Analysis of musculoskeletal system responses to pertrubations during standing posture

The purpose of the study is to examine the muscular responses of healthy adult subjects to support surface rotations in pitch and roll planes. We used an enhanced experimental approach compared to the traditionally used methods in the previous studies by other researchers where the support surface beneath the subject's feet randomly perturbed the equilibrium. Instead of a predetermined motion of the support surface that would cause the perturbation of the subject's equilibrium, we perturbed the equilibrium by a Stewart parallel platform. Mounted on the platform, a force plate was recording the motion of the centre-of-mass projection during the experiment. This setup enabled the subjects to actively control the orientation of the parallel platform by shifting the position of their centre-of-mass during the experiment. Using polar diagrams, we show the responses of four muscle groups during the stretch and proprioceptive reflex and the muscular responses to the visual stimuli. Compared to the results of the previous studies, muscular activities during the stretch and proprioceptive reflex are more equally distributed in all directions. Based on the motion of the subjects that we recorded using the motion capture system, we determined whether the muscles were stretching or extending during the responses to the perturbations. We also show the effectiveness of the muscles to compensate the perturbations in different directions.