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Inter‐individual Variability in Muscle Sympathetic Nerve Activity during Static Handgrip in Healthy Young Men
Author(s) -
Incognito Anthony V.,
Doherty Connor J.,
Lee Jordon B.,
Burns Matthew J.,
Millar Philip J.
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.708.8
Subject(s) - microneurography , supine position , medicine , isometric exercise , blood pressure , cardiology , heart rate , sympathetic nervous system , physical medicine and rehabilitation , baroreflex
Cardiovascular responses at the onset of static handgrip are thought to be governed primarily by activation of higher brain regions (central command) and stretch‐sensitive group III/IV skeletal muscle afferents (muscle mechanoreflex), whereas responses during more prolonged contractions also involve activation of chemically‐sensitive group III/IV afferents (muscle metaboreflex). However, the contribution of central command to the muscle sympathetic nerve activity (MSNA) response at exercise onset is unclear. Existing evidence suggests that central stressors (e.g. mental stress) can either increase or decrease MSNA, identifying two populations of responders. We hypothesized that considerable inter‐individual variability may exist also at the onset of static handgrip, when the net contribution of central command is greatest. Continuous supine measurements of multi‐unit MSNA (microneurography) and blood pressure (Finometer) were recorded in 28 healthy men (24±5 years) during 2 minutes of static handgrip (30% maximal voluntary contraction) and 3 minutes of post‐exercise circulatory occlusion (PECO). Participants were divided into high (n=14) and low (n=14) MSNA responders based on the median MSNA burst frequency response during the first minute of static handgrip. High responders demonstrated greater MSNA responses during the first (Δ6±5 vs. −3±3 bursts/min, p<0.001) and second (Δ13±5 vs. 5±6 bursts/min, p<0.001) minute of handgrip and during the last minute of PECO (Δ9±7 vs. 4±6 bursts/min, p<0.05) compared to low responders. However, the change in MSNA between the first and second minute of handgrip was similar between high and low responders (Δ6±7 vs. 7±4 bursts/min, p>0.05). The increases in mean arterial pressure during handgrip (minute one: Δ9±6 vs. 7±4 mmHg; minute two: Δ20±9 vs. 16±9 mmHg) and PECO (Δ15±9 vs. 12±8 mmHg) were similar between high and low responders (All p>0.05), as were changes in heart rate, stroke volume, cardiac output, and total vascular conductance (All p>0.05). Examining the individual data as a continuous variable, the changes in MSNA and mean arterial pressure were unrelated during the first minute of handgrip (r=0.04, p>0.05), but related positively during the second minute of handgrip (r=0.50, p<0.01) and last minute of PECO (r=0.50, p<0.01). In summary, MSNA responses at the onset of static handgrip in young healthy men are highly variable and exhibit a dissociation with the pressor response. Support or Funding Information This 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.), University of Guelph‐Humber Research Grant (P.J.M), and a Canadian Institutes of Health Research Fredrick Banting and Charles Best Canada Graduate Scholarship (A.V.I.).