Pelvis perturbations in various directions while standing in staggered stance elicit concurrent responses in both the sagittal and frontal plane

Walking very slowly increases the time spent in the double support phase, which could be resembled by the staggered stance posture. Maintaining balance in this posture is important in order to continue walking safely. We therefore aimed to increase the understanding of balance recovery in staggered stance. We studied balance responses on joint- and muscle level to pelvis perturbations in various directions while standing in this posture. Ten healthy individuals participated in this study. We used one motor beside and one behind the participant to apply perturbations in mediolateral (ML), anteroposterior (AP) and diagonal directions, with a magnitude of 3, 6, 9 and 12% of the participant’s body weight. Meanwhile motion capture, ground reaction forces and moments, and electromyography of the muscles around the ankles and hips were recorded. The perturbations caused movements of the centre of mass (CoM) and centre of pressure (CoP) in the direction of the perturbation. Furthermore, these were often accompanied by motions in a direction different from the perturbation direction. After ML perturbations and diagonal perturbations transverse to the line between both feet, large and significant CoM and CoP deviations were present in the sagittal plane. Also, stronger responses on joint and muscle level were present after these perturbations, compared to AP and diagonal perturbations collinear with the line between both feet. The hip, knee and ankle joints significantly responded to all perturbation directions, but in different manners and modes of cooperation. To conclude, standing in a staggered stance posture makes individuals more vulnerable to perturbations in ML direction and transverse to the line between both feet, requiring larger responses on joint level as well as contributions in the sagittal plane.