Muscle memory is a common term for neuromuscular facilitation, which is the process of the neuromuscular system memorizing motor skills.
When an active person trains movement, often of the same activity, in an effort to stimulate the mindís adaptation process, the end result is to induce a physiological change such as increased levels of accuracy through repetition. Even though the process is really brain-muscle memory or what some call motor memory, the nickname muscle memory is commonly used.
Individuals rely upon the mindís ability to assimilate a given activity and adapt to the training. As the brain and muscle adapts to training, the subsequent changes are a form or representation of its muscle memory.
There are two types of motor skills involved in muscle memory, fine and gross. Fine motor skills are small skills we perform with our hands such as brushing the teeth, combing the hair, using a pencil or pen to write, and playing video games. Gross motor skills are those actions that require large body parts and large body movements like throwing sports (bowling , American football , baseball , golfing, swimming, tennis, driving a car, and archery.
Gross motor skills are generally employed in activities, such as driving, while fine motor skills are generally used in activites, such as writing and sewing.
Muscle memory is fashioned over time through repetition of a given motor skill and the ability through brain activity to remember it. Activities such as brushing the teeth, combing the hair, or even driving a vehicle are not as easy as they look to the beginner. As one reinforces those movements day after day after day, the neural system learns those fine and gross motor skills to the degree that one no longer needs to think about them, but merely to react and to perform.
Essentially, when we don't practice, our brain forgets to tell the muscles what we want them to do.