It is difficult to resolve the strategies people use to skillfully stabilize their bodies during walking maneuvers. Stability, the tendency for a system to return to a consistent state, is generally considered beneficial during straight walking. However, it has been difficult to quantify stability during maneuvers given that the objective of a maneuver is to safely breach the current state and transition to an alternative stable state (e.g. straight walking in a path parallel but lateral to the previous one). Maneuvering, an essential skill for community ambulation, can be accomplished with varying strategies of foot placement, body movements, and ground-on-foot force control (1). However, our understanding of how people adapt stepping strategies to manage stability during maneuvers is poor.
The need to understand these stabilizing strategies among people who have sustained a motor-incomplete spinal cord injury (iSCI) is particularly pressing. iSCI disrupts balance and challenges one’s ability to safely and efficiently perform maneuvers. In addition, iSCI often limits strength and coordination, which may restrict the options for stabilization strategies. Difficulty maneuvering is likely a contributing factor to the reduced mobility (2) and high fall rate (3) observed among ambulatory individuals with iSCI. A greater understanding of how people manage stability during walking maneuvers could provide valuable insight for designing more effective interventions to enhance the ability to maneuver after iSCI.
To better understand how people perform lateral ‘lane change’ maneuvers during forward walking (4) (Fig. 1), velocity-dependent force fields can be applied in either the same (amplifying) or opposite (damping) direction as the lateral center of mass (COM) velocity. Maneuvering requires a lateral velocity-time profile of the COM that is different from straight walking, including a prolonged period of COM excursion in the direction of the maneuver and the subsequent arrest of that motion. Thus, lateral velocity-dependent force fields allow for richer characterization of the maneuver by altering the physical requirements for breaching and then reestablishing forward walking stability. Damping lateral COM velocity will increase frontal-plane stability during forward walking, which should resist the transition into a lateral maneuver but assist the arrest of the maneuver. Vice versa, amplifying lateral COM velocity will decrease frontal-plane stability during forward walking (5), which should assist the transition into a lateral maneuver but increase the challenge to arrest the maneuver. The current study introduces both Damped and Amplified fields to a lateral maneuver task to evaluate how different stability requirements affect the strategies people with and without iSCI use to maneuver.
Similar damping (6) and amplifying (5, 6) force fields have been valuable for understanding stability-related consequences of the stepping strategies adopted during straight walking. People with and without iSCI tend to modify lateral margins of stability (MOS), the distance between a velocity-adjusted COM position and the edge of an individual’s base of support (BOS), in response to changes made by external viscous force fields. By increasing or decreasing lateral MOS, the impulse needed to cause frontal plane instability (based on an inverted pendulum model of walking (7)) can be changed in accord with the challenges of a task. Thus, the adaptive stepping strategies and associated changes in lateral MOS used to first breach and then reestablish forward walking stability for a lateral maneuver are expected to manifest on steps initiating, executing, and terminating lateral maneuvers (Fig. 1) in the presence of force fields that damp and amplify lateral COM velocity.
To address gaps in understanding the stability and stepping during maneuvers, this study characterized strategies used by people with and without iSCI performing lateral “lane-change” maneuvers during forward walking in Damped, Amplified, and Null force fields. As observed in previous studies of straight walking (6), it was expected that participants with iSCI would maintain a larger MOS across fields in comparison to their peers without iSCI. Considering findings of maneuvering without force fields (1, 8) the MOS was expected to be smallest on the initiation step (Fig. 1) as participants bias their COM in the maneuver direction in anticipation of the impending movement and largest on the execution step as individuals generate a lateral impulse by pushing off of the limb contralateral to the maneuver direction. Given the adaptability of stepping behavior in previous work (5, 9), it was expected that people would adapt their stepping strategy to use the fields to aid their maneuvers when advantageous. The Damped field may be advantageous during the maneuver termination, while the Amplified field may be advantageous during the maneuver initiation/execution.
In the Damping field, we hypothesized that relative to the Null field, participants would 1) decrease the minimum lateral MOS (MOSmin) on the initiation step (Fig. 1) to facilitate the maneuver by biasing COM position towards the maneuver direction, 2) increase MOSmin on the execution step to increase potential for lateral ground-on-foot force in the direction of the maneuver to counter the opposing field, and 3) decrease MOSmin on the termination step to take advantage of the field reducing the need to brake. In the Amplified field, we hypothesized that relative to the Null field, participants would 1) increase MOSmin ipsilateral to the maneuver direction on the initiation step to afford increased stability, anticipating the assistance of the force field to overcome that stability on the subsequent execution step 2) decrease the MOSmin contralateral to the maneuver on the execution step to leverage assistance from the force field and 3) increase the lateral MOSmin ipsilateral to the maneuver direction on the termination step to prevent overshoot of the target end-position. We expected the termination step MOSmin increase to be especially evident in individuals with iSCI given their intensified cautious response to destabilizing fields in previous work (6). Step width, COM excursion, and COM peak velocity were also quantified to further unpack the strategies contributing to differences in MOSmin.