Faraday discovered in the XIX century that when an electrical current pass through a wire it generates a magnetic field. If a second wire is located nearby, an electrical current is generated.
In 1982 it was produced the first magnetic stimulator capable of nerve stimulation, and by 1985 it was firstly used to stimulate the human motor cortex in the brain, developing the transcranial magnetic stimulation (TMS). Using a coil with a rapidly changing magnetic field over the scalp, a series of weak electrical currents can excite the neural tissue. In 2008 the FDA (Food and Drugs Association) approved the TMS technique as a therapeutic approach for major depressive disorder.
A different cranial stimulation approach to the TMS is the transcranial direct current stimulation (TDCS), which uses a constant, low direct current delivered via electrodes on the head. Devices only need of two electrodes and an energy supply. Anodal stimulation is positive stimulation, while cathodal stimulation is negative. Unlike the TMS technique, TDCS has not been approved by the FDA, although it is approved in Europe to treat major depression.
Therefore, increases or decreases of neuronal activity can be achieved using the TMS or TDCS techniques. Neurons connecting to muscles have their location in the motor cortex, where pulses can be applied selectively at different locations, to act on specific muscle groups.
A motor evoked potential (or MEP), is an electrical potential recorded in a muscle after stimulation (of a certain intensity over a threshold) in the motor cortex. The size of the MEP response depends on the stimulus intensity and the excitability of cortical neurons and motoneurons. In a voluntary contraction neurons become more excitable, and the MEP size is larger than in resting conditions. In patients suffering from chronic fatigue syndrome, or depression, the MEP size is smaller than in control subjects, and their neurons may need a higher input to get activated. This would translate as an increased effort and fatigue sensation.
Muscle control is as important in sports as training and motivation. Approaches such as meditation, visualization, acupuncture and music are used by many athletes trying to maximize speed, power or effort duration. Transcranial stimulation could prove to be as useful as any of these techniques.
Twenty minutes of TDCS over the left temporal cortex (T3) in trained cyclists found improvements in peak power, and reduced heart rate and effort perception at submaximal workloads. In other study, also with cyclists, anodal stimulation on the motor cortex (M1), with the other electrode located in the contralateral shoulder, proved also useful in reducing the effort perception.
As fatigue not only affects muscular endurance, but also decision making, response time and skill, transcranial stimulation could also be used to enhance motor learning and performance. For example elite athletes improved cognitive performance and mood when receiving a current of 2 milliamps on the prefrontal cortex.
In the search of improving athletic capabilities beyond physiological limitations, a technological breakthrough as transcranial stimulation could surpass these performance barriers. Effort perception, endurance, fatigue and motor learning could be regulated to enhance performance. Its supplemental use will expand among athletes, as equipment becomes more accessible, raising new challenges for regulation among sports organisms.
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