thumb|Entering the same code into a keypad may, over time, become a muscle memory
Muscle memory is a form of procedural memory that involves consolidating a specific motor task into memory through repetition, which has been used synonymously with motor learning. When a movement is repeated over time, the brain creates a long-term muscle memory for that task, eventually allowing it to be performed with little to no conscious effort. This process decreases the need for attention and creates maximum efficiency within the motor and memory systems. Muscle memory is found in many everyday activities that become automatic and improve with practice, such as riding bikes, driving motor vehicles, playing ball sports, musical instruments, and poker, typing on keyboards, entering PINs, performing martial arts, swimming, dancing, and drawing.
History
The origins of research for the acquisition of motor skills stem from philosophers such as Plato, Aristotle and Galen. After the break from tradition of the pre-1900s view of introspection, psychologists emphasized research and more scientific methods in observing behaviours. Thereafter, numerous studies exploring the role of motor learning were conducted. Such studies included the research of handwriting, and various practice methods to maximize motor learning.
Retention
The retention of motor skills, now referred to as muscle memory, also began to be of great interest in the early 1900s. Most motor skills are thought to be acquired through practice; however, more observation of the skill has led to learning as well. Research suggests we do not start off with a blank slate with regard to motor memory although we do learn most of our motor memory repertoire during our lifetime. Movements such as facial expressions, which are thought to be learned, can actually be observed in children who are blind; thus there is some evidence for motor memory being genetically pre-wired. One of the earliest and most notable studies regarding the retention of motor skills was by Hill, Rejall, and Thorndike, who showed savings in relearning typing skills after a 25-year period with no practice.
Muscle memory encoding
The neuroanatomy of memory is widespread throughout the brain; however, the pathways important to motor memory are separate from the medial temporal lobe pathways associated with declarative memory. As with declarative memory, motor memory is theorized to have two stages: a short-term memory encoding stage, which is fragile and susceptible to damage, and a long-term memory consolidation stage, which is more stable.
The memory encoding stage is often referred to as motor learning, and requires an increase in brain activity in motor areas as well as an increase in attention. Brain areas active during motor learning include the motor and somatosensory cortices; however, these areas of activation decrease once the motor skill is learned. The prefrontal and frontal cortices are also active during this stage due to the need for increased attention on the task being learned. However, conflicting evidence suggests that a single plasticity mechanism is not sufficient and a multiple plasticity mechanism are needed to account for the storage of motor memories over time. Regardless of the mechanism, studies of cerebellar-dependent motor tasks show that cerebral cortical plasticity is crucial for motor learning, even if not necessarily for storage.
Muscle memory consolidation
Muscle memory consolidation involves the continuous evolution of neural processes after practicing a task has stopped. The exact mechanism of motor memory consolidation within the brain is controversial. However, most theories assume that there is a general redistribution of information across the brain from encoding to consolidation. Hebb's rule states that "synaptic connectivity changes as a function of repetitive firing." In this case, that would mean that the high amount of stimulation coming from practicing a movement would cause the repetition of firing in certain motor networks, presumably leading to an increase in the efficiency of exciting these motor networks over time.
While the exact location of muscle memory storage is not known, studies have suggested that it is the inter-regional connections that play the most important role in advancing motor memory encoding to consolidation, rather than decreases in overall regional activity. These studies have shown a weakened connection from the cerebellum to the primary motor area with practice, it is presumed, because of a decreased need for error correction from the cerebellum. However, the connection between the basal ganglia and the primary motor area is strengthened, suggesting the basal ganglia play an important role in the motor memory consolidation process. This is particularly beneficial with complex motor movements, where motor performance is improved following sleep.
Sleep duration and exercise also influence motor skill learning and memory. It has been proven through experiments that sleep after night training improves skill consolidation compared to morning training without sleep. This therefore implies that sleep is a time of heightened processing and consolidation of motor learning, allowing athletes and individuals maximizing their motor skills to attain maximum performance.
Furthermore, formal sleep therapies have also been discovered to enhance the performance of sports through enhanced reaction time, coordination, and overall execution of skills. Maintenance of proper quantities of sleep in addition to strict compliance to consistency in sleeping schedule can maximize the results of motor learning as well as support long-term memory for body skills. The application of sleep-based interventions, including following a constant sleeping pattern and minimizing disruptions to an absolute degree, can therefore be a significant assistant for the person who wants to optimize their motor capacity.
Strength training and adaptations
When participating in any sport, new motor skills and movement combinations are frequently being used and repeated. All sports require some degree of strength, endurance training, and skilled reaching in order to be successful in the required tasks. Muscle memory related to strength training involves elements of both motor learning, described below, and long-lasting changes in the muscle tissue.
Evidence has shown that increases in strength occur well before muscle hypertrophy, and decreases in strength due to detraining or ceasing to repeat the exercise over an extended period of time precede muscle atrophy. To be specific, strength training enhances motor neuron excitability and induces synaptogenesis, both of which would help in enhancing communication between the nervous system and the muscles themselves. This confirms that muscle strength is first influenced by the inner neural circuitry, rather than by external physiological changes in the muscle size.
Previously untrained muscles will acquire newly formed nuclei through the fusion of satellite cells preceding hypertrophy. Subsequent detraining will result in atrophy and the loss of myo-nuclei. While it was long believed that a muscle memory effect related to myo-nuclei permanence existed, current studies establish that during detraining, myo-nuclei will be lost.
Reorganization of motor maps within the cortex are not altered in either strength or endurance training. However, within the motor cortex, endurance induces angiogenesis within as little as three weeks to increase blood flow to the involved regions. Even a small amount of training may be enough to induce neural processes that continue to evolve even after the training has stopped, which provides a potential basis for consolidation of the task. In addition, studying mice while they are learning a new complex reaching task, has found that "motor learning leads to rapid formation of dendritic spines (spinogenesis) in the motor cortex contralateral to the reaching forelimb". However, motor cortex reorganization itself does not occur at a uniform rate across training periods. It has been suggested that the synaptogenesis and motor map reorganization merely represent the consolidation, and not the acquisition itself, of a specific motor task. Furthermore, the degree of plasticity in various locations (namely motor cortex versus spinal cord) is dependent on the behavioural demands and nature of the task (i.e., skilled reaching versus strength training). Indeed, previously untrained human participants experienced a chronic period of resistance exercise training (7 weeks) that evoked significant increases in skeletal muscle mass of the vastus lateralis muscle, in the quadriceps muscle group. Following a similar period of physical in-activity (7 weeks), where strength and muscle mass returned to baseline, participants performed a secondary period of resistance exercise. Importantly, these participants adapted in an enhanced manner, whereby the amount of skeletal muscle mass gained was greater in the second period of muscle growth than the first, suggesting a muscle memory concept. The researchers went on to examine the human epigenome in order to understand how DNA methylation may aid in creating this effect. During the first period of resistance exercise, the authors identify significant adaptations in the human methylome, whereby over 9,000 CpG sites were reported as being significantly hypomethylated, with these adaptations being sustained during the subsequent period of physical in-activity. However, upon secondary exposure to resistance exercise, a greater frequency of hypomethylated CpG sites was observed, where over 18,000 sites reported as being significantly hypomethylated. The authors went on to identify how these changes altered the expression of relevant transcripts, and subsequently correlated these changes with adaptations in skeletal muscle mass. Collectively, the authors conclude that skeletal muscle mass and muscle memory phenomenon is, at least in part, modulated due to changes in DNA methylation. Transitive movements have representations that become programmed to the premotor cortex, creating motor programs that result in the activation of the motor cortex and therefore the motor movements. Repetitive behaviors, such as typing on a computer from a young age, can enhance such abilities. Therefore, children who learn to use computer keyboards at an early age could benefit from the early muscle memories.
Music memory
thumb|right|alt=Bimanual synchronized finger movements play an essential role in piano playing.|Playing the piano requires complex actions.
Fine motor skills are very important in playing musical instruments. Muscle memory is relied on when playing the clarinet, specifically to help create special effects through certain tongue movements when blowing air into the instrument.
Certain human behaviours, especially actions like the finger movements in musical performances, are very complex and require many interconnected neural networks where information can be transmitted across multiple brain regions. It has been found that there are often functional differences in the brains of professional musicians, when compared to other individuals. This is thought to reflect the musician's innate ability, which may be fostered by an early exposure to musical training. When comparing professional musicians to a control group in complex bimanual movements, professionals are found to use an extensive motor network much less than those non-professionals. Solving these puzzles in an efficient manner requires the cube to be manipulated according to a set of complex sequences of turns, called algorithms. By building their muscle memory of each algorithm's movements, speed cubers can implement them at very fast speeds without conscious effort. This plays a role in major speedcubing methods such as Fridrich for the 3×3×3 Rubik's Cube and EG for the 2×2×2 Pocket Cube.
Gross motor memory
Gross motor skills are concerned with the movement of large muscles, or major body movements, such as those involved in walking or kicking, and are associated with normal development. The extent to which one exhibits gross motor skills depends largely on their muscle tone and the strength. The fact that the individuals could still exhibit two of the three original motor skills may have been a result of positive transfer in which previous exposure allows the individual to remember the motion, under the visual and verbal trial, and then later perform it under the verbal trial. This suggests that the use of self-instruction will increase the speed with which a preschooler will learn and remember a gross motor skill. It was also found that once the preschoolers learned and mastered the motor chain movements, they ceased the use of self-instruction. This suggests that the memory for the movements became strong enough that there was no longer a need for self-instruction and the movements could be reproduced without it. A study was created to test this assumption in which the patients were trained to throw a bean bag at a target. His impairment was specific to letters in the alphabet. He was able to copy letters from the alphabet, but he was not able to write these letters.