Small vs. Large Motor Units: Muscle Fiber Numbers Explained

Motor units are classified by the number of muscle fibers innervated by a single alpha motor neuron, determining precision vs. force.

Clinical Relevance

• ALS, polio: large units lost first → weakness disproportionate to atrophy.
• Myasthenia gravis: fatigable weakness affects all, but ocular (small units) often early.
• EMG: motor unit potential (MUP) size, duration, recruitment pattern diagnose neurogenic vs. myopathic.

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Feature	Small Motor Units	Large Motor Units
Fiber count	< 100 fibers (often 10–100)	> 100 fibers (up to 1,000–2,000)
Innervation ratio	Low (1:10–100)	High (1:1000+)
Muscles	Extraocular, intrinsic hand (thenar/hypothenar), laryngeal	Limb antigravity: gastrocnemius, quadriceps, gluteus maximus
Function	Fine, graded control (eye movement, finger manipulation, speech)	Gross force production (posture, locomotion, jumping)
Recruitment order	First (Henneman’s size principle)	Last (high-threshold)
Fatigability	Low (slow-twitch, oxidative)	High (fast-twitch, glycolytic)