Transient dynamics of ventilation and heart rate with step changes in work load from different load levels.

Broman, Svetlana and O. Wigertz, Transient dynamics of ventilation and heart rate with step changes in work load from different load levels. Acta physiol. scand. 1971. 81. 54–74. Transient dynamics of ventilation (V) and heart rate (HR) in response to 650 kpm/min step changes in work load were analyzed in six male athletes performing submaximal cycling exercise in the supine position, with the steps initiated at different levels of stable-state exercise, including “loadless” pedaling (0 kpm/min). By applying mathematical parameter identification, the responses of V to both positive and negative step changes in work load could be accurately described by first-order exponential models. With the positive steps initiated at 0, 300, and 650 kpm/min, the means of the time constant estimates for V ranged from 67 to 101 sec, and seemed to be independent of the initial work level. Shifting from rest to pedaling at zero load and back to rest caused abrupt changes in V, whereas this was rarely the case after step changes from one work load to another with the subjects already pedaling. The responses of HR required second-order models, with the two time constants ranging from 9.0 to 11.7 sec and from 1.8 to 3.7 min, and with the share of the slower component increasing with the initial work level. Pure time delays were negligible, indicating that the early readjustments in both V and HR are under neurogenic influence. The results support the notion that in exercise the respiratory and circulatory control systems exhibit dissociated dynamic properties.