The effect of an inverted body position on muscle force and activation

Paddock, Natasha Ruby (2008) The effect of an inverted body position on muscle force and activation. Masters thesis, Memorial University of Newfoundland.

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Abstract

Orthostatic pressure changes have been shown to significantly affect sympathetic nervous system responses in order to maintain balance and homeostasis. Neuromuscular responses have not been extensively, if at all, investigated in an inverted body position. Muscle force, activation and other neuromuscular factors are necessary, for instance, to successfully complete escape procedures from a secured inverted seated position of a overturned car or aircraft. -- It is known that both central and peripheral factors contribute to muscle force output. With an increase in pressure to levels above the heart in an inverted body position, cerebral blood pooling is likely. Even though there is evidence of a decrement in sympathetic functioning in similar circumstances to inversion, specific vestibulosympathetic responses during inversion are unknown, but possibly contribute to neuromuscular impairment. Peripheral factors such as lower levels of blood flow to the contracting muscles leading to decreased perfusion pressure and an oxygen deficit within the muscle results in a decreased force output. Decreased hydrostatic pressure in areas below the heart during inversion may also be a contributing hindrance to neuromuscular performance, but this has not been demonstrated. -- Based on the lack of literature in this area, the following experiment was implemented. Maximal and submaximal voluntary and evoked forces and EMG were recorded, and the contractions were analyzed for peak force, rate of force development and activation with upright and inverted seated positions. It was expected that inversion induced deficits in muscle force and activation would suggest impairment in neuromuscular efficiency in this tilt position. -- It was found that both quadriceps EMG activity during submaximal contractions, as well as instantaneous strength during maximal contractions, demonstrated a deficit in the inverted position. Therefore, during the inverted seated position it seems that neuromuscular function is impaired.

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