According to the all or none principle, what occurs when the electrical stimulation reaches the necessary threshold?

The all or none principle states that when an electrical stimulus reaches a certain threshold, all muscle fibers within a motor unit will respond by contracting fully. This means that once the threshold is crossed, there is no partial contraction or variable response; every fiber innervated by the activated motor neuron will either fully contract or not contract at all. This principle underscores the uniformity of muscular responses to stimulation, which is crucial for coordinated movements.

In this context, the correct answer highlights that when sufficient stimulation is provided, the entire motor unit is activated to its full capacity, leading to the full contraction of all its muscle fibers. This is essential for effective and powerful muscle contractions during physical activities.