What is the size principle?

• A. Motor units are recruited from largest to smallest
• B. All units recruited simultaneously
• C. Motor units are recruited from smallest to largest
• D. Only slow-twitch units recruited first

Answer: (C)

The size principle says that motor units are recruited in order of increasing size and recruitment threshold: small, slow, fatigue-resistant units are activated first, and as more force is needed, larger, faster, higher-threshold units are brought in. This arrangement lets the nervous system produce precisely graded force. Because small units are more excitable and generate less force, they handle light tasks smoothly and efficiently. When more power is required, the higher-threshold, larger units join in to provide the extra force. This sequence also helps distribute fatigue more evenly and allows quick, controlled increases in muscle tension.

Choosing the opposite order would make force come on too abruptly, with less fine control. Recruiting all units at once would remove the gradation that allows small adjustments in effort. And starting with only slow-twitch units would fail to meet demands that require rapid or high-force contractions, which fast-twitch units provide.