Pilot comparison of customized and generalized hip-knee-ankle exoskeleton torque profiles

Optimized assistance patterns have produced the greatest exoskeleton benefits to energy expenditure of any strategy to date. This strategy may be effective due to the customization of the applied torque profiles to the user as well as the locomotion condition; however, it is currently unclear how sensitive participants are to their unique torque profile. To investigate, we applied previously optimized hip-knee-ankle torque profiles to expert users (N=3; 1.25 m/s; 0 deg incline). The participants walked with the profile optimized to them, the two profiles optimized to the other two participants, and the average of the three torque profiles while we measured their energy expenditure. Relative to walking with the device turned off, on average, participants experienced a 47.5% (range 12%) metabolic reduction when walking with the torque profile optimized to them and a 46% (range 15%) reduction when walking with the other profiles. Interestingly, within-subject performance was more consistent than across subjects (P1: 52% range 5%, P2: 49% range 6%, P3: 39% range 3%) suggesting that, for expert users of some devices, there may be a range of nearly equally effective torque profiles to reduce the metabolic cost of walking. The torque timing was remarkably similar across the four torque profiles while the torque magnitude varied; participants may be much more sensitive to torque timing than torque magnitude, and there may be a set of torque timing parameters that are generally effective.