When a muscle changes from one state (balanced or unbalanced) to another (unbalanced or balanced). This definition is different from what many branches of Kinesiology used to say, especially during the early development of Kinesiology. In the past they used to say that a Muscle Change was when a Strong Muscle turned it state into a Weak Muscle or a Weak Muscle turned into a Strong Muscle. But this is not correct from a neuromuscular point of view, as Richard Utt, L.Ac, founder of Applied Physiology, has outlined.
We should say that a Muscle Change is when a balanced muscle (that is in an omeostatic state) can change into an unbalanced muscle (that can be either hypertonic or hypotonic), or when an unbalanced muscle (that can be either hypertonic or hypotonic), changes into balanced muscle (omeostatic state).