Question

Which of the following statements is NOT correct regarding the motor units and their activation?

A) A motor unit in muscles of gross movement can have thousands of muscle cells.
B) All muscle fibers in a single motor unit are of the same fiber type.
C) A weak contractile stimulus activates fast-twitch motor units.
D) The metabolic capacity of muscle fibers within a motor unit can be altered.
E) At its peak intensity, a contractile stimulus will activate muscle fibers that are easily fatigued.

Answer 1

The incorrect statement is that a weak contractile stimulus activates fast-twitch motor units; rather, it activates slow-twitch muscle fibers associated with small, low-threshold motor neurons.

Step-by-step explanation:

Regarding the incorrect statement about motor units and their activation, the statement "A weak contractile stimulus activates fast-twitch motor units." is NOT correct. Indeed, a weak contractile stimulus will initially activate the more excitable, smaller, lower-threshold motor neurons associated with slow-twitch muscle fibers, which are less prone to fatigue and more efficient for prolonged activities.

A large motor unit may be concerned with gross movements and can include thousands of muscle cells, and all muscle fibers within a motor unit will be the same type (slow-twitch or fast-twitch). It's also true that the metabolic capacity of the muscle fibers can change due to training and other factors, thus enhancing their performance characteristics. Moreover, at peak intensity, contractile stimuli activate muscle fibers which may fatigue more easily, typically the fast-twitch fibers that are utilized for short bursts of power or strength.