APA 6903
October 28, 2014
Olivia Zajdman
Michael Del Bel

• Muscle Anatomy Review
• What is a Motor Unit?
– Recruitment
– Fiber Type
• The Hill Model
– 3 Components
– Force-Length
– Force-Velocity
• Tendon Stretch-Shortening
• Musculoskeletal Model

• Fiber = Structural unit of muscle
– Consist of many myofibrils
• Myofibrils = Basic unit of contraction
– Consist of sarcomeres, which contain thin (actin), thick (myosin), elastic (titin) and inelastic (nebulin and titin) filaments.

• Sarcomeres extend between Z-Lines
– Actin filaments extend length of sarcomere
– Myosin filaments are located in the center of the sarcomere
– Titin and nebulin filaments form part of the intramyofibrillar cytoskeleton
Contracted
Relaxed

• Actin and myosin “slide” along one another to shorten the sarcomere.
• Myofilaments remain the same length.
• All sarcomeres per muscle fiber contract, in a wave-like manner, shortening the fiber as whole.

• The functional unit of skeletal muscle; allows for movement!
• Consists of…
– Single motor neuron
– All of the muscle fibers innervated
• Fibers may not be adjacent to each other.
• Contractile component of the Hill Model.

• All-or-nothing principal: All muscle fibers in MU are same type and all contract when stimulated.
• Motor Unit Action Potential: electrical twitch from MU recorded via EMG.
• Henneman Size Principal: smallest
MU recruited first.
– Use spatial or temporal summation to increase force produced .
• Slow twitch (slow oxidative) and fast twitch (fast glycolytic, fatigable) fibers.

(P+a)(V+b) = (P
0
+a)b
– P
0
= maximum isometric tension
– a = coefficient of shortening heat
– b = a* V
0
/P
0
– V
0
= maximum velocity (when P = 0).
• Primarily describes concentric contraction.
• Displays relationship between force and velocity in physiological environment.
Winter, 2009

• Contractile component (CC)
– Active
• Parallel elastic component (PEC)
– Passive
• Series elastic component (SEC)
– Passive
Nordin and Frankel, 2012

• Contractile component (CC)
-”Active” force component, generated by actin/myosin interactions (sarcomeres).
-Fully extended when inactive.
-Shortened when activated.
Nordin and Frankel, 2012

• Parallel elastic component (PEC)
– Consists of connective tissue
(fascia, epimysium, perimysium, endomysium) surrounding the muscle.
– Represents the passive muscle force that connective tissues are responsible for.
Nordin and Frankel, 2012

• Series elastic component (SEC)
– Consists of the tendon and elasticity of the intramyofibrillar cytoskeleton.
– Similar function as PEC.
– Lengthens as force increases, maintaining constant muscle length.
– Spring-like
Nordin and Frankel, 2012

• Force production proportional to number of actin-myosin cross bridges
• Lengthening or shortening to a degree decreases number of binding sites
• PEC contributes tension as it becomes taut as muscle lengthens
• PEC passive force always present, while CC voluntarily controlled
• Amplitude of force dependent on amount of excitation

• Concentric
– Force decreases as muscle shortens under load (cross bridges break & reform).
– Fluid viscosity (CC/PEC) creates friction -> requires force to overcome -> reduce tendon force.
• Eccentric
– Force increases as muscle lengthening velocity increases.
• Greater force to break cross-bridge links than to hold together.

Toe region : elongation reflect change in wavy pattern of relaxed collagen.
Elastic/linear region : increase stiffness in tissue.
Plastic region : some permanent damage after load removed
Yield point : intersection of stressstrain (max).
Failure point : fibers sustain irreversible damage.
Ultimate load : highest load structure can withstand before failure.
Slope: Elasticity modulus

• Representation of entire system’s movement
• Bones represent basis of modeling body (rigid segments)
– Important to be accurate!
• Muscle Architecture (structure reflects function)
– Pennation angle
– Physiological cross-section
– Fiber length/type
– Tendon morphology

• Inverse dynamics
– Bone segments are controlled by estimated resultant joint moments.
– Muscle forces are then estimated for sequences of motions, while behaviour can be attributed through inclusion of the Hill Model.

• Data from surface EMG electrodes may not fully represent muscle’s activity.
• Impossible to measure deep muscle activity.
• Models generally simplify reality
– Model = 6-8 muscles for a jump
– Reality = >40 muscles involved

• Nordin, M., & Frankel, V. (2012). Biomechanics of Tendons and
Ligaments and Biomechanics of Skeletal Muscle. In Basic
Biomechanics of the Musculoskeletal System(4th ed., pp. 102-180).
Baltimore, MD: Lippincott Williams & Wilkins.
• Robertson, D., Caldwell, G., Hamil, J., Kamen, G., & Whittlesey, S.
(2004). Muscle Modeling. In Research Methods in Biomechanics (pp.
183-207). United States: Human Kinetics.
• Winter, D. (2009). Muscle Mechanics. In Biomechanics and Motor
Control of Human Movement (4th ed., pp. 224-247). Hoboken, New
Jersey: John Wiley & Sons.