Mechanisms of Muscle Tension and Force

Factors Influencing Muscle Tension and Force

  • Mechanisms manipulating tension and force produced in muscles.

  • Why not all muscle fibers contract for movement?

  • Factors influencing muscle tension:

    • Muscle fiber diameter

    • Muscle fiber length

    • Contraction speed

    • Motor unit size and type

    • Motor unit recruitment

    • Stimulation frequency

Muscle Fiber Diameter

  • Larger diameter = greater force production.

  • Comparison: big vs. small muscle fiber.

Muscle Fiber Length

  • Longer muscle = greater force production.

  • Greater leverage capacity.

Contraction Speed

  • Speed impacts force produced.

  • Modulation of speeds based on activity requirements.

Motor Units

  • Motor unit: motor neuron + all muscle fibers it innervates.

  • Stimulus from central nervous system needed.

  • Multiple motor units innervate a muscle (10-30 depending on size).

  • Size and type influence force production.

Motor Unit Recruitment

  • Size Principle: how motor units are recruited.

  • Recruitment sequence: small to large?

Stimulation

  • Muscle requires action potentials.

  • Ramping up action potentials affects force production.

  • Muscle's saturation and exhaustion point.

  • Hertz (Hz): electrical impulses per second.

    • Higher Hz = more stimuli = increased discharge rate/frequency.

    • Lower Hz = less stimuli = less frequency.

    • Impact on muscle tension.

Muscle Fiber Usage

  • Why not all muscle fibers work at once?

    • Inefficient and ineffective.

    • Limits coordinated movement patterns.

Creating Tension

  • Muscle twitch (not a spasm).

  • Summation of stimulation.

  • Tetanus-based stimulus.

Muscle Fiber Diameter (cont.)

  • Greater diameter = more myofibrils.

  • Myofibrils: Myosin and Actin.

  • Larger motor units:

    • Need bigger motor neuron/impulse.

    • Innervate more fibers.

Interneurons and Motor Units

  • Interneuron: spinal cord to motor units.

  • Smaller motor unit: smaller number of fibers.

  • Bigger motor unit: more fibers.

  • More fibers = more myosin and actin = increased force production.

  • Greater motor units produce greater force production because there are more crossbridge cycling that can occur.

  • Smaller motor units have smaller amounts of cross bridges, less force production.
    Bigger diameter →\rightarrow more cross bridges →\rightarrow more force production.
    Smaller diameter →\rightarrow smaller amount of cross bridges →\rightarrow less force production.

Muscle Length and Optimal Overlap

  • Muscles have an optimal length/zone of overlap for maximal tension/force.

  • Sarcomere:

    • Myosin (thick filament).

    • Actin (thin filaments).

    • Nice overlap of myosin and actin = ability to shorten and produce tension.

  • Stretched muscle:

    • Minimal or no overlap of myosin and actin.

Tension in Pre-Contracted Muscle (e.g., Bicep)

  • Pre-contracted: decreased optimal zone of overlap.

  • Limited room for myosin and actin engagement.

  • Force production:

    • Ranges from 0 to 60% of total.

    • Easiest part of movement (e.g., bicep curl).

  • Increased length (90-15 degree mark in bicep curl):

    • Hardest part of movement.

    • Around 90% of maximal tension.

    • Optimal zone of overlap.

  • Lengthening muscle further (stretching sarcomeres):

    • Tension drops drastically.

    • No zone of overlap.

Optimal Zone of Overlap

  • Too compressed: minimal force production.

  • Optimal zone: maximal force production.

  • Stretched too far: no cross-bridge cycling, minimal/no force.

Velocity/Speed of Contraction

  • Slower movement = more force.

  • Faster movement = less time for cross-bridges to engage.

Velocity and Force Production

  • Zero velocity (isometric contraction):

    • 80% down to 50% force production.

  • Increasing speed:

    • Force declines (down to 25%).

    • Not enough time for myosin heads to attach to actin effectively.

    • Compromised cross-bridge cycling.

  • Slower movements:

    • More time for myosin to bind, pivot, detach, re-energize.

Muscle Size and Cross-Sectional Area

  • Bigger cross-sectional area = more potential for myosin and actin filaments.

  • Greater cross-bridge cycling = more force.

Example

  • Rectus femoris vs. bicep (cross-sectional area).

Formula

Force∝Cross-Sectional Area\text{Force} \propto \text{Cross-Sectional Area}

Graph Demonstration

Muscle length is still the same. Take a muscle at 12 millimeters, take a muscle 35 millimeters, both muscles are the same length. But if they have a different diameter in the muscle fiber, so we can see here, one is looking at going maybe to forty, fifty percent, one is going up to a %. Those with a larger physiological cross sectional area will then produce more force.