Saskia Baltrusch

119 Chapter 5 one single actuated joint is placed at the hip level or slightly above to allow movement in the sagittal plane. Further, a mechanical structure extends from this joint, to the trunk, and to the thigh. However, an important aspect, which sets these devices apart, is the degree, to which the kinematic structure of the exoskeleton is designed to align with the human. Misaligned joints can produce unwanted, parasitic forces and torques of up to 230 N and 1.5 Nm, respectively [15] which decrease the comfort of wearing a device. Because good alignment of an external exoskeleton structure is challenging, devices often compensate for misalignment [16,17], i.e., instead of trying to align the exoskeleton structure with the human, a certain amount of misalignment is accepted and compensated with the exoskeleton by introducing additional degrees of freedom (Figure 2C & D). Furthermore, proper misalignment compensation prevents relative movement between the device and the user and thereby indirectly also increases comfort [18]. The relatively big ranges of motion of the human trunk in the sagittal plane originates from the fact, that a human can flex or extend his hip joint as well as the lumbar joint(s). This leads to a certain redundancy, by which objects can be picked up. Two well-established cases include: a predominant use of the hip joint with small lumbar angles, which is commonly referred to as a squat lift, or a predominant use of the lumbar joint(s) with relatively small hip angles, commonly referred to as stoop lift [8]. These lifting styles have consequences on the torques that need to be provided at the lumbo-sacral joint: depending on lifting conditions, such as object height and size, moments at the L5-S1 joint can be substantially higher in either stoop or squat lifts [8]. The only exoskeleton of the above mentioned ones, that accounts for this additional degree of freedom in the sagittal plane with an additional joint, is the Back Support Muscle Suit [14]. This device has, next to the hip joint, an additional joint, which is placed at the bottom of the back. Unfortunately, no further evaluation of this additional joint was found. The other exoskeletons can still have a large range of motion of the trunk. However, they do not adopt their kinematics or support with variations in lifting style. 5