Influence of sex and age on the relationship between aerobic fitness and muscle sympathetic nerve activity in healthy adults.

We examined the influence of sex and age on the relationship between aerobic fitness and muscle sympathetic nerve activity (MSNA) in healthy adults. Data were assessed from 224 volunteers (88 females), aged 18-76 yr, in whom resting MSNA (microneurography) and peak oxygen uptake (V̇o 2peak ; incremental exercise test) were evaluated. When separated into younger (<50 yr) and older (≥50 yr) subgroups, there were inverse relationships between relative V̇o 2peak (mL·kg -1 ·min -1 ) and MSNA burst frequency in younger males ( R 2 = 0.21, P < 0.0001) and older females ( R 2 = 0.36, P < 0.01), but not older males ( R 2 = 0.05, P = 0.08) or younger females ( R 2 = 0.03, P = 0.14). Similar patterns were observed with absolute V̇o 2peak (L·min -1 ) and percent-predicted (based on age, sex, weight, height, and modality), and with burst incidence. Sex and age influence the relationship between aerobic fitness and resting MSNA, and, thus, must be considered as key variables when studying these potential associations; inverse relationships are strongest in younger males and older females. NEW & NOTEWORTHY Our data reveal for the first time that associations between aerobic fitness and resting muscle sympathetic nerve activity are sex and age specific; inverse relationships are evident in younger males (<50 yr) and older females (≥50 yr), but absent in younger females (<50 yr) and older males (≥50 yr).