The biomechanics of the upper limb have been examined extensively to understand neural control of movement, interactions of muscle structure and performance, and surgical strategies to improve function. In general, studies have examined individual upper limb joints. However, the actions of muscles crossing one joint are dependent on the posture and motion of adjacent joints. The goal of this dissertation was to develop computational tools and obtain experimental data that will further understanding of the multijoint function of the upper limb.

We developed a three-dimensional musculoskeletal upper limb model including 50 muscle-tendon actuators crossing the glenohumeral joint, elbow, forearm, and wrist. Moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics over a wide range of postures. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension.

We used our model to analyze the Steindler flexorplasty, a surgery that restores elbow flexion after paralysis by transferring the origin of the flexor-pronator mass proximally onto the humeral shaft. All simulated surgeries restored the elbow flexion moment-generating capacity to nearly two-thirds that of normal. However, more proximal transfers had the unwanted effect of greatly increasing the passive forces generated by overstretched muscles. The simulation revealed that transfers to the inferior border of feasible transfer sites restored elbow flexion function while limiting excessive passive forces that could lead to limited range of motion.

We measured maximum isometric joint moments and used magnetic resonance imaging to obtain muscle volumes at the shoulder, elbow, and wrist in 10 subjects. The distribution of muscle volume was conserved across subjects with a threefold difference in total upper limb muscle volume. On average, 83% of the variation across subjects in the measured joint moment was explained by the differences in physiologic crosssectional area of the muscles contributing to that moment. These data indicate that muscle size is the major determinant of joint moment-generating capacity in healthy adults.

This dissertation provides new tools for understanding upper limb biomechanics and highlights the importance of considering the upper limb as a whole when examining function.