To me, it is a motor skill that, once acquired, can be immediately acted on, even beyond the conscious level. The examples that come to my mind are riding a bicycle and swimming. Even if a person has not done either one of those activities in years, the basic skill is immediately there even if it lacks the fluidity of previous times.
Its exactly how Karin mentioned it, but to go go a little more in depth I would say muscle memory is the activation of motor neurons, and parts of the nervous system which lead to different bundles of muscle fibers, be it in one muscle group or a series ofmuscle groups. And of course this is also one of the main reasons you advance on your training be it strength, speed, agility etc.
Karin’s and Wayne’s answers are great. It’s not muscle memory. It’s a process in the motor cortex of the brain. Cycling and swimming are great examples. The more practice, the more repetition, the greater the ability to repeat the movement(s) even after long periods of absence from the activity.
While I agree that muscle memory has to do with repetition it also has to do with adaptation which is why I constantly change and mix up my workouts and my clients workouts as well.
One function of muscle tissue is to adapt which is a good thing for many things but if one wants to continue optimum fitness they need to keep surprising their muscles for strength and endurance.
I think Karin hit the nail on the head with this one. Her explanation is one that I often use with my clients. I’ve seen this in practice. I may introduce my client to a new exercise that’s moderately difficult to perform (usually balance or stability exercises), and we may not do that particular exercise for another two weeks. Every time… When the client does the exercise again, his or her body seems to “remember” how to perform the movement. My clients are always excited to see how easy it is for the human body to adapt to exercise. Little experiements like these can be real motivators for clients.