| Summary: | We investigated the role of intramuscular pressure on the blood volumes delivered to working muscle during contractions spanning the entire range of force output that could be elicited from our subjects. Our subjects included four male (age = 22 ± 14 yr, mass = 63 ± 1.5 kg) and four female (age = 23 ± 9 yr, mass = 81.4 ± 5.6 kg). Vascular occlusion is thought to occur at similar pressures between sexes, but these pressures are achieved at different fractions of the male vs female maximum voluntary contraction. We therefore used a triplex capable pulsed Doppler ultrasound to obtain non-invasive measures of blood velocity and femoral artery diameter during single leg knee extension exercise performed by male (n = 4) and female (n = 4) subjects on an instrumented custom ergometer. We observed that greater periods of muscle inactivity between subsequent analysis (i.e. lower duty cycles), provided enhanced opportunity for circulatory based clearance by as much as 60 % and 64 % in males and females respectively. The forces subjects applied to the ergometer were measured via single-element strain gauges as the subjects adhered to a constant investigator-imposed cadence. Subjects performed bouts of knee extension exercise ranging from no-external imposed force, up to the level that could be performed for only three consecutive contractions (male = 524.66 ± 71.24 N vs female = 326.90 ± 115.75 N) while matching the experimental cadence. Resistance is controlled by manual Monarch turns and cadence is controlled by dual column LED array. We measured peak oxygen uptake (male = 26.90 ± 5.17 ml O2 kg-1 min-1 vs female = 19.57 ± 2.47 ml O2 kg-1 min-1) and surface EMG to determine the forces necessary to elicit anaerobic metabolism and the onset of compensatory neuromuscular recruitment an indicator, respectively. These experiments determined whether short-duration performance decrements are an issue of oxygen availability or related to clearance of metabolic byproducts elicited by muscle contraction.
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