Effects of rapid and slower rotations to an inverted seated posture on human neuromuscular and cardiovascular functions

Abstract

An inverted body posture is not common. During unusual situations (e.g. overturned helicopters or motor vehicle accidents) when the body is inverted, the neuromuscular responses can change. In order to manage these situations, it is necessary to examine changes in muscle force output and activation. -- Although the exact mechanisms are unknown, it is believed that both central And peripheral factors can contribute to changes in muscle force output. Increase in cerebral blood pooling, increase in hydrostatic pressure, and decrease in sympathetic activity during rapid and slow transitions from upright to inverted seated position are considered to be the main central factors leading to decrease in inversion-induced muscle force output. Peripheral factors such as decreased blood flow to the contracting muscle resulting in decrease perfusion pressure and oxygen deficit within the muscle are most likely to summate along with central factors inducing neuromuscular impairments during rapid and slower inversion rotations. -- There is no evidence examining possible impairments in neuromuscular functioning with more rapid versus slower inverted rotations as compared to an upright seated position. Maximal voluntary contraction (MVC) and electromyographic (EMG) activity were recorded and analyzed in biceps and quadriceps, concurrently and individually, for maximal force output and activation with upright seated position, and inverted within 1s and 3s rotations. It was anticipated that changes in muscle force and activation within 1s and 3s inversion rotations would suggest impairments in the functioning of neuromuscular system, as compared to the upright position. In addition, heart rate (HR), systolic blood pressure (SBP), and diastolic blood pressure (DBP) were also recorded. -- Both biceps and quadriceps exhibited significant decreases in maximal voluntary force and EMG activity, both concurrently and individually, when inverted within 1s and 3s rotations compared to an upright position. HR, SBP, and DBP also demonstrated significant decreases when inverted within 1s and 3s rotations compared to upright position. This suggests both rapid and slower inversion impairs neuromuscular function.