Analysis of postural adjustment responses to perturbation stimulus by surface tilts in the feet-together position

Base of support (BOS) influences the postural control of upright standing in humans. Since a narrowed BOS in the upright position makes postural control difficult, it is important to evaluate and practice balance with a narrowed BOS. We performed an experimental study to clarify the postural control mechanism in a feet-together position in response to dynamic perturbations. Subjects were 10 healthy young adults. Using an electrically operated platform, tilting perturbation was applied to subjects standing in feet-together position. Perturbation was in four directions: backward, forward, right, and left. Using a motion analysis system and electromyography (EMG), we simultaneously measured the three-dimensional postural change and EMG of seven muscles of the trunk and legs. We obtained joint angular change from the motion analysis and the integrated values of muscle activity and latency of the lower extremities from the EMG. In terms of pitch tilt, the ankle strategy was observed: this involved simultaneous contraction of the ankle muscles as usual in the upright position. In contrast, in terms of roll tilt, "counterweighting" was observed. We also observed a change in postural control to respond to the narrowed BOS. We found that, unlike the feet-apart position, the feet-together position, narrowing the BOS in the left-right direction causes a prominent change and makes postural control strategy difficult only in response to roll tilt perturbations. In the future, we need to investigate the effectiveness of balance training including roll tilting in the feet-together position in improving balance ability in the left-right direction.