Introduction: Individuals with rotator cuff tears (RCT) have been found to compensate in their movement patterns by using lower thoracohumeral elevation angles during certain tasks, as well as increased internal rotation of the shoulder (Vidt et al., 2016). The leading joint hypothesis (LJH) suggests there is one leading joint that creates the foundation for the entire limb motion, and there are other subordinate joints that monitor the passive interaction torque (IT) and create a net torque (NT) aiding to limb motions required for the task. This experiment hopes to establish a better understanding of joint control strategies during a wide range of arm movements. Based off of the LJH, we hypothesize that when a subject has a rotator cuff tear, their performance of planar and three- dimensional motions should be altered not only at the shoulder, which is often the leading joint, but also at other joints on the arm such as the elbow and wrist.

Methods: There were 3 groups of participants: healthy younger adults (age 21.74 ± 1.97), healthy older adult controls (age 69.53 ± 6.85), and older adults with a RCT (age 64.33 ± 4.04). All three groups completed strength testing, horizontal drawing and pointing tasks, and three-dimensional (3D) activities of daily living (ADLs). Kinematic and kinetic variables of the arm were obtained during horizontal and 3D tasks using data from 13 reflective markers placed on the arm and trunk, 8 motion capture cameras, and Cortex motion capture software (Motion Analysis Corp., Santa Rosa, CA). During these tasks, electromyography (EMG) electrodes were placed on 12 muscles along the arm that affect shoulder, elbow, and wrist rotation. Strength testing tasks were measured using a dynamometer. All strength testing and 3D tasks were completed for three trials and horizontal tasks were completed for two trials.

Results: Results of the younger adult participants showed that during the forward portion of seven 3D tasks, there were four phases of different joint control mechanics seen in a majority of the movements. These phases included active rotation of both the shoulder and the elbow joint, active rotation of the shoulder with passive rotation of the elbow, passive rotation of the shoulder with active rotation of the elbow, and passive rotation of both the shoulder and the elbow. Passive rotation during movements was a result of gravitational torque (GT) on the different segments of the arm and IT caused as a result the multi-joint structure of human limbs. The number of tested participants for the healthy older adults and RCT older adults groups is not yet high enough to produce significant results and because of this their results are not reported in this article.

Discussion: Through the available results, multiple phases were found where one or both of the joints of the arm moved passively which further supports the LJH and extends it to include 3D movements. This article is a part of a bigger project which hopes to get a better understanding of how older adults adjust to large passive torques acting on the arm during 3D movements and how older adults with RCTs compensate for the decreased strength, the decreased range of motion (ROM), and the pain that accompany these types of tears. Hopefully the results of this experiment lead to more research toward better understanding how to treat patients with RCTs.