Physiology: Muscle Performance

Physiology: Muscle Performance

Muscle Fibre Types

Skeletal muscle fibres are classified into three types based on their contractile and metabolic properties:

• Type I (slow oxidative): many mitochondria, rich capillary supply, high myoglobin content (red), slow myosin ATPase and SERCA isoforms, fatigue-resistant. Used for sustained postural work and aerobic exercise (e.g., marathon running).
• Type IIA (intermediate): intermediate properties — oxidative and glycolytic capacity, moderately fatigue-resistant.
• Type IIB (fast glycolytic): few mitochondria, low myoglobin (pale/white), fast myosin ATPase, low fatigue resistance. Used for brief, powerful contractions (e.g., sprinting).

Force-Velocity and Length-Tension Relationships

Velocity-load relationship: as load increases, shortening velocity decreases. At zero load, maximum shortening velocity (Vmax). At maximum load (isometric), velocity = 0. Length-tension relationship: at optimal sarcomere length (~2.0–2.2 µm), there is maximum overlap of actin and myosin → maximum cross-bridge formation → maximum force. At shorter lengths, thin filaments overlap the H-zone (double overlap) → reduced force. At longer lengths, reduced filament overlap → fewer cross-bridges. Passive tension from titin (elastic protein) increases at long sarcomere lengths.

Proprioceptors

Muscle spindles: encapsulated mechanoreceptors containing intrafusal fibres (nuclear bag and chain fibres) sensitive to stretch (change in muscle length and velocity). Afferents: Ia (primary) and II (secondary). Efferents: gamma motor neurons control sensitivity. The stretch reflex (monosynaptic) uses the muscle spindle: tap → Ia afferent → alpha motor neuron → muscle contraction. Golgi tendon organs (GTOs): located in tendons, sensitive to tension (force), not stretch. Afferents: Ib fibres → inhibitory interneuron → alpha motor neuron (inverse myotatic reflex, autogenic inhibition). GTOs prevent muscle damage from excessive force.

Motor Units and Recruitment

A motor unit consists of one alpha motor neuron and all the muscle fibres it innervates. Force is graded by: (1) recruitment — activating more motor units (size principle: small slow motor units recruited first, then larger fast units); (2) frequency modulation — increasing firing rate → summation of twitches → tetanus. Recruitment is the more important mechanism for grading force. Motor unit size varies: small motor units (few fibres, fine control, e.g., extraocular muscles) vs large motor units (many fibres, coarse control, e.g., quadriceps).