Muscle Phys 2 AY2024


Part I: Muscle Properties

  • Focus on different muscle fiber types.

Muscle Fiber Types


    1. Slow-oxidative (Type I) fibers


    1. Fast-oxidative (Type IIa) fibers


    1. Fast-glycolytic (Type IIx) fibers

  • Distinction between slow twitch fibers and fast twitch fibers.

Characteristics of Skeletal Muscle Fibers Table

  • Fiber Types and Characteristics:

    • Slow-Oxidative (Type I): Low Myosin-ATPase activity, slow contraction, high fatigue resistance, high oxidative capacity, many mitochondria and capillaries, high myoglobin, red color, low glycogen content.

    • Fast-Oxidative (Type IIa): High Myosin-ATPase activity, fast contraction, intermediate fatigue resistance, high oxidative capacity, many mitochondria and capillaries, high myoglobin, red color, intermediate glycogen content.

    • Fast-Glycolytic (Type IIx): High Myosin-ATPase activity, fast contraction, low fatigue resistance, low oxidative capacity, few mitochondria and capillaries, low myoglobin content, white color, high glycogen content.

Comparing Muscle Fibers

  • Fast Versus Slow Fibers:

    • Higher Myosin-ATPase activity allows quicker ATP splitting for faster energy for crossbridge cycling.

  • Oxidative Versus Glycolytic Fibers:

    • Oxidative fibers are more fatigue-resistant.

  • Genetic Endowment:

    • Most muscles contain a mix of all three fiber types; specialization affects the percentage of each type, determining potential for activities like sprinting or long-distance running.

Chicken vs Cow - Muscle Color Comparison

  • Distinguishes between white (fast-twitch) and red (slow-twitch) meat based on muscle fiber types.

Muscle Adaptations (1)

  • Muscle fibers adapt to demands:

    • Improvement in oxidative capacity due to aerobic endurance exercise.

    • Muscle hypertrophy from anaerobic high-intensity resistance training.

    • Testosterone influences myosin and actin synthesis and assembly.

    • Jorgenson et al. study reference.

Muscle Adaptations (2)

  • Interconversion of Fiber Types:

    • Type IIa ↔ Type IIx.

    • Slow and fast fibers generally not interconvertible, except in specific cases (e.g., spinal injury).

  • Repair Mechanisms:

    • Satellite cells can become muscle-forming precursors post-damage.

  • Muscle Atrophy:

    • Caused by disuse, denervation, or aging ("Use it or lose it!").

Sarcopenia

  • Definition: Gradual muscle loss starting after age 40.

  • Rates of loss: Approximately 1% per year, with accelerated loss after age 50, especially in males.

  • Consequences: Permanent loss affects daily activities and bone mass.

Part II: Muscle Mechanics

  • Focus on contractions of muscles and related mechanics.

Contraction of Whole Muscles

  • Whole muscles made up of bundled fibers attached to bones via collagenous tendons.

  • Internal tension produced within sarcomeres, transmitted to bones through tendon.

Lever System

  • Description of skeletal muscle interaction with bones and joints:

    • Bones act as levers, joints as fulcrums, and muscles provide force.

Amplification of Velocity and Distance

  • Description with examples of muscle forces and lever mechanics, finding a common system for movement formulation.

Primary Types of Contraction

  • Isotonic: Load remains constant while muscle length changes.

  • Isometric: Muscle length remains the same while tension increases.

  • Isokinetic: Velocity remains constant as fibers shorten.

Types of Muscle Contraction

  • Concentric Contraction: Muscle shortens under load.

  • Eccentric Contraction: Muscle lengthens under load.

  • Some muscles (e.g., tongue) do not attach to bones at both ends yet still produce movement.

Exercise-Induced Muscle Damage

  • Eccentric exercises increase muscle damage risk.

  • Ultrastructural damage: sarcomere disruption, loss of Z lines, widening of filament distance.

  • Fast twitch fibers more susceptible to eccentric damage due to smaller proteins.

Load-Velocity Relationship

  • Inverse relationship observed in eccentric contractions; heavier loads result in slower lifts.

  • Energy expenditure during muscle contractions: 25% for external work, 75% lost as heat.

Graded Contractions

  • Muscle contraction strength depends on:

    • Number of fibers contracting

    • Amount of tension developed.

Motor Unit Recruitment

  • 1 motor unit consists of one motor neuron and all muscle fibers it innervates.

  • More motor units recruited lead to stronger contractions through increased fiber activation.

Factors Influencing Muscle Tension

  • Key factors:

    1. Frequency of stimulation

    2. Length of fiber at contraction onset

    3. Extent of fatigue

    4. Thickness of the fiber.

Twitch Summation

  • Increased stimulation frequency leads to twitch summation through sustained elevation of cytosolic Ca2+ and more time to stretch the series-elastic component.

Optimal Muscle Length for Maximal Tension

  • Optimal length (lo) enables maximum myosin cross-bridge interactions.

  • Length too short results in filament overlap; too long leaves cross-bridges unengaged.

Extent of Fatigue

  • Muscle fatigue can originate peripherally or centrally.

  • Peripheral fatigue: muscles can no longer respond effectively due to localized chemical changes.

  • Central fatigue occurs when CNS activation of motor neurons is inadequate.

Fiber Thickness and Muscle Strength

  • Influences include strength training, testosterone, and vigorous weight training that increase myofiber numbers and size.

Part III: Motor Control

  • Focus on muscle spindles, Golgi tendon organs, and reflexes.

Control of Motor Activity

  • Central nervous system (CNS) involvement:

    • Brain, spinal cord, and motor neurons coordinate movements.

  • Types of movements:

    1. Voluntary

    2. Reflexes

    3. Rhythmic actions (driven by central pattern generators).

Afferent Inputs

  • For effective motor control, CNS requires continuous input on muscle changes:

    1. Muscle length (detected by muscle spindles)

    2. Muscle tension (detected by Golgi Tendon Organs)

Muscle Spindles Structure

  • Description of muscle spindle components involved in sensing and responding to muscle length changes.

Neuronal Control of Muscle Spindles

  • Roles of afferent neurons, alpha motor neurons, and gamma motor neurons detailed in muscle spindle function.

Alpha-Gamma Coactivation

  • Mechanism ensuring the muscle spindle remains taut during muscle contraction to communicate length changes effectively.

Golgi Tendon Organ

  • Functions as a tension gauge, transmitting muscle tension to bones to facilitate movement.

Stretch Reflex

  • Example of patellar tendon reflex that senses and resists changes in muscle length for maintaining balance and posture.

Withdrawal Reflex

  • Mediated at spinal cord level

  • Reflex pathway description:

    • Response mechanism from sensory input to muscle contraction, promoting reflexive withdrawal from stimuli.

Crossed Extensor Reflex

  • Description of crossed reflex pathway that is “opposite muscular contraction” to bear weight on uninjured limb