Work Rate Decrease At A Fixed Heart Rate To Evaluate Exercise Tolerance In Microgravity.

Aerobic exercise prescription is usually set at specific heart rate (HR) values. Previous studies demonstrated that during exercise carried out at a HR slightly above that corresponding to the gas exchange threshold (GET), work rate (w') has to decrease in order to maintain HR constant. Simulated microgravity (bed rest, BR) determines a significant impairment of oxidative metabolism and exercise tolerance. PURPOSE: To test the hypothesis that the w' decrease at a fixed HR would be greater after BR. METHODS: Ten male volunteers (23±5 yr) were tested before (PRE) and after (POST) a 10-day horizontal BR, and performed on a cycle ergometer: a) an incremental exercise up to voluntary exhaustion (INCR); b) a 15-min "HRCLAMPED" exercise, in which w' was continuously adjusted to maintain a constant HR, slightly higher than that determined at GET. Pulmonary gas exchange was assessed breath-by-breath. Fractional O2 extraction (Δ[deoxy(Hb+Mb)]) was evaluated at different sites of the quadriceps by near-infrared spectroscopy (NIRS) RESULTS: During INCR peak pulmonary O2 uptake (...O2peak) (40.3±6.1 vs. 44.4±7.2 ml/kg/min, p<0.001) and Δ[deoxy(Hb+Mb)]peak (p<0.05) were significantly lower in POST vs. PRE. w' at GET was not different in POST (113±33 W) vs. PRE (118±33 W). During HRCLAMPED exercise (set at 145±11 b/min), the decrease in w' needed to maintain a constant HR was significantly more pronounced in POST vs. PRE (-39±10 vs. -29±14%, p<0.01), and it was associated with decreases (both in POST and in PRE) of V...O2 (-14 and -13%), Δ[deoxy(Hb+Mb)] (-41 and -18%), gas exchange ratio (R) and blood lactate ([La]b). Net mechanical efficiency decreased by 38 and 36%, in POST and PRE, respectively. CONCLUSIONS: During 15-min exercise initially set at a w' corresponding to a HR slightly above GET (as frequently done for aerobic exercise prescription), in order to keep HR constant w' had to decrease by ~29% and ~39%, respectively, before and after a 10-day BR. Exercise prescription at fixed submaximal HR is not associated with a specific w'; the problem is greater in microgravity. The w' decrease at a fixed HR is a useful tool to identify an impaired exercise tolerance. The w' decrease was more pronounced than that necessary to prevent "slow components" of ...O2, Δ[deoxy(Hb+Mb)], R and [La]b. Funding: ASI, MARS-PRE Project, n. DC-VUM-2017-006. [ABSTRACT FROM AUTHOR]