Which motor units are recruited as intensity increases beyond low levels?

• A. Slow-twitch high-threshold units
• B. Fast-twitch high-threshold units
• C. Cardiac motor units
• D. Medium-twitch units

Answer: (B)

The main idea here is the size principle of motor unit recruitment: as the demand for force increases, the nervous system activates motor units in order of increasing size and excitation threshold. At low intensities you mainly use small, slow-twitch units, and as intensity rises, you start recruiting larger, fast-twitch units that have higher thresholds.

Fast-twitch high-threshold motor units are those that require stronger neural input to be activated and can generate larger amounts of force quickly. When intensity goes beyond low levels, these high-threshold units kick in to boost total force output, even though they fatigue faster.

The other options don’t fit because slow-twitch units are low-threshold and are recruited first at light loads, cardiac motor units aren’t part of skeletal muscle recruitment, and medium-twitch isn’t the standard classification used for this recruitment pattern.