Purpose: Spontaneous postural sway during quiet standing has been considered a simple output error of postural control. However, as postural sway and inherent body orientation evoke compensatory activity of the plantar flexors, they might contribute to blood circulation under gravitational stress via the muscle pump. Hence, the present study employed an external support device to attenuate the plantar flexor activity in supported standing (SS), to experimentally identify its physiological impact on blood circulation.
Methods: Eight healthy young subjects performed two 5-min quiet standing trials (i.e., normal standing (NS) and SS), and the beat-to-beat interval (RRI) and blood pressure (BP) were compared between trials. We confirmed that postural sway and corresponding plantar flexor activity, quantified by the anteroposterior displacement of the foot center of pressure and lower back position with respect to the wall, and by the amplitude of electromyography and mechanomyography, respectively, were attenuated in SS, while mean body orientation angle and relative position of the BP sensor were comparable to NS.
Results: The 5-min averages of diastolic BP and mean arterial pressure during SS were significantly higher than during NS, while RRI and systolic BP did not change. These could be interpreted as an increase in peripheral vascular resistance; meanwhile, in NS, this effect was replaced by the muscle pump of the plantar flexors.
Conclusion: The muscle contractions related to spontaneous postural sway and body orientation produce substantial physiological impact on blood circulation during quiet standing.
Keywords: Blood circulation control; Body orientation; Muscle pump; Plantar flexion; Postural control; Postural sway.