The origin of the initial abrupt increase in ventilation at the onset of muscular exercise (phase 1) in man.

Ventilation and cardiac output in response to four different exercises, namely, volitional pedalling using a bicycle ergometer with a very mild (7 Watt) load, passive pedalling, electrically-induced isometric twitches of one leg, and voluntary twitches simulating the previous electrical twitches, were measured simultaneously during the transient phase from rest. Cardiac output was determined by automated impedance cardiography. A sudden increase in ventilation was observed immediately after the onset of the volitional and passive pedalling whereas cardiac output increased only gradually. Only a slight difference was observed between the cardio-ventilatory responses to volitional and passive exercises. Neither ventilation nor cardiac output changed significantly in response to volitional and electrical twitches of one leg. Conclusions were then drawn that the cardio-dynamic process could be ruled out as the origin of the initial ventilatory response, and instead, other neurogenic mechanisms mediated either centrally or peripherally, should be considered.