Saskia Baltrusch

185 Chapter 7 10 kg and the lifting rate were chosen to ensure that the task can be performed at a constant speed for at least 5 mins, without having a break to ensure steady- state oxygen uptake, which is important for data interpretation [26]. Ankle height was chosen, since we expected the exoskeleton to generate the biggest support at that height, based on its’ angle-torque relationship (shown in Figure 2). Also, at the worksite of our participants, luggage is often lifted from ankle height. Each lifting cycle started in an upright position and consisted of picking up the box, assuming an upright posture with the box, putting down the box, and assuming an upright posture without the box again. A metronome was used to impose lifting rate and participants were instructed to choose their own lifting technique. The instruction was: “Please pick up the box with every beat and put it down again at the next beat. Please adapt your speed to the beat of the metronome to have a smooth movement. You can choose the lifting technique you feel the most comfortable with.” This protocol was performed twice, one trial with the exoskeleton (exoskeleton condition) and one trial without the exoskeleton (control condition). In the exoskeleton condition the clutch was switched on to provide support. Also, great care was taken to avoid contact between the exoskeleton and the EMG electrodes to prevent from the potential confounding factor. The trial order was randomized and counterbalanced between participants. Breaks of at least 5 minutes were given between the trials. To reach steady state in the oxygen uptake, all trials lasted at least 5 minutes. Data on kinematics and EMG data were recorded over 5 minutes. 2.5 Data analysis All data collected in the study were processed using MATLAB (R2015b, The MathWorks, Inc., Natick, Massachusetts, United States). Metabolic cost Metabolic cost (J/kg/s) was calculated from oxygen uptake and respiratory quotient [27]. Flow rates were averaged over the final two minutes and normalized with respect to body mass. Net metabolic cost was found by subtracting resting metabolism from the metabolic cost during the lifting task. 7