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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.