Role of central and peripheral circulatory adjustments in oxygen transport at the onset of exercise

The t 1/2 V̇(O₂) on has been shown to range from 13 to 30 sec for isolated muscle, running dogs, and man. Under the same conditions, the circulatory response preceded the adjustment of V̇(O₂), and was judged not to limit V̇(O₂). More recently t 1/2 V̇(O₂) on has been shown to be significantly slower (t 1/2 = 70 to 120 sec) for untrained individuals especially in the supine posture. The purpose of the present study was to examine the role played by central, Q̇, and peripheral, MBF, circulatory adjustment in setting t 1/2 V̇(O₂) on. Q̇, HR, P̄a, V̇(O₂), and La for eight subjects were measured during rest and exercise at 1.0 for arms and legs and 1.8 and 2.3 l min for arm and leg pedaling in the supine posture. Three subjects exercised in the erect and supine posture at the same two work loads. In another study three fit and three unfit subjects exercised at the same work loads with arms and legs in the supine posture while MBF, V̇(O₂), and HR were determined continuously at rest and exercise. The t 1/2 V̇(O₂) on ranged from 24 sec to 84 sec depending on exercise and fitness. The t 1/2 Q̇ for the same conditions ranged from 6 to 15 sec and in virtually every case was less than t 1/2 V̇(O₂) on. The changes in HR and P̄a followed Q̇. The adjustment of MBF was complete in 6 to 18 sec for fit and 30 to 40 sec for unfit subjects, e.g., before V̇(O₂) reached t 1/2. Based on these observations, Q̇ and MBF do not play a role in setting V̇(O₂) kinetics. The limitation therefore must be distal to the capillary circulation.