IG

Peripheral Neuromuscular Mechanisms in Executing Movement - Study Notes

The Motor Unit

  • A motor unit (MU) consists of a single lower motor neuron (alpha motor neuron) and all the muscle fibers it innervates.
  • Innervation ratio: the number of muscle fibers per motor neuron. Typical range: 15 \le \text{innervation ratio} \le 2000.
  • All fibers within a single MU are of the same type (either slow-twitch or fast-twitch).
  • Muscle fibers are distributed throughout the muscle (not all clustered in one spot).
  • This chapter focuses on how MU properties and their control contribute to movement execution.

Motor Unit Characteristics

  • MU size is defined by the size of the neuron/axon; this size is linked to other properties:
    • Small MU: neuron/axon size is small; few muscle fibers; typically slow-twitch fibers; least energy for activation; recruited first; function: endurance/precision.
    • Medium MU: neuron/axon size medium; a moderate number of fibers; fiber type: slow-to-intermediate or mixed; energy demand moderate; recruitment second.
    • Large MU: neuron/axon size large; many muscle fibers; fast-twitch fibers; most energy for activation; recruited last; function: force/power.
  • Summary of typical MU properties by size:
    • Neuron/ax on size: Small → Medium → Large
    • Number of fibers: Few → Medium → Many
    • Fiber type: Slow twitch → Slow/Intermediate → Fast twitch
    • Energy for activation: Least → Moderate → Most
    • Recruitment order: First → Second → Third
    • Function: Endurance → Precision → Mixed/Force
  • Note: These relationships underpin how different units contribute to different aspects of movement (fine control vs high force).

Two Principles of Motor Unit Behavior

  • All-or-none principle: within a MU, all the muscle fibers fire together or none fire.
  • Size principle of recruitment: MUs fire in order from small to large and are derecruited in the reverse order. This principle arises because smaller neurons require less energy for activation and are easier to recruit.
    • Other factors that can delay activation of large MUs: synapse size and receptor sensitivity (morphological factors).

Motor Unit Behavior & Force Control

  • MUs are recruited to control small amounts of force within a single muscle and to enable broader limb and body movements.
  • The nervous system regulates three basic functions to control force output:
    • Recruitment of motor units
    • Rate coding (firing rate) of motor units
    • Coordination of motor units and muscles
  • These mechanisms operate across different muscles and movement tasks to produce smooth, adaptable force output.

MU Recruitment

  • Force output can be altered by increasing or decreasing the number of active MUs.
  • Recruitment follows the size principle: activating more MUs generally increases the total active muscle tissue.
  • Because larger MUs tend to be fast-twitch with more fibers, recruiting more MUs often yields a large jump in force/power.
  • Not all MUs may be activated until roughly 80\% \text{ to } 90\% of maximal force or effort (submaximal tasks may use only a subset).
  • Graphs in the lecture illustrate MU recruitment patterns across activities with different effort levels (e.g., sprinting vs lower-intensity tasks).
  • Note: Some sources label the precise thresholds differently, but the key idea is a progressive recruitment with increased effort.

MU Rate Coding

  • Fast MU firing rates contribute to force via summation of twitches, increasing the produced force.
  • Calcium remains in the muscle fiber cytosol longer when firing rate is high, delaying relaxation and increasing cross-bridge formation; this is part of the mechanism behind rate coding.
  • Slack in the muscle is taken up to allow force development.
  • Rate coding: increasing or decreasing firing rate to achieve moderate force levels.
  • Firing rates for MUs typically range from 5\,\text{Hz} \le f \le 120\,\text{Hz} depending on MU type and contraction type.
  • On a typical contraction, firing rates start slow and then increase; there is variation across muscles and contractions.
  • Time-varying firing patterns (e.g., ballistic vs steady contractions) are reflected in rate coding curves.

Neuromuscular Coordination

  • Neuromuscular coordination is arguably the most important way the nervous system controls muscle force output.
  • Coordination can be classified as:
    • Intermuscular coordination: coordination between different muscle groups or body segments.
    • Intramuscular coordination: coordination within a muscle, or across muscle task groups, or across multiple muscles involved in a task.

Intermuscular Coordination

  • Intermuscular coordination is observed in patterning and role-playing among different muscles.
  • A muscle can act as an agonist, stabilizer, or neutralizer depending on the movement moment.
  • Outcomes are often measured via EMG patterns and biomechanical efficiency.
  • The concept is that the nervous system orchestrates multiple muscles to achieve a smooth, efficient movement.

Intramuscular Coordination

  • Intramuscular coordination refers to the patterning and use of motor units within a single muscle or across muscle task groups.
  • It governs how firing of MUs and their recruitment are integrated to produce the required force.

Intramuscular Coordination: Recruitment & Rate Coding

  • Normal contraction force output results from a combination of MU recruitment and rate coding.
  • Rate coding (increasing/decreasing MU firing rate) is used to modulate force without adding or removing active units.
  • The combination of recruitment and rate coding depends on several factors (e.g., muscle size, contraction type).
  • Example concepts:
    • Small muscles may achieve full MU recruitment at around 30\% of maximum force.
    • Large muscles may rely more on rate coding early and may not recruit all MU until 80\% \text{ to } 90\% of maximum effort.
    • Static vs dynamic contractions and concentric vs eccentric contractions may alter the relative contribution of recruitment vs rate coding.

Intramuscular Coordination: Discharge Patterning

  • Discharge patterning is the targeted manipulation of firing rate to meet task demands; includes:
    • Sequences of 2–3 rapid action potentials to increase tension output with relatively low metabolic cost.
    • Rapid onset of high firing rates to generate ballistic contractions.
  • Muscle wisdom: during fatiguing conditions, MUs may slow their discharge rate to maintain force output, a mechanism not tied to ATP depletion or waste buildup.
  • This is thought to be an automatic energy-conservation response.
  • Synchronization of firing rates across MUs can maximize force output for short bursts.

Intramuscular Coordination: Discharge Patterning (continued)

  • Discharge patterning can also involve synchronization strategies and timing adjustments that optimize force production for quick or powerful movements.

Intramuscular Coordination: Compartmentalization

  • Compartmentalization refers to the presence of smaller, independently controlled groups of muscle fibers within a single muscle or across a group of muscles (e.g., quadriceps femoris).
  • Compartments can cross over from one muscle to another, creating potential “task groups” for coordination.
  • Compartmentalization provides the nervous system with more flexibility in coordinating movement.

Compartmentalization: How Compartments Are Defined

  • Based on:
    • Muscle morphology (e.g., slow vs fast twitch)
    • Neural recruitment (certain parts activated only for specific movements or force demands)
    • Biomechanical functions (angle of pull; proximal vs distal compartments acting on joints)
  • Example: Deltoid compartments during adduction and flexion illustrate how different fiber groups contribute differently depending on movement.
  • The exact role of compartments is not fully known, but they give the nervous system more control options.

Motor Unit Behavior: Adaptations to Training

  • Strength and power training can lead to:
    • Increased maximal MU activation to 100% if neural drive was not already maximal.
    • Increased average or maximal firing rates.
    • Earlier onset of high-threshold MUs without violating the size principle.
    • Ability to reach high firing rates sooner or to start contractions at a higher firing rate.
    • More doublets produced; possible increases in synchronization.

Training-Related Changes in MU Behavior (Strength/Power)

  • After strength training, there may be:
    • Fewer MU recruited for a given submaximal effort, suggesting higher firing rates in active units.
    • Delayed recruitment of other units; changes in intramuscular coordination.
    • MU discharge rates become less variable as movement becomes steadier and more controlled.
  • Detraining or immobilization tends to reverse these adaptations.

Neural Adaptations to Training: Intermuscular Coordination

  • Training can alter the timing and sequencing of muscle activation, including synergists and antagonists.
  • Training can reduce agonist–antagonist cocontraction and enhance activation of synergists and stabilizers.

Training Effects on Intermuscular Coordination

  • Empirical observations (e.g., a leg pedaling practice protocol) show changes in leg muscle activity after practice that reflect improved intermuscular coordination.

Muscle Properties and Neuromuscular Mechanics

  • The muscle–tendon complex has three fundamental mechanical properties:
    • Extensibility: ability to stretch
    • Elasticity: ability to recoil from a stretch
    • Contractility: ability to shorten to produce force
  • The muscle–tendon complex can be modeled as a mechanical device with:
    • Tendons as Serial Elastic Elements (SE)
    • Epimysium, perimysium, and endomysium as Parallel Elastic Elements (PE)
    • Muscle as the Contractile Element (CE)
    • Fluid as the Viscous Element (VE)
  • The combination of SE and PE constitutes the elastic elements (EE).
  • This model helps explain how mechanical properties interact with neural control to produce movement.

The Elastic Elements in Action

  • CE produces force; EE stores force and releases force via recoil.
  • Stretch and recoil depend on shortening/lengthening velocity, tissue length, thickness, and health.
  • The nervous system regulates stiffness, force absorption, and recoil by changing the timing and magnitude of muscle contraction.
  • Mechanical properties are used to distribute force for different tasks (as illustrated in the course materials).

Task–Body–Muscle–Tendon Energy and Power Concepts

  • A schematic representation shows how energy flows through body, muscle, and tendon to accomplish tasks like energy conservation, power production, and energy absorption across activities (running, hopping, walking, jumping, etc.).

Neuromechanics and the Length–Tension Relationship

  • The amount of isometric force a muscle can generate and store is dependent on muscle length.
  • The length–tension curve includes contributions from both contractile and elastic elements.
  • Variations in CE force along the curve are largely due to the number of cross-bridges formed from actin–myosin overlap.

Force–Velocity Relationship

  • The force produced by the contractile element (CE) decreases as sarcomere shortening velocity increases.
  • Higher shortening velocity leads to fewer cross-bridges forming in the available time, reducing force.
  • At low speeds, greater force can be developed; at high speeds, force potential is reduced.
  • Conceptually represented on a force–velocity curve: high force at low velocity, decreasing force with increasing velocity.

Stretch-Shorten Cycle (SSC)

  • A stretch–shorten movement is an eccentric contraction followed by a concentric contraction.
  • The SSC produces more force than a concentric contraction alone due to:
    • More time for force buildup during the stretch
    • Potential reflex potentiation
    • Elastic energy recoil stored during the stretch (and possibly preload effects)
  • Practical example: a jumper performing a countermovement jump uses the SSC for enhanced force output.

The Stretch-Shorten Cycle: Illustrations

  • Descriptive examples include squat jump and countermovement jump, where the eccentric phase precedes the concentric propulsion.

Exercise Training and Neuromechanics

  • Both strength training and flexibility training can alter the mechanical profile of the muscle–tendon complex.
  • Acute flexibility exercises may transiently increase joint ROM.
  • Long-term flexibility training can lead to chronic increases in joint ROM.

Flexibility Training and Neuromechanics

  • Increased joint ROM may result from:
    • Decreased stiffness of the muscle–tendon complex
    • Greater pain tolerance to stretching
    • A longer muscle–tendon complex
    • Relaxation of the contractile element
  • Stretching before strength and power work may reduce performance due to the musculotendinous system becoming more compliant and absorbing/ dissipating force rather than transmitting it.
  • Real-world applicability of such data is debated; laboratory findings may not fully translate to everyday training.
  • The evidence on flexibility training and injury prevention is varied and inconclusive overall.
  • Acute stretching can reduce muscle–tendon injuries during exercise; stretching can help alleviate symptoms in various musculoskeletal disorders; stretching is a tool to maintain muscle health, especially in aging populations.

Strength Training and Neuromechanics

  • Strength training can increase musculotendinous stiffness, partly due to hypertrophy and possible increases in tissue density.
  • Hypertrophy may cause muscle fascicles to become more angled, altering the angle of force application.

Plyometric Training and Neuromechanics

  • Plyometric training targets the stretch–shorten cycle and emphasizes a forceful eccentric phase followed by an explosive rapid reversal of the concentric phase.
  • Performed at high speed and at the high-velocity portion of the force–velocity curve.
  • Aims to increase tissue stiffness, maximize elastic energy recoil, and improve neural coordination mechanisms that optimize rapid contractions.
  • Potential outcomes include:
    • Altered muscle architecture
    • Increased neural drive
    • Increased muscle and tendon stiffness
    • Improved intermuscular coordination
    • Enhanced stretch–shorten performance due to improved stretch tolerance at high velocities, leading to more rapid and complete force transfer to mechanical work.

Other Musculoskeletal Properties Influencing Control

  • Tendon length and thickness:
    • Long tendons increase the range of motion and enable dampening and energy storage; thicker tendons are stiffer and transmit force more quickly with less energy storage.
  • Muscle insertion moment arms:
    • Long moment arms enable greater force production; short moment arms favor speed and range of motion.
  • Fiber type:
    • Fast-twitch fibers enable fast and powerful contractions; slow-twitch fibers provide fatigue resistance.
  • Muscle fiber length and arrangement:
    • Long fibers support speed and ROM; oblique/pennated fiber arrangements can alter force vectors compared with longitudinal fibers.
  • Muscle fiber cross-sectional area (CSA):
    • Larger CSA supports greater force production.

Summary

  • MU size is defined by neuron size; this size correlates with muscle fiber type, recruitment order, and innervation ratio, shaping how the MU is used.
  • MUs control force through recruitment, rate coding, and coordination.
  • Coordination can be intramuscular (within a muscle) or intermuscular (between muscles).
  • Practice and training bring about changes in MU behavior, including recruitment, rate coding, and coordination.
  • Skeletal muscles can be modeled as a system of contractile elements (CE) and elastic elements (SE and PE) that interact with the viscous element (VE).
  • Force–velocity and length–tension relationships describe how mechanical properties influence muscle force production across speeds and lengths.
  • Training (strength, power, flexibility, plyometric) can alter the mechanical properties of the muscle–tendon complex and neuromuscular coordination, affecting movement quality and performance.
  • Morphological and architectural changes in tissue structure can accompany training adaptations, influencing stiffness, force direction, and energy transfer.

Endnote

  • Please stay up to date on the course schedule and due dates for continued study and assessment preparation.