Neurophysiological and perceptual responses to consecutive days of exhaustive high-intensity interval cycling are dissociated

The impact of performing consecutive high-intensity interval (HIIT) exercise sessions on psychophysiological responses remains unclear. We hypothesized that consecutive HIIT sessions would exacerbate neuromuscular function impairments, metabolic disruptions, and perceptual responses. Sixteen healthy participants (7 females) performed five consecutive days of HIIT (4:4 min cycling:rest intervals) to task failure. On Days 1, 3, and 5, transcranial magnetic and femoral nerve stimuli were delivered in the final minute of each interval to characterize corticospinal excitability. Immediately upon completion of each interval, neuromuscular fatigue was assessed by a maximal isometric voluntary knee extension coupled with superimposed and resting twitches. Cardiorespiratory and metabolic responses were measured throughout the exercise. Perceived fatigue, pain, breathlessness, and effort were recorded at the end of each work bout, using psychophysical scales. The total intervals completed were unchanged across the five days (P=0.415). The decline in quadriceps resting twitch force was attenuated on Day 5 compared to Day 3 (P=0.038) and Day 1 (P=0.020). Motor evoked potential normalized to maximal muscle compound action potential (i.e., corticospinal excitability) was lower on Day 5 compared to Day 1 (P=0.044). Blood lactate concentration was lower (P<0.001), and perceived fatigue (P<0.001), effort (P<0.001), pain (P=0.002), breathlessness (P<0.001) were all higher on Day 5 compared to Day 1. Overall, the five consecutive HIIT sessions resulted in less decline in resting twitch, decreased corticospinal excitability, and exacerbated perceptual responses. These results suggest increased perceptual responses observed after five consecutive days of HIIT exercise reflect fatigue-related central alterations rather than alterations at the muscle level.