Saskia Baltrusch

118 Chapter 5 either by external forces that run parallel to the human back, or moments that help extend the back. Applying these forces and moments to the torso and below the lumbo-sacral joint (L5-S1) mechanically unloads the lower back. Figure 1: Back muscle forces and exoskeleton forces and moments (A) . In order to balance the gravitational forces acting on on the trunk, the back muscles (FMuscle) contract. Since these forces act over a small lever arm of only a few centimeters (rLever ), almost parallel to the spine, the spine is subject to large compression forces. Exoskeletons aim at reducing these forces, by applying forces on the trunk (FExoTrunk ), pelvis (FExoPelvis), and thigh (FExoThigh). Together they produce an extension moment (MExo); Passive spexor exoskeleton (B) . The exoskeleton unloads the back by applying a force at the torso, pelvis, and the thighs. The user has a large range of motion, when wearing the passive exoskeleton, due to advanced misalignment compensating mechanisms like the flexible beams in parallel to the spine. Written informed consent was obtained from the participant to publish this picture. Since the properties of these back support exoskeletons and exosuits differ significantly with the way they are constructed, they are further subdivided into two groups: rigid and soft. In this context, rigid means, that the exoskeleton structure can transmit compression as well as extension forces, whereas soft refers to, that only compression forces can be transmitted. Classical rigid exoskeletons such as Laevo [9], Robomate [10], Bending Non- Demand Return (BNDR) [11], Wearable Moment Restoring Device (WMRD) [12], BackX [13], and Back Support Muscle Suit [14] have in common, that at least