# Shoulder Kinematics Assessment towards Exoskeleton Development
## 6 Exoskeleton Alignment and Its Effect on the Shoulder Muscles: Case Study  
Through an experiment, shoulder movement is studied in a pick-and-place task performed by a person without upper arm movement disability. A 2 lb weight placed on top of a table is used for this purpose as the person is seated behind the table. The trajectory of the right arm is recorded by an integrated motion capture (MoCap) system. The collected data are processed to replicate the same movement in software which provides a better understanding of the mechanism driving the arm and muscles contributing to generate sufficient force as well. In this section, the effect of exoskeleton intervention in upper arm and specifically shoulder rehabilitation is assessed through introducing a certain type of non-anatomical shoulder joint rotation due to interactive forces between exoskeleton and upper arm that could directly affect shoulder joint. Traditionally, exoskeletons are designed to align with the human joint axes of motion by assigning each human joint with an equivalent exoskeleton joint (e.g., a hinge joint for the elbow). This assumes that the location of the axis can be accurately known, and that such a fixed axis exists for the range of motion of the joint or set of joints, which is not always the case. For example, the human shoulder permits complex motions, in which the shoulder joint's center of rotation moves with different arm motions. Misalignment can create large stresses on the attached systems and underlying human anatomy. Upper limb exoskeletons are modern devices meant to enhance upper-limb functionalities through providing adequate support for people with disabilities in their ADL [42], rehabilitation for patients suffering post-stroke trauma [43], power amplification in an industrial [44] or medical settings [45], or people with neuromuscular injuries [46]. High demands of such profitable machines compelled researchers and the private sector to develop more efficient control systems and mechanism to optimize exoskeletons functionality through intelligent human robot interaction [47]. There are several aspects to investigate the interaction between human and robot counterpart, namely, the design of the mechanism and antropometric characteristics considered in design; actuation of motors and transmissibility of the device while adjusting necessary stiffness or compliance; control type selection [48]. The anatomy of shoulder girdle encompasses several articulations or joints connecting the upper limb to the skeleton. There exist three bones, including scapula (shoulder blade), clavicle, and humerus (upper arm bones) at the shoulder site. Major joints at shoulder girdle are glenohumeral (GH), acromioclavicular (AC), and sternoclavicular (SC) joint (Figure 1). The GH joint is also known as ball and socket joint, which produces a substantial portion of the shoulder movement [49]. While GH is often considered to have three DOF [50], its axis of rotation changes as shoulder moves; hence in simulation, it is crucial to implement a clearance area so as to let GH anatomically moves as human moving his/her arm. There are studies where various methods are applied in order to replicate shoulder movements under different conditions with various level of impairment [51; 52]. Arm rotation that originates at the shoulder joint can also be affected through any simplification in modeling, or external pressure from an outside agent, such as exoskeletons. Robotics has been center of attention in rehabilitation and the way it facilitates training has been tremendously helpful to physicians; however, there are some technical issues involved, which makes equipping controlling such devices challenging at times [53]. Exoskeletons as a popular type of robotic devices in clinical settings may introduce unintended external force or misalignment.