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.

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.

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.

In the early stages of empirical research of motor memory Edward Thorndike, a leading pioneer in the study of motor memory, was among the first to acknowledge learning can occur without conscious awareness. 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. Findings related to the retention of learned motor skills have been continuously replicated in studies, suggesting that through subsequent practice, motor learning is stored in the brain as memory. This is why performing skills such as riding a bike or driving a car are effortlessly and 'subconsciously' executed, even if someone had not performed these skills in a long period of time.

When first learning a motor task, movement is often slow, stiff and easily disrupted without attention. With practice, execution of the motor task becomes smoother, there is a decrease in limb stiffness, and the muscle activity necessary to the task is performed without conscious effort.

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.

The main area involved in motor learning is the cerebellum. Some models of cerebellar-dependent motor learning, in particular the Marr-Albus model, propose a single plasticity mechanism involving the cerebellar long-term depression (LTD) of the parallel fiber synapses onto Purkinje cells. These modifications in synapse activity would mediate motor input with motor outputs critical to inducing motor learning. 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.