Fig. 4

Evaluation of transparency in unimpaired participants: raw data. Trajectories and interaction forces for the right lower leg for two unimpaired participants (U1 & U2) during treadmill locomotion with and without a robotic arm attached to the leg. In a control condition (no robot; NR), the subjects walked without the robot attached to their legs (top; black trajectories). In the other condition (red trajectories), a robotic arm was attached close to the center-of-mass of the leg (with robot; WR), and the robot was programmed to follow the leg and not interfere. The measured interaction forces (\({\varvec{F}}_{{\varvec{R}}}\)) are shown from the reference frame of the subjects. A positive anterior–posterior (AP) force means the robotic arm was pushing anteriorly on the leg; a positive vertical force means that the robotic arm was pushing upwards on the leg. Force data are not interpolated to the step-cycle; therefore, the timing of the swing cycle varies slightly from step-to-step (hence, the gradient shading)