Fig. 1.

Regions of interaction portrait (IP). Each quadrant of the circle corresponds to different human–exoskeleton interaction modes determined by the variation of the normalized total muscle activation ($_{c_1}^{c_2}\mathrm{\Delta }\mu$) with respect to the normalized total interaction torque ($_{c_1}^{c_2}\mathrm{\Delta }\tau$) between controllers $c_1$ and $c_2$, respectively. The first quadrant (red) indicates increased disagreement between the user and exoskeleton, resulting in an increase in both muscle effort and the total interaction torque. The second quadrant (green) determines the co-adaptation of the user toward participating in the motion as much as possible and leading the motion. The third quadrant (blue) denotes the decrease in total interaction torque and the total muscle effort, associated with the decrease in human–exoskeleton disagreement. Finally, the fourth quadrant (orange) denotes the condition at which the user yields control of the motion to the exoskeleton and minimally activates their muscles. In this case, muscle activation decreases while the total interaction increases since the exoskeleton has to carry the user’s body (passive dynamics) in addition to the exoskeleton dynamics