Decrease in work rate in order to keep a constant heart rate: biomarker of exercise intolerance following a 10-day bed rest.

Aerobic exercise prescription is often 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 (WR) has to decrease in order to maintain HR constant. We hypothesized a greater WR decrease at a fixed HR after simulated microgravity/inactivity (bed rest, BR). Ten male volunteers (23 ± 5 yr) were tested before (PRE) and after (POST) a 10-day horizontal BR and performed on a cycle ergometer 1 ) incremental exercise; b ) 15-min HR _CLAMPED exercise, in which WR was continuously adjusted to maintain a constant HR, corresponding to that at 120% of GET determined in PRE; 3 ) two moderate-intensity constant WR (MOD) exercises. Breath-by-breath O ₂ uptake (V̇o ₂ ), HR, and other variables were determined. After BR, peak V̇o ₂ (V̇o _2peak ) and GET significantly decreased, by ∼10%. During HR _CLAMPED (145 ± 11 beats·min ^-1 ), the decrease in WR needed to maintain a constant HR was greater in POST versus PRE (-39 ± 10% vs. -29 ± 14%, P < 0.01). In six subjects the decreased WR switched from the heavy- to the moderate-intensity domain. The decrease in WR during HR _CLAMPED , in PRE versus POST, was significantly correlated with the V̇o _2peak decrease ( R ² = 0.52; P = 0.02). A greater amplitude of the slow component of the HR kinetics was observed during MOD following BR. Exercise at a fixed HR is not associated with a specific WR or WR domain; the problem, affecting exercise evaluation and prescription, is greater after BR. The WR decrease during HR _CLAMPED is a biomarker of exercise intolerance after BR. NEW & NOTEWORTHY During a 15-min exercise carried out at a heart rate (HR) slightly above that corresponding to the gas exchange threshold, to keep HR constant work rate significantly decreased; the decrease was more pronounced after a 10-day horizontal bed rest. The work rate decrease at a fixed HR can be considered a systemic biomarker of exercise intolerance during microgravity/inactivity and could also be easily and reliably determined during spaceflights or in patients.