The experimental results are shown in Figure 4b. The endurance was maximized around K ~0.25 for each sub- ject. Further, the endurance with an exoskeleton increased by 1.5-fold to 2.5-fold compared to the endurance when no exoskeleton assist was employed. Using a mathemati- cal model of the human arm and exoskeleton, researchers [59] related overall muscle efficiency to exoskeletal stiff- ness. The model predicted that muscle efficiency was max- imized at the same dimensionless stiffness where endurance reached its maximum (K~0.25 in Figure 4b), suggesting that the endurance changes were a conse- quence of changes in the efficiency with which the body performed the required work for each cycle.