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 increase 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 of pedal movement within an RRI. Changes in RRI were expressed as the difference of the interval in which the trigger was activated (onset RRI) and the average of resting intervals (48 intervals). Volitional unloaded 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 the 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 the 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 when the movement was initiated within the first one-third of the interval. These findings suggest that heart rate changes elicited by central command (latency <300 ms) can occur at least twice as fast as responses produced by the muscle-heart reflex (latency ~600 ms) but that the muscle-heart reflex can play a role in RRI shortening, if the contraction is initiated early enough in the interval to incorporate this reflex mechanism.