Exercise Science 7

Muscular System Overview

  • This chapter, presented by Nicole Mendola, MS, RCEP, C-EP, GEI, EIM III, focuses on the muscular system in detail.

Objectives

  • Discuss different types of muscles in the human body with a focus on skeletal muscle.

  • Cover detailed aspects including:

    • Muscle fiber architecture

    • Skeletal muscle organization

    • Major principles of muscular function:

      • Sliding filament theory

      • All or None Principle

      • Types of contractions

      • Muscle fiber types

Types of Muscles in the Human Body

  • Smooth Muscle:

    • Found in internal organs

    • Involuntary control

  • Cardiac Muscle:

    • Found in the heart

    • Involuntary control

  • Skeletal Muscle:

    • Primarily attached to bones

    • Voluntary control

    • Responsible for moving the skeletal system and stabilizing the body

    • Comprises more than 600 muscles, including 100 primary movement muscles

    • Generally attached to the skeleton by tendons

      • Tendons: Dense cords of connective tissue that attach a muscle to the periosteum

Skeletal Muscle Architecture

  • Classification of skeletal muscles by fiber architecture:

    • Architecture: Arrangement of muscle fibers relative to the line of pull of the muscle

      • Parallel Arrangement: Muscle fibers run in line with the pull of the muscle

        • Fusiform: A spindle-shaped arrangement tapering at each end

        • Longitudinal/Strap: Strap-like, with parallel fibers

        • Quadrate: Four-sided, flat, consisting of parallel fibers

        • Fan-shaped: Fibers radiate from a narrow attachment at one end to a broad attachment at another

      • Pennate Arrangement: Fibers run obliquely or at an angle to the line of pull

        • Unipennate: Fibers on one side of the tendon

        • Bipennate: Fibers on both sides of a centrally positioned tendon

        • Multipennate: Two or more fasciculi attach obliquely and combine into one muscle

Classification Based on Role in Movement

  • Agonist: Muscle responsible for the primary movement; referred to as the "prime mover".

  • Antagonist: Opposing muscle that relaxes to permit primary movement and contracts to act as a brake at the completion of movement.

  • Synergists: Additional muscles involved in the movement to prevent unwanted movements and assist prime movers.

  • Fixators/Stabilizers: Stabilize joints to provide a stable base for movement; can involve co-contraction where both agonist and antagonist contract simultaneously.

Muscle Properties

  • Irritability: Ability to respond to electrical or mechanical stimuli.

  • Extensibility: Capacity of a muscle to be stretched like an elastic band.

  • Elasticity: Ability of a muscle to return to its resting length after a stretching force is discontinued.

  • Contractility: Unique property of muscle tissue to shorten and produce tension.

Structure of Skeletal Muscle

  • Hierarchical Structure:

    • Whole muscle consists of fascicles, which contain muscle fibers (cells), which in turn consist of myofibrils, sarcomeres, and myofilaments.

      • Muscle Components:

        • Tendon: Connective tissue attaching muscle to bone

        • Muscle Belly

        • Fasciculus: Bundle of muscle fibers covered by perimysium

        • Myofibril: Part of muscle fiber; responsible for contraction

        • Sarcomere: Functional unit of muscle contraction

        • Myofilaments: Composed of actin (thin) and myosin (thick)

        • Endomysium: Connective tissue enveloping individual muscle fibers

        • Perimysium: Connective tissue surrounding fascicles

        • Epiysium: Outermost layer surrounding the muscle

      • Sarcoplasm: Cytoplasm of a muscle cell containing organelles like nuclei, mitochondria, and stored nutrients (myoglobin and glycogen).

Muscle Contraction Principles

  • Sliding Filament Theory:

    • Muscle contraction occurs via cross bridge cycling where:

      • Myosin heads attach to actin in response to a nerve impulse.

      • The heads cock towards the center of the sarcomere, pulling the actin towards the center, resulting in muscle shortening.

  • All-or-None Principle: An action potential in a muscle cell will lead to maximal contraction or no contraction, depending on whether the stimulus reaches the threshold.

Neuromuscular Activation

  • Motor Neuron: Functional unit of the neuromuscular system that transmits signals to muscle fibers through:

    • Dendrites which collect signals

    • Axon which passes signals

    • Axon terminals that form neuromuscular junctions with muscle fibers.

  • Motor Unit: A motor neuron and the muscle fibers it innervates; varies in size and requires activation of all available motor units for maximal force.

Types of Muscle Contraction

  • Static Contractions (Isometric):

    • Muscle maintains constant length with no change in joint position.

    • Strength gains noted to be limited to specific joint angles; useful in rehabilitation.

  • Dynamic Contractions (Isotonic):

    • Involves movement of joints.

      • Concentric Contraction: Muscle shortens while producing force.

      • Eccentric Contraction: Muscle lengthens under load which can produce greater force.

Force-Velocity Relationship

  • As the speed of a muscular contraction increases, the force it can exert decreases.

Muscle Fiber Types

  • Muscle fibers are categorically different but not mutually exclusive. Changes in muscle types due to training remain a question of research.

Type I Muscle Fibers

  • “Slow Twitch” Fibers:

    • Primarily aerobic in nature.

    • Highly fatigue resistant.

    • Suited for low-intensity, long-duration activities.

    • Average person has roughly 50% slow-twitch fibers.

Type II Muscle Fibers

  • “Fast-Twitch” Fibers:

    • Primarily anaerobic.

    • Designed for power and high-intensity speed tasks.

  • Two Subtypes:

    • Type IIA: Transitional between type I and type II fibers.

    • Type IIx: Develop tension and shorten faster than type I fibers.

Effects of Training on Muscle Fibers

  • It is not possible to increase the number of muscle fibers we possess.

  • Training can possibly increase the size of muscle fibers.

  • Training may also shift the type of some muscle fibers from fast to slow-twitch and vice versa depending on the nature of the training undertaken.