ACE Personal Trainer Practice Exam

According to the All-or-None Principle, what occurs when a motor unit is activated?

1. Some muscle fibers contract

2. All muscle fibers will maximally contract

3. No muscle fibers will contract

4. Only slow-twitch muscle fibers contract

The correct answer is: All muscle fibers will maximally contract

The All-or-None Principle states that once a motor unit is activated, all of its associated muscle fibers will contract to their maximum potential. This principle underscores the idea that muscle fibers do not partially contract; they either fully engage or remain inactive. When a motor neuron stimulates a motor unit, the electrical impulse leads to a response from all the muscle fibers within that unit, ensuring that they all contract simultaneously and with maximum force. This principle plays a crucial role in muscle performance and coordination during various activities. It also serves as a foundation for understanding muscle responses during different levels of physical exertion—while it governs the response within an individual motor unit, multiple motor units can be recruited to achieve various degrees of force generation across a muscle group.