A physiotherapist notes that a patient can sustain posture for prolonged periods without fatigue. This reflects the predominance of which motor unit type in postural muscles?

• A Type II-X (fast-twitch, fatigable) fibers
• B Type II-A (fast-twitch, fatigue-resistant, oxidative-glycolytic) fibers
• C Large motor units with high innervation ratio
• D Type I (slow-twitch, fatigue-resistant, oxidative) fibers ✓

Explanation

Type I (slow-twitch, oxidative) fibers are characterized by high myoglobin content (red color), dense mitochondria, high aerobic enzyme activity, small motor unit size, and resistance to fatigue. They are the predominant fiber type in postural muscles (e.g., soleus, paraspinal). Type II-X fibers fatigue rapidly; Type II-A are intermediate. The 'size principle' of Henneman states that smaller, Type I motor units are recruited first during low-level sustained activity.

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

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