When an active person repeatedly trains movement, often of the same activity, in an effort to stimulate the mind’s adaptation process, the outcome is to induce physiological changes which attain increased levels of accuracy through repetition. Even though the process is really brain-muscle memory or motor memory, the colloquial expression "muscle memory" is commonly used.
Individuals rely upon the mind’s 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 very minute and small skills we perform with our hands such as brushing teeth, combing hair, using a pencil or pen to write, touch typing or even playing video games. Gross motor skills are those actions that require large body parts and large body movements as in the throwing sports such as bowling, American football, and baseball, sports such as archery, basketball, golfing, judo, swimming, and tennis, and activities such as driving a car (especially one with a manual transmission), playing a musical instrument, and marksmanship.
Muscle memory is fashioned over time through repetition of a given suite of motor skills and the ability through brain activity to inculcate and instill it such it they become automatic. 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 through repetition, 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 perform appropriately. In this sense the muscle memory process is an example of automating an OODA Loop insofar as one learns to Observe, Orient, Decide, and Act.
When one picks up a hair brush, one automatically has a certain motion, style, number of strokes, and amount of pressure as the hair is brushed without requiring conscious thought about each movement. Other forms of rather elaborate motions that have become automatic include speech. As one speaks, one usually does not consciously think about the complex tongue movements, synchronisation with vocal cords and various lip movements that are required to produce phonemes, because of muscle memory. In speaking a language that is not one's native language, one typically speaks with an accent, because one's muscle memory is tuned to forming the phonemes of one's native language, rather than those of the language one is speaking. An accent can be eliminated only by carefully retraining the muscle memory.
Muscle memory starts with a visual cue. A classic example are chords while playing instruments such as the piano or guitar. The beginner must think and interpret these chords, but after repetition, the letters and symbols on the page become cues to the muscle movements. As the brain processes the information about the desired activity and motion such as a golf swing, one then commits to that motion thought as correct. Over time, the accuracy and skills in performing the swing or movement improve.
Muscle memory is the control center of the movement. In maximizing muscle memory to learn a new motion, practicing that same motion over a long enough period makes it become automatic. This learning process could take months, even years, to perfect, depending on the individual's dedication to practice, and their unique biochemical neuromuscular learning system to retain that practice.
In detail, inside the brain are neurons that produce impulses, which carry tiny electrical currents. These currents cross the synapses between neurons with chemical transporters called neurotransmitters to carry the communication. Neurotransmitters are the body’s communicative mechanisms and one of their many functions is to travel through the central nervous system and carry the signal from visual cue to the muscle for the contraction.
Acetylcholine is the major neurotransmitter used in memory, focus, concentration, and muscle memory. It is the substance that transports messages from one nerve cell to another. Acetylcholine is critical to the process of creating and remembering the muscle contraction. It achieves this through motor neurons.
Serotonin is imperative in the muscle memory process. Serotonin has multiple physiological actions at neuromuscular junctions where communication crosses over. This includes facilitation of transmitter release from nerve terminals and an increase in the communication to muscle fibers.
When a motor neuron depolarizes, an electrical current is passed down the nerve fiber and the impulse causes the neurotransmitter acetylcholine to be released to the muscle cell. Acetylcholine then binds with receptors on the muscle membrane to create the contraction. Over time, with acetylcholine the brain-muscle learns the chosen motion and induces its own form of memory. This process is also called neuromuscular facilitation. Once muscle memory is created and retained, there is no longer need to actively think about the movement and this frees up capacity for other activities.