Heart rate variability during exercise can quantify cardiorespiratory deconditioning in 55- to 65-yr-old men and women exposed to head-down bed rest.

The heart rate variability (HRV) features HRV_0.75 and HRV_0.5, obtained by detrended fluctuation analysis during incremental cycle ergometer tests, have been shown to correlate with indices of cardiorespiratory fitness. Here, we evaluate the ability of these features to track individual changes in cardiorespiratory fitness induced by head-down bed rest (HDBR) among late middle-aged adults. Twenty-two healthy participants (55-65 yr old, 11 women) were exposed to 14-day HDBR. Eleven participants were assigned to a group performing daily aerobic and resistive exercises during HDBR, whereas the others were assigned to a control group with no countermeasure intervention. We measured gas exchange and electrocardiography during incremental cycle ergometer tests pre- and post-HDBR. The countermeasure preserved oxygen uptake (V̇o₂) at the two ventilatory thresholds (VT1 and VT2), HRV_0.75 and HRV_0.5. However, these four features decreased in the control group after HDBR. A large and proportional bias was evidenced between V̇o₂ at VT1 and HRV_0.75, which was less the case for V̇o₂ at VT2 versus HRV_0.5. However, the intraindividual changes in the control group were correlated (r_rm = 0.66 for VT1 vs. HRV_0.75 and r_rm = 0.82 for V̇o₂ at VT2 vs. HRV_0.5, both P < 0.001). Overall, this study indicated that HRV_0.75 and HRV_0.5 can be used to longitudinally monitor changes in cardiorespiratory fitness in late middle-aged adults with physical deconditioning. Future research can apply these methods to other study populations.NEW & NOTEWORTHY We report the applicability of two heart rate variability features (HRV_0.75, HRV_0.5) computed during exercise to assess changes in cardiorespiratory fitness. Although previous cross-sectional studies suggested that these features were highly correlated to the ventilatory thresholds, we highlight the fact that they should not be considered as equivalent. More easily obtained than ventilatory thresholds, HRV_0.75 and HRV_0.5 could be used as fitness markers along a continuum of the physiological response between rest and maximal exercise.