The effect of acute exercise with increasing workloads on inactive muscle blood flow and its heterogeneity in humans.

The distribution of blood flow between active and inactive skeletal muscles has been sparsely studied in humans. Here we investigated non-exercising leg blood flow in six healthy young women during intermittent isometric one leg knee extension exercise with increasing workloads. Positron emission tomography was used to measure blood flow in hamstring muscles of the exercising leg, and whole thigh muscles as well as its knee extensor and hamstring compartment of the resting leg. Mean blood flow to the hamstrings of the exercising leg (5.8 ± 2.6 ml/100 g/min during the highest exercise workload) and whole thigh muscle of the resting leg (7.1 ± 3.8 ml/100 g/min) did not change significantly from rest (4.0 ± 0.7 and 4.7 ± 1.9 ml/100 g/min, respectively) to exercise, but flow heterogeneity increased substantially at increasing workloads. Importantly, during the highest exercise workload, mean blood flow in the knee extensors of the resting leg decreased (5.5 ± 3.0 ml/100 g/min at rest and 3.4 ± 2.0 ml/100 g/min during exercise, p < 0.01) while flow heterogeneity increased (28 ± 8% at rest and 83 ± 26% during exercise, p < 0.05). Conversely, in hamstring muscles of the resting leg blood flow increased from 3.9 ± 1.0 ml/100 g/min at rest to 11.5 ± 6.8 ml/100 g/min during exercise (p < 0.05) while flow heterogeneity increased from 30 ± 7 to 58 ± 19% (p < 0.05). In conclusion, while mean whole thigh muscle blood flow of the resting leg remains at resting level during one leg exercise of the contralateral leg, redistribution of blood flow between muscle parts occurs within the thigh. Based on previous studies, nervous constraints most probably act to cause this blood flow distribution.