Saskia Baltrusch

183 Chapter 7 2.3 Instrumentation Metabolic cost For assessing metabolic cost, a breathing gas analysis system was used (Cosmed srl, Quark CPET, Rome, Italy), measuring breathing volume, the rate of oxygen consumption and carbon dioxide production. Kinematics and mechanical joint work Ground reaction forces were measured using a single custom-made force plate (1.0 x 1.0 m) at a sample rate of 200 Hz. The force plate is regularly checked for linearity and has a centre of pressure (COP) error of < 4mm. 3D Kinematics were recorded with an optoelectronic motion capture system (Optotrak, Northern Digital Inc., Waterloo ON, Canada) at a sample rate of 50 Hz. The measurement error of the Optotrak device is < 0.05 mm. Segment kinematics were quantified using a dynamic 3D linked segment model [24]. Cluster markers were attached to lower and upper leg, pelvis, trunk (T10), upper and lower arm, head and box. Given the fact that participants performed symmetric lifting, we only recorded kinematics from the right side of the body. In addition, four cluster markers were attached to the exoskeleton (pelvis frame, hip actuator, beam base, beam top) to measure the ‘hip’ and ‘lumbar’ flexion of the exoskeleton. Prior to the measurements, cluster markers were related to anatomical landmarks using pointer measurements [25]. Muscle Activity Muscle activity was recorded at a sample rate of 2000 Hz, using surface Electromyography (Porti, TMSi, Oldenzaal, The Netherlands). Bipolar surface electrodes were placed at 5 bilateral sites on the skin after abrasion and cleaning with alcohol (Ag-AgCl electrodes; interelectrode distance, 20 mm). The recording sites were: m. longissimus thoracis (LT) at the T9 level (4 cm lateral), m. iliocostalis lumborum (IL) at the L2 level (6 cm lateral), m. longissimus lumborum (LL) at the L3 level (3 cm lateral), m. external oblique muscles (EO) about 15 cm above the SIAS and m. rectus abdominis (RA), 3 cm lateral from the umbilicus. The complete experimental setup is shown in Figure 3. 7