Loss of exercise training-induced bradycardia with continued improvement in fitness.

In sedentary individuals, the decrease in heart rate (HR) at a respiratory exchange ratio (R) of unity (HR/R = 1) is lost after 8 weeks of moderate-intensity exercise training. The purpose of this investigation was to examine the effect of training on HR/R = 1, using the exercise intensity (EI) at R = 1 (EI/R = 1) and the HR at a set EI (HR/EIS). In nine healthy volunteers, these indices showed small measurement variability and detected changes associated with 16 weeks of training in that EI/R = 1 and VO2/R = 1 increased, whereas HR/EIS, HR/R = 1 and HR/VO2 decreased. All of these changes were reversed when training ended. In a second trial in a further nine volunteers, the training programme was extended to 20 weeks, and again showed that the decrease in HR/R = 1 failed to progress after 8 weeks despite continued changes in other indices (increases in VO2/R = 1 and decreases in HR/VO2 elevation). The loss of HR/R = 1 decrease was attributed to equal and opposite effects on HR/EIS and EI/R = 1. However, the decreases in HR/EIS showed the same trend as HR/R = 1, with a lack of progression beyond 8-16 weeks. These findings suggest that different indices of cardiorespiratory fitness have different patterns of change and limitations during training. Those which rely on decreases in submaximal HR fail to detect adaptation during continued training. Using submaximal exercise test indices, detection of such changes is better achieved by the measurement of decreases in HR/VO2 elevation and increases in VO2/R = 1 values.