Interleg coordination is not strictly controlled during walking
In human walking, the left and right legs move alternately, half a stride out of phase with each other. Although various parameters, such as stride frequency, stride length, and duty factor, vary with walking speed, the antiphase relationship of the leg motion remains unchanged. This is the case even during running. However, during walking in left-right asymmetric situations, such as walking with unilateral leg loading, walking along a curved path, and walking on a split-belt treadmill, the relative phase between left and right leg motion shifts from the antiphase condition to compensate for the asymmetry. In addition, the phase relationship fluctuates significantly during walking of elderly people and patients with neurological disabilities, such as those caused by stroke or Parkinsons disease. These observations suggest that appropriate interleg coordination is important for adaptive walking and that interleg coordination is strictly controlled during walking of healthy young people. However, the control mechanism of interleg coordination remains unclear. In the present study, we derive a quantity that models the control of interleg coordination during walking of healthy young people by taking advantage of a state-of-the-art method that combines big data science with nonlinear dynamics. This is done by modeling this control as the interaction between two coupled oscillators through the phase reduction theory and Bayesian inference method. However, the results were not what we expected. Specifically, we found that the relative phase between the motion of the legs is not actively controlled until the deviation from the antiphase condition exceeds a certain threshold. In other words, the control of interleg coordination has a dead zone like that in the case of the steering wheel of an automobile. Such forgoing of control presumably enhances energy efficiency and maneuverability during walking. Furthermore, the forgoing of control in specific situations, where we expect strict control, also appears in quiet standing. This suggests that interleg coordination in walking and quiet standing have a common characteristic strategy. Our discovery of the dead zone in the control of interleg coordination not only provides useful insight for understanding gait control in humans, but also should lead to the elucidation of the mechanisms involved in gait adaptation and disorders through further investigation of the dead zone.