Assessing functional sympatholysis during rhythmic handgrip exercise using Doppler ultrasound and near-infrared spectroscopy: sex differences and test-retest reliability

The effects of sympathetic activity on vasoconstriction are dampened in active skeletal muscle during exercise, a phenomenon termed functional sympatholysis. Limited work has examined the influence of sex on the magnitude of sympatholysis or the test-retest reliability of measurements. In 16 women and 15 men, forearm blood flow (FBF; Doppler ultrasound), muscle oxygenation (near-infrared spectroscopy, NIRS), and beat-to-beat mean arterial pressure (MAP; photoplethysmography) were measured during lower-body negative pressure (LBNP; -20 mmHg) at rest and simultaneously during rhythmic handgrip exercise (30% maximum contraction). Measures were taken twice within the same visit (separated by 15 min) and repeated on a second visit. Forearm vascular conductance (FVC) was calculated as FBF/MAP. The magnitude of sympatholysis was calculated as the difference of LBNP-induced changes between handgrip and rest. LBNP decreased FBF (Delta-45 +/- 15%), FVC (Delta-45 +/- 16%), and muscle oxygenation (Delta-14 +/- 11%); however, these responses were attenuated when LBNP was applied during rhythmic handgrip exercise (Delta-7 +/- 9%, +/- 10%, and Delta-6 +/- 9%, respectively). The magnitude of sympatholysis was not different between men and women (FBF: 40 +/- 16% vs. 35 +/- 9%, P = 0.37; FVC: 38 +/- 16% vs. 35 +/- 11%, P = 0.53; muscle oxygenation: 5 +/- 9% vs. 11 +/- 10%, P = 0.11). Furthermore, sympatholysis measurements demonstrated good to excellent intraday (intraclass-correlation coefficients; ICC >= 0.85) and interday (ICC >= 0.72) test-retest reliability (all P <= 0.01) in both sexes. The coefficients of variation were larger with NIRS (68-91%) than with Doppler ultrasound (16% -22%) assessments of functional sympatholysis. Collectively, these findings demonstrate that assessments of functional sympatholysis are not impacted by biological sex and that Doppler ultrasoundderived measures of sympatholysis have better within-subject reliability than NIRS-derived measures in young healthy adults.

Automated summary

The influence of sex on the magnitude of sympatholysis and the test-retest reliability of measurements during exercise was examined in this study. The findings demonstrated that the effects of sympathetic activity on vasoconstriction are dampened in active skeletal muscle during exercise, regardless of biological sex. Doppler ultrasound-derived measures of sympatholysis have better within-subject reliability than NIRS-derived measures.