Recovery from exercise at varying work loads. Time course of responses of heart rate and systolic intervals.

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Abstract
The time course of the recovery period was characterized by non-invasive measurements after 4 min bicycle exercise at 3 separate (50, 100 and 150 W) work loads in 10 volunteers in whom the peak responses during exercise of heart rate and systolic time intervals were consistent with previous investigations of comparable subjects. Most responses started immediately to return toward resting control values. Despite rapidly falling post-exercise heart rates, left ventricular ejection times fell further from their exercise nadirs for the first 5-15 s of recovery (depending on load) probably because of diminished stroke volume caused by venous pooling and by continued rapid ejection rate. The stroke volume change was indicated by a concomitant precipitous fall in ejection time index, which then stabilized in tandem with the subsequently normal (i.e., discordant) left ventricular ejection time-heart rate relation. For the first 30 s heart rate fell from load-related exercise peaks, but at equal slopes of change for all loads; thereafter, rates of return toward resting control were strictly load-related. Changes in pre-ejection period were determined by changes in isovolumic contraction time; the degree of return toward control of pre-ejection period and isovolumic contraction time from their exercise nadirs were load-dependent throughout recovery; load-dependent differences in their rate of return were pronounced in the first 30 s. The rates of change in the ejection time index and in the ratio pre-ejection period/left ventricular ejection time (PEP/LVET) were virtually independent of load throughout most of recovery.