Muscle Mechanics APA 6903 October 28, 2014 Olivia Zajdman Michael Del Bel Outline • 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 Organization/Structure of Muscle • 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. Organization/Structure of Muscle Organization/Structure of Muscle • 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 Sliding Filament Theory • 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. What is a Motor Unit? • 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. Recruitment • 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. Hill Model (P+a)(V+b) = (P0+a)b – – – – P0 = maximum isometric tension a = coefficient of shortening heat b = a* V0/P0 V0 = maximum velocity (when P = 0). • Primarily describes concentric contraction. • Displays relationship between force and velocity in physiological environment. Winter, 2009 Hill Model • Contractile component (CC) – Active • Parallel elastic component (PEC) – Passive • Series elastic component (SEC) – Passive Nordin and Frankel, 2012 Hill Model • Contractile component (CC) -”Active” force component, generated by actin/myosin interactions (sarcomeres). -Fully extended when inactive. -Shortened when activated. Nordin and Frankel, 2012 Hill Model • 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 Hill Model • 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 vs. Length • 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 Force vs. Velocity • • 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. Tendon Stretch-Shortening 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 Viscoelasticity Characteristics Load Relaxation Creep phenomenon Musculoskeletal Model • 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 Musculoskeletal Model • 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. Problems with EMG-Driven Models • 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 References • • • 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.