Influence of active muscle size on sympathetic nerve discharge during isometric contractions in humans

We tested the hypothesis that sympathetic nerve discharge to nonactive skeletal muscle (MSNA) is influenced by active muscle size during isometric contractions performed at a constant submaximal force in humans. In six subjects, MSNA (peroneal microneurography), arterial pressure, heart rate, and ratings of perceived effort were recorded before (resting control) and during isometric contractions of either a small hand muscle [1st dorsal interosseus (FDI); 2 trials (FDI1 and FDI2)] or the fore- and upper arm muscles (handgrip; 1 trial) sustained to the point of exhaustion and normalized to endurance time (te). There were no differences in resting control levels. MSNA, heart rate, arterial pressure, and perceived effort all increased similarly during the two FDI contractions. During handgrip, the rates of rise and peak increases MSNA, heart rate, and arterial pressure were much greater than during FDI (all P < 0.05), but peak levels of perceived effort were not different. These findings indicate that during voluntary isometric efforts sustained to the same performance and perceptual end points the rates of increase and final levels of MSNA, heart rate, and arterial pressure are greater during contraction of a larger muscle mass and that these responses appear to be similar during successive trials of a task when normalized to te. Thus, active muscle size can be an important factor in the regulation of sympathetic nervous system discharge and cardiovascular function during isometric muscle activity in the human.