Whole muscle → fascicles → muscle fibers (cells) → myofibrils → sarcomeres → myofilaments. Connective-tissue coverings are epimysium around muscle, perimysium around fascicles and endomysium around individual fibers.
The sarcomere extends Z disc to Z disc. Thin filaments contain actin plus regulatory proteins troponin/tropomyosin. Thick filaments contain myosin. During contraction, filaments slide rather than physically shortening themselves.
A motor neuron releases acetylcholine at the neuromuscular junction. The muscle action potential travels along sarcolemma and T-tubules, triggering calcium release from sarcoplasmic reticulum. Calcium binds troponin, shifting tropomyosin so myosin can interact with actin. ATP is required for cross-bridge cycling and calcium handling.
A motor unit is one motor neuron plus all muscle fibers it innervates. Recruitment of additional motor units increases force. Fine-control muscles generally use smaller motor units than muscles designed for larger gross forces.
Concentric: muscle produces force while shortening. Eccentric: produces force while lengthening. Isometric: produces force without appreciable joint movement/muscle-length change. Isotonic is a broader traditional term for dynamic contractions against load.
Type I fibers are relatively fatigue-resistant and oxidative. Type II fibers generally produce faster/more powerful contractions with subtypes differing in metabolic characteristics and fatigue resistance. Real muscles contain mixtures.
ATP is the immediate energy currency. Phosphagen, anaerobic glycolytic and aerobic oxidative pathways all contribute depending on intensity/duration. Lactic acid is not a persistent “toxin” stored in muscles waiting for massage to flush it out.
Muscle spindles detect muscle length/change in length and participate in stretch reflexes. Golgi tendon organs detect tension/force at musculotendinous regions. They are sensory receptors, not switches that massage therapists manually turn on and off.