Heart rate rather than heart rate variability is better associated with cardiorespiratory fitness in adults

This study aimed to examine the association of heart rate (HR) and heart rate variability (HRV) with cardiorespiratory fitness (CRF) in adults. This cross-sectional study included 204 participants: 132 young adults (67% women; 22.3 +/- 2.3 years) and 72 middle-aged adults (53% women; 53.2 +/- 5 years). HR and HRV were measured using a Polar RS800CX heart rate monitor. CRF was assessed using a metabolic cart during an incremental test. HR was negatively associated with CRF relative to body weight (BW) in young adults adjusting for sex, age, and physical activity (beta ranged from -0.200 to -0.195; all P <= 0.022). These associations disappeared after including fat mass as a covariate (all P >= 0.106) while remained significant after adjusting for fat-free mass (FFM; all P <= 0.008). Neither HR nor HRV variables were associated with CRF expressed as absolute values or relative to FFM in young and middle-aged adults after adjusting for the abovementioned covariates (all P >= 0.074). Our results suggest that HR rather than HRV is better associated with CRF relative to BW in young adults. However, this association seems to be explained by the inclusion of fat mass in the calculation of CRF relative to BW.