Do Muscle Contractile Mechanics Alter Blood Flow and Explain the Onset of Muscle Fatigue?

During brief all-out muscular activity eliciting failure between 3 and 300 s, the level of performance available from muscle decreases exponentially between the musculoskeletal maximum and the performance supported by the body’s aerobic power(i.e.VO2 peak). Here, the muscle duty cycle, the ratio of the durations of muscle activity to the entire movement cycle, was experimentally manipulated to alter the level of sustainable force at the condition-specific aerobic limit. We hypothesized that the expected greater levels of sustainable force output achieved in the mode with longer rest periods between contractions, were due to the greater opportunity for vascular perfusion or muscle clearance during the longer inactive periods. Therefore, we continuously measured femoral artery blood velocity via pulsed Doppler ultrasound and artery diameter by sonogram, to calculate blood flow throughout each of the 10 exhaustive bouts of knee extension exercise administered in each experimental by every subject(n = 8). This novel data acquisition technique allowed us to obtain what we believe to be the first-ever continuous measurements of blood flow by quantifying the femoral artery diameter and blood velocity during exhaustive trials. Applied muscle forces and powers were measured continuously from a custom knee-extension ergometer with strain gauges and an incremental encoder. We measured non-steady rates of oxygen uptake in Douglas bags evaluating whether the hypothesized unequal blood volumes conferred greater rates of aerobic metabolism or greater rates of muscle clearance. Our results showed the forces eliciting this common exercise intensity differed nearly 2-fold(60% duty cycle 95 ± 46N vs 30% duty cycle 184 ± 46N). Rates of oxygen uptake by muscle were similar between conditions but measured blood velocities were greater in the 30% duty cycle condition, suggesting that during high intensity exercise sustainable levels of performance are achieved through removal of accumulated metabolites rather than increased aerobic respiration