Transient Dynamics of Ventilation and Heart Rate with Step Changes in Work Load from Different Load Levels

Abstract

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 (V̇) 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 V̇ 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 V̇ 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 V̇, 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.

Exponential Hyperbolic Sine Function Fitting of Heart Rate Response to Constant Load Exercise.

“…The exponential hyperbolic sine function is useful for the quantitative evaluation of the HR responses to different constant loads. The HR trends in response to three different loads observed in the present study were in agreement with those in previously reported studies [5][6][7][8][9][10], and HR trends can thus be analyzed and discussed. The findings of the successful fitting of the exponential hyperbolic sine curve to the HR changes in response to lowload exercise indicated that the HR response includes an oscillation component (i.e., an inductor component).…”

“…Moreover, the relation of each coefficient to physiological indices such as sympathetic tone, etc. could not be determined, as was also the case in the previous fittings [5][6][7][8][9].…”

“…Impaired response of heart rate to exercise has recently been demonstrated to be predictive of increased mortality and coronary heart disease incidence [3,4]. Quantitative analyses of the HR response are thus important with respect to cardiac accidents.In high-intensity bicycle ergometer-exercise (120 W), the HR at first increased rapidly followed by a continuous and gradual increase, and this response was fitted to a second-order exponential function [5][6][7][8][9]. In moderate-intensity exercise (50-75 W), the HR increased and reached a plateau level, and this response was fitted to a first-order exponential function [5][6][7][8][9].…”

“…In high-intensity bicycle ergometer-exercise (120 W), the HR at first increased rapidly followed by a continuous and gradual increase, and this response was fitted to a second-order exponential function [5][6][7][8][9]. In moderate-intensity exercise (50-75 W), the HR increased and reached a plateau level, and this response was fitted to a first-order exponential function [5][6][7][8][9].…”

“…In moderate-intensity exercise (50-75 W), the HR increased and reached a plateau level, and this response was fitted to a first-order exponential function [5][6][7][8][9]. In unloaded or low-intensity exercise, the HR increased transiently and then declined, and an adequate equation fitting this response has not yet been obtained [10].…”