Foot Placement in balance recovery - complex humans vs simple model
Mark is PhD Student in the department of Medical Engineering in The Biomechanical Engineering Research group. His supervisor is Professor Herman van der Kooij from the Engineering Technology Faculty
Maintaining balance in daily life is very common to us. For a healthy individual, a fall is simply not supposed to happen. Unfortunately, various conditions such as stroke, spinal cord injury, or aging can lead to balance problems and affect a person's mobility. Robotic devices such as powered orthoses, often referred to as exoskeletons, might provide an outcome in case of these balance problems.
In case of a lower-extremity exoskeleton, the user wears a construction around the legs that should provide support during standing and walking. This support can be to various purposes, such as to reduce the energetic costs of walking, to assist in gait rehabilitation, or to fully take over the walking motion. Although the purpose of the device might differ, most lower-extremity exoskeletons have one thing in common: they have no sense of balance. The exoskeleton cannot react to unexpected disturbances. Because of that, the user has to take the lead in making a balance recovery. This is especially troublesome when the user has balance impairments.
To tackle these issues, the control of exoskeletons needs to be improved. Specifically, if the device can assist in balance control in a way that feels natural and intuitive to the user, the device is less likely to conflict with the user's intention. To realize such human-like balance controllers, we must first understand what human balance is, and investigate the way healthy humans regain their balance when it is lost. This might be investigated by applying perturbations to experimental subjects. Disturbances will lead to a balance recovery response involving various balance strategies, such as adjustments in foot placement, or modulation of ankle and hip moments. Especially foot placement adjustments are considered a major balance strategy in human walking.
Though literature already provides a broad range of perturbation experiments and conditions, it is nearly impossible to investigate all possible forms of perturbations, applied to various body parts, during various circumstances. It is therefore crucial to converge towards models that can mimic and explain the observed human behavior when balance is disturbed. One of these models is the inverted pendulum, which describes the motion of a point mass on top of a single leg. This resembles the single support phase of human walking. Using experimental outcomes to validate simple models of human balance could provide a basis for the design of balance controllers for lower-extremity orthoses.
The focus of this thesis is on human balance recovery in response to external perturbations during walking. Because we mainly deal with walking, foot placement adjustments are expected to be a major, crucial strategy in balance control. This strategy might be replicated using simple inverted pendulum models of walking. To make comparison with such models more straightforward, perturbations are applied to the approximate location of the whole-body center of mass of the human, which is the pelvis. In particular, the thesis deals with the following questions:
I) How do humans adjust their foot placement in terms of location and time when subjected to horizontal pelvis perturbations during walking?
Ia) How do physical constraints affects the foot placement strategy for these perturbations?
Ib) How is the foot placement strategy complemented by other balance strategies for these perturbations?
II) Can the foot placement strategy be predicted using simple low dimensional models of walking, and concepts derived from these models?
The results presented in this work provide insight in how the location and timing of foot placement change following horizontal perturbations applied at the level of the center of mass. The most prominent foot placement modulation occurs for sideways perturbations, given that there is sufficient time to realize foot placement adjustments. The closer in time a disturbance occurs before foot contact, the less the step will be modulated. In those cases, adjustments partially transfer to the second step.
It also becomes clear that foot placement adjustments are not always the primary strategy to recover from perturbations during walking, even if it is an available option. Other strategies might be addressed instead. An example is the ankle strategy following fore-aft perturbations. Disabling the ankle strategy through a physical constraint elicits foot placement adjustments that were otherwise not present. This suggests that the ankle provides an alternative recovery mechanism.
The work furthermore reveals joint-level and muscle-level responses in reaction to disturbances. Such results can function as a benchmark for orthoses- and controller design in terms of actuator degrees-of-freedom and output. In addition, the muscle responses show that reactive balance control likely involves centralized, supra-spinal mechanisms, without the specific need for joint-level afferent feedback.
Finally, a concept derived from a simple inverted pendulum model is shown to have predictive value in human balance control. This concept provides a relation between the horizontal velocity of the body's center of mass, and the distance between the center of mass and the origin of the net ground reaction force following foot contact. When this concept is incorporated in a simple inverted pendulum model and is combined with two energetic costs, one for leg swing and one for leg transition, the model can yield human-like modulation of step location and step time in response to disturbances. These model results are in line with experimental results in perturbed human subjects, which brings us one step closer to replicating human-like balance in walking.
