Spatiotemporal gait characteristics change during very slow walking, a relevant speed considering individuals with movement disorders or using assistive devices. However, we lack insights in how very slow walking affects human balance control. Therefore, we aimed to identify how healthy individuals use balance strategies while walking very slow. Ten healthy participants walked on a treadmill at an average speed of 0.43 m s−1, while being perturbed at toe off right by either perturbations of the whole-body linear momentum (WBLM) or angular momentum (WBAM). WBLM perturbations were given by a perturbation on the pelvis in forward or backward direction. The WBAM was perturbed by two simultaneous perturbations in opposite directions on the pelvis and upper body. The given perturbations had magnitudes of 4, 8, 12 and 16 % of the participant's body weight, and lasted for 150 ms. After perturbations of the WBLM the centre of pressure placement was modulated using the ankle joint, while keeping the moment arm of the ground reaction force (GRF) with respect to the centre of mass (CoM) small. After the perturbations of the WBAM a quick recovery was initiated, using the hip joint and adjusting the horizontal GRF to create a moment arm with respect to the CoM. These findings suggest no fundamental differences in the use of balance strategies at very slow walking compared to normal speeds. Still as the gait phases last longer, this time was exploited to counteract perturbations in the ongoing gait phase.@en

BACKGROUND: In the last two decades, lower-limb exoskeletons have been developed to assist human standing and locomotion. One of the ongoing challenges is the cooperation between the exoskeleton balance support and the wearer control. Here we present a cooperative ankle-exoskeleton control strategy to assist in balance recovery after unexpected disturbances during walking, which is inspired on human balance responses. METHODS: We evaluated the novel controller in ten able-bodied participants wearing the ankle modules of the Symbitron exoskeleton. During walking, participants received unexpected forward pushes with different timing and magnitude at the pelvis level, while being supported (Exo-Assistance) or not (Exo-NoAssistance) by the robotic assistance provided by the controller. The effectiveness of the assistive strategy was assessed in terms of (1) controller performance (Detection Delay, Joint Angles, and Exerted Ankle Torques), (2) analysis of effort (integral of normalized Muscle Activity after perturbation onset); and (3) Analysis of center of mass COM kinematics (relative maximum COM Motion, Recovery Time and Margin of Stability) and spatio-temporal parameters (Step Length and Swing Time). RESULTS: In general, the results show that when the controller was active, it was able to reduce participants' effort while keeping similar ability to counteract and withstand the balance disturbances. Significant reductions were found for soleus and gastrocnemius medialis activity of the stance leg when comparing Exo-Assistance and Exo-NoAssistance walking conditions. CONCLUSIONS: The proposed controller was able to cooperate with the able-bodied participants in counteracting perturbations, contributing to the state-of-the-art of bio-inspired cooperative ankle exoskeleton controllers for supporting dynamic balance. In the future, this control strategy may be used in exoskeletons to support and improve balance control in users with motor disabilities.@en

Healthy individuals highly regulate their whole body angular momentum (WBAM) during walking. Since WBAM regulation is essential in maintaining balance, a better understanding is required on how healthy individuals recover from WBAM perturbations. We therefore studied how healthy individuals recover WBAM in the sagittal plane. WBAM can be regulated by adjusting the moment arm of the ground reaction force (GRF) vector with respect to the whole-body centre of mass (CoM). In principle this can be done by centre of pressure (CoP) modulation and/or adjustments of the GRF direction. Two simultaneous perturbations of the same magnitude were applied in opposite direction to the pelvis and upper body (0.34m apart) to perturb WBAM but not the whole body linear momentum (WBLM), while participants walked on a treadmill. The perturbations were given at toe off right, had a magnitude of 4, 8, 12 and 16% of the participant's body weight, and lasted for 150ms. A recovery of the WBAM was seen directly after the perturbations, induced by adapting the moment arm of the GRF with respect to the CoM. The hip joint of the stance leg played an important role in achieving the WBAM recovery. A change in the direction of the GRF vector and not a contributing CoP modulation, caused the change in moment arm. However, the change in GRF direction came from a change in the horizontal GRF, which also affects the WBLM. This suggest that regulating WBAM may take precedence over the WBLM in early recovery.@en

Humans prioritize regulation of the whole-body angular momentum (WBAM) during walking. When perturbed, modulations of the moment arm of the ground reaction force (GRF) with respect to the centre of mass (CoM) assist in recovering WBAM. For sagittal-plane perturbations of the WBAM given at toe off right (TOR), horizontal GRF modulations and not centre of pressure (COP) modulations were mainly responsible for these moment arm modulations. In this study, we aimed to find whether the instant of perturbations affects the contributions of the GRF and/or CoP modulations to the moment arm changes, in balance recovery during very slow walking. Perturbations of the WBAM were applied at three different instants of the gait cycle, namely at TOR, mid-swing (MS), and heel strike right (HSR). Forces equal to 16% of the participant's body weight were applied simultaneously to the pelvis and upper body in opposite directions for a duration of 150 ms. The results showed that the perturbation onset did not significantly affect the GRF moment arm modulation. However, the contribution of both the CoP and GRF modulation to the moment arm changes did change depending on the perturbation instant. After perturbations resulting in a forward pitch of the trunk a larger contribution was present from the CoP modulation when perturbations were given at MS or HSR, compared to perturbations at TOR. After backward pitch perturbations given at MS and HSR the CoP modulation counteracted the moment arm required for WBAM recovery. Therefore a larger contribution from the horizontal GRF was needed to direct the GRF posterior to the CoM and recover WBAM. In conclusion, the onset of WBAM perturbations does not affect the moment arm modulation needed for WBAM recovery, while it does affect the way CoP and GRF modulation contribute to that recovery.@en

Increasing knowledge on human balance recovery strategies is important for the development of balance assistance strategies using assistive devices like a powered lower-limb exoskeleton. One of the postures which is relevant for this scenario, but underexposed in research, is staggered stance, a posture with one foot in front. We therefore aimed to gain a better understanding of balance recovery in staggered stance. We studied balance responses at joint- and muscle levels to pelvis perturbations in various directions while standing in this posture. Ten healthy individuals participated in this study. We used one actuator beside and one behind the participant to apply 150 ms perturbations in mediolateral (ML), anteroposterior (AP) and diagonal directions, with a magnitude of 3, 6, 9 and 12% of the participant’s body weight (BW). 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. These were often accompanied by motions in a direction different from the perturbation direction. After perturbations perpendicular to the line between both feet, large and significant AP deviations were present of the CoM (-0.27 till 0.40 cm/%BW, p < 0.029) and CoP (-0.99 till 0.80 cm/%BW, p < 0.001). 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 contributed differently to the balance responses after the different perturbation directions. To conclude, standing in a staggered stance posture makes individuals more vulnerable to perturbations perpendicular to the line between both feet, requiring larger responses on joint level as well as contributions in the sagittal plane@en