More is not necessarily better: A comparison of 2-mA and 4-mA prefrontal cortex anodal tDCS on endurance performance, head oxygenation, cognition, and psychophysiological responses in cyclists.

We evaluated the effects of 2-mA and 4-mA anodal transcranial direct current stimulation (tDCS), targeting the left dorsolateral prefrontal cortex (DLPFC), on endurance performance, prefrontal head oxygenation, cognition, and psychophysiological responses in trained cyclists. Eighteen trained male cyclists (age: 36.0 ± 8.52 years; V̇O2max: 58.82 ± 8.16 ml·kg-1·min-1) performed 20-km time trials (TT) after receiving 20-min 2 mA, 4 mA, or sham tDCS in a randomized, double-blind, crossover design. Head oxyhemoglobin (HbO2) and deoxyhemoglobin (Hbb), and cognitive performance (Stroop test) were assessed pre-tDCS, post-tDCS, and post-TT. Power output, heart rate (HR), and HbO2/Hbb were also monitored, with ratings of perceived exertion (RPE) and affective response collected during TT. The intervention was well-tolerated with successful blinding. Relative to sham, 2 mA increased average power and reduced completion time, with no difference from 4 mA; it also raised HR above both sham and 4 mA. During exercise, 2 mA raised left-hemisphere HbO2 relative to 4 mA and Hbb relative to both sham and 4 mA. The 2-mA power gain was greater with slower baseline reaction time. No condition affected cognition, RPE, or affective responses. A single session of 2 mA anodal tDCS produced small, practically relevant improvements in endurance performance relative to sham alongside intensity-dependent changes in prefrontal head oxygenation, with the largest performance benefit associated with poorer baseline cognitive performance; 4 mA did not differ from sham on either outcome. These findings suggest a non-monotonic intensity-response relationship, where higher intensity does not necessarily produce a larger effect.