Transcranial electrical stimulation (TES) in human motor Optimization: Mechanisms, safety, and emerging applications

Abstract

Non-invasive brain stimulation (NIBS) has emerged as a rapidly advancing field, offering promising therapeutic interventions for a range of neurological disorders while effectively bridging the gap between laboratory research and clinical applications. Among NIBS technologies, transcranial electrical stimulation (TES) stands out as a notable example, utilizing electrodes of varying sizes to deliver low-intensity electrical currents to specific regions of the cerebral cortex. This technique facilitates the modulation of neuronal excitability, regulation of brainwave activity, promotion of neural remodeling and repair, enhancement of cerebral blood flow, and improvement of brain-muscle connectivity. Despite its potential, current research on the effects of TES on motor function across diverse populations, particularly from a central nervous system perspective, remains limited. This review seeks to establish a theoretical framework for the future advancement of TES technology in sports science, elucidate the neurophysiological mechanisms underlying various TES modalities, and synthesize the most recent experimental findings from the past two decades regarding its impact on physical fitness, motor skill acquisition, and recovery in different populations.

Keywords: Neuromodulation; Safety; Sports performance; TES.

Fig. 1

Electrode placement, current waveform and influencing factors for different TES types.

Fig. 2

The development history of TES [7,8,9,10,11,12,13].

Fig. 3

Summary of applications of TES electrode placement in different brain regions based on Functional Magnetic Resonance Imaging (fMRI) [28,29,30,31,32,33,34,35,36], Electroencephalogram (EEG) [37,38,39,40,41,42], Functional Near-Infrared Spectroscopy (fNIRS) [43,44,45,46,47,48]and Magnetoencephalography (MEG) imaging techniques [49,44,50,51].

Fig. 4

Schematic representation of the different types of tDCS and their physiological mechanisms.(a)Conventional tDCS [52].(b)Individualized High-Definition tDCS [47].(c)High-Definition tDCS [53].(d)Differential polarization of cortical pyramidalneuron dendrites through weak extracellular fields [54].(e)The effects of tDCS on individual neurons are neurochemically modulated to include LTP and LTD [55].(f)Astrocytes as a target of tDCS to treat depression [56,57,58,52]. (g)tDCS-induced changes in brain synchronization and topological functional organization [52,59].

Fig. 5

Effects of tDCS on cerebral haemodynamics(a)Cerebral oxygenation percent change from baseline. Early Group (stimulation at 10–20 min); Late Group (stimulation at 30–40 min) [74].(b)Changes in rCBF over time in a typical subject fitted with the anodal montage [71].(c)Regional oxygen saturation results [75].(d)Brain perfusion changes during stimulation compared with baseline Red areas represent areas where cerebral perfusion increases during anodic stimulation, and blue areas represent areas where perfusion decreases during cathodic stimulation [69].(e)Spatiotemporal representation of CHbO2 obtained under both real stimulus (top) and sham stimulus (mid) conditions [73].

Fig. 6

Schematic representation of different types of tACS and their physiological mechanisms.(a)Conventional TACS [80].(b)High-Definition TACS [81].(c)Individualized High-Definition TACS [75].(d)Schematic diagram of the physiological mechanism of tACS [80].

Fig. 7

Schematic representation of the different types of tRNS and their physiological mechanisms.(a)Conventional tRNS [110].(b)High-Definition tRNS [109].(c)Conceptual representation of how electrical RNS may enhance the neural signal and influence neural response according to the SR phenomenon [111].(d)The method to analyze the effects of electrical RNS on the peak amplitude of Na + currents elicited by a voltage-clamp-ramp protocol in dissociated cortical neurons of Wistar rats. Left panel, pictures of two pyramidal cells from the auditory and somatosensory cortex. Right panel, voltage-clamp ramps and the associated Na + currents for these cells in conditions of zero RNS and five different levels of RNS as indicated above. Note that there is an increase in the peak amplitude of the Na + current for intermediate intensities of RNS (red recordings) [112].(e)Regions of decreased activity for hf-tRNS. Contrast sham- Hfreq (left) revealed changes in the left frontal cortex. Contrast Lfreq-Hfreq (right) revealed additional changes in right frontal cortex and precuneous [113].(f)Boxplots of the activation volumes resulting from the movement after the diVerent stimulation conditions compared to the REST in the sensorimotor, premotor and SMA [114].