Dynamics of the heart rate response to sinusoidal work in humans: influence of physical activity and age

The purpose of the present study was to define the influence of age and exercise training on the heart rate (HR) dynamic response (i.e. kinetics) to sinusoidal work. A total of 63 healthy subjects (31 men and 32 women; age range 19-69 years) underwent a three-step incremental work test, during which peak oxygen uptake (.V(O)(2peak)) was estimated by the YMCA method. Sinusoidal work varying between 20% and 60% of HR(reserve) was employed for periods of 1, 3, 6, 9 and 12 min. HR was monitored in a beat-by-beat manner with a cardiotachometer. The kinetics of the HR response were analysed by frequency analysis and estimated by a first-order transfer function with time constant (tau) and time delay (TD). Physical training status was estimated as stepping frequency, as measured with a pedometer during the daytime, and averaged over seven consecutive days. The mean response time of HR kinetics (HR(MRT): tau pulse TD) tended to increase gradually with age (0.36 s.year(-1)), and linear regression analysis revealed that the correlation between HR(MRT) and age was significant (r=0.31, P<0.05), although not as highly significant as that between HR(MRT) and physical activity (r=-0.48, P<0.0001). HR(MRT) was not related to the S.D. of HR variation (an indicator of parasympathetic mediation) at rest. In addition, .V(O)(2peak) showed a significantly greater correlation with age (r=-0.60, P<0.0001) than with physical activity (r=-0.14, not significant). In conclusion, these findings suggest that HR dynamics, which may depend on sympathetic nervous activity, are more sensitive to physical activity than to age, but that .V(O)(2peak), as estimated by the age-associated decline in maximum HR, is unrelated to physical training status.