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OPEN A portable articulated

dynamometer for ankle
dorsiflexion and plantar flexion
strength measurement: a design,
validation, and user experience
study
Seung Yeon Cho 1,8, Youho Myong 2,3,8, Sungwoo Park 1,4, Minwoo Cho 5,6,9* &
Sungwan Kim 1,2,7,9*

Monitoring ankle strength is crucial for assessing daily activities, functional ability, and preventing
lower extremity injuries. However, the current methods for measuring ankle strength are often
unreliable or not easily portable to be used in clinical settings. Therefore, this study proposes a
portable dynamometer with high reliability capable of measuring ankle dorsiflexion and plantar
flexion. The proposed portable dynamometer comprised plates made of aluminum alloy 6061 and
a miniature tension–compression load cell. A total of 41 healthy adult participants applied maximal
isometric dorsiflexor and plantar flexor forces on a Lafayette Handheld Dynamometer (HHD) and
the portable dynamometer. The results were cross-validated, using change in mean, and two
independent examiners evaluated the inter-rater and test–retest reliabilities in separate sessions
using intraclass correlation coefficients, standard error of measurement, and minimal detectable
change. Both dorsiflexion and plantar flexion measurements demonstrated a strong correlation with
the HHD (r = 0.827; r = 0.973) and showed high inter-rater and test–retest reliabilities. Additionally, the
participant responses to the user experience questionnaire survey indicated vastly superior positive
experiences with the portable dynamometer. The study findings suggest that the designed portable
dynamometer can provide accurate and reliable measurements of ankle strengths, making it a
potential alternative to current methods in clinical settings.

Maintaining muscle strength is crucial for overall health and physical fitness, as it plays a vital role in performing
daily activities and maintaining functional ­ability1–3. The decline in muscle strength is associated with neuro-
muscular disorders, and considered a risk factor for all-cause mortality in healthy p ­ opulations4–6. Inadequate
muscle strength in the ankle can increase the risk of ankle sprain and other lower extremity injuries since ankle
­ alance7–9. For instance, ankle dorsiflexor strength, an important determinant
strength is essential for gait and b
7
of walking e­ ndurance , is often impaired in stroke, cerebral palsy, and other neuromuscular disorders, such as
myotonic dystrophy type 1­ 10. Moreover, ankle dorsiflexion was found to be one of the primary predictors of loss

1
Interdisciplinary Program in Bioengineering, The Graduate School, Seoul National University, Seoul, South
Korea. 2Department of Biomedical Engineering, Seoul National University College of Medicine, 103 Daehak‑Ro,
Jongno‑Gu, Seoul 03080, South Korea. 3Department of Rehabilitation Medicine, Seoul National University
Hospital, Seoul, South Korea. 4Institute of Innovative Medical Technology, Seoul National University Hospital
Biomedical Research Institute, Seoul, South Korea. 5Department of Transdisciplinary Medicine, Seoul National
University Hospital, 101 Daehak‑Ro, Jongno‑Gu, Seoul 03080, South Korea. 6Department of Medicine, Seoul
National University College of Medicine, Seoul, South Korea. 7Institute of Bioengineering, Seoul National
University, Seoul, South Korea. 8These authors contributed equally: Seung Yeon Cho and Youho Myong. 9These
authors jointly supervised this work: Minwoo Cho and Sungwan Kim. *email: windsblues@snu.ac.kr; sungwan@
snu.ac.kr

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of ambulation in Duchenne muscular ­dystrophy11, with its weakness also influencing motor function in Charcot-
Marie-Tooth ­disease12. Therefore, monitoring the muscle strength of ankles may provide valuable information to
clinicians and individuals seeking to improve lower extremity strength and stability, necessitating the appropriate
measurement of ankle s­ trength13.
Manual muscle testing (MMT) has been the preferred method for ankle-strength measurements in most
clinical settings, as it is quick and easy to ­perform14,15. However, its reliability is low because its grading depends
on the examiner’s muscle strength, and quantifying temporal changes is c­ hallenging16. Hand-held dynamom-
eters (HHDs) or the isokinetic dynamometers are commonly used for muscle strength assessment in clinical
­settings17–20. HHD is advantageous over MMT because it is fully quantitative and more sensitive for detecting
small ­changes21; however, its reliability is also limited by the examiner’s s­ trength22. Furthermore, HHDs require
different subject positioning for different m ­ easurements23,24. While isokinetic dynamometers are often used as
the gold standard for muscle strength assessment, they are significantly larger and less portable, making routine
clinical practice or bedside monitoring ­difficult20. Additionally, isokinetic dynamometers are typically more
expensive, requiring trained technical expertise owing to the complexity of the software and the setup ­process18.
The existing methods of ankle strength assessment have limitations, with previous studies having attempted
to develop a dynamometry system specifically for the measurement of ankle dorsiflexion and plantar flexion
strengths. However, some portable dynamometers were limited to only ankle plantar flexion measurements,
while others that allowed both measurements demonstrated low ­reliability25,26. Another study developed a novel
portable iso-damping dynamometer that was comparable to the gold standard but required a subject-specific
pre-parameter set for each assessment, and test–retest validation was a­ bsent27. Therefore, a portable, highly
accurate, and reliable dynamometer that can measure ankle dorsiflexion and plantar flexion is required. Recently,
the authors have developed a portable dynamometer with high accuracy and reliability for measurement of
knee extensor ­strength28, and the concept was further developed to extend its application to ankle in this study.
This study aimed to design and validate a portable and reliable isometric dynamometer for measuring ankle
dorsiflexion and plantar flexion strengths. The accessibility, portability, and reliability of the device were evalu-
ated through cross-validation with healthy adult subjects, comparing the results with those obtained using the
Lafayette HHD and followed by a user experience questionnaire survey (UEQ-S). To provide stability during
isometric measurements, a Lafayette Support Stand was used in conjunction with the Lafayette HHD.

Results
A total of 41 healthy adults were included in this study, and the cohort size was sufficient for the validation of
proposed device, taking account of the population recruited in similar previous ­works17,29. All subjects per-
formed ankle dorsiflexion and plantar flexion assessments for both ankles in two independent sessions, all of
which were evaluated by two different examiners. The results of this study provided 656 observations in total:
328 dorsiflexions and 328 plantar flexions.
Table 1 summarizes the results of a cross-validation analysis conducted to assess the agreement between the
HHD and the portable dynamometer. The maximum force of ankle dorsiflexion measured with the portable
dynamometer exhibited a high correlation (r = 0.827, p < 0.001) than that measured with the HHD (mean differ-
ence: 1.48 kg f, 95% CI 0.62, and 2.35 kg f). Similarly, the maximum force of ankle plantar flexion measured with
the portable dynamometer demonstrated a higher correlation (r = 0.973, p < 0.001) than the measurements from
the HHD, with no significant difference (mean difference: − 0.04 kg f, 95% CI –4.27, and 4.18 kg f). The intraclass
correlation coefficient (ICC) between the HHD and portable dynamometer was 0.785 and 0.972 for the dorsiflex-
ion and plantar flexion, respectively, indicating good and excellent reliability, respectively. The minimal detect-
able change (MDC) was 7.93 kg f in dorsiflexion and 13.03 kg f in plantar flexion. Figure 1 illustrates the linear
relationship between the measured values of the two dynamometers, along with the Bland–Altman ­analysis30,31.
Two types of error sources were observed in the result, which can be categorized into objective and subjective
error sources. The objective error sources primarily encompass systematic errors, such as calibration errors in
the portable dynamometer, as shown in Supplementary Fig. 1. The amount of deviation for a best fit line in the
calibration curve indicated the presence of the objective error source. The impact of such objective error source
became evident in the linear regression analysis of dorsiflexion measurement (Fig. 1c), wherein a slight force
offset was observed.
The subjective error sources would include the variability in measurement setup and data recording per-
formed by examiners, variability in the characteristics of individuals, and potential learning effects over mul-
tiple trials. These errors can be analyzed with evaluation of inter-rater reliability and test–retest reliability. The
inter-rater reliability of the portable dynamometer was evaluated by having two independent examiners assess
each participant. The data collected from both examiners showed a strong correlation with both the portable
dynamometer (r = 0.931, p < 0.001 in dorsiflexion; r = 0.969, p < 0.001 for plantar flexion) and the HHD (r = 0.941,
p < 0.001 in dorsiflexion; r = 0.987, p < 0.001 in plantar flexion). No significant differences between the examin-
ers were observed for either device, with mean differences of 0.16 (dorsiflexion), 0.23 (plantar flexion) and 0.43
(dorsiflexion), 0.46 (plantar flexion) kg f for the HHD and the portable dynamometer, respectively. Both devices
demonstrated excellent relative reliability, with ICC values of 0.941 (dorsiflexion), 0.986 (plantar flexion) and
0.929 (dorsiflexion), 0.969 (plantar flexion) for the HHD and the portable dynamometer, respectively. The MDC
was 3.41 kg f for the HHD and 4.52 for the portable dynamometer in dorsiflexion, and 8.98 kg f for the HHD
and 13.80 kg f for the portable dynamometer in plantar flexion.
The study additionally evaluated test–retest reliability by including a retest session after 24 h of the ini-
tial session The measurements obtained from the retest session showed a strong correlation with those from
the initial session for both the HHD (r = 0.878, p < 0.001 in dorsiflexion; r = 0.968, p < 0.001 in plantar flexion)
and the portable dynamometer (r = 0.871, p < 0.001 in dorsiflexion; r = 0.961, p < 0.001 in plantar flexion). The

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CIM (kg·f)
Measurement (kg·f) (95% CI) Pearson’s r p-value ICCa SEM (kg·f) MDC (kg·f)
Agreement
Dorsiflexion
HHD 17.69 ± 5.06
1.48 (0.62, 2.35) 0.827 < 0.001 0.785 2.86 (14.92%) 7.93 (41.35%)
Ours 19.17 ± 6.16
Plantar flexion
HHD 59.98 ± 27.00
− 0.04 (− 4.27, 4.18) 0.973 < 0.001 0.972 4.70 (7.84%) 13.03 (21.72%)
Ours 59.94 ± 28.11
Inter-rater reliability
Dorsiflexion Examiner A Examiner B
HHD 17.61 ± 5.08 17.77 ± 5.06 0.16 (− 0.95, 1.26) 0.941 < 0.001 0.941 1.23 (6.92%) 3.41 (19.19%)
Ours 18.96 ± 6.20 19.39 ± 6.13 0.43 (− 0.90, 1.78) 0.931 < 0.001 0.929 1.63 (8.41%) 4.52 (23.30%)
Plantar flexion
HHD 59.87 ± 26.65 60.10 ± 27.42 0.23 (− 5.65, 6.10) 0.987 < 0.001 0.986 3.24 (5.39%) 8.98 (14.94%)
Ours 59.71 ± 28.01 60.17 ± 28.29 0.46 (− 5.66, 6.57) 0.969 < 0.001 0.969 4.98 (8.28%) 13.80 (22.94%)
Test–retest reliability
Dorsiflexion Session 1 Session 2
HHD 17.68 ± 5.27 17.70 ± 4.87 0.02 (− 1.09, 1.12) 0.878 < 0.001 0.876 1.71 (9.66%) 4.74 (26.78%)
Ours 19.08 ± 6.49 19.27 ± 5.84 0.19 (− 1.15, 1.53) 0.871 < 0.001 0.867 2.13 (11.05%) 5.90 (30.64%)
Plantar flexion
HHD 60.35 ± 27.06 59.62 ± 27.02 − 0.73 (− 6.60, 5.14) 0.968 < 0.001 0.968 4.83 (8.00%) 13.39 (22.18%)
Ours 59.73 ± 27.80 60.15 ± 28.50 0.42 (− 5.70, 6.53) 0.961 < 0.001 0.961 5.56 (9.24%) 15.41 (25.62%)

Table 1.  Summary of overall results: agreement, inter-rater reliability, and test–retest reliability of hand-held
dynamometer and portable dynamometer (ours). CIM change in mean, ICC intraclass correlation coefficient,
SEM standard error of measurement, MDC minimal detectable change, HHD hand-held dynamometer.
a
ICC(2,k) was used for the agreement and inter-rater reliability, and ­ICC(3,k) was used for the test–retest
reliability.

between-day measurements for both devices did not have a significant difference, with mean differences of 0.02
(dorsiflexion), − 0.73 (plantar flexion) and 0.19 (dorsiflexion), 0.42 (plantar flexion) kg f for the HHD and port-
able dynamometer, respectively. Both devices demonstrated excellent relative reliability, with ICC values of 0.876
(dorsiflexion) and 0.968 (plantar flexion) for the HHD, and 0.867 (dorsiflexion) and 0.961 (plantar flexion) for
the portable dynamometer. The MDC was 4.74 kg f for the HHD and 5.90 kg f for the portable dynamometer
in dorsiflexion, and 13.39 kg f for the HHD and 15.41 for the portable dynamometer in plantar flexion. Table 1
summarizes the results of the inter-rater and test–retest reliabilities, while Figs. 2 and 3 provide a visual repre-
sentation of the results.
Out of 41 participants in this study, 38 completed the UEQ-S survey. The results reported that the overall
user experience with the portable dynamometer was significantly more positive than with the HHD (p < 0.001).
Specifically, five items showed statistically significant differences between the two devices. Participants found
the portable dynamometer to be easier to handle (p = 0.004), more efficient (p < 0.001), clearer (p = 0.032), more
inventive (p < 0.001), and more leading-edge (p = 0.004) than the HHD. Table 2 presents additional details on
the survey.

Discussion
The measurement of maximal ankle dorsiflexion and plantar flexion strength is crucial for assessing gait, bal-
ance, and other daily a­ ctivities1,7,8. To address the need for accurate and accessible measurements, a portable
dynamometer was developed in this study, which aimed to provide portability while improving accessibility
and reliability. A sample of 41 healthy adult subjects was considered to test the dynamometer, and its validity
was evaluated by assessing the level of agreement with the gold standard, inter-rater reliability, test–retest reli-
ability, and user experience. Isometric measurements were conducted using the HHD, which was fastened to the
Lafayette Support Stand to improve stability and objectivity. The HHD showed high reliability in the inter-rater
and test–retest assessments, confirming its validity. Although valid, since HHD requires different arrangement
or position of either the device or the subject for measurement of different force types, the position of HHD had
to be rearranged for each assessment of dorsiflexion and plantar flexion. However, the size (610 × 610 × 880 mm
in dimension) and weight (approximately 20 kg) of the stand made rearrangements transportation challenging.
Additionally, some subjects experienced discomfort with use of HHD because the point of contact with the
device was minimal.
Several studies have previously attempted to increase the portability and reliability of existing ankle muscle
strength assessment methods. For instance, a study designed a unique dynamometer that could measure both
ankle dorsiflexion and plantar flexion, offered easy stabilization of the ankle joint for strength measurement, and

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Figure 1.  Bland–Altman analyses of the agreement between the portable dynamometer and HHD for (a)
dorsiflexion and (b) plantar flexion measurements; linear regression analyses of the agreement between the
portable dynamometer and HHD for (c) dorsiflexion and (d) plantar flexion measurements.

exhibited low variability in healthy ­subjects32. However, the device lacked portability, with its maximal range
of measurement too low to cover the plantar flexor force, which is usually larger than that of the subjects they
­evaluated33. Another study developed a custom-built electronic dynamometer for ankle joint torque measurement
that used a simple load cell to assess both dorsiflexion and plantar flexion. However, it also lacked portability
and had stability problems, where the optimal pivotal position must be determined for every subject, and its
validation was conducted using only 4 human ­subjects34. The portable dynamometer developed in this study
utilizes a low-cost high-capacity load cell that can sufficiently measure the plantar flexor force of a healthy adult.
Additionally, by using inelastic Velcro belts, the ankle joint can be easily stabilized on the device, contributing
to the precision of measurements. While another study aimed to develop a portable dynamometry system and
validate it on a wider range of subjects from 5 to 80 years of age, the possible positioning of the subject was limited
to only one posture, and the test–retest agreement and reliability found in the current study were significantly
­higher26. It is also noteworthy that some studies have measured force of ­ankles24,35, while others have focused on
torque ­measurements26,34. Despite such difference, they were all applied for similar diagnostic purposes. This
suggests that in clinical settings, force and torque are sometimes interchangeable. However, from an engineering
perspective, it is important to consider that torque sensors are typically more expensive, bulkier, and heavier
compared to load cells. Given the primary objective of this study, which was to design a portable dynamometer,
the proposed device employed a load cell to measure ankle force, allowing for lightweight and portability.
The portable dynamometer used in this study was designed to address the limitations of previous studies.
It has succeeded in demonstrating strong agreement with the clinical gold standard HHD, excellent inter-rater
reliability, and excellent test–retest reliability for maximal strength measurement of ankle dorsiflexion and plan-
tar flexion. Furthermore, the participants reported significantly more positive experiences with the portable
dynamometer in the user experience survey. Compared to an HHD, the portable dynamometer allowed for both
ankle dorsiflexion and plantar flexion assessments to be performed in a single position once it was attached to
the ankle, eliminating the need for separate positions and arrangements. Moreover, the device was designed
to be intuitive and compact, measuring 256 × 94 × 85 mm in dimension when completely folded and weighing

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Figure 2.  Bland–Altman analyses of the inter-rater reliability for (a) HHD and (b) portable dynamometer;
linear regression analyses of the inter-rater reliability for (c) HHD and (d) portable dynamometer.

only 2.3 kg. This weight is significantly lighter than other portable dynamometer designs previously attempted,
which ranged from 3 to 30 ­kg27,32. The use of aluminum material contributed to a lighter weight while maintain-
ing rigidity.
The designed portable dynamometer demonstrated excellent accuracy and reliability in ankle strength meas-
urements in both dorsiflexion and plantar flexion, despite ankle plantar flexion being generally more difficult to
measure due to the extent of its s­ trength33. However, the agreement for dorsiflexion was relatively lower, which
may be due to discomfort in the positioning of the subject, reflecting potential subjective error sources. Ankle
dorsiflexion is known to be maximal when the ankle is plantarflexed by 10°–15°36,37. However, the portable
dynamometer stabilizes the ankle in a neutral position (0°), which may not be the most comfortable position for
some subjects, while the HHD may have allowed the subject to assume the most comfortable position because
it did not completely secure the ankle in a fixed position. Despite the stretching sessions being preceded by the
assessments, some subjects expressed hamstring strains during the dorsiflexion measurement in a long-sitting
position, with the ankle in a neutral position, which may have hindered their maximal force production. In
addition, a few subjects with larger foot sizes expressed discomfort with the location of the Velcro strap that
runs over the dorsum of the foot. Interestingly, some of the dorsiflexion data with a lower agreement with HHD
were observed in male subjects with large foot sizes.
In future studies, it is important to address the limitations of this study. One limitation is the customization
of the footplate and location of the strap over the dorsum of the foot. Few subjects with relatively larger foot sizes
have expressed discomfort with the location of the strap, but the location of strap was able to be adjusted within
only a limited range due to given size of the footplate. The strap was limited for large range of adjustment also to
strongly fasten the foot, but since the strap has provided sufficient fastening, it could be considered for allowing
more adjustment afterwards. This can be improved by customizing the height of the footplate to support larger
foot sizes and widening the range of the strap’s location to cover the optimal dorsal region of the foot based on its
size. Another limitation is the need for additional validation regarding other postures to explore the optimized
positions of the ankle and knee for dorsiflexion and plantar flexion measurements. Given that the portable

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Figure 3.  Bland–Altman analyses of the test–retest reliability for (a) HHD and (b) portable dynamometer;
linear regression analyses of the test–retest reliability for (c) HHD and (d) portable dynamometer.

HHD Oursa CIM [95% CI] p-value

Supportive 1.08 ± 1.57 1.66 ± 1.19 0.58 [− 0.06, 1.21] 0.064
Easy 0.79 ± 1.95 2.00 ± 1.34 1.21 [0.45, 1.97] 0.004
Efficient 0.39 ± 1.95 1.84 ± 1.13 1.45 [0.72, 2.18] < 0.001
Clear 0.84 ± 1.84 1.63 ± 1.51 0.79 [0.02, 1.56] 0.032
Exciting 0.79 ± 1.40 1.05 ± 1.41 0.26 [− 0.38, 0.91] 0.287
Interesting 0.95 ± 1.43 1.18 ± 1.43 0.23 [− 0.42, 0.89] 0.152
Inventive 0.05 ± 1.77 1.55 ± 1.41 1.50 [0.77, 2.23] < 0.001
Leading edge 0.00 ± 1.76 0.92 ± 1.38 0.92 [0.20, 1.64] 0.004
Total score 4.89 ± 0.41 11.83 ± 0.39 6.94 [6.55, 7.33] < 0.001

Table 2.  User experience questionnaire survey score (N = 38). HHD hand-held dynamometer, CIM change in
mean, CI confidence interval. Significant values are in bold. a Ours: the portable dynamometer.

dynamometer allows measurements in seated or supine positions as well, it would be valuable to consider the
possible variance in ankle strength measurements in other postures. Potential difference in force characteristics
could be observed in other postures for different individuals. Thus, comparisons with HHD in other positions will
further enhance the validity of this work. This study only recruited healthy adults aged between 20 and 39 years,
so further validation with a more diverse population, including children, elderly individuals, and patients with
weak ankle strength, can improve the device’s accuracy and reliability.

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Conclusion
The portable dynamometer developed in this study addresses the clinical need for a reliable and accessible device
for accurate ankle strength measurements. With its strong agreement with the gold standard HHD and high
inter-rater and test–retest reliabilities, this device may be a valuable tool for routine muscle strength monitoring
in clinical settings. It has the potential to provide accurate and reliable measurements of muscle strength and can
be used in various populations, including children, elderly individuals, and patients with weak ankle strength,
after further validation.

Methods
Study subjects
This study recruited 41 healthy adults (21 males and 20 females) aged between 20 and 39 years who had pro-
vided written informed consent (Table 3). Individuals who had undergone lower extremity joint surgery, had
neurological, musculoskeletal, or orthopedic conditions, or were unable to understand the instructions were
excluded. The study adhered to the principles of the Declaration of Helsinki, with the study protocols approved
by the Institutional Review Board Committee of Seoul National University Hospital (IRB number: H-2201-025-
1288), Seoul, Korea. The participant appearing in Fig. 4 provided written informed consent to publish the figure
in an online open-access publication.

Male (N = 21) Female (N = 20)

Age group (years)
20–29 20 (49%)
30–39 21 (51%)
Age (years) 28.81 ± 4.35 29.95 ± 3.90
Height (cm) 174.31 ± 4.64 162.28 ± 6.34
Weight (kg) 73.86 ± 10.01 53.71 ± 8.93
Foot size (mm)
Right 249.57 ± 10.22 228.7 ± 9.69
Left 250.33 ± 9.21 228.1 ± 9.75
Grip strength (kg)
Right 44.29 ± 7.18 27.46 ± 5.00
Left 42.68 ± 7.43 25.95 ± 4.80

Table 3.  Demographics and anthropometry of study participants (N = 41). cm centimeter, kg kilogram, mm

millimeter.

Figure 4.  (a) 90° formation and (b) folded formations of the designed portable dynamometer; subject position
for (c) ankle dorsiflexion strength measurement and (d) ankle plantar flexion strength measurement using the
HHD; (e) long sitting, (f) supine, and (g) seated positions for ankle strength measurement using a portable
dynamometer. (Figure of HHD is presented within (c) and (d)).

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Anthropometry
The anthropometric data, including height, weight, and foot size, along with birth date and sex, were recorded
for each participant (Table 3). This allowed the examiners to identify individuals, while all data were anonymized
during the analyses. Height measurements were taken using an ultrasonic stadiometer to the nearest 0.1 cm, with
weight measured using a digital scale to the nearest 0.1 kg. Foot sizes were measured (with the subject barefoot)
as the distance from the outermost part of the heel to the tip of the longest toe, using a plastic tape measure.
Additionally, the maximum grip strength was measured to the nearest 0.1 kg for each hand using a hand-held grip
dynamometer (EH-101; Camry Scale, California, USA). The participants were instructed to squeeze the device
as hard as possible in an upright posture for approximately 3 s. The foot size and the maximum grip strength
were measured for each foot and hand.

Design and development

The purpose of this study was to design and validate a portable isometric dynamometer that specifically measures
ankle dorsiflexion and plantar flexion. The device possessed a simple mechanical structure comprising three main
plates made of anodized aluminum alloy 6061, which were joined at a single frame box. A couple of stainless
cylinders were included to allow for the device to be folded into a completely compact formation or configured
into a 90° angle, with the distal and proximal limbs serving as a footplate and a calf support, respectively. This
foldable structure allows for easier storage and enhanced portability. Additionally, a silicone pad was attached
at both ends of the calf support plate to maximize wearability and protect the heel and calf from friction during
the measurement process, which could potentially cause pain and hinder the subject from applying maximum
strength.
A miniature tension–compression load cell (CBF30-100, CASKOREA) was integrated between the two distal
limb plates at a distance corresponding to the location of the first metatarsophalangeal joint, as determined by
the average foot size of young a­ dults38. The signal was acquired at sampling frequency of 10 Hz using an HX711
amplifier and an Arduino Uno microcontroller with analog-to-digital conversion resolution of 10-bit, which
amplified and converted the load cell output into a digital signal that could be processed by the microcontroller
unit. The acquired data were then transferred to a computer via a serial port connection through an integrated
development environment (Arduino IDE 1.8.19) with a baud rate of 9600 bits per second and visualized through
a graphical user interface created by Processing 4.0b2. A real-time signal graph was presented for visual assistance
in monitoring the signal acquisition, and the maximum value of the acquired data was selected for the final data
to record. The overall system had approximately 97.7 g of resolution, and the system flow is illustrated in Fig. 5.
Prior to conducting the clinical trials in human subjects, the load cell was calibrated for force measurements.
The calibration was performed by applying a series of M1 class test weights ranging from 0.1 to 20 kg to the
dynamometer at the location of the load cell and recording the corresponding readings of the measured value.
The applied forces were then correlated with the experimentally measured forces from the device to validate the
calibration of the load-cell-embedded device. The results demonstrated strong agreement between the applied
and measured forces (r = 0.999, p < 0.001), indicating accurate force measurements. The calibration curve is
shown in Supplementary Fig. 1.

Figure 5.  Graphical diagram of the overall flow of the dynamometry system.

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Maximal ankle Dorsi‑/plantar flexion measurement

To secure the device when the subject exerted force, the HHD (Lafayette Instrument Company, USA, Model
01163) was attached to the Hand-Held Dynamometer Support Stand (Lafayette Instrument Company, USA,
Model 01166), hereinafter referred to as the stand, allowing more objective and reliable measurement of the
isometric force. The participants were seated on a non-slip floor in a long-sitting position with their back against
the wall. The stand was configured to position the HHD in front of the dorsal region of the foot for dorsiflexion
measurement, and behind the plantar region of the foot for plantar flexion measurement (Fig. 4c,d). The height
of the HHD was adjusted for each participant to align the stirrup with their first metatarsophalangeal joint. The
examiner stood on the footplate of the stand and held onto the handlebars to provide additional stability during
the measurement. For measurements using the portable dynamometer, a subject’s foot was fastened to the device
with Velcro belts, and they sat in a long-sitting position with their feet against the wall (Fig. 4e). Although this
study conducted the assessments in a long-sitting position, the design of the portable dynamometer allowed
measurements in a supine or seated position, as shown in Fig. 4f,g. In the case of measurements in a seated posi-
tion, the zeroing function of the system, which resets the measurement to zero once the subject is positioned,
can eliminate the effect of foot weight. Such feature ensured that measurements can be taken with minimal
interference from the external loads in any measurement position.
For both dynamometers, the subjects were instructed to perform maximal dorsiflexion and plantar flexion
for a duration of 3 s per force direction. Verbal guidance was provided for the counts, and subjects were encour-
aged to exert maximum force in each trial. The inter-rater reliability was assessed by two independent examiners
(who measured both ankles), and test–retest reliability was evaluated by repeating identical protocols for two
sessions, 24 h apart. To prevent muscle fatigue, subjects were given a break of 10 min between the trials of each
dynamometer, and a break of 3 min when switching examiners. Additionally, all subjects underwent a stretching
session of 3 min to relax their muscles prior to the assessments. The assessments were supervised by a physiatrist
with seven years of clinical experience. The overall protocol is illustrated in Supplementary Fig. 2.

User experience study

After the assessments, the participants were asked to complete the short version of the UEQ-S voluntarily to
evaluate their experience with both dynamometers. The questionnaire is a reliable tool for measuring subjective
­ roduct39. The UEQ-S comprised eight items, each rated on a 7-point
impressions of the user experience with a p
Likert scale ranging from − 3 (most negative experience) to + 3 (most positive experience); all responses were
anonymous.

Statistical analysis
The purpose of this study was to evaluate the performance of the designed portable dynamometer in compari-
son to the HHD, as well as to assess inter-rater reliability and test–retest reliabilities. Statistical analyses were
conducted using Pearson’s correlation coefficient, intraclass correlation coefficient (ICC), and the standard error
of measurement (SEM), where ICC and SEM represented relative and absolute reliabilities, r­ espectively40. ICC
values of less than 0.5, between 0.5 and 0.7, between 0.75 and 0.9, and greater than 0.9 represented poor, moder-
ate, good, and excellent relative reliability, r­ espectively41. A two-way random-effects model (­ ICC2,k) was used to
evaluate the agreement between the two dynamometers and inter-rater reliability, and a two-way mixed effects
model ­(ICC3,k) was used to assess
√ the test–retest reliability. The SEM was calculated using the standard devia-
tion (SD), where SEM = SD 1 −√ ICC , and minimal detectable change at 95% confidence level was computed
using the formula MDC = 1.96 · 2 · SEM . Additionally, changes in the mean (CIM) with 95% confidence
intervals and paired t-tests were used to examine systematic bias. The UEQ-S responses between the HHD and
the designed dynamometer were compared using a paired t-test. The normal distribution of all data was assessed
using the Shapiro–Wilk test. All statistical analyses were performed using the Python package SciPy (v.1.10.1) and
its subpackage designed for statistical analyses, scipy.stats42. The level of statistical significance was set at p < 0.05.

Data availability
The data acquired in this study are not openly available due to the sensitive nature of human data (e.g. age, sex,
height, and weight). A de-identified dataset containing the full demographic and clinimetric data is available
from the corresponding authors (M.C., S.K.) upon reasonable request.

Received: 15 August 2023; Accepted: 6 December 2023

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Acknowledgements
This research was supported by a Grant from the National Research Foundation of Korea (NRF) funded by
the Korean government (MIST) (No-2021R1C1C2095529), the MD-PhD/Medical Scientist Training Program
through the Korea Health Industry Development Institute (KHIDI) funded by the Ministry of Health and Wel-
fare, Republic of Korea, and a Grant (No-1920230020) from the SNUH Research Fund.

Author contributions
S.Y.C., Y.M., and S.P., conceptualized and designed the research. S.Y.C., Y.M., S.P., and M.C. acquired data.
S.Y.C., Y.M., and M.C. analyzed and interpreted data. S.Y.C., Y.M., M.C., and S.K. wrote, reviewed, and edited
the manuscript. All authors reviewed the manuscript.

Competing interests
The authors declare no competing interests.

Additional information
Supplementary Information The online version contains supplementary material available at https://​doi.​org/​
10.​1038/​s41598-​023-​49263-2.
Correspondence and requests for materials should be addressed to M.C. or S.K.
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