Quantifying internal shoulder loading

QC 261105

Abstract

Quantifying internal joint loading may help explain joint overuse, especially in vulnerable populations such as wheelchair users. Wheelchair users rely heavily on upper body function to perform repetitive and mechanically demanding tasks such as propulsion and transfers, and they therefore face a high risk of overuse. Invasive instrumentation is, however, needed to measure internal joint loading directly, which makes experimental measurement difficult. Musculoskeletal simulations offer an alternative for estimating these loads, but their validation against in vivo measurements remains limited. In addition, musculoskeletal models often oversimplify spinal articulation. This causes a loss of quantitative information on coordinated upper body motion, particularly in seated exercises. This thesis aimed to develop a comprehensive inverse musculoskeletal simulation framework to quantify internal shoulder loading in people with and without physical disabilities.

Study A investigated what degree of glenohumeral stability is necessary to reasonably estimate joint contact forces compared with in vivo measurements. The study evaluated multiple glenohumeral stability formulations that either restricted the contact force direction to specific stability-perimeter shapes or applied a penalty that discouraged deviation from the glenoid cavity center. Nearly all formulations matched the in vivo contact force magnitudes, except the most restrictive one, which forced the joint contact force directly toward the glenoid center and performed poorly. Models using hard constraints or conditional penalties tended to push force vectors toward the perimeter boundaries. In contrast, continuous penalty models produced more realistic force directions within the glenoid cavity. These results indicate that the penalty-based formulations offer a more reasonable representation of glenohumeral stability than the alternative approaches examined.

To assess how posture affects shoulder biomechanics, Study B introduced the spine-integrated thoracoscapular (SITS) model with cervical and lumbar articulations. The penalty formulation from Study A was implemented in a muscle redundancy solver that was integrated with the SITS model. Inverse simulations were performed on ten non-disabled adults performing 2 kg dumbbell raises in upright and slouched postures. The study first assessed the sensitivity of model estimates to locked versus unlocked spinal articulation in the SITS model. The unlocked model reproduced spinal kinematics more accurately and significantly changed the scapular elevation angle. Effects on glenohumeral contact force were, however, non-significant, suggesting the locked model may suffice when that force is the outcome of interest. The unlocked model, however, revealed posture-induced trunk compensation. The slouched posture limited the reachable workspace, requiring posterior trunk tilt. This tilt was driven mainly by lumbar rather than pelvic motion, likely to preserve sitting balance. Slouching also increased task demands, particularly during lateral and anterior raises, producing significantly higher peak joint contact forces.

Wheelchair transfer is a common and load-demanding daily task that is needed for mobility. Study C applied the framework from Studies A and B to quantify internal shoulder loading during wheelchair transfers to different heights. The participants were manual wheelchair users with chronic motor-complete thoracic spinal cord injury. Estimated forces on the trailing arm were higher than on the leading arm. The subscapularis and supraspinatus were estimated to stabilize the glenohumeral joint while the pectoralis major to drive motion. Transfer height non-significantly affected joint contact forces but significantly influenced individual muscle forces. Leading arm muscles often showed lower forces during downward transfer than upward transfer. These findings suggest that changing the transfer surface height may modulate the estimated loading demands on the shoulders.

Forward trunk lean is a common clinical recommendation during wheelchair transfers to increase pelvis elevation. However, how this technique affects internal shoulder mechanics has not been established. Study D investigated how a verbal instruction to lean the trunk forward affects shoulder loading during same-level wheelchair transfer. The study used the inverse musculoskeletal simulation approach from Studies A and B and the same study population as Study C. Two conditions were compared: participants' preferred transfer technique and a version with verbal instruction to lean forward as much as possible. With instruction, peak pelvis elevation increased significantly. Estimated forces in several muscles, notably the trailing arm biceps long head and the leading arm anterior deltoid, also increased significantly with instruction. In contrast, estimated forces in other muscles decreased significantly, most notably in the inferior subscapularis of both arms. Estimated glenohumeral contact forces did not differ significantly between conditions. Forward trunk lean may therefore increase pelvis elevation without a detectable change in glenohumeral contact forces, while redistributing loading among shoulder muscles.

These findings position the proposed simulation framework as a promising tool for quantifying internal shoulder loading and detecting changes between conditions. The framework may provide quantitative metrics for testing hypotheses in people with and without physical disabilities. However, rigorous validation against accurate measurements of loading is still needed before clinical recommendations can be made.

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