Muscle Mechanics and Motor Unit Control

Length-Tension Relationship

• Sarcomere Level:

  • Sarcomeres are typically $2-3$ microns in length.
  • Too Short: If the sarcomere is significantly shortened, the filaments (actin and myosin) become completely overlapped. This limits further shortening and results in very low tension generation because there's nowhere left for the filaments to slide.
  • Too Long: If the sarcomere is stretched to a very long length, there is minimal to no overlap between the actin and myosin filaments. This reduces the opportunity for active myosin cross-bridges to engage with actin, leading to very little tension generation.
  • Optimal Length: There is an optimal sarcomere length at which maximum active tension can be generated, as it allows for the greatest number of myosin heads to optimally bind to actin.

• Whole Muscle Level (Active and Passive Tension):

  • The length-tension relationship at the whole muscle level differs from the sarcomere level due to additional components like connective tissue, elastic properties, and overall biomechanics.
  • Active Tension Curve: This curve reflects the tension generated primarily by the actin-myosin cross-bridge cycling within the muscle fibers. Similar to the sarcomere, it has an optimal length for maximum active force production. As the muscle is stretched beyond this optimal length, active tension decreases.
  • Passive Tension: As a whole muscle is stretched beyond its resting (often optimal) length, passive elements such as connective tissue (e.g., perimysium, epimysium, tendons) and elastic components (e.g., titin) begin to resist the stretch. This resistance generates passive tension.
  • Total Tension Curve: The total tension produced by a whole muscle at any given length is the sum of its active tension and passive tension (Total Tension = Active Tension + Passive Tension). When the muscle is stretched significantly beyond its optimal contractile length, the passive tension becomes a dominant contributor to the overall force, causing the total tension curve to rise sharply.
  • Take-Home Message: The force production of a muscle is significantly affected by its initial length status (optimal, shorter, or longer) when contraction is initiated.

Force-Velocity Relationship

• This relationship describes how the force a muscle can generate is inversely related to its velocity of shortening.
• Maximum Velocity of Shortening ($V_{max}$): Occurs when there is minimal to no resistance or load on the muscle. The muscle can shorten at its fastest possible speed under these conditions.
• Curve Characteristics: The relationship is curvilinear.
  • Concentric Contraction (Shortening): As the load on the muscle increases, the velocity of shortening decreases. The highest force is generated at zero shortening velocity (isometric contraction).
  • Isometric Contraction: Occurs when the load is so large that the muscle cannot shorten, and there is no joint movement. This is a midpoint on the force-velocity curve, where force generation is high, but velocity is zero.
  • Eccentric Contraction (Lengthening): When the external load exceeds the muscle's maximal concentric force, the muscle lengthens while still trying to contract. During eccentric contractions, a muscle can generate greater force than during isometric or concentric contractions. However, the curve flattens out at very high lengthening velocities/loads because there's a limit to how much force the muscle can withstand before potential damage (e.g., ripping or tearing).
• This relationship is observed in both single muscle fibers and whole muscles.

Power Relationship

• Definition: Power is a functional measure of muscle performance that combines force and velocity. It represents the rate at which work is done.
• Formulas:
  • Power = Force x Velocity
  • Work = Force x Distance
  • Therefore, Power = (Force x Distance) / Time
• Peak Power: Optimal or peak power output is typically achieved at approximately 50 ext{%} of a muscle's maximum shortening velocity and 50 ext{%} of its maximum force/strength. Power output decreases on either side of this optimal point.
• Power Curves: These curves can be plotted and analyzed under various conditions such as training, retraining, fatigue, disease, and aging. These conditions can cause power curves to shift (e.g., up or down, indicating improvements or decrements in power).

Motor Unit Function and Electromyography (EMG)

• Motor Unit: Consists of a single motor neuron and all the muscle fibers it innervates. Muscles contain hundreds of motor units, each connected to hundreds of muscle fibers.

• Voluntary Contraction: Involves activating numerous motor units, leading to the contraction of their associated muscle fibers. The overall muscle force is roughly the summation of individual action potentials from all active motor units.

• Surface Electromyography (sEMG):

  • Method: Records the summed electrical activity (action potentials) of asynchronously activated motor units using electrodes placed on the skin over the target muscle.
  • Usefulness:
    • Indicates muscle activation (absence of activity at rest).
    • Shows a general relationship between increased electrical activity and increased force generation in a healthy system.
  • Limitations: Provides a global, summed signal that doesn't allow for the discrimination of individual motor units or their specific firing patterns.

• Intramuscular Electromyography (IMEMG / Needle EMG):

  • Method: Involves inserting a fine wire or needle electrode directly into the muscle tissue.
  • Usefulness:
    • Offers a much more selective recording, picking up signals from individual motor units in close proximity to the electrode.
    • Allows for the decomposition or extraction of firing rates from individual motor units by analyzing distinct action potential waveforms.
  • Advantages over sEMG: Provides detailed information about specific motor unit activation, recruitment thresholds, and firing rates, which is crucial for understanding neural control.
  • Limitations: Cannot directly determine if a recorded motor unit is fast-twitch, slow-twitch, large, or small in anesthetized preparations; it's also more invasive.

Grading Muscle Force: Two Mechanisms of Neural Control

To control the force output of a muscle, the central nervous system primarily utilizes two mechanisms:

1. Recruitment (Spatial Summation) - Activating More Motor Units

• Henneman's Size Principle: This fundamental principle, established by Elwood Henneman in the $1950s$ (building on earlier $1920s$ research), explains the orderly recruitment of motor units.
  • The Principle: Motor neurons are recruited in a fixed, predictable order from smallest to largest size. This size refers to the motor neuron's cell body (soma) in the spinal cord.
  • Small Motor Units:
    • Possess small cell bodies.
    • Innervate relatively few muscle fibers (e.g., slow-twitch, Type I fibers).
    • Generate a small amount of force.
    • Are highly excitable: Due to their small size, the synaptic current from descending neural input is concentrated on a smaller membrane surface. This makes them easier to depolarize and reach action potential threshold (analogized to a