Abstract
Studies investigating the effects of transcranial direct current stimulation (tDCS) on fatigue and muscle activity have elicited measurable improvements using stimulation intensities ≤2 mA and submaximal effort tasks. The purpose of this study was to determine the effects of 2 mA and 4 mA anodal tDCS over the primary motor cortex (M1) on performance fatigability and electromyographic (EMG) activity of the leg muscles during a maximal isokinetic task in healthy young adults. A double-blind, randomized, sham-controlled crossover study design was applied. Twenty-seven active young adults completed four sessions, each spaced by 5–8 days. During session 1, dominance was verified with isokinetic strength testing, and subjects were familiarized with the fatigue task (FT). The FT protocol included 40 continuous maximum isokinetic contractions of the knee extensors and flexors (120°/s, concentric/concentric). During Sessions 2–4, tDCS was applied for 20 min with one of three randomly assigned intensities (sham, 2 mA or 4 mA) and the FT was repeated. The anode and cathode of the tDCS device were placed over C3 and the contralateral supraorbital area, respectively. A wireless EMG system collected muscle activity during the FT. The 2 mA tDCS condition had significantly less torque (65.9 ± 32.7 Nm) during the FT than both the sham (68.4 ± 33.9 Nm, p < 0.001) and 4 mA conditions (68.4 ± 33.9 Nm, p = 0.001). Furthermore, the 2 mA condition (33.8 ± 11.7%) had significantly less EMG activity during the FT than both the sham (39.7 ± 10.6%, p < 0.001) and 4 mA conditions (40.5 ± 13.4%, p = 0.001). Contrary to previous submaximal isometric fatigue investigations, the 2 mA tDCS condition significantly reduced torque production and EMG activity of the leg extensors during a maximal isokinetic FT compared with the sham and 4 mA conditions. Also, torque production and EMG activity in the 4 mA condition were not significantly different from sham. Thus, the effects of tDCS, and the underlying mechanisms, might not be the same for different tasks and warrants more investigation.
Highlights
Transcranial direct current stimulation is a non-invasive means of increasing brain excitability (Nitsche and Paulus, 2000)
It was hypothesized that anodal transcranial direct current stimulation (tDCS) would decrease leg muscle fatigability, which would be accompanied by a modulated neural drive (EMG) to these same muscles
The main and novel results of this study are: (1) 2 mA tDCS reduced torque production of the knee extensors during a maximal isokinetic fatigue task (FT), accompanied by reduced EMG activity; and (2) 4 mA tDCS did not affect torque production and EMG activity compared to sham
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive means of increasing brain excitability (Nitsche and Paulus, 2000). It has been used for several years and in many populations to improve physical and psychological outcomes (Chhatbar et al, 2017). Studies examining the safety of tDCS for human subjects used careful and moderate stimulation approaches (Bikson et al, 2016). That the safety and tolerability of tDCS at higher intensities is better established, work exploring the performance differences between moderate (i.e., 2 mA) and higher (i.e., 4 mA) intensities is necessary to determine if increasing intensity further enhances outcomes
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