The TMS is a noninvasive method to investigate the CNS in the human [9]. Since its introductions close [13], TMS, has been used to study intracortical, cortico-cortical, and cortico-subcortical interactions [14]. In 1982, Polson, Barker and Freeston produced the first magnetic stimulator capable of stimulating peripheral nerves and in 1985 Barker, Jalinous and Freeston were the first to describe magnetic stimulation of the human motor cortex [13]. The information described above led to the development of the TMS. With this device, through a coil held on the scalp, magnetic fields are generated capable of inducing weak currents able to excite the underlying neural tissue. These currents cause activity in specific parts of the brain, with minimal discomfort, allowing us to study neural functions and interconnections in the intact human being. Brain stimulation techniques, as well as those of the peripheral nerve, trigger a series of events which, by depolarizing the neuron membranes, trigger the action potential. Experience from invasive stimulation during neurosurgery or epilepsy monitoring shows that stimulation parameters for the CNS are similar to those needed for peripheral nerve: short pulses with a duration of less than 1 ms and with an amplitude of few milliamperes. TMS methods for brain stimulation face the problem of delivering such a stimulus across the high resistance barrier of the periencephalic ‘layers’, including scalp, skull, meninges and cerebrospinal fluid [15]. The first brain stimulation studies were conducted using high voltage electrical stimuli through the use of electrodes placed directly on the scalp. This technique is known as transcranial electrical stimulation (TES) [16]. The TES did have the huge merit of introducing a neurophysiological technique for studying for the first time excitability and propagation properties along CNS fibers in intact and cooperative human beings [15]. However, the fields of application declined rapidly with the introduction of TMS because high-voltage TES is uncomfortable [13]. The ability of TMS to stimulate deep neural structures, such as the motor cortex, has enabled researchers to investigate the integrity of the brain to muscle pathway and the functionality of cortical networks [17]. To appreciate the potential of TMS, it is necessary to characterize the neuromuscular responses to cortical stimulation (Figure 1). The MEPs were elicited by positioning the coil tangentially to the scalp with the handle of the coil pointing backward and 45° laterally from the interhemispheric line (Figure 2).

Since neurons connecting to muscles in distinct regions of the body have their own geographical location across the motor cortex [18], it is possible to deliver magnetic stimuli to discrete collections of neurons relating to specific muscle groups. The TMS has been used also to study the human nervous system within clinical populations [19,20,21]; mechanisms of fatigue in small, isolated muscle groups [22,23,24,25]; corticospinal contributions during human gaitand acute neural adaptations following strength training [26,27,28,29]. In neurostimulation studies using TMS, magnetic pulses are delivered directly to the motor cortex and their MEPS is recorded on the muscle by surface electromyography. The intensity of magnetic stimuli is typically given as a multiple or a percentage of the resting motor threshold, which is the intensity to evoke members of a certain amplitude in a specified fraction of a series of consecutive demonstrations in a hand muscle. However, several studies show that the resting motor threshold in humans varies from subject to subject, so to obtain significant results it would be advisable to have an adequate number of subjects involved in the studies. The resting motor threshold (rMT) is the intensity of the stimulus necessary to evoke a muscular response; the rMT is used as target intensity for the following stimulations [30]. TMS induces electrical currents in the brain via Faraday’s principle of electromagnetic induction [31]. Faraday has shown that an electrical impulse that runs through a wire wound in a coil generates a magnetic field, and the speed variation of this magnetic field causes the induction of a secondary current in a nearby conductor. This is what happens with TMS, where an electrical stimulus, which reaches peak strength and decreases to zero in a short period of time (<1ms), is sent through the conductive wiring inside the TMS coil. The rapid fluctuation of this current produces a magnetic field perpendicular to the plane of the coil that similarly rises (up to about 2.5 T) and falls rapidly in time. This rapidly fluctuating magnetic field passes unimpeded through the subject’s scalp and skull and induces a current in the brain in the opposite direction of the original current [31,32,33,34,35]. When TMS is performed with the target muscle steadily contracting, it shows different results than when the muscle is relaxed. Muscle contraction has three main effects [36]: The threshold for evoking the motor response is reduced, the latency of the MEP is shortened, and the amplitude of the MEP is markedly increased [37]. A subthreshold stimulus followed by a suprathreshold test stimulus (S1) at interstimulus interval (ISI) of 1–6 ms, the MEP generated by the S1 is inhibited and this is known as short interval intracortical inhibition (SICI). On the other hand, the MEP generated by S1 is facilitated at ISI of 8–30 ms and this is termed intracortical facilitation (ICF) [38]. The underlying mechanisms for facilitation are not entirely understood but likely include increased cortical and spinal excitability [39]. With voluntary contraction, the resting potential of the anterior horn cell (AHC) is closer to a threshold, requiring less temporal summation of descending volleys, which means that the discharge can occur at an earlier I or D wave, thus shortening the onset latency. The increase of the compound muscle action potential amplitude indicates the recruitment of a greater number of spinal motoneurons. This could also be due to increased spinal excitability, increased synchronization of spinal motoneuron firing, or an increasing number of I waves bringing more AHCs to the threshold. In the past years, the deep brain stimulation (DBS) technique has been used to investigate movement disorders. DBS involves the implantation of electrodes in certain areas of the brain. These electrodes produce electrical pulses that regulate abnormal pulses. The amount of stimulation in deep brain stimulation is controlled by a pacemaker-like device placed under the skin in the upper chest. A wire that travels under the skin connects this device to electrodes in the brain. Neuroscience investigations have revealed that DBS may be correlated to several mechanisms including functional changes with neuronal activation or inhibition, neurotransmitter release, and long-term plastic changes in target and remote areas [40]. The DBS technique is more invasive than tms; moreover, DBS is mainly indicated for the treatment of some pathologies. Future studies exploiting the combined use of TMS and DBS in patients with movement disorders could lead to new treatment strategies for these patients.

In recent decades, in order to understand how the brain networks build and optimize motor programs, responsible for the different types of muscle activity and related coordination [41], numerous studies have been performed that included the use of neuroimaging and TMS [42,43]. The ability of TMS to stimulate deep neural structures, such as the motor cortex, has enabled researchers to investigate the integrity of the brain to muscle pathway and the functionality of cortical networks [44]. Since MEP are readily measurable by electromyographic recordings on peripheral muscles, the investigation of cortical excitability has become the focus of numerous studies. The brain reorganization in human is highly dependent on the specific behavioral demands of the training experience.

The use of the TMS for research purposes in the motor and sports field is of great interest as it is applied to investigate post-exercise facilitation, central fatigue, sensorimotor integration, motor coordination, and neuronal plasticity. For example, with TMS, it was possible to demonstrate that when a subject performs a voluntary non-maximal muscle contraction, the corticospinal path to the muscle is facilitated [74]. Additionally, other neurostimulation studies with TMS have shown greater improvement in MEP for precision movement than for general gripping tasks, and this seems to be due to greater recruitment of pyramidal neurons [75]. Instead, there are conflicting arguments regarding the facilitating effects during a voluntary contraction of the ipsilateral orneighboring homonyms muscles [43]. However, in addition to the acute effects of motor activity, long-term effects of MEP enhancement can also be appreciated. In fact, Brasi-Neto et al. show that 10-s activation could lead to post-exercise facilitation, which decayed to the baseline over 2 to 4 min [76]. Since these effects were not present after the magnetic stimulations, the researchers hypothesized that these are the changes in the intracorticular plastics. Hollge et al. (1997) were the first to apply TMS to the study of dynamic exercise [58]. Those authors found significant decreases in MEP amplitude evoked in the primary muscles associated with exhaustive 400 m running, press-ups and dumbbell holding. This decrease were described as a central failure because responses to peripheral nerve stimulation were unchanged [58]. Confirming this CNS impairment, reduced intracortical facilitation was found after pull-ups to task failure, reflecting a decreased excitability of interneuronal circuits within the motor cortex [58]. Others authors shoed reduced MEP amplitudes of both the quadriceps and diaphragm after maximal incremental treadmill exercise, with no change in the response to peripheral nerve stimulation [77]. Transcranial magnetic stimulation has also been used to assess supraspinal fatigue of small muscle groups working in isolation. Goodall et al. (2012) used TMS to evaluate supraspinal fatigue of the knee-extensor muscles in response to sustained, high-intensity cycling in normoxia and acute severe hypoxia. Cortical voluntary activation declined after exercise in both conditions, but the decline was two-fold greater in hypoxia. Recently, Moscatelli et al. (2016), investigated the relationship between blood lactate and cortical excitability in taekwondo athletes. In this study, the authors show that blood lactate seems to have a greater influence in athletes compared to untrained subjects. It seems that, during extremely intensive exercise in athletes, lactate may the onset of fatigue not only by maintaining the excitability of muscle but also by increasing the primary motor cortex excitability more than in non-athletes [41]. Collectively, these findings suggest that TMS has the potential to quantify the contribution of central processes to fatigue of limb locomotor muscles. A recent investigation showed that, after 8 weeks of aerobic training, there was a significant increase of distance covered during Cooper’s test and a significant increase of VO2max; there was also an improvement in resting motor threshold, MEP latency and ME amplitude improvement [78]. Transcranial magnetic stimulation can be used to investigate physiological states other than fatigue. For example, it is well established that neuromuscular adaptation readily occurs as a result of resistance exercise training [17]. The M1 is heavily involved in voluntary contraction of skeletal muscle and shows a high degree of plasticity, or capacity to change quickly, with motor practice [42,43,44]. In a classic example, Muellbacher et al. (2002) showed that 20 min practice of a ballistic pinching task elicited a significant improvement in task performance [79]. The improvement in task performance was accompanied by an immediate increase in the corticospinal response, demonstrating that M1 has an adaptive role in the consolidation of motor tasks (Table 1).

Therefore, TMS enables a greater understanding of the behavior of the corticospinal tract in ‘top-down’ paradigms, where the effect of motor skills on corticospinal plasticity and neuromuscular adaptation can be examined. The remainder of this section will explore some potential applications of TMS for the investigation of M1 plasticity during and following different experimental paradigms, including task-specific contractions and resistance exercise training.

An interesting study was recently published that shows the effects of tDCS using the Halo Sport device on repeated sprint cycling ability and cognitive performance. The authors found that by using this device, the power delivered by repeated sprint cycles was improved. Interest in the possible ergogenic effect of noninvasive brain stimulation is growing and therefore in the future it could be useful to conduct new experiments to evaluate the impact on learning and motor performance [80,81,82].