effusion and no instability of the knee/graft) [159] optimal loading, it is important to monitor the specific work
and so for clinicians this provides little indication of when and the knee’s response to loading. Pain and swelling can be
they can actually introduce running. The other possible use- used to determine exercise-based progressions as these fac-
ful criteria are an isometric knee extension and flexion LSI tors will relate to the loading stress experienced by the knee
of 70%, which should be adopted as this can provide some [161]. Measurement of knee circumference at the patella has
objective criteria to guide this decision-making process. been shown to be clinically relevant, with good reliability
These criteria though fail to understand a patients’ closed and sensitivity to change [162]. Changes greater than 1 cm
kinetic chain strength, strength about the adjacent joints, as were reported to be clinically significant, indicating pos-
well as movement quality during basic movement tasks. In sible exercise overload. Other techniques include assessing
addition, a patient should be exposed to running gait re-edu- knee effusion via the stroke test [163]. Furthermore, regular
cation using running type activity at lower loads (e.g. deep measurement of joint range of motion can facilitate progres-
water running or running on a trampoline and alter-G). They sion, and changes in range of motion may reflect the level of
should then follow an introduction to running programme joint effusion [164]. Pain can be monitored using a 10-point
which should include a walk-run and then gradual increase numeric rating scale (0 no pain, 10 worst imaginable pain),
in running speeds and time of running. We suggest achiev- which has been shown to be sensitive to changes in pain
ing a single leg closed kinetic chain peak strength of at least which affect function [165] with a reduction or increase by
1.25 times body mass, an LSI for knee extensors and flexors 1 point being regarded as the minimal clinically important
of greater than 70% [159], as well as good single leg squat change [166]. Furthermore, assessing/recording joint sore-
and bilateral landing movement quality as part of the return ness during the warm up can support more optimised within
to run decision-making process. This provides the patient session loading [34].
with guidance and targets, as opposed to time-based trial It is also important to have specific criteria or ‘targets’ to
and error. achieve by the end of the stage to allow for effective transi-
It is also important that mid-stage rehabilitation activ- tion into the late-stage rehabilitation and RTS programme
ity is effectively planned to ensure an optimal stimulus for (where applicable). When establishing criterion-based reha-
adaptation, as well as sufficient load variation and recovery bilitation it is important to understand the must haves ver-
within and between weeks (e.g. periodisation). For recre- sus nice to haves. Although certain aspects of function are
ational athletes, programme planning is often simpler, as important to be trained, they may not limit progression to
they typically train a maximum of 2–3 times per week, and late-stage rehabilitation. For example, CV training is impor-
as such require a focus on priorities, as well as typically tant, as too is upper body strengthening for athletes; how-
session replication. This would normally involve 2–3 days ever, if these aspects are not specifically trained and devel-
between sessions allowing sufficient recovery. Fitness re- oped within mid-stage rehabilitation, the patient can still
conditioning is less of a priority as they have much lower satisfactorily progress to more intensive rehabilitation, they
 M. Buckthorpe, F. Della Villa

Table 3  A breakdown of the typical week in the first half of mid-stage rehabilitation with a professional athlete
Time/session Mon Tues Wed Thurs Fri Sat Sun

Session 1/AM Gym: LB RT Gym: LB RT Gym: recovery, Gym: LB RT Gym - LB RT Gym recovery, Off
(muscle endur- (muscle endur- non-weight-bear- (muscle endur- (muscle endur- non-weight-
ance loads/iso- ance loads/iso- ing conditioning ance loads/iso- ance loads bearing condi-
metric strength- metric) and UB RT metric) dynamic/isomet- tioning and UB
ening) Additional modali- Additional modal- ric strengthen- RT
Additional modali- ties: BFRT knee ities: NMES ing)
ties: NMES extension Additional modali-
quads ties: BFRT leg
press
Session 2/PM Hydrotherapy Hydrotherapy Off Hydrotherapy Hydrotherapy Off Off

LB lower body, RT resistance training, NMES neuromuscular electrical stimulation, BFRT blood flow restrictive training, UB upper body

Table 4  A breakdown of the typical week in the second half of mid-stage rehabilitation with a professional athlete

Time/session Mon Tues Wed Thurs Fri Sat Sun

Session 1/AM Gym/MR—(LPH Gym (massage, Gym—(core/ Gym (massage, Gym—(core/ Optional—Gym Off
correctives and flexibility, gen- motor pattern- flexibility, motor pattern- (massage, flex-
motor patterning/ eral condition- ing/neuromuscu- general CV ing/neuromuscu- ibility, general
neuromuscular ing, UB RT) lar control) conditioning, UB lar control) conditioning,
control) RT) UB RT)
Session 2/PM Gym—LB RT Hydrotherapy Gym—LB RT Hydrotherapy— Gym—LB RT Off Off
(rehab, mechani- (movement and (rehab, meta- (movement and (rehab, muscle
cal stimulus/ CV) bolic stimulus/ CV) damage/ECC
higher loading) high volume strength training)
fatiguing)

MR movement room, LPH lumbo-pelvic-hip, UB upper body, RT resistance training, LB lower body, CV cardiovascular, ECC eccentric

just do so at a lower level of physical fitness. However, fail- relating to mid-stage rehabilitation after ACLR and should
ure to recover knee extensor strength or closed kinetic chain be considered alongside content on other stages (e.g. [11]).
strength will negatively affect their ability to perform high Mid-stage activity can be divided across (1) muscle strength-
load functional tasks, and potentially result in knee overload ening, (2) movement training and (3) fitness re-conditioning.
or even re-injury. Table 5 presents our recommended crite- It is important to plan and prioritise the activity according
ria, based on both evidence from the literature, as well as to the individual, and the main priority should be the safe
substantial clinical experience. The criteria are focused on and optimal recovery of knee extensor and flexor muscle
understanding if patients have achieved a minimum level of strength. The programme should be planned according to
knee function, muscle strength and neuromuscular control/ the functionality of the athlete, and we recommend splitting
movement quality to be prepared for entry into a late-stage the programme into 2 halves, with a greater focus on isolated
rehabilitation and RTS programme [11]. and non-weight-bearing corrective exercise during the first
stage, with support from hydrotherapy and a more functional
higher load land-based movement programme in the second
5 Conclusions half. Incorporating physical re-conditioning where possible
can reduce the detraining effect and provide a stronger physi-
There is a need to optimise the rehabilitation process after cal fitness foundation to commence more high-intensity late-
ACLR. This includes addressing the whole of the functional stage rehabilitation, supporting the achievement of higher
recovery process. This paper discussed important concepts levels of athletic physical performance than before the injury.
Table 5  Recommended criteria for progression from the mid-stage to the late-stage of rehabilitation after anterior cruciate ligament reconstruction. Each outcome measure, the specific test and
published reference are included, as well as the goal to achieve in order to transition to the next stage
Outcome measure Test Goal Reason for inclusion

Knee effusion Stroke test [163] Zero effusion with minimal activity related effu- Changes in knee joint effusion are frequently associ-
and Knee circumference measurements [162] sion (< 1 cm change patella) ated with irritation of intra-articular structures
Changes of greater than 1 cm in knee circumfer- and articular disorders in clinically active knees
ence at the patella are clinically significant, [167]. Swelling can result in AMI, cause pain and
indicating the levels of load applied were causes prevent optimal range of motion. It is also typi-
of joint stress cally a sign of joint overload and a joint reaction
to loading. The impact of loading can be measured
and monitored by changes in knee circumference
[167]. Thus, if the knee is reactive to mid-stage
rehabilitation, it may not react well to more
intense rehabilitative loading
Optimising the Mid-Stage of ACL Reconstruction

Knee joint ROM As for mid-stage entry criteria Full See Table 1
Knee extensor and flexor strength Isokinetic testing [176]—alternating knee exten- LSI > 80% for flexors and extensors and ideally Weakness in knee extensor strength will alter
sors and flexor concentrically at: a > 0.60 F/E ratio biomechanics, reduce functional performance, and
90° s−1 for 4 repetitions is linked to elevated re-injury risk and poorer RTS
180° s−1 for 20 repetitions outcomes [5, 30]. Achieving 80% is important
prior to commencing high load (e.g. single leg
deceleration, landing and plyometric) drills, as
lower values may result in movement compensa-
tions and interfere with the movement retraining
programme
Closed chain muscle strength Leg press strength test [25] At least 125% body mass for 8 reps or 1.5 × BM Having sufficient closed kinetic chain strength is
Leg press 90° knee flexion and seat at 45 degrees predicted 1 RM important. Weakness will result in insufficient
Maximal weight achieved for 8 RM test Peak force > 150% body mass strength for acceptance and propulsion during
or landing and jumping tasks and may result in
Isometric single leg squat [110, 177] altered movement quality [30]
Single leg stance 60° knee flexion on force plate
below a fixed bar as part of testing rig
Gluteal muscle capacity Single leg bridge test (variation [178]) Greater than 20 reps and within 5 of each side, Gluteal muscle weakness is associated with altered
Supine with knee angle at 90° flexion and feet on with no cramping of hamstring or adductor motor patterns specifically dynamic knee valgus
the floor and arms across the chest, lift up the magnus [106, 107] and weakness of external rotation
hips from the floor to neutral hip flexion position strength is a prospective risk factor for ACL
and down again until buttocks touch the ground. injury [98]. Gluteus maximus is typically weaker
Test conclude when subject cannot reach the after injury and re-activating and strengthening
height or gives up this muscle is important [104, 105]. Assessing
muscle work capacity and activation is important.
Cramping of the hamstrings or adductor magnus
may indicate inhibition of gluteus maximus and
synergistic dominance [179] which would need
to be corrected prior to progression to more func-
tional exercise
Table 5  (continued)
Outcome measure Test Goal Reason for inclusion

Calf muscle capacity Single leg calf raises [180] Greater than 20 reps and within 5 repetitions Calf muscle strength is important for acceptance
Subjects stand on one foot on the edge of the step versus other side and propulsion. The soleus muscle contributes
and perform a calf raise through full range of the most muscle force production during running
motion. Calf raises are performed at 1 repetition less than 7 m s−1 [91] and the ankle eccentrically
every 2 s. The test concludes when subjects are accepts up to 50% of the impact forces from land-
unable to move through full range or slow below ing [92]. Reduced strength may limit absorption
the cadence outlined above and possibly alter motion, resulting in reduced
knee loading and compensatory patterns
Single leg balance Single leg balance in stance [181] A (eyes open) Poor balance/muscle coordination will alter neuro-
Subjects stand on one leg with other leg raised and 43 s muscular control and may limit the expression of
arms crossed over the chest. The assessor uses a B (eyes closed) muscle strength [60, 61]. Multi angle static bal-
stopwatch to time how long stance is maintained 9 s (normative data for 18–39 year olds) ance forms the pre-requisite to any dynamic activ-
on one leg with (a) eyes open, and (b) eyes ity [20]; without good static balance performance
closed during dynamic tasks is likely to be significantly
Time ends when; compromised. Having sufficient balance with eyes
Arms are used (uncrossed) open and closed is important to progress but also
Use of the raised foot (touches down or other leg) ensure sufficient visual-motor retraining and joint
Movement of the stance foot position sense
45 secs has elapsed (maximum time)
Eyes opened and eyes closed trials
Movement quality Single leg squat test [182] Good movement quality (no zeros and score The squat forms the foundation exercise for many
Squat to at least 60° knee flexion greater than 6) sporting type tasks. Poor squat is associated with
Minimal trunk motion poor biomechanics in more complex tasks as it
Minimal pelvic motion forms the motor pattern foundation for many tasks
No hip adduction or internal rotation involving single leg exercise and triple flexion and
extension
Running gait Assessment of running gait at 8–10 km h−1 [140, Sufficiently normalised running gait and ability to Running represents a functional task which all peo-
176] run for > 10 min at 8 km h−1 ple should do and a milestone mark for the ACLR
Qualitative and/or quantitative assessment of patient. Assessing running gait on a treadmill may
running mechanics including where possible allow the clinician to provide feedback (visual
good control of frontal plane alignment (minimal or immediate or delayed feedback with video
dynamic knee valgus, lateral trunk lean, pelvic recording) cues to support the improvement of the
drop) and sagittal plane loading (optimal triple athlete’s running technique [161]. Poor running
flexion angles, e.g. no knee avoidance) gait at low speeds will be exaggerated at high
speeds and restoring running mechanics prior to
progressing to multi-directional movement retrain-
ing is important

ROM range of motion, AMI arthrogenic muscle inhibition, F/E flexor: extensor ratio, RTS return-to-sport, RM repetition maximum, reps repetitions, BM body mass
M. Buckthorpe, F. Della Villa
Optimising the Mid-Stage of ACL Reconstruction

Compliance with Ethical Standards 13. Waldén M, Hägglund M, Magnusson H, Ekstrand J. ACL injuries
in men’s professional football: a 15-year prospective study on
time trends and return-to-play rates reveals only 65% of play-
Funding No sources of funding were used to assist in the preparation
ers still play at the top level 3 years after ACL rupture. Br J
of this article.
Sports Med. 2016;50(12):744–50. https​://doi.org/10.1136/bjspo​
rts-2015-09595​2.
Conflict of interest Matthew Buckthorpe and Francesco Della Villa 14. Welling W, Benjaminse A, Seil R, et al. Low rates of patients
declare they have no conflicts of interest relevant to the content of this meeting return to sport criteria 9 months after anterior cruciate
review. ligament reconstruction: a prospective longitudinal study. Knee
Surg Sports Traumatol Arthrosc. 2018;26(12):3636–44. https​://
doi.org/10.1007/s0016​7-018-4916-4.
15. Nagelli CV, Hewett TE. Should return to sport be delayed
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