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Muscle Anatomy and Function

Major Muscles Overview

  • Pectoralis Major: Large muscle contributing to upper body movement; involved in shoulder adduction and internal rotation.
  • Deltoid: Covers the shoulder; responsible for shoulder abduction.
  • Trapezius: Upper back muscle; involved in scapular stabilization and movement.
  • Biceps Brachii: Flexes the elbow and shoulder.
  • Triceps Brachii: Opposes biceps; extends the elbow and shoulder.
  • Latissimus Dorsi: Responsible for shoulder extension, adduction, and internal rotation.

Muscular Functions

  • Body Movement:

  • Interaction of bones, skeletal muscles, and joints enables movement. Skeletal muscle contractions exert force on tendons, pulling bones.

  • Smooth muscles aid in internal movements (e.g., food through the GI tract).

  • Cardiac muscle pumps blood; skeletal muscle assists in venous return.

  • Stabilization & Body Posture:

  • Postural muscles help maintain positions (e.g., neck muscles hold head upright).

  • Muscles can be active even without visible movement.

Properties of Muscle Tissue

  • Elasticity: Ability to return to resting length.
  • Extensibility: Ability to stretch beyond resting length.
  • Contractility: Ability to generate tension and shorten with an applied force.

Muscle Contraction Types

  1. Isotonic Contraction: Muscle length changes under tension.
  • Concentric: Muscle shortens (e.g., lifting weight).
  • Eccentric: Muscle lengthens while under tension (e.g., lowering weight).
  1. Isometric Contraction: Muscle generates force without changing length (e.g., holding a plank).
  2. Isokinetic Motion: Rare in exercise; involves dynamic movement at a constant speed under controlled conditions.

Neuromuscular Function

  • Central Nervous System (CNS): Comprises brain and spinal cord, controls motor function.
  • Peripheral Nervous System (PNS): Includes motor neurons sending signals from CNS to muscles.
  • Energy for Muscle Contraction: Powered by ATP (Adenosine Triphosphate) and phosphocreatine, which rapidly generate energy during physical activity.

Motor Units and Muscle Fibers

  • A motor unit consists of a motor neuron and the muscle fibers it innervates.
  • Types of motor units:
  • Type I: Slow-twitch fibers; endurance-oriented, fatigue resistant.
  • Type IIa: Intermediate fibers; moderate forces with some fatigue resistance.
  • Type IIx: Fast-twitch fibers; produce high forces quickly but fatigue rapidly.

Muscle Recruitment During Movements

  • Muscle Roles:
  • Agonist: Primary mover (e.g., biceps in elbow flexion).
  • Antagonist: Opposing muscle that lengthens (e.g., triceps in elbow flexion).
  • Fixator: Stabilizes the origin of the muscle (e.g., trapezius stabilizing the scapula).
  • Synergist: Assists the agonist to enhance movement (e.g., brachialis in a bicep curl).

Reciprocal Inhibition

  • Fundamental concept where the contraction of the agonist muscle is accompanied by the relaxation of the antagonist muscle, promoting efficient movement and preventing injury.

Delayed Onset Muscle Soreness (DOMS)

  • Occurs 1-2 days post-exercise, often due to high levels of eccentric contractions. It results from sub-cellular damage and inflammation within the muscle fibers.

Muscle Fiber Types and Exercise

  • Type I (Slow-Twitch): Suited for endurance and aerobic activities (e.g., long-distance running).
  • Type IIa (Fast-Twitch): Suited for sustained power and aerobic/anaerobic activities (e.g., cycling).
  • Type IIx (Fast-Twitch): Suited for explosive power activities (e.g., sprinting).

Summary of Muscle Actions

  • Common Movements:
  • Bicep Curl: Agonist - Biceps Brachii; Antagonist - Triceps Brachii.
  • Squats: Agonist - Quadriceps; Antagonist - Hamstrings.
  • Push-Ups: Agonist - Pectoralis Major; Antagonist - Latissimus Dorsi.