Pasquale J. Succi1, Taylor K. Dinyer-McNeely2, Caleb C. Voskuil3, Brian Benitez1, Minyoung Kwak1, Mark G. Abel1, Jody L. Clasey, FACSM1, Haley C. Bergstrom1. 1University of Kentucky, Lexington, KY. 2Oklahoma State University, Stillwater, OK. 3Texas Christian University, Fort Worth, TX.

BACKGROUND: Recommendations for endurance exercise prescription are based on percentages of an individual’s heart rate (HR) or volume of oxygen consumption (V̇O2) maximum or reserve. These intensities are then extrapolated to a power output (P) or velocity. Previous work has demonstrated a dissociation of the physiological, perceptual, and neuromuscular responses to exercise anchored to the critical heart rate (CHR) compared to the P associated with CHR (PCHR). However, it is unclear if a similar dissociation, due to reductions in P to maintain the designated intensity, would be present during exercise anchored to the V̇O2 associated with CHR (V̇O2CHR). The purpose of this study was to examine the patterns in physiological (V̇O2, HR, P, respiration rate [RR], muscle oxygen saturation [%SmO2]), neuromuscular (electromyographic and mechanomyographic amplitude [EMG AMP, MMG AMP] and mean power frequency [EMG MPF, MMG MPF]), and perceptual (rating of perceived exertion [RPE]) responses during exercise at V̇O2CHR (V̇O2-clamp). METHODS: On separate days, ten participants (Age=25±4yrs;Height=173.1±11.9cm;Mass=74.7±8.5kg) performed a graded exercise test, 4 constant P trials at 85-100% of peak P (PP) to derive CHR and V̇O2CHR from the total heartbeats vs. time to exhaustion (TLim) and the V̇O2 vs. HR relationships, respectively. Responses were recorded during a trial to exhaustion at V̇O2CHR (32.86±7.12mL·kg-1·min-1; TLim=31.31±21.37min) and normalized in 10% intervals of TLim to their respective values at PP. RESULTS: The one-way repeated-measures ANOVA with follow-up Student Newman-Keuls tests indicated there were differences (p<0.001) among timepoints for HR (mean±SD %change=8±3%), RR (43±38%), P (-15±5%), EMG MPF (10±8%), and RPE (65±38%), but no differences (p=0.077-0.955) for %SmO2 (-17±53%), EMG AMP (-3±16%), MMG AMP (40±61%), and MMG MPF (1±7%). CONCLUSIONS: The loss in performance observed during V̇O2-clamp exercise may provide a quantification of the inefficiency associated with the V̇O2 slow component phenomenon. The EMG, MMG, and %SmO2 responses suggested continued muscle activation levels despite the reductions in P, but the HR, RR, and RPE responses suggested a combination of feedforward and feedback mechanisms regulating TLim. V̇O2-clamp exercise may present a novel methodology to examine mechanisms of fatigue and future studies should examine responses to V̇O2-clamp exercise at a uniform threshold.

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