Muscle memory is a common phrase associated with the martial arts as well as in other sports, playing a musical instrument, riding a bike or in the acquisition of any other psycho-motor skill for that matter. It is a useful way of trying to understand what is going on: through repetition of a set of muscular actions, that muscle (or group of muscles) will eventually react to a stimulus in a predictable and reliable way. It is as if those muscles have ‘remembered’ what to do and the movements become automatic without the need for conscious control.
Unfortunately the phrase ‘muscle memory’ grates on me a little! It may feel as if your muscles just know what to do all by themselves and you aren’t consciously sending them messages to contract or relax at a given moment but your unconscious brain is working very hard to tell your muscles what to do in any given situation. Clearly memory resides in the brain not the muscles. In my opinion a more accurate phrase to describe what is happening is motor memory.