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Lower body negative pressure (LBNP) of 100 mm Hg generates a footward force approximating one body weight for most subjects, yet LBNP may also produce syncope by pooling blood in the lower body. We hypothesized that dynamic leg exercise would improve LBNP tolerance by reducing lower body fluid accumulation. Seven healthy male subjects underwent four supine LBNP tolerance tests: 1) the control condition: conventional, resting LBNP with a saddle (SADL); 2) resting LBNP against a footplate instead of a saddle (FP); 3) cyclic, non-load-bearing ankle plantar- and dorsiflexion movements during LBNP with a saddle (SADLEX); and 4) LBNP with cyclic ankle plantar- and dorsiflexion exercise against the footplate (FPEX). Tolerance tests consisted of reducing chamber pressure in 10 mm Hg decrements every 3 min to -100 mm Hg or presyncope. Tolerance was assessed by integrating the area under the LBNP x time step function. Exercise doubled LBNP tolerance relative to resting LBNP (SADL: 771 +/- 162 mm Hg x min; FP: 819 +/- 212; SADLEX: 1461 +/- 175; FPEX: 1656 +/- 160; p < 0.05). During FPEX, footward force oscillations averaged 215 +/- 14 N (21.9 +/- 1.4 kg), and mean peak force of 872 +/- 26 N (88.9 +/- 2.7 kg; n = 5) was produced by plantarflexing against 100 mm Hg LBNP. Calf volume increased slightly less when exercise accompanied LBNP (SADLEX and FPEX), while heart rate increased more. Therefore, exercise improved tolerance in part by skeletal muscle pumping of venous blood from the legs.(ABSTRACT TRUNCATED AT 250 WORDS)
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