The biomechanics of human skeletal muscle delves into the intricate mechanisms that governs the movement and function of body’s primary contractile tissues. It combines principles from biomechanics, physiology, and anatomy to elucidate how muscles generate force, control movement and adapt to various physiological demands. One fundamental aspect of skeletal muscle biomechanics is the structure-function relationship within muscle fibres each containing myofibrils comprised of overlapping actin and myosin filaments.The biomechanics of human skeletal muscle is a multifaceted field that underpins our understanding of movement, performance, and injury prevention. By elucidating the intricate interplay between structure, function and control mechanisms within muscles, researchers can enhance athletic performance, rehabilitate injuries and optimize human movements across various contexts.

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The Biomechanics
of Human Skeletal
Muscle
D r. S h u m a i l a
M S - O M P T

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Learning Objectives
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• Describe the skeletal muscle function
• State the factors affecting muscular force generation
• Define muscular strength, power and endurance
• Briefly explain the common musculoskeletal injuries

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SKELETAL MUSCLE FUNCTION
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• Amount of tension is constant throughout in an activated muscle
• The net torque at a joint is the vector sum of the muscle torque and
the resistive torque

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Recruitment of Motor Units
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• CNS matches the speed and magnitude of muscle contraction to
requirements of movement
• Slow-twitch motor units always produce tension first, whether the
resulting movement is slow or fast.
• Requirement changes the fiber type

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Change in Muscle Length with Tension Development
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• When muscular tension produces a torque larger than the resistive
torque at a joint, the muscle shortens, causing a change in the angle
at the joint.
• Concentric: describing a contraction involving shortening of a muscle
• Isometric: describing a contraction involving no change in muscle
length
• Eccentric: describing a contraction involving lengthening of a muscle

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Roles Assumed by Muscles
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An activated muscle can do only one thing: Develop tension.
• Agonist: role played by a muscle acting to cause a movement
• Antagonist: role played by a muscle acting to slow or stop a
movement

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Example
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• During the elbow flexion phase of a forearm curl, the brachialis and
the biceps brachii act as the primary agonists, with the
brachioradialis, extensor carpi radialis longus, and pronator teres
serving as assistant agonists.
• The triceps could act as antagonists by developing resistive tension.
Conversely, during elbow extension, when the triceps are the
agonists, the brachialis and biceps brachii could perform as
antagonists.

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• Stabilizer: role played by a muscle acting to stabilize a body part
against some other force
• The rhomboids act as stabilizers by developing tension to stabilize
the scapulae against the pull of the tow rope during waterskiing.

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• Neutralizer: role played by a muscle acting to eliminate an unwanted
action produced by an agonist
• For example, if a muscle causes both flexion and abduction at a
joint but only flexion is desired, the action of a neutralizer. causing
adduction can eliminate the unwanted abduction.

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• Human movements require combined action of various muscle
groups
• For example, even the simple task of lifting a glass of water from a
table requires several different muscle groups to function in different
ways.

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Two-Joint and Multijoint Muscles
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• Many muscles in the human body cross two or more joints.
Examples:
• biceps brachii
• long head of the triceps brachii
• Hamstrings
• Rectus femoris
• a number of muscles crossing the wrist and all finger joints

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• Muscles affect motion at both or all of the joints crossed
simultaneously
• One-joint muscles produce force directed primarily in line with a
body segment, two-joint muscles can produce force with a
significant transverse component
• Two joint muscles convert rotational to translatory motion of COG

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Disadvantages associated with the function
of two-joint and multijoint muscles
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• active insufficiency: limited ability of a two-joint muscle to produce
force when joint position places the muscle on slack
• passive insufficiency: inability of a two-joint muscle to stretch to the
extent required to allow full range of motion at all joints crossed

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Example
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FACTORS AFFECTING MUSCULAR FORCE GENERATION
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• Force-velocity relationship
• Length-tension relationship
• Stretch-shortening cycle
• Electromechanical delay

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Force–Velocity Relationship
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• The maximal force that a muscle can develop is governed by the
velocity of the muscle’s shortening or lengthening
• The force–velocity relationship does not imply that it is impossible to
move a heavy resistance at a fast speed
• The force–velocity relationship also does not imply that it is
impossible to move a light load at a slow speed

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The force–velocity relationship
for muscle tissue
• When the resistance (force)
is negligible, muscle
contracts with maximal
velocity
• As the load progressively
increases, concentric
contraction velocity slows to
zero at isometric maximum
• As the load increases further,
the muscle lengthens
eccentrically

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• Eccentric strength training involves the use of resistances that are
greater than the athlete’s maximum isometric force generation
capability
• As soon as the load is assumed, the muscle begins to lengthen
• Research shows this type of training to be more effective than
concentric training in increasing muscle size and strength
• As compared with concentric and isometric training, however,
eccentric training is also associated with delayed onset muscle
soreness

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Length–Tension Relationship
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• The amount of maximum isometric tension a muscle is capable of
producing is partly dependent on the muscle’s length
• Force generation capability increases when the muscle is slightly
stretched
• Research indicates that following eccentric exercise there may be a
slight, transient increase in muscle length that impairs force
development when joint angle does not place the muscle in
sufficient stretch

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• The total tension present
in a stretched muscle is
the sum of the active
tension provided by the
muscle fibers and the
passive tension provided
by the tendons and
muscle membranes.

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Stretch-Shortening Cycle (SSC)
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• More forceful contraction occurs in an actively tensed muscle if it is
pre-stretched
• This pattern of eccentric contraction followed immediately by
concentric contraction is known as the (SSC)
• Mechanism is not so clear

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• Baseball pitchers initiate a forceful
stretch of the shoulder flexors and
horizontal adductors immediately
before throwing the ball
• The stretch reflex then contributes to
forceful tension development in these
muscles

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Electromechanical Delay (EMD)
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• The time between the arrival of neural stimulus and tension
development by the muscle
• Length of EMD varies from 20-100 msec
• Factors affecting EMD???
• The time required for a muscle to develop maximum isometric
tension may be a full second following EMD
• Shorter maximum force development times are associated with a
high percentage of FT fibers in the muscle and with a trained state

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MUSCULAR STRENGTH, POWER, AND ENDURANCE
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• These characteristics of muscle function have significant
implications for success in different forms of strenuous physical
activity
• Among senior citizens and individuals with neuromuscular disorders
or injuries, maintaining adequate muscular strength and endurance
is essential for carrying out daily activities and avoiding injury

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Muscular Strength
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• How do we measure muscular strength?
The amount of torque a muscle group can generate at a joint

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• The component of muscular force that produces torque at the joint
crossed (Ft) is directed perpendicular to the attached bone.

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• Contraction of the biceps brachii produces a component of force at
the elbow that may tend to be stabilizing or dislocating, depending
on the angle present at the elbow when contraction occurs

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• Muscular strength is derived both from the amount of tension the
muscles can generate and from the moment arms of the contributing
muscles with respect to the joint center
• The tension-generating capability of a muscle is related to its cross-
sectional area and its training state
• The force generation capability per cross-sectional area of muscle is
approximately 90 N/cm2

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Muscular Power
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• Mechanical power is the product of force and velocity
• Muscular power is therefore the product of muscular force and the
velocity of muscle shortening
• Explosive movements require muscular power
• Muscular power is mostly generated by Type IIb fibers
• The ratio for mean peak power production by Type IIb, Type IIa, and
Type I fibers in human skeletal muscle is 10:5:1

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Muscular Endurance
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• Muscular endurance is the ability of the muscle to exert tension over
time
• Training for muscular endurance typically involves large numbers of
repetitions against relatively light resistance
• This type of training does not increase muscle fiber diameter

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Muscle Fatigue
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• Muscle fatigue has been defined as an exercise-induced reduction
in the maximal force capacity of muscle
• Characteristics of muscle fatigue include reduction in muscle force
production capability and shortening velocity, as well as prolonged
relaxation of motor units between recruitment
• FG fibers fatigue more rapidly than FOG fibers, and SO fibers are
the most resistant to fatigue

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Effect of Muscle Temperature
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• As body temperature elevates, the speeds of nerve and muscle
functions increase
• Muscle function is most efficient at 38.5°C (101°F)

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COMMON MUSCLE INJURIES
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• Muscle injuries are common, with most being relatively minor
• Fortunately, healthy skeletal muscle has considerable ability to self-
repair
• Strains
• Contusions
• Cramps
• Delayed-Onset Muscle Soreness
• Compartment Syndrome

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