Exoskeletons that act in parallel with a human limb for endurance augmentation. An example is the MIT climbing exoskeleton  shown in 4a. As shown in 4b, when the stiffness of the mechanism was optimally tuned, endurance was increased from 1.5-fold to 2.5-fold across the six human subjects evaluated. The mean number of cycles to exhaustion (), or the endurance, normalized by the mean value at zero stiffness (), is plotted in Fig. 4b versus the dimensionless arm spring stiffness (K). K is defined as the measured stiffness of the added spring (k) multiplied by the maximum distance the spring was stretched (Xm), and divided by the subject's body weight (W). For each subject, a cubic spline curve passes through the mean of the normalized cycle values (± SE) at each of the five stiffness values. Endurance is maximized around K ~0.25 for each subject.