Answer:
When the electrical potential reaches a threshold, an actual potential occurs in the muscle fibers.
Step-by-step explanation:
A muscle fiber is a special type of a cell which can change its shape due to actin chains sliding across each other. This process then causes changes in the electrical potential of the muscle.
There are few types of muscle fiber that designed for specific type of muscles activity. Some of them involved in endurance exercises whereas others are best for the strength exercises. These muscles fibers are attached to bones allowing them to move.