# Shoulder Kinematics Assessment towards Exoskeleton Development
## 5 Design of Exoskeletons to Align with Shoulder Kinematics  
As the mathematical modeling of the shoulder is very complex, more simplified models are being used for exoskeleton design and alignment. Often, researchers approximate the shoulder mechanism as a pure spherical joint with its center of rotation at the head of the humerus [17; 33; 34]. This type of design assumes the shoulder girdle motion can be neglected, and the center of rotation of the GH joint is accurately known and fixed over the whole range of motion of the shoulder. However, this is not the case, as studied in [35], where the human joint shifts away its mechanical counterpart. Any misalignment between the exoskeleton and the human shoulder can cause discomfort and produce reaction forces that lead to negative effects over time. Additionally, from the mechanical perspective, spherical joint mechanism is modeled by 3 revolute joints, which include singularity configurations to reduce the exoskeleton performance. Reference [36] proposes a redundant 4R mechanism in order to avoid singularities; theoretically, this design can operate over the whole workspace of the shoulder, thus making it a simple exoskeleton to model and build. Nonetheless, it does not address the misalignment that was provoked by the shoulder girdle motion or how to accurately locate the center of rotation of the shoulder over the the range of motion of the shoulder. Relatively, a more realistic designs have been proposed and discussed in [37; 38; 39], where the position of the humerus head is estimated. These exoskeletons take into account the motion of the humerus head due to shoulder girdle. For example, the MEDARM [37] is a rehabilitation robot with five DOF at the shoulder complex, three DOF for the GH joint, and two DOF for the ST joint to compensate the translation of the humerus head. Notably, the ARMIN III [39] is an exoskeleton that has been used for rehabilitation. In this case, it has been designed to compensate for shoulder girdle motion by approximating the humerus head motion by a circle. However, it assumes that the motion of the humerus head is the same for individuals of the same height. Even though these exoskeletons address the translation that the humerus head suffers during the shoulder motion, they still have to be aligned with their human counterpart, which is very challenging and tedious, and it varies with each individual. Another challenge for exoskeleton design is the collection of data for the construction of these rehabilitation robotic devices. In particular, one popular approach to collect information about a limb trajectory is with the used of motion capture systems, see [6]. By placing IMU sensors over the skin at specific locations of the limb being studied, it is possible to reconstruct the trajectory and orientation, information that can be used as an input for exoskeleton design algorithm. However, because these sensors are placed on the skin, the measurement will have errors due to the natural motion of the skin over the trajectory being performed. This error will be carried out to the exoskeleton design, which further increases the misalignment between the mechanism and joint understudied. A similar limitations may exist when using MoCap systems as markers that are placed on bony part of the limb are still subject to slight movement, and this small displacement will be transferred to simulation model in OpenSIM, which is discussed in further details in this paper. It is clear that another approach has to be taken in order to produce exoskeletons that can fully reproduce the motion of human joints, especially the shoulder complex range of motion. Reference [40] suggests that probably researchers should embrace redundancy instead of avoiding it as new kinematic representation for exoskeleton design, in order to build mechanism that identically reproduce limbs workspace. In fact, in [41], a novel task-based exoskeleton design is proposed.