Fig. 1. Experimental set-up. Subjects walked on a motorized treadmill for 7 min with exoskeletons unpowered, then rested for 3 min, then walked for 7 min with exoskeletons powered, while a metronome enforced their preferred step frequency (from unpowered walking at 1.25 m s^–1). Treadmill belt speed was set to achieve speed/step-length conditions of 0.8, 1.0, 1.2 and 1.4x the preferred step length at 1.25 m s^–1 (L^*; i.e. 1.00, 1.25, 1.50 and 1.75 m s^–1). Conditions were presented in randomized order. The boxes indicate periods when data were collected (minutes 4–6) in both unpowered and powered conditions. We collected joint kinematics using motion capture and reflective markers, O₂ consumption and CO₂ production using a metabolic cart, ankle muscle activation patterns using surface electromyography and artificial muscle forces using series load transducers.