Muscle Mechanics PDF

MUSCLE MECHANICS PHYTM8125 Kinesiology & Biomechanics I Dr. Wing Fu, PT, PhD, MA Structural Organization of Skeletal Muscle Muscle belly Fascicle Muscle fiber Myofibril...

MUSCLE MECHANICS PHYTM8125 Kinesiology & Biomechanics I Dr. Wing Fu, PT, PhD, MA Structural Organization of Skeletal Muscle Muscle belly Fascicle Muscle fiber Myofibril Myofilament Extracellular Connective Tissues within Muscle Muscle belly epimysium Fascicle perimysium Muscle fiber endomysium Myofibril Myofilament  Contractile and Non- Contractile Muscle Contractile Non-Contractile Elements Elements Active Extracellular Proteins Structural Proteins Connective Tissues Actin Myosin Muscle Morphology ¨ Describes the basic shape of a whole muscle ¨ Shapes influence muscular function Muscle Architecture ¨ Two important architectural features ¤Physiologic cross-sectional area (PCSA) ¤Pennation angle ¨ These features significantly affect the amount of force transmitted to the skeleton  Physiologic Cross- Sectional Area (PCSA) ¨ PCSA of a whole muscle ¤ Perpendicular to the muscle fibers ¤ Reflects the amount of active proteins available to generate a contraction force ¤ PCSA of a fusiform Biceps brachii muscle on the right If one of the guys below is going to punch you, who do you choose and why? PCSA of Pennate Muscles Green line: PCSA Blue line: anatomical cross-sectional area (Ignore this)  Pennation Angle ¨ θ in the picture ¤ The bigger the angle, the lesser the contractile force gets transmitted to the tendon ¤ 0 degree transmits 100% of the contractile force n Which type of muscle in terms of shape? Fusiform Cosine 300 = Force in tendon/Force in muscle fiber Fusiform versus Pennate If the above two muscles have the same volume, which one is able to produce more force? Why? Fusiform versus Pennate If the above two muscles have the same volume, which one is able to produce more force? Pennate Why? Pennate has a larger PSCA; a pennation angle of 30 degrees still enables transmission of 86% of the contractile force Fusiform versus Pennate Activating Muscle via the Nervous System Video: https://www.youtube.com/watch?v=gsEF8l3xuyQ  Activating Muscle via the Nervous System A motor unit consists of the (alpha) motor neuron and the muscle fibers it innervates  Activating Muscle via the Nervous System ¨ The nervous system recruits a motor unit by altering the voltage potential across the membrane of the cell body of the alpha motor neuron ¨ At a critical voltage, an action potential is generated which is propagated down the axon of the alpha motor neuron to the motor endplate at the neuromuscular junction ¨ All or none response  Activating Muscle via the Nervous System ¨ How to generate more force? ¤ By recruiting more motor units ¨ Motor units are generally recruited in order of smallest to largest motor neurons ¤ Henneman Size Principle  Activating Muscle via the Nervous System: ¨ After a specific motor neuron has been activated, how to modulate the force produced by the associated muscle fibers? ¤ Rate coding ¨ Motor units activated at high rates à greater overall force Activating Muscle via the Nervous System: Types of Muscle Activation (Contraction) 1. Isometric 1. Concentric 1. Eccentric Isometric Activation ¨ Occurs when a muscle is producing a pulling force while maintaining a constant length ¨ The internal torque produced within a given plane at a joint is equal to the external torque Concentric Activation ¨ Occurs when a muscle produces a pulling force as it contracts (shortens) ¨ The internal torque produced within a given plane at a joint exceeds the opposing external torque ¨ The contracting muscle creates a rotation of the joint in the direction of the pull of the muscle Eccentric Activation ¨ Occurs when a muscle produces a pulling force as it is being elongated by another more dominant force ¨ The external torque around the joint exceeds the internal torque ¨ The joint rotates in the direction dictated by the relatively larger external torque  Classifications of Muscle by Function ¨ Agonist ¤ The muscle or muscle group that is most directly related to the initiation and execution of a particular movement ¤ The prime mover ¨ Antagonist ¤ The muscle or muscle group that is considered to have the opposite action of a particular agonist  Classifications of Muscle by Function ¨ Synergist ¤ The muscle or muscle group that cooperates during the execution of a particular movement n Assisting prime mover n Controlling undesired action(s) of the prime mover n Triceps  Classifications of Muscle by Function ¨ Stabilizer or Fixator ¤ The muscle or muscle group that will steady, anchor or support a bone or body part so that another muscle will pull from a firm base ¤ Proximal stability for distal mobility  Muscle Tension ¨ Magnitude of force developed by a muscle ¨ Two types ¤Active tension nTension developed by the contractile elements of a muscle ¤Passive tension nTension developed by stretching the non- contractile elements of a muscle Passive Length-Tension Curve Productive Antagonism ¨ The body’s ability to convert passive tension into useful work Active Length-Tension Curve Active Insufficiency ¨ Decreased ability of a 2-jointed muscle to produce or maintain active tension as it reaches its shortest possible length over the 2 joints simultaneously ¨ Let’s do an exercise on ONE leg ¤ Standing; hold onto something stable 1. Keep the knee straight. Only extend the hip as hard as you can. Remember how forceful you can extend the hip. 2. Bend the knee as much as possible and keep it flexed; then extend the hip as hard as you can. Compare the force you can generate with that in #1 ¨ Hamstrings Total Length-Tension Curve Force-Velocity Curve Total Force Generated Max eccentric > Max isometric > Any velocity of max concentric Able to lift 100 lb once through the entire ROM slowly ¨ How much weight (more or less than 100 lb) can be lifted more rapidly? ¨ What happens if given slightly more than 100 lb? ¨ What happens if given even more weight? Total Force Generated Max eccentric > Max isometric > Any velocity of max concentric Able to lift 100 lb once through the entire ROM slowly ¨ How much weight (more or less than 100 lb) can be lifted more rapidly? Less ¨ What happens if given slightly more than 100 lb? May switch to isometric ¨ What happens if given even more weight? May switch to eccentric and even drop the weight  References ¨ Neumann DA. Kinesiology of the musculoskeletal system: foundations for rehabilitation. 3rd ed. St. Louis: Elsevier; 2017. ¨ Moore KL, Agur AMR, Dalley AF. Essential clinical anatomy. 4th ed. Baltimore: Lippincott Williams & Wilkins; 2011.