The Tolerability and Efficacy of 4 mA Transcranial Direct Current Stimulation on Leg Muscle Fatigability

Abstract

Transcranial direct current stimulation (tDCS) modulates cortical excitability and affects a variety of outcomes. tDCS at intensities ≤2 mA is well-tolerated, but the tolerability and efficacy of tDCS at intensities >2 mA merits systematic investigation. The study objective was to determine the tolerability and effects of 4 mA tDCS on leg muscle fatigability. Thirty-one young, healthy adults underwent two randomly ordered tDCS conditions (sham, 4 mA) applied before and during an isokinetic fatigue test of the knee extensors and flexors. Subjects reported the severity of the sensations felt from tDCS. Primary outcomes were sensation tolerability and the fatigue index of the knee extensors and flexors. A repeated-measures ANOVA determined statistical significance (p < 0.05). Sensation severity at 4 mA tDCS was not substantially different than sham. However, two subjects reported a moderate–severe headache, which dissipated soon after the stimulation ended. The left knee flexors had significantly greater fatigability with 4 mA tDCS compared with sham (p = 0.018). tDCS at 4 mA was well-tolerated by young, healthy subjects and increased left knee flexor fatigability. Exploration of higher intensity tDCS (>2 mA) to determine the potential benefits of increasing intensity, especially in clinical populations with decreased brain activity/excitability, is warranted.

Transcranial Direct Current Stimulation at 4 mA Induces Greater Leg Muscle Fatigability in Women Compared to Men

“…To examine the effect of different tDCS intensities on leg muscle fatigue, a torque-derived fatigue index (FI-T) was calculated for both the knee extensors and flexors on each leg. The FI-T was computed using the peak torque from each repetition of the FT as follows: ([mean of the first five repetitions − mean of last five repetitions]/[mean of first five repetitions]) × 100 [14,15,26,27,29,30]. The first two repetitions of each FT were considered as adaptations to the FT and were excluded from the FI-T calculation [14,15] (Figure 2).…”

“…The FT commenced with the same submaximal warm-up as the strength test (15 reps, concentric/concentric, 60 • /s) and consisted of 40 continuous concentric/concentric maximal contractions of the knee extensors and flexors at 120 • /s [25]. This FT protocol has been well established in both healthy people and people with neurological disorders [14,15,[26][27][28][29][30]. At minute 15 of the tDCS protocol (see "tDCS sessions" below), the R-FT was performed, followed by the L-FT.…”

“…After the completion of the FT and tDCS, participants were asked to describe the sensations they experienced from stimulation (e.g., itching, tingling, burning, etc.) and to rate stimulation severity on a 10-point (1 = "barely perceptible" and 10 = "most I could possible stand") Likert-type scale [14,15,42]. Participants were also asked to guess which stimulation intensity (sham, 2 mA, or 4 mA) they received in their session.…”

“…In addition, performance fatigability is the magnitude or rate of change in performance over a given time relative to a reference value. The effects of tDCS on performance fatigability in healthy participants [14][15][16] and in people with neurological impairments [17][18][19][20][21][22] has been previously investigated and has yielded ambiguous findings. The mixed results might be from a lack of standardized protocols and inconsistent definitions of fatigue, which makes comparison of fatigue outcomes between studies more challenging.…”

“…Nevertheless, a majority have used isometric contractions, tDCS intensities less ≤ 2 mA, and administered stimulation before a given task. Only two studies [14,15] applied different intensities of tDCS (2 mA and 4 mA) during an isokinetic fatigue task in healthy young adults. Surprisingly, the authors found that both lower and higher intensity tDCS resulted in greater leg muscle fatigability compared to sham.…”