Muscle Sympathetic Nerve Responses to Passive and Active One‐legged Cycling: Insight into the Contributions of Central Command

Abstract

Prior studies using neuromuscular blockade to increase perception of effort during mild or moderate intensity handgrip exercise have demonstrated that activation of higher brain regions, termed central command, contribute importantly to cardiac parasympathetic withdrawal but have limited influence on muscle sympathetic activation. The purpose of the present study was to investigate the contribution of central command to the control of muscle sympathetic nerve activity (MSNA) during dynamic one‐legged cycling. Heart rate (HR), mean arterial pressure (MAP), stroke volume, and MSNA (contralateral leg microneurography) were measured in 18 healthy participants (9 men; 22±3 yr of age) at baseline and during two minutes of upright passive cycling movement and active unloaded cycling, each separated by 10 minutes of rest. The increases in HR were greater during the first (Δ3±3 vs. Δ11±7 bpm, p<0.0001) and second (Δ3±4 vs. Δ9±6 bpm, p<0.0001) minute, and MAP during the second minute (Δ4±3 vs. Δ7±5 mmHg, p<0.01), of active unloaded cycling compared to passive cycling movement. Stroke volume increased over time (p<0.001) but was not different between passive and active unloaded cycling (p>0.05). MSNA burst frequency decreased similarly during the first (Δ−4±5 vs. Δ−3±5 bursts/min, p>0.05) and second (Δ−6±4 vs. Δ−4±4 bursts/min, p>0.05) minute of passive and active unloaded cycling. In contrast, total MSNA demonstrated attenuated reductions during the first (Δ−3.2±3.0 vs. Δ−0.6±3.3 arbitrary units/min, p<0.05 [Δ−18±16% vs. Δ0±31%, p<0.05]) and second (Δ−4.5±3.3 vs. Δ−1.5±3.5 arbitrary units/min, p<0.01 [Δ−28±17% vs. Δ−11±24%, p<0.05]) minute of active unloaded cycling. These results support the concept that muscle sympathetic burst occurrence and strength can be regulated independently and suggest that during dynamic leg exercise central command primarily alters vasoconstrictor outflow through modulations in muscle sympathetic burst strength.Support or Funding InformationThis research was supported by a Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant (P.J.M), the Canada Foundation for Innovation (P.J.M.), and a Canadian Institutes of Health Research Fredrick Banting and Charles Best Canada Graduate Scholarship (A.V.I.).