Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC84
Effects of an Inverted Body Position on Arm Maximal Voluntary Contract Force and Cardiovascular Parameters
J. Neary1, D. M. Salmon1, E. Pritchett2, D. G. Behm2
1. Kinesiology & Health Studies, University of Regina, Regina, Saskatchewan, Canada. 2. School of Human Kinetics & Recreation, Memorial University, St John's, Newfoundland, Canada.
Some recent research has suggested that differences in maximal voluntary contraction (MVC) in an upright vs. inverted postural position are related to alterations in sympathetic nerve stimulation, which could alter cardiovascular responses (Paddock & Behm 2009). Our purpose was to determine the effects of postural changes on cardiovascular responses during upper body (arm) MVC. We hypothesized that inversion would elicit a significantly different effect on selected cardiovascular parameters. Twelve healthy male human subjects (age= 22.5±1.6yr; mass= 82.1±17.1kg; BMI= 22.3±3.5) completed three postural trials in random order on separate days: upright seated (U), supine seated (S), and inverted seated (I) position in a specially designed inversion chair. At baseline, a five-second elbow flexion MVC was performed with the elbow positioned at 90 degrees. The subject was then positioned for 150 sec in each posture, followed by a 30sec MVC (MVC30). During each trial, stroke volume (SV), cardiac output (Q), heart rate (HR) and mean arterial blood pressure (MAP) measurements were recorded using continuous monitoring finger plethysmography (Finometer). MVC force (N) was averaged over the 30sec. The ANOVA results showed statistical differences in MVC force from baseline to post MCV30 for all postures (U=190.9 ± 55.7N vs. 154.2 ± 57.5N; S=191.7 ± 94.0N vs. 167.2 ± 73.9; I=176.7 ± 42.1N vs. 158.2 ± 37.5), but no significant force differences between postures at MCV30. Statistical differences were found for the cardiovascular variables HR, SV, and MAP from baseline to MVC30. Only HR and Q were significantly different between postures for U vs. S (HR=15.7%; Q=7.6%) and U vs. I (HR=15.6%; Q=7.6%). Collectively, the cardiovascular results presented here suggest that the significant reduction in HR and Q between postures may be related to an inversion-induced inhibition of sympathetic stimulation, supporting previous research (Bosone et al. 2004; Hearn et al., 2009). Further research is warranted to investigate whether prolonged inversion has an effect on neuromuscular and cardiovascular parameters. This has important implications for scenarios where individuals may have to perform muscle contractions under inverted conditions (e.g., overturned submerged helicopter or motor vehicle, military operations).
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