To add the the above responses, it is important think about training adaptations to movement–specifically what happens when a muscle tissue receives a new stimulus.
Types I, II and their subgroups respond differently to repeated stimuli. Not to be forgotten are acute and chronic adaptations to physical activity.
So, one may learn a skill such as bike riding or swimming, however, even though the skill is not forgotten, the muscle loses the training adaptations gained as a consequence of regular physical activity. It is not only the muscle tissue but all the bodily systems involves in movement (cardiovascular, respiratory, endocrine, musculoskeletal, etc) that undergo training adaptations.
So in a nutshell what I am saying is “muscle memory” is dependent upon all the body’s physiological systems (central nervous system, peripheral nervous system, musculoskeletal system, the energy system involved in the task at hand). As a consequence of the entire human organism working together, the muscles involved in the renewed action is resumed with greater ease.