Trajectories of cardiopulmonary adaptation and executive function in elite endurance athletes receiving transcranial direct current stimulation with interval training

Abstract

This study examined how transcranial direct current stimulation (tDCS) interacts with high-intensity interval training (HIIT) to shape cardiopulmonary fitness and executive function in elite endurance athletes across an 8-week protocol. Sixty-six nationally ranked athletes were stratified by training tenure, competitive level, and baseline maximal oxygen uptake, then prospectively allocated, with participant and outcome-assessor blinding, to one of three intervention arms: real-tDCS plus HIIT, sham-tDCS plus HIIT, or HIIT-only; 59 completed the programme. Both stimulation arms alternated between a left dorsolateral prefrontal montage and a bilateral motor montage, and cortical target was carried through the analysis as a within-subject factor rather than folded into a single stimulation label. Cardiopulmonary indices, executive function (Stroop, N-back, Flanker), prefrontal hemodynamics via functional near-infrared spectroscopy, and heart rate variability were measured at four assessment waves, with female athletes tested inside the early follicular window at every wave. Nonlinear mixed-effects and generalized additive mixed models resolved dynamic trajectories at the individual level, while a second-order partial derivative quantified the stimulation × load interaction. End-of-study allocation guesses did not depart from chance in either masked arm, so blinding held. Because masking held only for the two stimulation arms, the real- versus sham-tDCS comparison carries the causal weight of the trial, and the HIIT-only arm enters as a partially unblinded reference rather than a second control. Against sham, the real-tDCS arm showed a delayed but steeper rise in maximal oxygen uptake and an earlier separation in Stroop interference. A modelled cumulative exposure-response surface exhibited a non-monotone shape with an interior maximum whose geometry, but not whose coordinates, survived leave-one-subject-out cross-validation; because intensity and duration were fixed and exposure varied only with session completion, we present that surface as exploratory. Cross-system coupling between prefrontal oxygenation and ventilatory threshold strengthened under active stimulation, consistent with, though not proof of, a bidirectional central-peripheral regulation account. Adherence captured the largest share of individual response variance, with BDNF and ACTN3 variants contributing modestly. As exploratory findings, these patterns are consistent with a stratified, trajectory-aware view of neuromodulation in elite training, though claims about an optimal dose must await confirmatory trials that deliberately vary stimulation intensity.