Shoulder Muscles Moment Arm Contribution to Glenohumerol Joint Motion and Stability

The objective of this chapter is to define further the clinical value of moment arms and provide a descriptive approach based on experimental investigation into the calculation of moment arm for the rotator cuff muscles spanning the glenohumeral joint during abduction. With the advent of imaging techniques quantifying the muscle torque capacity can be a reliable tool for clinical assessment of RC repairs as well as GH joint arthroplasty. Joint kinematics and muscle force estimation are essential for quantification of patient outcome and rehabilitation. Methods used in the calculation of moment arms usually referred to as the geometric approach and excursion method are directly related through principle of virtual work and moment equations where both require a center of rotation and a measureable distance to the line of action of the muscle force or simply the rate of change in the excursion of the muscle force length. In this chapter experimental data is provided for two muscles supraspinatus and subscapularis, respectively where a combination of a 3D model and digitized points along the tendons attachment and insertion surface area are used in the analysis.