The Henneman size principle of motor unit recruitment states that during graded muscle contractions:
- A Fast-twitch Type IIb motor units are recruited first due to their high force output
- B Slow-twitch Type I (slow oxidative) motor units, innervated by smaller alpha motor neurons, are recruited first ✓
- C All motor units in a muscle are recruited simultaneously to grade force output by rate coding
- D Motor unit recruitment follows an inverse size principle: large units first, small units last
Correct answer: B. Slow-twitch Type I (slow oxidative) motor units, innervated by smaller alpha motor neurons, are recruited first
Explanation
Henneman's size principle states that motor units are recruited in order from smallest (lowest threshold) to largest. Small alpha motor neurons (Type I, slow-twitch, high fatigue resistance) have higher input resistance, so a given synaptic current produces greater depolarisation, making them recruit first. As force demand increases, progressively larger fast-twitch (Type IIa, then IIb) units are recruited. This orderly recruitment provides smooth graded contractions and reserves powerful but fatigue-prone units for high-force tasks.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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