Power reserve following ramp-incremental cycling to exhaustion: implications for muscle fatigue and function

Abstract

In ramp-incremental cycling exercise, some individuals are capable of producing power output (PO) in excess of that produced at their limit of tolerance (LoT) whereas others cannot. This study sought to describe the 1) prevalence of a "power reserve" within a group of young men ( n = 21; mean ± SD: age 25 ± 4 yr; V̇o_2max 45 ± 8 ml·kg^-1·min^-1); and 2) muscle fatigue characteristics of those with and without a power reserve. "Power reserve" (ΔPReserve) was determined as the difference between peak PO achieved during a ramp-incremental test to exhaustion and maximal, single-leg isokinetic dynamometer power determined within 45 s of completing the ramp-incremental test. Between-group differences in pre- vs. postexercise changes in voluntary and electrically stimulated single-leg muscle force production measures (maximal voluntary contraction torque, voluntary activation, maximal isotonic velocity and isokinetic power; 1-, 10-, 50-Hz torque; and 10/50-Hz ratio), V̇o_2max, and constant-PO cycling time-to-exhaustion also were assessed. Frequency distribution analysis revealed a dichotomy in the prevalence of a power reserve within the sample resulting in two groups: 1) "No Reserve" (NRES: power reserve <5%; n = 10) and 2) "Reserve" (RES: power reserve >15%; n = 11). At the LoT, all participants had achieved V̇o_2max. Muscle fatigue was evident in both groups, although the NRES group had greater reductions ( P < 0.05) in 10-Hz peak torque (PT), 10/50 Hz ratio, and maximal velocity. Time to the LoT during the constant PO test was 22 ± 16% greater ( P < 0.05) in RES (116 ± 19 s; PO = 317 ± 52 W) than in NRES (90 ± 23 s; PO = 337 ± 71 W), despite similar ramp-incremental exercise durations and V̇o_2max between groups. Compared with the RES group, the NRES group accrued greater peripheral muscle fatigue at the LoT, suggesting that the mechanisms contributing to exhaustion in a ramp-incremental protocol are not uniform. NEW & NOTEWORTHY This study demonstrates that the mechanisms associated with the limit of tolerance during ramp-incremental cycling exercise differ between those who are capable of generating power output in excess of that at exercise termination vs. those who are not. Those without a "power reserve" exhibit greater peripheral muscle fatigue and reduced muscle endurance, supporting the hypothesis that exhaustion occurs at a specific level of neuromuscular fatigue. In contrast, those with a power reserve likely are limited by other mechanisms.

Keywords: central fatigue; muscle function; peripheral fatigue; ramp-incremental exercise.

Fig. 1.

Schematic of exercise protocol. A: visit 1 (familiarization). Ramp-incremental (RI) exercise test (50 W baseline, 25 W/min ramp). B: visit 2. RI-muscle fatigue intervention. t = −20 to t = −5 illustrates the pre-RI neuromuscular assessment; t = 0–17 illustrates the RI-muscle fatigue intervention, and t = 17–20 illustrates P_70POST and post-RI neuromuscular assessment. C: visit 3. RI and 95% PO_peak for V̇o_2max validation and time to exhaustion (muscle function). PO_peak, peak power output; V̇o_2max, maximal oxygen uptake; LoT, limit of tolerance; WR, work rate.

Fig. 2.

Schematic of neuromuscular testing. Pre-RI: doublet stimulation to maximal knee extensor twitch torque, MVC, 1-Hz, 50-Hz, 10-Hz, isokinetic knee extensions (maximal power/P_70PRE), and isotonic knee extensions (maximal velocity). Post-RI: isokinetic knee extensions (P_70POST), isotonic knee extensions, 1-Hz, 50-Hz, 10-Hz, MVC. RI, ramp-incremental exercise; MVC, maximal voluntary contraction.

Fig. 3.

Distribution of the prevalence of isokinetic power reserve vs. RI PO_peak within a single group of recreationally active young men. The absence of isokinetic power reserve between 5% and 15% allowed for differentiation of 2 distinct populations: 1) NRES (closed circles; <5%; n = 10), and 2) RES (open circles; >15%; n = 11). The open square represents the mean PO_peak and ΔPReserve for RES, whereas the closed square represents the mean PO_peak and ΔPReserve for NRES.

Fig. 4.

Group mean pre- to postexercise muscle response (as represented by % change in torque measurements during 1 s at 1-Hz (1-Hz PT), 1 s at 50-Hz (50-Hz PT), and 1 s at 10-Hz (10-Hz PT) electrical stimulation, potentiated twitch (PoT), and 10/50-Hz ratio. Black bars represent NRES, while gray bars represents RES. Significant differences (¥) exist between groups in 10-Hz PT and 10/50-Hz. Statistical significance was accepted at α < 0.05. PT, peak torque.