Anodal transcranial direct current stimulation does not influence the neural adjustments associated with fatiguing contractions in a hand muscle

Abstract

Purpose: The objective of the current study was to investigate the mechanisms responsible for the briefer time to failure of a submaximal contraction (C2) when performed 60 min after a similar contraction (C1), and the influence of anodal transcranial direct current stimulation (a-tDCS) applied over the motor cortex on these mechanisms.

Methods: In two sessions, ten adults sustained two isometric contractions (35% of maximum) to failure with the abductor pollicis brevis (APB). Before C2, either a-tDCS or sham stimulation was applied over the motor cortex. Fatigue-related changes in Hoffmann (H) and long-latency (LLR) reflexes, motor-evoked potential (MEP) induced by transcranial magnetic stimulation and associated silent period (SP), maximal motor wave (M_max), voluntary activation (VA), electromyographic (EMG) activity and peak force (PT₃) evoked by a 3 pulse-train (100 Hz) were investigated.

Results: The results indicate that regardless of session, the time to failure was briefer (- 13%, p < 0.05) for C2 than C1, with no a-tDCS effect. During C1, MEP amplitude, SP duration and LLR amplitude increased, H-reflex amplitude did not change, and M_max, VA and PT₃ decreased (p < 0.05). Except for EMG activity that was greater during C2 than C1 (p < 0.001), all variables were similar in C1 and C2 (p > 0.05), and recovered their initial values after the 60-min rest, except PT₃.

Conclusions: The results of the current study indicate that a-tDCS did not influence corticospinal excitability and time to failure of C2 when performed with the APB. These observations may reflect a peripheral origin of the briefer C2 time to failure in the APB.

Keywords: Electromyography; Fatigue; H reflex; Long-latency reflex; Motor-evoked potential; Transcranial magnetic stimulation.