What is true about tonic muscle contraction?

Tonic muscle contraction is characterized by a state where muscles maintain a slight contraction even when at rest. This tonic state is essential for maintaining posture and stability in the body, as well as for ensuring readiness for movement. It provides a baseline level of tension that allows for quick responses to any external stimuli, as opposed to being completely relaxed. This continuous low-level contraction contrasts with other types of muscle contractions that occur in response to specific activities or demands.

This is why the correct understanding revolves around the idea that muscles are always slightly contracted at rest. The other options misrepresent the nature of tonic contractions: they do not solely occur during sleep, and muscles are not completely relaxed in this state. Furthermore, muscles contracting only in response to external stimuli pertains more to phasic muscle contractions rather than tonic ones. Understanding these distinctions is vital in grasping how the body maintains stability and reacts to its environment.