A Subject-Specific Analysis of the Kinematic Constraint Imposed by the Relink Trainer

The relink trainer (RLT) is a novel end-effector device designed for gait-retraining poststroke. The user's foot is constrained to a specific kinematic trajectory relative to the trainer, while the hip and knee are unconstrained. As the RLT only fixes the footplate trajectory, the expected constraint on the hip and knee angles will be subject-specific due to individual lower limb geometries. This study had two objectives (1) to calculate the subject-specific theoretical joint angle trajectories, the RLT should constrain the hip and knee angle to using computer simulation, assuming a fixed hip position relative to the RLT, and (2) experimentally determine the actual hip and knee joint angle trajectories of healthy users walking in the RLT, and compare them to the theoretical joint angle trajectories. The root-mean-square (RMS) error between joint trajectories obtained from motion capture and simulation ranged from 4.31 deg to 20.51 deg for the hip and between 4.48 deg and 22.58 deg for the knee, suggestive of moderate to poor accuracy and distinct kinematic adaptation strategies when using the RLT. A linear fit method (LFM) was used to determine the similarity between the obtained and simulated joint angle trajectories. LFM results would suggest that users' hip and knee joint angles follow the simulated joint angle trajectories when walking in the RLT; however, the actual joint angle trajectories are offset from the simulation trajectories. Post hoc analyses suggest hip motion when using the RLT influences the hip and knee angle trajectory differences for participants.