The physical limits of the human performance have been the object of study for a considerable time. Most of the research has focused on the locomotor muscles, lungs, and heart. As a consequence, much of the contemporary literature has ignored the importance of the brain and brain stimulation in the regulation of exercise performance.
With the introduction and development of new non-invasive devices, the knowledge regarding the behavior of the central nervous system during exercise has advanced.
A first step has been provided from studies involving neuroimaging techniques where the role of specific brain areas have been identified during isolated muscle or whole-body exercise. Furthermore, a new interesting approach has been provided by studies involving non-invasive techniques to manipulate specific brain areas (brain stimulation).
These techniques most commonly involve the use of an electrical or magnetic field crossing the brain. In this regard, there has been emerging literature demonstrating the possibility to influence exercise outcomes in healthy people following stimulation of specific brain areas.
Specifically, transcranial direct current stimulation (tDCS) has been recently used prior to exercise in order to improve exercise performance under a wide range of exercise types.
During sustained submaximal contraction, the excitability of spinal motoneurons and the contractile capacity of the muscle fibers are reduced (Butler et al., 2003; Allen et al., 2008), so that in order to maintain the required force or power, the input to the spinal motoneurons must increase (Taylor et al., 1996).
This input (also called descending drive) is likely to originate from the corticospinal pathway, and previous experiments have demonstrated a number of factors which may moderate this (Gandevia, 2001; Enoka et al., 2011). In this regard, a failure to generate output from the motor cortex (M1) has been defined as supraspinal fatigue, and together with peripheral mechanisms, participates in muscle fatigue (Gandevia, 2001).
Previous studies have suggested that the development supraspinal fatigue is accompanied by changes in motor cortex excitability (Taylor et al., 1996). Interventions that increase M1 excitability might increase the output from M1 (increase descending drive) thus delaying the development of supraspinal fatigue and therefore improving exercise capacity (Cogiamanian et al., 2007; Williams et al., 2013).
In this regard, a neuromodulatory technique called transcranial direct current stimulation (tDCS) or brain stimulation has been widely used to modulate the excitability of a targeted brain area through the application of a weak electrical current across the scalp. The electrical current alters the resting membrane potential of the targeted neurons, with the anodal electrode being excitatory and the cathodal being inhibitory (Nitsche et al., 2008; George and Aston-Jones, 2010).
These effects can persist for up to 90 min following 9–13 min of stimulation (Nitsche and Paulus, 2001).
Studies have demonstrated that acute tDCS is a safe neuromodulatory brain technique, with no or only minor side effects (Fregni et al., 2006; Poreisz et al., 2007; Frank et al., 2010) and is both cheap and easy to administer. Therefore, interest in tDCS’ ergogenic potential has grown considerably wondering how maximizing sports performance in the future might look like.
Research has only recently started to investigate the effect of tDCS on physical performance and, given the prominent role of the motor and premotor brain regions in the development of supraspinal fatigue (Gandevia, 2001), most studies have attempted to target these areas.
To date, there are a limited number of studies, showing inconsistent results and often with flawed methodological design. Nevertheless, the balance of evidence suggests that tDCS might have a positive effect on exercise capacity.
Source: The Ergogenic Effects of Transcranial Direct Current Stimulation on Exercise Performance (Front Physiol, 2017 Feb 14;8:90. doi: 10.3389/fphys.2017.00090. eCollection 2017)