Peripheral circulatory factors limit rate of increase in muscle O2 uptake at onset of heavy exercise

We used an exercise paradigm with repeated bouts of heavy forearm exercise to test the hypothesis that alterations in local acid-base environment that remain after the first exercise result in greater blood flow and O-2 delivery at the onset of the second bout of exercise. Two bouts of handgrip exercise at 75% peak workload were performed for 5 min, separated by 5 min of recovery. We continuously measured blood flow using Doppler ultrasound and sampled venous blood for O-2 content, PCO2, pH, and lactate and potassium concentrations, and we calculated muscle O-2 uptake ((V) over dot O-2). Forearm blood flow was elevated before the second exercise compared with the first and remained higher during the first 30 s of exercise (234 +/- 18 vs. 187 +/- 4 ml/min, P < 0.05). Flow was not different at 5 min. Arteriovenous O-2 content difference was lower before the second bout (4.6 +/- 0.9 vs. 7.2 +/- 0.7 ml O-2/dl) and higher by 30 s of exercise (11.2 +/- 0.7 vs. 10.8 +/- 0.7 ml O-2/dl, P < 0.05). Muscle (V) over dot O-2 was unchanged before the start of exercise but was elevated during the first 30 s of the transition to the second exercise bout (26.0 +/- 2.1 vs. 20.0 +/- 0.9 ml/min, P < 0.05). Changes in venous blood PCO2, pH, and lactate concentration were consistent with reduced reliance on anaerobic glycolysis at the onset of the second exercise bout. These data show that limitations of muscle blood flow can restrict the adaptation of oxidative metabolism at the onset of heavy muscular exertion.