Balance recovery schemes following mediolateral gyroscopic moment perturbations during walking

Abstract

Maintaining balance during human walking hinges on the exquisite orchestration of whole-body angular momentum (WBAM). This study delves into the regulation of WBAM during gait by examining balance strategies in response to upper-body moment perturbations in the frontal plane. A portable Angular Momentum Perturbator (AMP) was utilized in this work, capable of generating perturbation torques on the upper body while minimizing the impact on the center of mass (CoM) excursions. Ten participants underwent upper-body perturbations during either the mid-stance or touch-down moment in both ipsilateral and contralateral directions in the frontal plane. Our findings emphasize the predominant role of the hip strategy and foot placement as primary mechanisms for recovering from WBAM perturbations, regardless of the perturbation's timing or direction. Specifically, hip add/abduction torque and step width were significantly modulated following perturbations during the stance and swing phases, respectively, to reject frontal-plane balance threats. The knee and ankle torque modulation were not found to be effective in the recovery process. Additionally, we observed that recovery from WBAM perturbations occurs promptly within the same stride in which the perturbation occurs, unlike other perturbation scenarios, such as platform translation. These insights have the potential to enhance the development of assistive devices and more robust controllers for bipedal robots.

Fig 1. Experimental setup and perturbation protocol.

(a) Participant equipped with the portable angular momentum perturbator (AMP), capable of applying pure moments to the upper body. (b) Participants initiated walking with their left leg, walking a raised platform for three steps before encountering the first and second force plates, where perturbations were applied. An additional 3 to 4 steps were taken before stopping. For safety, participants were secured with a safety harness. (c) Participants experienced four different perturbations: MI (Midstance-Ipsilateral) and MC (Midstance-Contralateral) at the midstance of the right leg on the first force plate, and TC (Touchdown-Contralateral) and TI (Touchdown-Ipsilateral) at the touchdown of the left leg on the second force plate. Stick figures visually represent each perturbation experimental condition, with the middle figure illustrating the application of perturbation and the side figures depicting the effect of perturbations.

Fig 2. Perturbation profiles and their effect.

Grand mean ± standard deviation of the perturbation profiles and their resulting effects on trunk angle (θ), whole-body angular momentum (WBAM, H), and time derivative of WBAM (dH/dt) in the frontal plane, averaged across all subjects and trials. In all figures, the time point 0 indicates the moment when the left leg comes into contact with the second force plate. Horizontal lines overlaid on the graphs indicate regions where signals in the perturbed cases exhibit significant differences compared to control trials (p < 0.05). CTRL: Control Trials, MI: Midstance-Ipsilateral, MC: Midstance-Contralateral, TC: Touchdown-Contralateral, TI: Touchdown-Ipsilateral.

Fig 3. Analysis of joints’ reactive moments to mediolateral upper-body gyroscopic perturbations.

Each row displays the grand mean of moment time series for each joint (hip, knee, and ankle) for both the left and right legs across 10 subjects. Each figure depicts moments in five experimental conditions: Control (CTRL), Midstance-Ipsilateral (MI), Midstance-Contralateral (MC), Touchdown-Contralateral (TC), and Touchdown-Ipsilateral (TI). The time point 0 indicates the moment when the left leg comes into contact with the second force plate. Vertical dashed black lines mark the instance of perturbation occurrence. Horizontal lines overlaid on the graphs indicate regions where signals in the perturbed cases exhibit significant differences compared to control trials (p < 0.05). The shaded grey background depicts the estimated double support phase. The results are presented using coordinates following the convention established by OpenSim.

Fig 4. Analysis of foot positioning.

(a) ML CoM velocity for all five experimental conditions; CTRL: Control Trials, MI: Midstance-Ipsilateral, MC: Midstance-Contralateral, TC: Touchdown-Contralateral, TI: Touchdown-Ipsilateral. The time point labeled as 0 indicates the moment when the left leg touches the second force plate. Horizontal lines overlaid on the graphs indicate regions where signals in the perturbed cases exhibit significant differences compared to control trials (p < 0.05). (b) step width at heel strike with respect to that of control trials for three steps following the perturbation step. Solid black lines and white circles in the violin plot represent the mean and median values, respectively. The asterisk symbol (*) denotes statistical significance at the p < 0.05 level with respect to the control trials.