Recovering linear and angular momentum during walking

Abstract

For most individuals maintaining balance during walking goes natural. We are not continuously thinking of how to maintain balance, and still we usually do not fall. However, maintaining balance is not a given for everyone. Both aging and various neuromuscular disorders affect the ability to maintain balance, resulting in an increased incidence of falls and the associated consequences. Also, in the situation of someone walking with an assistive device such as a powered lower-limb exoskeleton, for example due to a spinal cord injury (SCI), maintaining balance may be challenging. To provide better care and training programs and to improve balance support with assistive devices, a better understanding is needed of human balance recovery. Previous research often focused on the recovery of linear perturbations disturbing the body's linear momentum. However, in daily life we also encounter perturbations resulting in a rotational effect, disturbing the body's angular momentum. The aim of this work was to gain insights in the use of human balance strategies to recover whole-body linear and angular momentum. To provoke balance recovery responses we performed experiments in which perturbations were applied during standing and treadmill walking. These studies were performed with healthy participants, since they can serve as a source of inspiration for how balance recovery strategies are being used successfully. Perturbations of the linear and/or angular momentum were induced by the application of forces to the body at shoulder and/or pelvis height, provided by a haptic robot. This allowed for a controlled duration, magnitude and onset of the perturbations. Thereafter we analyzed how modulations of the ground reaction force and its point of application were used in order to maintain balance. We studied this in 1) situations that are relevant for SCI individuals walking with a powered lower limb exoskeleton and 2) situations that have not extensively been studied before, and therefore fill a gap in the current knowledge on human balance recovery. In several chapters we address the topic of balance recovery during very slow walking, since this is a relevant speed for walking with a lower limb exoskeleton. Walking very slowly increases the time spent in the double support phase. Studying the responses to perturbations of the whole-body linear momentum (WBLM) while standing in a static double support phase, also called a staggered stance posture, provided insights in the coupling between the frontal- and sagittal-plane. A large base of support (BoS) enables opportunities for centre of pressure (CoP) modulation. Therefore, the large dimension of the BoS in the anteroposterior direction during staggered stance could also be used in the recovery from perturbations that were perpendicular to this direction. Focusing on the double support phase, with simulations based on a simple linear inverted pendulum model, we showed the effects of modulations of the CoP trajectory on the control of the centre of mass position and velocity. Comparing the simulated opportunities with the strategies that healthy individuals used, it turned out that we do not fully exploit the available options for a quick balance recovery. A specific type of perturbation that we used for several studies is a perturbation of the whole-body angular momentum (WBAM). This was obtained by applying two perturbations at the same time in opposite direction on the pelvis and upper body respectively. The responses to these perturbations revealed a high priority in recovery of the WBAM. This was done even at the expense of the WBLM. The WBAM recovery comprised a modulation of the horizontal ground reaction force, affecting the WBLM while this was not perturbed initially. This effect was independent of the instant of the gait cycle at which the perturbation was given and holds for very slow and normal walking speeds. The results emphasize the importance and prioritization of WBAM regulation in balance recovery. To conclude, the studies presented in this thesis provide insights into the human balance strategies used to recover from perturbations of the WBLM and WBAM during walking at very low and normal speeds. These insights can be considered in the development of controllers to assist balance or to improve balance training for those experiencing difficulties with balance control. Finally, the recorded data itself is valuable for validating whether proposed recovery strategies are human-like.