Fig. 2

Preference identification. A Representative preference identification trial. Subjects used a mechanical dial to directly control VSPA stiffness while walking on a treadmill. Subjects were encouraged to explore the full range of stiffness values before identifying their preferred stiffness. B Inter-subject average preferred stiffness at different treadmill speeds. A LMEM showed a significant second-order relationship (p = 0.0046). Treadmill speed is normalized to each subject’s self-selected treadmill speed. Error bars show Standard Error of the Mean (SEM). C Individual preference selections. Six of the seven subjects preferred lower VSPA stiffness at their self-selected treadmill speed, compared to speeds above or below the self-selected speed. Each subject’s selections are shown in a different color. Light lines connect each subject’s mean preferred stiffness at each treadmill speed