Heart rate dynamics during sinusoidal exercise: comparison of the control system between children and adults.

The aim was to model the dynamics of heart rate (HR) response to sinusoidal work rate (WR) forcing in children and adults. Seven pre-pubertal boys (aged 10-13) and five adult males (aged 22-37) were studied. Continuous ECG recordings were obtained during the following physiological manoeuvres: five constant amplitude ergometer exercises with WR varying sinusoidally with periods of 0.75, 1, 2, 3.5, and 5 min duration, and one step exercise at a constant WR equal to the midpoint of the sinusoid amplitude. The amplitude ratio (AR; standardized by WR) of the fundamental harmonic of the HR response and the phase shift (phi) between the WR to HR were calculated by Fourier analysis. The HR dynamic parameters (gain and time constant (tau)) of a first order model with or without delay (Td) were also estimated. The AR in children was always higher than that in adults, in absolute terms, but not as a function of body weight. The phi was more delayed in the children than the adults only for the shortest period, i.e. 0.75 min. The tau for the first order model, either without or with Td, was found to be no difference between children and adults (44.7 vs. 45.9 s (without Td), 34.9 vs. 42.3 s (with Td)). Td, however, was longer in the children (6.6 vs. 2.3 s). The goodness of fit for the first order model with Td was better than that without Td in children, i.e. due to the difference of phi for 0.75 min period, whereas the HR dynamics in adults was appropriately described by first order model without Td. It is concluded that the fundamental control of HR to sinusoidal exercise between children and adults was not appreciably different, except for a small Td difference at high sinusoidal frequency.