Muscle Ergoreflex Activity and Autonomic Balance Assessed in the Vertical and Horizontal Body Positions in Young Healthy Men

We investigated whether indices of the reflex regulation of the cardiovascular system, i.e., muscle ergoreflex activity (ERGO) and characteristics of the autonomic balance, differ from each other while measured in the vertical vs. horizontal positions in healthy humans. Ten young healthy men (mean age 28 ± 1 years; BMI 23.6 ± 0.8 kg/m²) were examined. In each subject, the ERGO (ventilatory response to a control handgrip exercise, with repetitive isometric contractions at 50% of the maximal strength), baroreflex sensitivity (BRS, the sequence method), heart rate variability (HRV), and a few hemodynamic parameters were measured in two positions, horizontal and vertical. In the latter position, the heart rate, and diastolic and mean blood pressures (BP) were higher, while the stroke volume was smaller (all P < 0.05) with no differences in the systolic BP, cardiac output, and systemic vascular resistance (all P > 0.2). The HRV time domains, as well as BRS, decreased, whereas HRV-LF increased (all P < 0.05) in the vertical position as compared to the horizontal one. There were no differences between other HRV parameters (all P > 0.2) in the two examined positions. Minute ventilation during the exercise increased, while ERGO was reduced in the vertical position (all P < 0.05). Thus, the body position noticeably affects both reflex regulation of the cardiovascular system and autonomic balance. This observation has to be taken into account during physiological testing.

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