Is the time to task failure during severe intensity exercise associated with muscle, blood, and respiratory changes?

Purpose: The study aimed to verify the physiological and metabolic parameters associated with the time to task failure (TTF) during cycling exercise performed within the severe-intensity domain. Methods: Forty-five healthy and physically active males participated in two independent experiments. In experiment 1, after a graded exercise test, participants underwent constant work rate cycling efforts (CWR) at 115% of peak power output to assess neuromuscular function (Potentiated twitch) pre- and post-exercise. Experiment 2 was similarl to experiment 1, but with physiological (respiratory parameters, energetic pathway contribution) and metabolic parameters in the blood (gasometry and blood lactate responses) and vastus lateralis muscle tissue (target metabolomic analysis, glycogen content, muscle pH and buffering capacity in vitro) measured instead of neuromuscular function. Results: Experiment 1 evidenced a significant decrease in muscle force with instauration of peripheral fatigability indices and no change in central fatigue indices. Severe-intensity domain exercise in Experiment 2 was accompanied by changes in physiological and metabolic parameters and in blood and muscle parameters. However, the TTF was associated with oxidative contribution (r=0.811, p<0.001), as well as anaerobic capacity (r=0.554, p=0.027), muscle buffering capacity (r=0.792, p=0.035), phosphagen energy contribution (r=0.583, p=0.017), and carnitine changes (r=0.855, p=0.016), but no correlated with electromyographic response, blood acid-base balance, and muscular glycogen content and pH. Conclusion: TTF during CWR exercise within the severe-intensity domain is likely explained by a combination of interacting mechanisms, with oxidative and phosphagen contributions, and muscle buffering capacity suggested as the main peripherals limiting factors to exercise within this exercise intensity domain.