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Motor Priming is the use of an excitatory or inhibitory stimulus prior to or in conjunction with the performance of a skilled movement in order to to affect performance, learning, or neurophysiological response. Motor priming is derived from the psychological concept of priming, in which one stimulus unconsciously impacts response to a second stimulus.

Proposed Physiological Mechanisms
In most circumstances goal of motor priming is to use an initial or concurrent excitatory stimulus to depolarize neurons involved in motor pathways and reduce the activation threshold for subsequent stimuli. This depolarization places the targeted neurons in a state of greater excitability, which is believed to motor learning through neuroplastic change. Alternatively the use of inhibitory priming stimuli may be used to reduce interference from less functionally important neural pathways. Initial inhibitory stimuli may also be used exploit homeostatic metaplasticity. Proponents of this mechanism argue that plasticity brain is determined by the strength of the relevant stimulus relative to previous stimuli, rather than the absolute strength of the relevant single stimulus. Thus, an inhibitory stimulus provided prior to training may lower the amount of activation required to induce neuroplastic change.

Forms of Motor Priming
Motor priming can be grouped into five major categories: stimulation-based priming, motor imagery and action observation, sensory priming, movement-based priming, and pharmacological priming.

Stimulation Based Priming
In stimulation based priming a non-invasive electrical or magnetic stimulus is administered to the nervous system with the intention of increasing movement dependent neuroplasticity. The two most common forms of stimulation-based priming are transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). Together these techniques are referred to as non-invasive brain stimulation or NIBS as they stimulate targeted areas by generating electric or magnetic fields outside the skull. Usually, NIBS targets the primary motor cortex or M1, however stimulation of other key movement related areas of the brain has been investigated.

Other forms of stimulation-based priming include peripheral nerve stimulation (PNS) which functions in a similar manner to non-invasive brain stimulation, but targets a peripheral nerve, and paired associative stimulation (PAS) which involves the concurrent provision of PNS and NIBS.

Motor Imagery and Action Observation
Motor imagery(or mental practice) involves the mental performance of a target task without the required physical movements, while action observation involves watching another individual perform the target activity in order to improve the performance of the observer. Neuroimaging has shown activation in key motor areas in the brain during motor imagery and mental practice despite the lack of descending input to afferent peripheral nerves. These forms of priming are thought to be most beneficial reducing physical workload for individuals with limited movement capacity and for allowing athletes a chance to prepare themselves for a skilled movement (i.e. a golfer may use motor imagery before attempting a difficult putt).

Sensory Priming
Sensory priming involves the augmentation or reduction of external sensory stimuli with the intention of modulating motor performance. Sensory augmentation techniques include, but are not limited to vibration, and peripheral electrical stimulation (which is considered a category of both sensory and stimulation-based priming). Vibration based techniques are generally applied to the antagonist muscle in an attempt to stimulate inhibitory neural pathways and reduce interference in agonist activities. Peripheral nerve stimulation is generally applied to the nerve supplying the agonist muscle in order to enhance sensory feedback to the brain which in turn is thought to increase motor excitability.

Sensory deafferentation involves blocking of sensory signals from peripheral nerves generally using a tourniquet or anesthetic cream. These techniques are have been applied to involved and uninvolved body areas with the intention of modulating neural plasticity. A common target of sensory deafferentation is the contralateral extremity which is targeted to reduce interhemispheric inhibition.

Movement Based Priming
Movement based priming involves the performance of an additional motor task prior to practice of the target task. There is a large amount of variability in movement based priming protocols, but they generally involve repetitive or continuous movement performed immediately prior to the target activity. A common form of motor priming used in rehabilitation is bimanual which involves the performance of identical symmetric movements in the more and less effected limb. This approach attempts to take advantage of the symmetry constraint or the propensity of toward symmetrical movements in both upper limbs.

Pharmacological Priming
In pharmacological priming drugs are provided with the intention of neuroplasticity during motor training. The aim of these drugs is to upregulate excitatory signaling in the brain by either by directly increasing the quantity of excitatory neurotransmitters in the nervous system or by increasing the potency of such neurotransmitters by preventing their removal from the synaptic cleft. Excitatory neurotransmitters targeted in in pharmacological priming include dopamine, norepinephrine, serotonin, and acetylcholine.

Use and Efficacy in Clinical Populations
In clinical settings, motor priming is most often used prior or in conjunction with to rehabilitative exercises in an attempt to facilitate recovery from neurological injury. Due to variability in the parameters of priming techniques as well as differences in patient response due the variability of brain physiology at an individual level, results of individual motor priming studies have been mixed. A 2020 metanalysis of 36 upper extremity motor priming techniques performed prior to task oriented training found moderate quality evidence of functional improvements due stimulation based priming and sensory priming. The same metanalysis found low quality evidence of functional improvements due to movement-based priming, and inconclusive results for action observation. A 2017 systematic review showed that non-invasive transcranial brain stimulation positively impacted motor function in individuals with Parkinson’s disease. A 2007 systematic review found the use of amphetamines for motor priming no significant benefit in stroke. The use of norepinephrine inhibitors following stroke has been supported by multiple studies. A 2010 systematic review found that selective serotonin reuptake inhibitors(SSRIs) may improve motor recovery from stroke, but evidence was not strong enough to make recommendations for clinical practice.

Use and Efficacy in Non-clinical Populations
Motor priming has been studied in a diverse array of non-clinical contexts including but not limited to athletics and military training. However, because there are fewer studies available for these populations more research is needed. Like in clinical research motor priming studies targeting unimpaired individuals are subject to greatly variable results due to individual differences as well as differences in priming parameters. The United States Airforce has performed multiple studies assessing the efficacy of non-invasive brain stimulation in improving cognitive and motor performance, but research in this area is still ongoing. Noninvasive brain stimulation has also been used by athletes during training. Early research in the use of priming techniques to improve athletic performance is promising, but there is less data available for these populations.