The Orthostatic Stress of Quiet Sitting

Abstract

Sitting is not commonly considered to be a significant orthostatic stress, and yet in the absence of leg muscle activity, this common activity could result in substantial fluid pooling in the lower limbs. Here, we assess the development of peripheral edema in adult women during extended quiet sitting and investigate the role of calf muscle pump (CMP) during this activity. Methods: Fluid volume was evaluated by air plethysmography in the dominant leg of 56 adult women (mean age: 46.7 +/− 10.6 yrs) who underwent 30 mins of quiet sitting, followed by 30 mins of sitting with CMP stimulation via micromechanical stimulation of plantar mechanoreceptors at 45 Hz/50 μm. Beat-to-beat blood pressure (BP) and heart rate of the subjects was acquired continuously throughout the protocol. Results: Leg volume changes during sitting demonstrated a distinct bimodal distribution, indicating two subpopulations, those who pool substantially during sitting (40% of the study group), and those who did not. Correspondingly, diastolic BP values also demonstrated a bimodal distribution with one subpopulation demonstrating a significant drop in BP, consistent with inadequate fluid return. CMP stimulation was observed to reverse the fluid build-up in the leg and significantly increase BP. Conclusions: The data reported here suggests that for ~40% of adult women, substantial pooling does appear to occur, and this edema may largely be due to inadequate calf muscle tone, as exogenous stimulation of the calf muscle eliminates the pooled fluid and returns normal right atrial filling. Research supported by NY State Office of Science, Technology and Academic Research.