Muscular contractions and their effect on the vertical ground reaction force during quiet stance--Part II:...

A one-dimensional lumped-parameters mathematical model of the standing human was developed to specifically investigate how the muscular activity and the cardiovascular forces originated by the heart movement and the blood ejection in the ascending aorta affect the ground reaction force in the head-to-toes direction. The forces produced by the cardiovascular activity were modeled by a time-varying force actuator connected to the torso whose characteristics were obtained from the literature. The muscles identified as active during Part I of this work, mainly the erector spinae and oblique abdominal muscles, were modeled as a single force actuator acting with equal and opposite force between the pelvis and the thoracic spine. The force versus time data were obtained from the electromyographic signals obtained in Part I. Although the model of the human body used in this investigation was simple, the results of the simulations clearly showed that the cardiovascular forces alone are not sufficient for generating the large negative (i.e., upward) peak observed in the vertical ground reaction force. The peaks were mainly the result of a very modest muscular contraction of the spinal muscles. The simulations offer further evidence to support the hypothesis that trunk muscles contractions are capable of generating vertical ground reaction force oscillations that are consistent with the experimental results. These oscillations are apparently generated by a combination of cardiovascular and muscular forces.