Heart rate transient at the onset of active and passive dynamic exercise.

At the onset of dynamic exercise there is an almost instantaneous heart rate (HR) acceleration caused by neural activation of central motor areas (central command) and stimulation of mechanoreceptors located in the moving limbs. Aiming to identify the independent contribution of the peripheral mechanism to the initial HR response to exercise, 29 subjects performed two 4-s bouts of unloaded cycling (active (AEx) and passive (PEx)) on an adapted commercial tandem bicycle. PEx was accomplished by having a staff member pedal while sitting on the rear seat. The HR was continuously measured from electrocardiographic tracings. Records of electromyography (EMG) were obtained in a small sample of subjects during the exercise tasks. The number of pedal rotations was very similar (mean +/- SE) (AEx = 7.4 +/- 0.3, PEx = 7.5 +/- 0.2, P = 0.455), determining significant HR changes (P < 0.001) that were similar in the two types of exercise (AEx from 92 to 125 bpm: 35.9% increase; PEx from 87 to 111 bpm: 27.6% increase; P = 0.185). Contrasting to AEx, no muscle contraction was observed by EMG during PEx, suggesting that central command was absent. We concluded that independent activation of mechanoreceptors can promote HR acceleration at the onset of dynamic exercise.