A motor unit consists of a single alpha motor neuron and all the muscle fibres it innervates. Slow-twitch (Type I) motor units differ from fast-twitch (Type IIx) motor units in which of the following respects?

• A Type I units have smaller motor neurons with lower threshold for recruitment, higher fatigue resistance (more mitochondria, myoglobin, oxidative enzymes), and slower twitch speed; Type IIx have larger motor neurons, higher recruitment threshold, fast twitch speed, and fatigue rapidly ✓
• B Type I units have a higher peak twitch force than Type IIx units because they are recruited first and sustain contraction
• C Type I units depend on glycolytic metabolism and have higher glycogen content than Type IIx oxidative fibres
• D Type IIx motor units are recruited first (lower threshold) and Type I are recruited only during maximal effort

Explanation

According to the Henneman size principle, motor units are recruited in order of increasing motor neuron size: small slow-twitch (Type I/S) units first, then fast fatigable-resistant (Type IIa/FR), then fast fatigable (Type IIx/FF) units at highest effort. Type I fibres have: small soma (easier to depolarise → lower threshold), abundant mitochondria and myoglobin (high oxidative capacity → fatigue-resistant), slow myosin ATPase, and produce low but sustained force for postural and endurance tasks. Type IIx fibres have large soma, rely primarily on glycolysis, have fast myosin ATPase, produce high peak force but fatigue rapidly. Option B is incorrect: Type II units generate higher peak force. Option C reverses the metabolic profiles. Option D reverses the recruitment order.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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