Muscle Structure & Function 2024 PDF

Muscle Structure & Function  Objectives Describe factors impacting muscle force & tension Discuss general structural components of muscles Discuss and apply the various types of muscle contraction Understand the length-tension relationship Understand how velocity impacts muscle force...

Muscle Structure & Function  Objectives Describe factors impacting muscle force & tension Discuss general structural components of muscles Discuss and apply the various types of muscle contraction Understand the length-tension relationship Understand how velocity impacts muscle force production Recognize the characteristics of open chain vs closed chain muscle action Describe the concepts of passive and active insufficiency Muscle Structure: Quick Review! CT Layers of Muscle Epimysium Surrounds entire muscle Contains fascicles (bundles of muscle fibers) Perimysium Surround each fascicle Endomysium Surrounds each muscle fiber Muscle Structure: Quick Review! Myofibrils are composed of thick & thin myofilaments Myofilaments are formed by protein molecules: Actin, myosin, troponin, & tropomyosin Sarcomere = contractile unit of the muscle Types of Muscle Action Isometric Isometric Contraction of a muscle fiber w/o changing length Concentric Muscle contraction resulting in active shortening of a muscle Eccentric Muscle contracting as it lengthens in response to an external force (gravity, weights)  Types of Muscle Action Concentric Eccentric “Positive work” is performed by Considered “negative work” bc the muscle as the limb moves the work is done on the muscle Muscle creates the motion External torque > internal Internal muscle torque > muscle torque external torque Muscle controls the motion Expends the greatest amount of Muscle expends the least energy of the 3 types of actions amount of energy as compared to other types of actions A. Sarcomere at resting length B. Concentric contraction Thin myofilaments are pulled toward the thick myofilaments, & cross-bridges are formed, broken, and re-formed C. Eccentric contraction Thin myofilaments are pulled away from thick myofilaments, & cross- bridges are broken, re-formed, and broken again D. Isometric contraction Length is constant Muscle Tension Ability to develop tension & exert force on a bone is key characteristic of muscle Tension can be active or passive Total muscle tension is related to active AND passive components  Determinants of Muscle Tension Passive tension Developed in parallel elastic components of muscle Created by lengthening a muscle beyond the slack length of the tissues In a shortened muscle Parallel elastic elements are on slack No significant contribution to total muscle tension Passive Tension Parallel elastic components: connective tissues surrounding muscle, the sarcolemma, & other Passive Tension Curve structures (i.e., nerves & blood vessels)  Determinants of Muscle Tension Active tension Developed by contractile elements of muscle Generated whenever cross-bridges are formed Depends on neural factors & mechanical properties of muscle fibers Active Muscle Tension Factors impacting active tension Neural factors Related to characteristics of the motor unit Mechanical properties Isometric length-tension relationship Force-velocity relationship 13 Active Tension: Neural Factors # of muscle fibers in motor unit Diameter of axon innervating a motor unit # of motor units firing at any one time Frequency of motor unit firing Motor unit: alpha motor neuron and all the muscle fibers it innervates  Neurophysiologic Factors Golgi tendon organ (GTO) Sensory organ located near musculotendinous junction Sensitive to changes of tension caused by passive stretch or active muscle contractions Autogenic inhibition Stimulation of GTO has inhibitory effect on muscle tension Esp in response to a prolonged stretch force Contributes to reflexive muscle relaxation during stretching interventions Enables muscle to be elongated against less muscle tension Activation leads to inhibition of alpha motor neuron activity and decreases tension in the muscle-tendon unit  Neurophysiologic Factors Muscle spindle Sensitive to quick stretch Detects & conveys information about change in muscle length & speed of those changes When stimulated, causes excitation of the muscle fibers Stretch reflex Application of a quick stretch force stimulates muscle spindle Facilitates muscle contraction & increases active tension in muscle Clinical applications Slowly applied, low intensity stretching helps reduce activation of stretch reflex Can be used to enhance contraction in weak muscles Length-Tension Relationship There is an optimal sarcomere length at which a muscle fiber produces maximal total isometric tension Related to actin & myosin being in position that maximizes potential # of cross-bridges Amount of active tension a muscle can generate decreases when muscle fiber is lengthened or shortened beyond optimal length Total Muscle Tension a) At shortened lengths, all tension is generated actively b) As muscle fiber is stretched beyond resting length, passive tension begins to contribute to total tension c) As the muscle approaches max stretch, passive tension accounts for most of the total tension  Muscle Classifications: Role in Movement Agonist Muscle that produces desired motion at a joint Prime movers – muscles that are first recruited or produce the action with greatest potential force Antagonist Muscle(s) that have the potential to perform the opposite motion of a desired movement Synergists Muscles that assist the agonist (prime mover) to perform a desired motion Directly assisting by performing the desired action Indirect assistance Stabilizing a segment or by preventing an undesired action Co-contraction: purposeful simultaneous contraction of an agonist & antagonist Reverse Action of Muscle Often think of muscle actions as the distal segment moving on a “fixed” proximal segment Reverse action Motion occurring when the distal segment is “fixed” and the proximal segment moves Pull-up: forearm is “fixed” and contraction of the brachialis and biceps causes the humerus (and the body) to move Open Chain Vs Closed Chain Variability in definitions! Open chain Distal segment is free to move Closed chain Distal segment is fixed Often goes hand in hand with a WBing position Motion occurs at multiple 24 joints Open Chain Quadriceps Action Closed Chain Quadriceps Action Muscle Function & Location of Attachments Line of action relative to jt determines motion a muscle will perform May function differently depending upon jt position Distal attachment closer to jt axis produces a greater ROM of the bony segment Distal attachment farther from jt axis provides more stability for the jt Most of the force is directed toward the joint (FX > FY) Location of Muscle Attachments B. Attachment of brachioradialis (BR) on radius is further from elbow joint. Creates a small rotary component & smaller arc of motion of forearm. The greater compressive component makes BR the BR a stronger stabilizer A. Attachment of biceps brachii (BB) on radius is close to elbow joint. Creates more substantial rotary component (Fy) and produces greater ROM  Muscle fiber length Muscles with long fibers have greater shortening capability & can create greater ROM Muscle Architecture Physiological cross-sectional area (CSA) Muscle force production is proportional to # of sarcomeres in parallel (arranged side-by-side) w/in muscle fiber Large # of fibers packed into a muscle or if the fiber increases in size (addition of myofibrils) → increased force production potential Muscle Architecture Fascicle arrangement Fusiform (strap): fascicles run parallel to the long axis of the muscle Unipennate, bipennate, or multipennate: fascicles are at an oblique angle to long axis of the muscle Shorter and more fibers as compared to fusiform Unipennate Fibers fan out on one side of a central tendon Bipennate Fibers fan out on 2 sides of central tendon Multipennate Fascicles/fibers converge on several tendons Muscle Architecture A. Fusiform (strap) examples: SCM, sartorius B. Unipennate examples: flexor pollicis longus, tib post. C. Bipennate examples: biceps femoris, tib ant D. Multipennate examples: soleus, subscapularis, deltoids  Intrinsic & Extrinsic Factors Involving Active Muscle Tension Contraction Recruitment order of motor units Units with slow conduction velocities are generally recruited first velocity is impacted by: Type II fibers can attain max tension Type of muscle fibers in the motor units more rapidly than type I Muscle fiber length Long fibers have a higher shortening velocity than do shorter fibers Magnitude of resistance For a given muscle force, a greater resistance to the muscle action will result in a slower contraction  Intrinsic & Extrinsic Factors Involving Active Muscle Tension Factors Size of motor units Larger units produce greater impacting tension magnitude # of motor units firing Greater the # of motor units firing in a of active muscle, the greater the tension tension: Higher frequency of firing Frequency of firing of motor units produces greater tension Sarcomere length The closer to optimal length, the greater the isometric tension that can be generated Type of contraction Concentric < isometric < eccentric Intrinsic & Extrinsic Factors Involving Active Muscle Tension Factors # and size of the Larger CSA, the impacting muscle fibers in a greater the tension a magnitude of CSA muscle may produce active tension Fiber arrangement Pennate arrangement: Greater potential for greater # of muscle active tension as fibers & potentially > compared to parallel CSA muscle (fusiform/strap) Velocity speed of concentric contraction, decreases tension speed of eccentric contraction, tension Passive Insufficiency Occurs with insufficient length in a passive muscle Passive insufficiency can limit ROM and muscle function Rare for a single jt muscle to have insufficient extensibility significant enough to limit full ROM More commonly related to 2-joint or multi-joint muscles Passive Insufficiency Example When the wrist is flexed, the finger When the wrist is extended, the extensors are stretched over the finger flexors are stretched over wrist with passive tension causing the wrist with passive tension finger extension causing finger flexion Active Insufficiency Decrease in muscle torque production when multi-joint muscle attempts to act simultaneously at all jts it crosses Muscle is unable to shorten sufficiently to complete the full available range at all joints crossed Related to the length-tension curve, change in MA, & passive restraint of lengthened antagonists Active insufficiency can limit AROM, but not PROM Questions?