Muscular Considerations for
Movement
Kinesiology
RHS 341
Lecture 4
Dr. Einas Al-Eisa
Role of muscles
Prime mover Agonist Stabilizers
Synergist Antagonist Neutralizers
Role of muscles
• Agonist:
¾Muscles producing the movement
¾Primary or prime movers
¾Contract actively to produce a concentric,
isometric, or eccentric contraction
Role of muscles
• Antagonist:
¾More susceptible to injury because the
muscle contracts to slow the limb (or control
the movement) while being stretched
Example
When the thigh swings forward:
¾Agonists: hip flexors (iliopsoas, rectus
femoris, sartorius, pectineus, gracilis)
¾Antagonists: hip extensors (hamstrings
& gluteus maximus)
Role of muscles
• Synergists:
¾Neutralizers
¾Stabilizers / Fixators
Role of muscles
• Synergists (Neutralizers):
Muscles that contract to assist the prime movers,
either by:
¾adding force to the movement and making it
more refined
Or
¾eliminating undesired movement
Role of muscles
• Synergists (Neutralizers):
– Some prime movers cross several joints and
cause movements at all those joints, but
synergists act to cancel some of these
movements.
– Example: making a fist without flexing the
wrist, although the muscles that flex the
fingers also flex the wrist.
Role of muscles
• Stabilizers (Fixators):
¾Muscles that fix or stabilize one segment to
allow another segment to move smoothly &
efficiently
¾Example: muscles that fix the scapula when
the arm moves
Type of muscles
Cardiac Smooth Skeletal
muscles muscles muscles
Skeletal muscles
• Attach to and move the skeleton
• Makes up 40% of the total body weight
• Under voluntary control
• Cells show stripes or striations
• The cells are elongated and called fibers
• Contraction depends on myofilaments
which fill most of the cytoplasm:
¾actin & myosin = proteins generating
contractile force
Skeletal muscles
• Several sheaths of connective tissue
surround the muscle fibers within a muscle
• Those sheaths of connective tissue are
continuous with the tendons that join
muscles to bones
Skeletal muscles
• When muscle fibers contract
pull on the connective tissue sheaths
transmit the force to the bone being moved
Muscle strength
• = the maximum amount of force produced by
a muscle at the site of attachment on the
skeleton
• Usually measured by moving the heaviest
possible external load through one repetition
of a specific range of motion
• Affected by the size & structure of the muscle
Muscle strength
• Affected by the physiologic cross-section (PCS) =
the perpendicular section that cuts all muscle fibers
at its thickest part while the muscle is in midway
between complete contraction and complete stretch
• Weight training
Greater cross-sectional area (hypertrophy)
associated with an increase in the size of the
muscle fibers
Muscle strength
Affected by the following factors:
¾Arrangement of the muscle fibers
¾Width of the muscle (circumference)
¾Gender (muscle force is greater in males?)
¾Age (muscle force decreases with age)
Arrangement of fascicles in muscles
• Fascicles = bundles of fibers enclosed in
a sheath of connective tissue
• The action of each muscles is dependent
(in part) on the arrangement of its fascicles
• The power of a muscle depends on the
total number of fibers it contains
Arrangement of fascicles
• Parallel arrangement:
¾The long axes of the fascicles run parallel to
the long axis of the muscle itself
¾Strap-like; e.g., sternocleidomastoid
Or:
¾Spindle-shape (fusiform) with an expanded
central belly, e.g., biceps brachii
Arrangement of fascicles
• Convergent pattern:
¾The origin of the muscle is broad and the
fascicles converge toward the tendon of insertion
¾Can be triangular or fan-shaped
¾Example: pectoralis major
Arrangement of fascicles
• Pennate pattern:
– Fascicles are short and attach obliquely to a tendon
that runs the whole length of the muscle
– Unipennate: if the muscle inserts into only one side
of the tendon; e.g., flexor pollicis longus
– Bipennate: if the fascicles insert into the tendon from
both sides; e.g., rectus femoris
– Multipennate: looks like many feathers situated side
by side, with all their quills inserting into one large
tendon; e.g., deltiod
Arrangement of fascicles
• Circular (sphincter) pattern:
¾Surround external body openings, which the
muscle closes by contraction
¾Example: Orbicularis oris (mouth), Orbicularis
oculi (eye)
Muscle strength
• = magnitude of the muscle force
• Changes according to the PCS of the muscle
• Force exerted by multipennate muscle is
more than fusiform (because it has greater
PCS?..)
Effect of muscle structure on force
• The force a muscle can exert is proportional
to its PCS
• A broad, thick, longitudinal muscle exerts
more force than a thin one
Effect of muscle structure on force
• A penniform (pennate) muscle of the same
thickness as a longitudinal muscle can exert
greater force (because the oblique arrangement of
the fibers allows for a larger number of fibers in
comparable sizes of the other classifications)
• Pennate muscles are the most common type
of skeletal muscles & predominate when
forceful movements are needed
Muscle length-tension relationship
• The greatest amount of tension can be
developed when a muscle is stretched
(between 100-130% of its resting length)
• The amount of force that can be exerted
by the muscle if it is shortened or if it is
over stretched (beyond 100-130% of its
resting length)
Stretch-shortening cycle
• = proceeding a concentric contraction phase
with an eccentric phase
• Putting the muscle under stretch in the
eccentric phase enables the muscle to store
potential energy
• Example: vertical jumps
Electromyography (EMG)
• = the study of electrical activity of muscles
• Provides insight into:
¾which muscles are active during a task
¾when the muscles initiate and stop their
activity
EMG: Ergonomic application
• Effect of sitting posture on the activity of the neck
and shoulder muscles
• Effect of carrying a load (with respect to magnitude
& duration) on the activity of the shoulder, back, and
leg muscles to develop proper lifting techniques
• Effect of exercises on back muscles in rehabilitation
of low back pain
