AHS 22002 Clinical Kinesiology - Muscular Structure and Function

Learning Objectives

  • Discuss muscle structure and function, including properties, mechanics and roles of skeletal muscle
  • Identify the different types of muscle fibers, shapes, and contractions
  • Define key terms in muscle mechanics:
    • Agonist
    • Antagonist
    • Reciprocal inhibition
    • Synergist
    • Co-contraction
  • Differentiate between two muscle types:
    • Tonic
    • Phasic
  • Differentiate between types of insufficiency:
    • Active
    • Passive
  • Describe how muscle contractions are elicited by the nervous system
  • Describe the passive elastic component of muscle
  • Explain the factors that influence the strength of a muscle contraction
  • Define key sensory receptors:
    • Golgi tendon organ
    • Muscle spindle
  • Discuss effects of immobilization, injury, and age on muscle tissue

Definition of Muscle

  • A muscle is defined as a bundle of red contractile fibers held together by a sheath of connective tissue.

Types of Muscle Tissue in the Body

  1. Smooth Muscle
    • Involuntary
    • Non-striated
  2. Cardiac Muscle
    • Involuntary
    • Striated
  3. Skeletal Muscle
    • Voluntary
    • Striated
    • Forms over 1/3 of the body mass

Functions of Skeletal Muscle

  1. Stability
    • Resisting movement
    • Joint approximation
  2. Mobility

Muscle Structure

  • Muscle is composed of individual muscle fibers.
  • Endomysium: Delicate connective tissue covering fibers.
  • Fasciculi: Bundles of parallel muscle fibers.
  • Perimysium: Denser connective tissue binding fasciculi together.
  • Groups of fasciculi form muscles, surrounded by epimysium.
  • Connective tissue of muscles attaches to tendons.

Contractile Unit of the Muscle

  • Each muscle fiber contains bundles of smaller units called myofibrils.
  • Myofibrils are divided into segments known as sarcomeres.
  • Each sarcomere consists of actin and myosin fibers, the contractile elements of the muscle.

Properties of Muscular Tissue

  1. Extensibility: The muscle can be stretched.
  2. Contractility: The muscle can develop tension.
  3. Irritability: The muscle responds to stimulus.

Passive Elastic Component of Muscle

  • The connective tissue in muscle changes length as the muscle itself shortens and lengthens.
  • Tendons are in series with muscles; tension is experienced during muscle shortenings/lenthenings.

Types of Muscle Fibers

  1. Fast Twitch Glycolytic
    • Contract quickly.
    • Fatigue quickly.
  2. Fast Twitch Oxidative/Glycolytic
    • Intermediate fiber type.
  3. Slow Twitch Oxidative
    • Contract slowly.
    • Fatigue slowly.

Muscle Attachments

  1. Insertion: The more mobile end of the muscle.
  2. Origin: The more stable end of the muscle.

Reversal of Muscle Action

  • Occurs when the origin moves toward the insertion in closed-chain situations.

Muscle Fiber Arrangement

  1. Parallel Muscle Fibers
    • Longer, providing greater range of motion.
  2. Oblique Muscle Fibers
    • Shorter, providing greater power.

Types of Parallel Muscle Fibers

  1. Strap: Long and thin fibers running the length of muscle.
  2. Fusiform: Wider in the middle and tapered on the ends.
  3. Rhomboidal: Four-sided and flat with broad attachments.
  4. Triangular: Flat, narrow attachment on one end, broad at the other.

Types of Oblique Muscle Fibers

  1. Unipennate: Fibers fan out on one side of a central tendon.
  2. Bipennate: Fibers fan out on both sides of a central tendon.
  3. Multipennate: Fibers fan out from several tendons.

Types of Muscle Contractions

  1. Isotonic: Tension remains constant while the muscle length changes.
  2. Isometric: Force is produced without a change in muscle length.
  3. Isokinetic: Speed of joint movement is kept constant.

Isotonic Muscle Contractions

  1. Concentric Contraction: Muscle ends move closer together (shortening contraction).
  2. Eccentric Contraction: Muscle ends move further apart (lengthening contraction); usually occurs under gravity (lowering) and during deceleration activities.

Strength of Contraction

  • Concentric: Greater speed results in less force produced.
  • Eccentric: Greater speed of lengthening results in higher force production.

Kinetic Chains

  1. Open Kinetic Chain: The distal segment of a limb is free to move (e.g., bending elbow).
  2. Closed Kinetic Chain: The distal segment is fixed, moving all joints in the extremity simultaneously (e.g., standing from a chair).

Muscle Mechanics

  • Muscles typically span one joint and attach to two consecutive bones.
  • Without opposing force, contracting muscle would move bones toward each other.
  • Muscle body is usually proximal to the segment moved, affecting joint type and muscle position.

Role of Skeletal Muscle

  • Prime Mover (Agonist): Muscle primarily responsible for motion at a joint.
  • Antagonist: Muscle producing action opposite the agonist.
  • Synergist: Muscle that assists in enhancing the action of the agonist.

Functions of a Synergist

  1. Assist the agonist in movement.
  2. Neutralize any unwanted movements.
  3. Stabilize a part to allow movement of another part.

Muscle Function Terms

  1. Reciprocal Inhibition: When a muscle contracts, its antagonist relaxes (is inhibited).
  2. Co-Contraction: Agonist and antagonist contract simultaneously for stability.

Muscle Location Indicators

  • Anterior Joint Aspects: Indicate flexor actions.
  • Posterior Joint Aspects: Indicate extensor actions.

Tonic vs. Phasic Muscles

  • Tonic Muscles: Maintain joint stability, compressive force; respond quickly to training, slowly to immobilization; shorten with overuse.
  • Phasic Muscles: Provide movement, react slowly to training, quickly to immobilization; weaken with overuse.

Length-Tension Relationship Terms

  1. Tension: Force buildup in a muscle (active or passive).
    • Active: Force produced during contraction.
    • Passive: Force produced during stretching.
  2. Tone: Slight tension present in muscle, indicating readiness.
  3. Excursion: The distance from maximum lengthening to maximum shortening.

Active and Passive Insufficiency

  1. Active Insufficiency: Due to excessive shortening, the force production of muscle decreases.
  2. Passive Insufficiency: Inactivity of antagonist may not allow full range of motion for agonist; commonly occurs in two-joint muscles.

Tenodesis

  • Refers to grip created through passive insufficiency of long finger tendons; when the wrist extends, fingers curl to create grip, and when wrist flexes, fingers open passively.

Excitation of Nerve and Skeletal Muscle Fibers

  • Movement originates at the CNS; the stimulus is electrochemical.
  • The stimulus transfers to the muscle at the myoneural junction.

Myoneural Junction

  • Contact point between nerve cell and muscle fiber where the nerve reaches the end plate.
  • Release of acetylcholine leads to increased muscle membrane permeability, allowing ions to flow across and cause contraction through bonding of actin and myosin.

Motor Unit

  • Defined as the final common pathway consisting of an alpha motor neuron, its axon, and the muscle fibers it innervates.
  • One neuron can innervate between 5 and 1000 muscle fibers.

Factors Influencing Strength of Muscular Contraction

  1. Number of muscle fibers innervated by the motor unit.
  2. More activated motor units equate to stronger contraction.
  3. Faster rate of stimulus leads to stronger contraction.
  4. Smaller motor neurons recruited first.

Additional Factors

  1. More cross-bridges lead to greater tension.
    • Muscles with larger cross-sectional areas have more actin and myosin for cross-bridge formation.
    • The length-tension relationship demonstrates an optimal length for maximum cross-bridge formation.

All or None Law

  • States that when a muscle fiber is stimulated to contract, it will do so maximally.

Sensory Receptors in the Muscle

  1. Golgi Tendon Organs: Sense tension in the muscle, located near the myotendinous junction; they inhibit muscle action to protect against overexertion.
  2. Muscle Spindle: Detects length changes and the speed of those changes; located within the muscle and functions to stimulate muscle contraction; the receptor for the stretch reflex (Deep Tendon Reflex - DTR).

Effects of Immobilization on Muscle

  • Causes loss of sarcomeres and increased connective tissue within the muscle lead to muscle atrophy.

Effects of Injury on Muscle

  • Results in reduced maximal force production, loss of coordination, delayed onset muscle soreness, and post-exercise soreness.

Effects of Aging on Muscle

  • Increased connective tissue concentration leads to muscle stiffness; limited response to training with advancing age.