Power Reserve At Intolerance In Ramp-Incremental Exercise Is Dependent On Incrementation Rate

Introduction: The mechanism(s) of exercise intolerance at (V)over dotO(2max) remain poorly understood. In health, standard ramp-incremental (RI) exercise is limited by fatigue-induced reductions in maximum voluntary cycling power. Whether neuromuscular fatigue also limits exercise when the RI rate is slow and RI peak power at intolerance is lower than standard RI exercise, is unknown. Methods: In twelve healthy participants, maximal voluntary cycling power was measured during a short (similar to 6 s) isokinetic effort at 80 rpm (P-iso) at baseline and, using an instantaneous switch from cadence-independent to isokinetic cycling, immediately at the limit of RI exercise with RI rates of 50, 25, and 10 W.min(-1) (RI-50, RI-25, and RI-10). Breath-by-breath pulmonary gas exchange was measured throughout. Results: Baseline P-iso was not different among RI rates (analysis of variance; P> 0.05). Tolerable duration increased with decreasing RI rate (RI-50, 411 +/- 58 s vs RI-25, 732 +/- 93 s vs RI-10, 1531 +/- 288 s; P < 0.05). At intolerance, (V)over dotO(2peak) was not different among RI rates (analysis of variance; P > 0.05), but RI peak power decreased with RI rate (RI-50, 361 +/- 48 W vs RI-25, 323 +/- 39 W vs RI-10, 275 +/- 38 W; P < 0.05). P-iso at intolerance was 346 +/- 43 W, 353 +/- 45 W, and 392 +/- 69 W for RI-50, RI-25, and RI-10, respectively (P < 0.05 for RI-10 vs RI-50 and RI-25). At intolerance, in RI-50 and RI-25, P-iso was not different from RI peak power (P> 0.05), thus there was no "power reserve." In RI-10, P-iso. was greater than RI peak power at intolerance (P< 0.001), that is, there was a "power reserve." Conclusions: In RI-50 and RI-25, the absence of a power reserve suggests the neuromuscular fatigue-induced reduction in P-iso coincided with (V)over dotO(2max). and limited the exercise. In RI-10, the power reserve suggests neuromuscular fatigue was insufficient to limit the exercise, and additional mechanisms contributed to intolerance at (V)over dotO(2max).