Static asymmetrical whole body exertion in humans
The asymmetrical postures of subjects were recorded simultaneously with the force applied by one hand on a handle at three heights during exertion in specific directions, including many with a left or right directional component. A biomechanical analysis of the exertions was made in the horizontal plane and in the vertical plane containing the force vector. Analysis in the vertical plane showed that subjects achieved greater forces by both increased muscular effort and more effective deployment of body weight. Analysis in the horizontal plane revealed the existence of a horizontal moment at the foot-base. This moment was found to be small or negligible when a person exerted in the fore-aft plane, but was of considerable magnitude when exertion was carried out in directions with a lateral component. To investigate the generation of this moment, two further experiments were conducted. The first used surface electromyography to explore the roles of the flexors and extensors of the lower limb, while the second utilized a force plate to examine in detail the forces and moments at the foot-base. It was found that for laterally directed exertions, approximately half of the moment was generated by one foot exerting a force on the floor in a direction opposite to the other, and half was produced by each foot exerting a horizontal torque individually. The quadriceps, hamstrings and tibialis anterior were implicated in the first mechanism. The horizontal turning moment at the feet has not previously been recognised and should be incorporated into models of asymmetrical exertion.