INSTANTANEOUS HEART-RATE INCREASE WITH DYNAMIC EXERCISE - CENTRAL COMMAND AND MUSCLE-HEART REFLEX CONTRIBUTIONS

R-R interval (RRI) changes were recorded from 15 healthy volunteers in response to volitional unloaded cycling and passively induced cycling (PC). PC was also combined with electrical stimulation (n = 5) to inc rease muscle mechanoreceptor activation. The electrocardiogram and leg electromyographic activity were continuously sampled by computer at 1 ,000 Hz, and an electronic trigger was used to designate the instant o f pedal movement within an RRI. Changes in RRI were expressed as the d ifference of the interval in which the trigger was activated (onset RR I) and the average of resting intervals (4-8 intervals). Volitional un loaded cycling produced the greatest decrease in the onset RRI [907 +/ - 11 (SE) to 855 +/- 10 ms; -5.4 +/- 0.4%; P < 0.01] when movement was initiated within the first one-third of the interval. A shortening of the onset RRI was also detected when trigger activation occurred in t he last one-third of the interval (906 +/- 12 to 875 +/- 11 ms; -3.1 /- 0.4%; P < 0.01). There were no significant effects of PC alone on t he onset RRI. However, PC + electrical stimulation shortened the onset RRI (906 +/- 12 to 883 +/- 11 ms; -2.5 +/- 0.2%; P < 0.05) but only w hen the movement was initiated within the first one-third of the inter val. These findings suggest that heart rate changes elicited by centra l command (latency <300 ms) can occur at least twice as fast as respon ses produced by the muscle-heart reflex (latency similar to 600 ms) bu t that the muscle-heart reflex can play a role in RRI shortening, if t he contraction is initiated early enough in the interval to incorporat e this reflex mechanism.