Duty Cycle Impairs Functional Sympatholysis During Moderate Intensity Hand‐grip Exercise

Abstract

Functional sympatholysis is a well‐known mechanism that blunts the sympathetic vasoconstrictor signal in working muscle, enabling blood flow to be maintained during exercise in the moderate intensity domain. Recent work has demonstrated that exercise intensity and contraction frequency effects functional sympatholysis via changes in metabolic rate. However, less is known about the effects duty cycle (DC) on functional sympatholysis, which alters the time that intramuscular pressure is developed throughout a contraction cycle independent of changes in frequency and intensity. Given that a higher duty cycle (i.e., 50% DC) attenuates muscle blood flow to the working limb, potentially leading to a greater metabolite buildup, we hypothesized that a 20% DC would show greater percent change in forearm vascular conductance (%Δ FVC) and forearm blood flow (%Δ FBF) when compared to 50% duty cycle.METHODSEight healthy male subjects (age: 25 ± 1 yrs; height: 176.8 ± 1.5 cm; weight: 84 ± 5 kg) agreed to participate in the current investigation. Subjects began with a four‐minute resting protocol, two‐minutes of supine blood pressure and blood flow and two‐minutes utilizing lower‐body negative pressure (LBNP; ‐30 mmHg) to increase sympathetic outflow. Subjects then underwent two exercise conditions in a randomized order consisting of 20% and 50% DC (20%:50% or 50%:20%) with ~10 min between trials to allow blood pressure and blood flow to return to baseline. All hand‐grip exercise was performed at 20 contractions per minute at a workload of 20% of subject's maximal voluntary contraction.RESULTSAbsolute FVC during the 20% duty cycle condition was significantly higher than the 50% duty cycle (20%: 398 ± 34; 50%: 327 ± 37 ml min−1 mmHg−1; P < 0.01). FBF was also significantly higher in the 20% duty cycle condition (20%: 392 ± 21; 50%: 330 ± 29 ml min−1; P < 0.01). During steady state hand‐grip exercise coupled with LBNP, %Δ FVC, an indicator of sympathetically‐mediated vasoconstriction, showed a larger percent reduction during 20% duty cycle (20%: −12.6 ± 1.5%; 50%: 1.4 ± 3.9%; P < 0.01).CONCLUSIONThe results of the current investigation suggest that the 20% DC had a greater sympathetically‐mediated vasoconstrictor response compared to the 50% duty cycle.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.