Saskia Baltrusch

93 Chapter 4 and walking with the low-cam Exoskeleton. These trials were performed at two different walking speeds: Preferred Walking Speed without the exoskeleton (PWS) and Preferred Walking Speed with the exoskeleton (PWSX), resulting in 4 different walking trials. The order of these trials was randomized for each participant. Note that only the low-cam Exoskeleton was used in the walking experiments, since this type of exoskeleton should in theory provide the least hindrance during walking, based on the fact that it provides support at high angles. A walking period of 5 minutes was chosen to achieve steady-state walking. Three minutes of rest were provided between conditions to prevent fatigue. Before starting the lifting protocol, participants had a break of 30 minutes to prevent fatigue. After attaching the cluster marker and fitting the oxygen mask again, participants performed the repetitive lifting task. They were required to lift and lower a 10-kg box (0.39 x 0.37 x 0.11m, with 2.5 cm diameter handles) at a rate of 6 lifts per minute. Each lift consisted of picking up the box, assuming an upright posture, putting down the box, and assuming an upright posture again. The participants were instructed to choose their own lifting technique. In that way we took into account potential changes in lifting technique that will occur when people use this exoskeleton in practice. The lifting rate was imposed by a metronome that sounded in each upright position. Movement speed was left free, as participants were allowed to pause the movement when standing upright. Hence, the movement was performed at a natural speed. Each lifting trial lasted for 5 minutes and was performed in 3 conditions: lifting without the exoskeleton (Control Condition), lifting with the high-cam exoskeleton, and lifting with the low-cam exoskeleton. These lifting trials were conducted from two different heights: ankle height and knee height. The order of the trials was block randomized per exoskeleton condition. Approximately 30 s of rest was given between trials. 2.5 Data Processing Metabolic energy expenditure (J/min) was calculated from O 2 (ml/s) and respiratory exchange ratio (RER; [26]). Flow rates were averaged over the final 2 minutes to ensure a steady state condition and were normalized to body mass. Net metabolic energy expenditure was calculated by subtracting resting 4