Muscle sympathetic nerve responses to passive and active one-legged cycling: insights into the contributions of central command

Abstract

The contribution of central command to the peripheral vasoconstrictor response during exercise has been investigated using primarily handgrip exercise. The purpose of the present study was to compare muscle sympathetic nerve activity (MSNA) responses during passive (involuntary) and active (voluntary) zero-load cycling to gain insights into the effects of central command on sympathetic outflow during dynamic exercise. Hemodynamic measurements and contralateral leg MSNA (microneurography) data were collected in 18 young healthy participants at rest and during 2 min of passive and active zero-load one-legged cycling. Arterial baroreflex control of MSNA burst occurrence and burst area were calculated separately in the time domain. Blood pressure and stroke volume increased during exercise ( P < 0.0001) but were not different between passive and active cycling ( P > 0.05). In contrast, heart rate, cardiac output, and total vascular conductance were greater during the first and second minute of active cycling ( P < 0.001). MSNA burst frequency and incidence decreased during passive and active cycling ( P < 0.0001), but no differences were detected between exercise modes ( P > 0.05). Reductions in total MSNA were attenuated during the first ( P < 0.0001) and second ( P = 0.0004) minute of active compared with passive cycling, in concert with increased MSNA burst amplitude ( P = 0.02 and P = 0.005, respectively). The sensitivity of arterial baroreflex control of MSNA burst occurrence was lower during active than passive cycling ( P = 0.01), while control of MSNA burst strength was unchanged ( P > 0.05). These results suggest that central feedforward mechanisms are involved primarily in modulating the strength, but not the occurrence, of a sympathetic burst during low-intensity dynamic leg exercise. NEW & NOTEWORTHY Muscle sympathetic nerve activity burst frequency decreased equally during passive and active cycling, but reductions in total muscle sympathetic nerve activity were attenuated during active cycling. These results suggest that central command primarily regulates the strength, not the occurrence, of a muscle sympathetic burst during low-intensity dynamic leg exercise.

Arterial baroreflex regulation of muscle sympathetic nerve activity at rest and during stress

“…; Doherty et al . ). To extend this work, we assessed whether static handgrip exercise or mental stress altered sBRS using both the absolute DBP and ∆DBP inputs.…”

“…To examine the effects of stress on arterial baroreflex function, we calculated the change in sBRS, as well as the change in the input threshold required to elicit a MSNA burst incidence of 50% (T 50 ) (Kienbaum et al 2001;Wehrwein et al 2010), from baseline to the last minute of static handgrip and mental stress. This epoch duration was selected to align with prior investigations (Kamiya et al 2001;Ichinose et al 2006;Doherty et al 2018) and to permit for a sufficient number of strong regression relationships to be obtained for statistical comparison using absolute DBP (the current standard).…”

“…; Doherty et al . ) and to permit for a sufficient number of strong regression relationships to be obtained for statistical comparison using absolute DBP (the current standard).…”

“…The spontaneous sBRS method was developed under resting conditions (Sundlöf & Wallin, 1978); however, a number of studies have applied these analyses to periods of stress, mainly exercise (Kamiya et al 2001;Ichinose et al 2006Ichinose et al , 2008Ogoh et al 2007;Greaney et al 2013;Doherty et al 2018) and mental stress (Durocher et al 2011). A common observation is that the strength of the negative linear relationship between absolute DBP variations and MSNA is weakened (or absent) during stress, particularly during periods associated with progressive (i.e.…”

Muscle sympathetic outflow during exercise: a tale of two limbs

“…Additionally, our laboratory recently compared passive versus unloaded (minimal metaboreflex activation) one‐legged cycling to probe the contributions of central command and demonstrated increases in multi‐unit MSNA burst amplitude, but not occurrence, in the non‐active leg during the first and second minute in the dynamic exercise condition (Doherty et al . ). Given that the authors established in a previous investigation that their measure of MSNA using spike frequency was most closely representative of MSNA burst amplitude, as opposed to burst frequency or total MSNA, the absence of a rapid increase in MSNA in the non‐active limb during a static contraction is in contrast to our findings.…”