Saskia Baltrusch

123 Chapter 5 Kinematic Compatibility and Fitting In the process of designing back support exoskeletons, the degrees of freedom introduced by the lumbar spine are not always taken into account or are merged with the degrees of freedom of the hip [9-13]. Simple models of the human spine, suggest, that the lumbar spine can be modeled as one additional joint in the sagittal plane (Figure 2A). More complex models, consist of five separate spherical joints to model the lumbar spine [28]. The angular displacement in the lumbar spine can occur in the sagittal plane by flexing or extending the spine. Lateral bending in the frontal plane and axial rotation in the transverse plane constitute the rest of the possible angular displacements. Peak values for the range of motion of the human hip, spine and trunk can be found in Table 1. The relatively large amount of lumbar flexion of around 60° is not accounted for in most exoskeleton designs. This is astounding, since approximately 33% of the range of motion of the trunk flexion, comes from the lumbar spine (Table 1). The hip joint is commonly modeled as a a ball joint [29]. Its degrees of freedom are comprised of flexion and extension in the sagittal plane, abduction and adduction in the frontal plane and internal external rotation in the transverse plane. The main reason, why the degrees of freedom of the human are important, is the fact, that a misaligned exoskeleton joint can cause some unwanted forces and moments [18]. Additionally it can lead to unwanted relative movements between the device and the wearer. This problem is well-documented in Schiele and van der Helm (2006) [18] and is common in all exoskeleton joints that are not aligned (Figure 2C). However, an exoskeleton joint can be misaligned without causing harm, as long as it is properly compensated. There are several ways to solve this. One that is used in a number of devices [10, 15, 16, 18], is the introduction of additional degrees of freedom (Figure 2D). Anthropomorphic data [30] suggests that significant differences exist in hip and waist width (Figure 2B). These differences are in the range of several centimeters. If not properly accounted for, these differences can be a source of discomfort and misalignment. 5