Can all fibers, within a skeletal muscle, be stimulated during the same interval?

• A. Not all of them
• B. Yes, all can be stimulated together
• C. Only when relaxed
• D. Only during maximal contraction

Answer: (A)

Activation of skeletal muscle fibers relies on motor units. Each motor unit is a single motor neuron and all the muscle fibers it innervates, and those fibers contract together when that neuron fires. A muscle contains many motor units, and they are controlled independently, not all at once by a single neural impulse.

Because only the motor units that are recruited respond at any moment, not every fiber in the muscle contracts in the same interval. The nervous system adds force by recruiting more motor units in a graded fashion, rather than firing every unit simultaneously. In normal function, you can’t stimulate all fibers at once; achieving near-total activation would require an artificial, maximal stimulation of the entire nerve, which isn’t how voluntary contraction works.