Quantifying Manual Adjustment of Foot Placement Under a Fixed Robotic Trajectory in Lower Limb Exoskeletons

Controlling foot placement is a key challenge in the use of assistive lower limb exoskeletons designed for those with motor impairments. Due to the mechanical flexibility of exoskeletons, users can intentionally manipulate the resulting step length without alteration of the exoskeleton’s reference trajectory. This is generally achieved by manually applying wrench upon the exoskeleton and the ground using crutches. This work sought to investigate this mechanism as a deliberate means to control foot placement. Ten nondisabled participants were asked to pilot a user-balanced exoskeleton to target step lengths of 0.1 to 0.4 m, with the exoskeleton trajectory unchanged throughout the experiment. Performance was evaluated by mean absolute error (MAE) and standard deviation (SD) of resulting step lengths. To explore the degree that these results might apply to users with impairments, participants were asked to minimize leg muscle activations during the experiment. Simultaneously, surface electromyography (sEMG) data were collected and normalized between resting (0.0) and unassisted walking (1.0). Activations ranged between 0.014 and 2.853, and were used to categorize participants into High muscle activation (HMA) and Low muscle activation (LMA) groups. The LMA group (median MAE 0.026 m, SD 0.028 m) performed differently compared to the HMA group (median MAE 0.021 m, SD 0.021 m), however, most participants achieved acceptable performance across all target step lengths, compared to a 0.05 m guideline. The results confirm that step length can be controlled through exoskeleton users’ manual efforts. Whilst the range of adjustments may vary with device and user, this could facilitate simplified exoskeleton control strategies and an intuitive method of user control.