Anodal tDCS over the left DLPFC but not M1 increases muscle activity and improves psychophysiological responses, cognitive function, and endurance performance in normobaric hypoxia: a randomized controlled trial

Abstract

Background: Transcranial direct current stimulation (tDCS) has been shown to have positive effects on exercise performance and cognitive function in the normal ambient condition. Hypoxia is deemed a stressful situation with detrimental effects on physiological, psychological, cognitive, and perceptual responses of the body. Nevertheless, no study has evaluated the efficacy of tDCS for counteracting the negative effects of hypoxic conditions on exercise performance and cognition so far. Hence, in the present study, we investigated the effects of anodal tDCS on endurance performance, cognitive function, and perceptual responses in hypoxia.

Participants and methods: Fourteen endurance-trained males participated in five experimental sessions. After familiarization and measuring peak power output in hypoxia, in the first and second sessions, through the 3rd to 5th sessions, participants performed a cycling endurance task until exhaustion after 30 min hypoxic exposure at resting position followed by 20 min of anodal stimulation of the motor cortex (M1), left dorsolateral prefrontal cortex (DLPFC), or sham-tDCS. Color-word Stroop test and choice reaction time were measured at baseline and after exhaustion. Time to exhaustion, heart rate, saturated O₂, EMG amplitude of the vastus lateralis, vastus medialis, and rectus femoris muscles, RPE, affective response, and felt arousal were also measured during the task under hypoxia.

Results: The results showed a longer time to exhaustion (+ 30.96%, p₌0.036), lower RPE (- 10.23%, p ₌ 0.045) and higher EMG amplitude of the vastus medialis muscle (+ 37.24%, p₌0.003), affective response (+ 260%, p₌0.035) and felt arousal (+ 28.9%, p₌0.029) in the DLPFC tDCS compared to sham. The choice reaction time was shorter in DLPFC tDCS compared to sham (- 17.55%, p₌0.029), and no differences were seen in the color-word Stroop test among the conditions under hypoxia. M1 tDCS resulted in no significant effect for any outcome measure.

Conclusions: We concluded that, as a novel finding, anodal stimulation of the left DLPFC might provide an ergogenic aid for endurance performance and cognitive function under the hypoxic condition probably via increasing neural drive to the working muscles, lowering RPE, and increasing perceptual responses.

Keywords: Circumplex model of affect; Electromyography; Non-invasive brain stimulation; Perceived exertion; Perceptual responses; Time to exhaustion.

Fig. 1

Mean values of TTE, HR, SpO₂, and EMG under 3 experimental conditions. Endurance A cycling time to exhaustion (TTE), B heart rate (HR), C blood oxygen saturation (SpO₂), electromyography (EMG) amplitude of D the vastus lateralis (VL), E vastus medialis (VM), and F rectus femoris (RF) muscles under hypoxia, with transcranial direct current stimulation targeting the primary motor cortex (M1), dorsolateral prefrontal cortex (DLPFC), and sham conditions. * = Significantly different from Sham

Fig. 2

Mean values of RPE, affective responses, and FA under 3 experimental condition. A Rating of perceived exertion (RPE), B affective response, and C felt arousal during the endurance cycling task in hypoxia after transcranial direct current stimulation targeting the motor cortex (M1), dorsolateral prefrontal cortex (DLPFC), and sham conditions. * = Significantly different from Sham

Fig. 3

Mean values of CRT and CWST score after endurance exhaustion under 3 experimental conditions. A Choice reaction time (CRT) and B color-word Stroop test (CWST) scores immediately after endurance time to exhaustion test in hypoxia with transcranial direct current stimulation targeting the motor cortex (M1), dorsolateral prefrontal cortex (DLPFC), and sham conditions. * = Significantly different from Sham

Fig. 4

The two-dimension Circumplex Model of Affect under 3 experimental conditions. The mean value of affective response and felt arousal (FA) were used to create a two-dimension circumplex model of affect during the endurance cycling task in hypoxia after transcranial direct current stimulation targeting the motor cortex (M1), dorsolateral prefrontal cortex (DLPFC), and sham conditions. FS feeling scale; FAS felt arousal scale

Fig. 5

Schematic of the whole study procedure and details of three experimental sessions (M1, DLPFC, and Sham). PPO Peak Power Output; CWST Color-Word Stroop Test; CRT Choice Reaction Time; tDCS Transcranial Direct Current Stimulation; TTE Time to Exhaustion; HR Heart Rate; SpO2 Blood Oxygen Saturation; RPE Rating of Perceived Exertion; EMG Electromyography; PA Pleasure Sensation FA Felt Arousal

Fig. 6

tDCS-induced electric field on brain areas for the study montages. tDCS-induced electric field on brain areas for the montages targeting the left dorsolateral prefrontal cortex (top line) and primary motor cortex representation of the lower limbs (bottom line). Anodal (red rectangle) and cathodal (blue rectangle) electrodes placed over the scalp (A, B, F, and G). Figures are color-coded according to the electric field strength so that hot colors (e.g., red) represent stronger electric fields and cold colors (e.g., blue) represent weaker electric fields. Frontal (C and H) and top (D and I) view of the electric current distribution in gray and white matter. Diagonal (E) and sagittal (J) view of the electric current distribution in gray matter with arrows roughly over the nominal targets (blue = left dorsolateral prefrontal cortex; red = primary motor cortex representation of the lower limbs). Note: Because the anatomical model does not include a shoulder for the M1 tDCS montage, the cathode electrode was placed on the lower part of the neck, which provides a good approximation of the should placement