Final answer:
The speed at which muscle action is performed is defined by velocity, which indicates both the rate of movement and its direction. Velocity is distinct from acceleration, force, and power.
Step-by-step explanation:
The speed at which each muscle action is performed is primarily defined by velocity. Velocity is the physical quantity that describes how fast an object is moving and in what direction over time. It's important to understand that velocity is different from acceleration, which is the rate of change of velocity, and force, which can cause acceleration depending on the mass of the object. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. When discussing muscle actions in the context of physiology, the velocity of muscle contraction would determine how quickly a muscle can move a limb or generate movement. In contrast, power refers to the rate at which work is done, which can relate to the efficiency and intensity of the muscle action, but it does not solely define the speed of the action.