Myo-Biomechanics

Introduction to KIN2230: Introductory Exercise Physiology

  • Course offered by Western Health Sciences, School of Kinesiology.

Learning Outcomes

  • Students will have the opportunity to:

    • Review muscle contraction.

    • Learn about mechanical properties of muscle:

    • Traits of Force Generation.

    • States of Force Generation.

  • Focus on:

    • What factors affect the ability of the muscle to generate force.

    • The states of muscle necessary for force generation.

Muscle Action and Mechanisms

Muscle "Magic"

  • Excitation-Contraction Coupling:

    • The muscle action potential depolarizes transverse (T) tubules at the A-I junction of the sarcomere.

    • Acetylcholine (ACh) is released from sac-like vesicles within the terminal axon, diffusing across the synaptic cleft and attaching to ACh receptors on the sarcolemma.

    • This wave of depolarization leads to the release of Ca2+ from lateral sacs of the sarcoplasmic reticulum.

    • Ca2+ Mechanism:

    • Ca2+ binds to the troponin-tropomyosin complex on actin filaments, releasing the inhibition of actin combining with myosin.

    • Myosin ATPase splits ATP, releasing energy that facilitates muscle action.

    • The energization of myosin results in the binding to actin and the formation of the crossbridge.

    • When ATP binds to the myosin crossbridge, it breaks the actin-myosin bond allowing the dissociation of the crossbridge from filament actin.

    • Ca2+ Restoration:

    • Upon cessation of muscle stimulation, Ca2+ returns to the sarcoplasmic reticulum through active transport via ATP hydrolysis, restoring the inhibitory action of troponin-tropomyosin.

    • With ATP present, actin and myosin remain in a relaxed state when dissociated.

Crossbridge Theory of Muscle Mechanics

  • Crossbridge Cycle:

    1. Energized myosin head binds to actin.

    2. ADP is released during the power stroke: the crossbridge rotates pulling the thin filament toward the sarcomere center.

    3. ATP binds to myosin, causing detachment from actin.

    4. ATP is hydrolyzed into ADP and inorganic phosphate (Pi), resetting the crossbridge to an energized state.

    • The cycle repeats with energetic interactions alongside muscle shortening.

Determinants of Muscle Force Generation

Key Factors Affecting Muscle Contraction

  • Force is determined by the number of active crossbridges:

    • Each crossbridge produces:

      • 3.4 pN of force.

      • 10 nM of shortening.

    • Each crossbridge consumes:

      • 1 ATP per cycle.

    • Force Production Examples:

      • 1 crossbridge: 3.4 pN of force.

      • 2 crossbridges: 6.8 pN of force.

      • 3 crossbridges: 10.2 pN of force.

      • 4 crossbridges: 13.6 pN of force, and so on.

Traits of Force Generation

  • Muscle Performance Metrics:

    • Muscle strength (max force generation).

    • Muscle size (cross-sectional area, CSA).

    • Muscle pennation (angular orientation of muscle fibers).

    • Muscle composition (density and type of muscle fibers).

    • Muscle endurance (capacity to sustain force).

  • Capillarization:

    • More capillaries enable better oxygen supply, affecting endurance.

  • Mitochondrial Density and Function:

    • Increased mitochondrial density enhances muscle metabolism, sustaining force.

  • Effect of Muscle Size on Strength:

    • Larger muscles correlate with greater force due to increased crossbridges.

    • Essential studies cited: Marusic et al. (2021), Balshaw et al. (2021) highlighting muscular adaptations.

States of Muscle Generation

Length Dependence

  • Muscle force output depends on the sarcomere length affecting overlap:

    • Active Crossbridges:

    • Short length can lead to fewer active crossbridges due to interference from thin filaments.

    • Optimal overlap needed for maximal force production.

    • Overlap Conditions:

    • Too much stretching can reduce force production, whereas optimal lengths maintain the best conditions for maximum tension.

Velocity Dependence

  • Velocity of Shortening:

    • Vector Analysis in Muscle Actions:

    • Concentric (muscle shortening), isometric (no change in muscle length), and eccentric (muscle lengthening) define the varying states of velocity.

    • Positive velocity indicates concentric contractions, zero indicates isometric, and negative indicates eccentric contractions.

  • Crossbridge Kinetics in Different Contractions:

    • In isometric conditions, crossbridges attach and generate force without changing the length.

    • In concentric conditions, force diminishes as actin slides past myosin.

    • In eccentric contractions, crossbridges remain attached while generating force, which leads to greater force requirements due to stretching.

Conclusion on Force Output

  • Total force output of muscle is influenced by:

    • Traits (muscle strength and endurance) and states (length and velocity).

    • Optimal manipulation of crossbridge dynamics is crucial for performance efficacy in physical activities.

Questions and Discussion

  • End of materials for KIN2230 with an invitation for student questions.

  • Course affiliation: Western Health Sciences, School of Kinesiology.