Saskia Baltrusch

186 Chapter 7 Kinematics and mechanical joint work Forces and kinematics were bi-directionally filtered with a 2 nd order Butterworth filter and a cut-off frequency of 10 Hz and 5 Hz, respectively. To assess a potential change in movement behaviour between the control and exoskeleton conditions, we calculated joint angles and the range of motion of the centre of mass (CoM). Joint flexion-extension angles in the knee, hip, trunk and L5S1 were calculated by Euler decomposition of the orientation matrices of the segment local reference frames [28]. Since all participants adopted a squat or semi-squat lifting style, peak knee angles were used to define the start and end of a single lifting cycle. The body’s centre of mass (CoM) was calculated based on the mass and CoM position of all segments, estimated according to Zatiorsky (2002) [29]. Range of motion of the CoM was obtained from the vertical distance travelled by the body centre of mass over a full lifting cycle. Flexion-extension net joint moments around knee, hip and L5S1 were calculated from ground reaction forces (GRF) and body kinematics with a bottom-up inverse dynamics model [24]. Angular velocities of the knee, hip and L5S1 joint were defined as the derivative of the joint angles. By multiplying the calculated net joint moments with the calculated angular velocities, we arrived at the joint power for each of the joints. To estimate positive and negative mechanical work generated around the joints, phases of positive and negative joint power were integrated separately over time. To assess mechanical work generated by the exoskeleton, we calculated the moment generated by the exoskeleton during lifting, by using the measured exoskeleton angles and the known angle torque relation of the exoskeleton [23]. By multiplying the estimated support moments of the exoskeleton around the hip and the L5S1 with the joint angular velocities, mechanical power generated by the exoskeleton was calculated. After integrating negative and positive joint power episodes over time, we arrived at the negative and positive work generated by the exoskeleton (exowork) at the hip and the L5S1 joint. Finally, the mechanical work generated by the muscles (muswork) was found by subtracting the work generated by the exoskeleton (exowork) from the joint work (generated by muscles and exoskeleton).