Inter-individual variability in muscle sympathetic responses to static handgrip in young men: Evidence for sympathetic responder types?

Negative and positive muscle sympathetic nerve activity (MSNA) responders have been observed during mental stress. We hypothesized that similar MSNA response patterns could be identified during the first minute of static handgrip and contribute to the inter-individual variability throughout exercise. Supine measurements of multi-unit MSNA (microneurography) and continuous blood pressure (Finometer) were recorded in 29 young healthy men during the first (HG1) and second (HG2) minute of static handgrip (30% maximal voluntary contraction) and subsequent post-exercise circulatory occlusion (PECO). Responders were identified based on differences from the typical error of baseline total MSNA; 7 negative, 12 positive, and 10 non-response patterns. Positive responders demonstrated larger total MSNA responses during HG1 (p<0.01) and HG2 (p<0.0001), however, the increases in blood pressure throughout handgrip exercise were similar between all groups, as were the changes in heart rate, stroke volume, cardiac output, total vascular conductance, and respiration (All p>0.05). Comparing negative and positive responders, total MSNA responses were similar during PECO (p=0.17) but opposite from HG2 to PECO (40±46 vs. -21±62%, p=0.04). Negative responders also had a shorter time-to-peak diastolic blood pressure during HG1 (20±20 vs. 44±14 seconds, p<0.001). Total MSNA responses during HG1 were associated with responses to PECO (r=0.39, p<0.05), the change from HG2 to PECO (r=-0.49, p<0.01), and diastolic blood pressure time-to-peak (r=0.50, p<0.01). Overall, MSNA response patterns during the first minute of static handgrip contribute to inter-individual variability, and appear to be influenced by differences in central command, muscle metaboreflex activation, and rate of loading of the arterial baroreflex.

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