Analysis of shoulder loading can shed light on injury mechanisms, but direct measurement of loading remains challenging. Musculoskeletal simulations offer alternative estimation methods, provided they are validated. A recent open-source thoracoscapular shoulder model can reproduce scapulothoracic kinematics accurately, but its validity in estimating joint loading has been unknown. Previous attempts to use this model with  muscle redundancy solvers to estimate shoulder joint loading have been made, but whether the solutions fulfilled conditions for glenohumeral stability has been relatively little explored. The existing thoracoscapular model, moreover, does not allow any articulation in the spine, whereas muscles that span the shoulder are influenced by spinal movement.

The first aim of the thesis was to explore what degree of glenohumeral stability is adequate in a musculoskeletal model to accurately estimate shoulder joint forces. We used available kinematics and in vivo glenohumeral joint contact forces from the Orthoload dataset to evaluate the criterion validity of the proposed stability formulations. Different formulations for shoulder joint stability were introduced based on the computed direction of the joint contact force. This force was constrained to be directed into or close to the glenoid cavity, described with different geometric shapes or penalties in the muscle redundancy solver. We found that restricting the force direction towards a specified shape resulted in unrealistic force vectors that were directed along the shape borders. A less strict approach that encouraged joint contact forces to be directed centrally in the glenoid cavity estimated relatively more accurate force magnitudes and contact force directions, though some differences with the in vivo measurements still exist. 

The second aim of the thesis was to validate the use of a spine-integrated thoracoscapular shoulder (SITS) model, which includes cervical and lumbar spine articulation, to estimate shoulder biomechanics in seated activities. Specifically, aims of the second study were to evaluate the model's content validity, then to study how sitting posture can affect shoulder muscle activation and joint loading. We estimated shoulder loading during captured movements of subjects performing simple dumbbell lifting tasks in two different sitting postures—slouched and upright. We compared estimated muscle and joint loading with the rigid (locked) spine and with vertebral articulation (unlocked), and found that the customized model with an unlocked spine reproduced the actual movement more accurately. We then found that sitting postures influenced muscle activation and joint loading; compared to an upright posture, the dumbbell lateral and anterior lifting in a slouched posture involved greater glenohumeral joint movement, increased ligament lengthening, more muscle activation, and higher joint contact forces. These findings suggest that performing dumbbell lifts in a slouched posture places more load on the glenohumeral joint and increases strain on soft tissues, specifically glenohumeral ligaments.

These findings support the proposed enhanced shoulder model and stability formulations as benchmark methods for comprehensive shoulder biomechanical analysis. 

Analys av axelbelastning kan ge insikt i skadeorsaker. Direkt mätning av belastning är dock fortfarande en utmaning. Som ett alternativ kan muskel- och ledbelastningar uppskattas med muskuloskeletala simuleringar, men modellerna måste valideras. Modeller har utvecklats för att uppskatta rörelse och belastning i axelns olika leder och muskler. Validitet vid skattning av belastning har dock inte rapporterats. Tidigare modeller tillåter inte heller rörelse i ryggraden, trots att hållningen påverkar muskler som spänner över axeln. Det första syftet med avhandlingen var att undersöka vilken grad av stabilitet runt axelleden som är tillräcklig i en muskuloskeletal modell för att uppskatta axelledens krafter. Utifrån tillgängliga in vivo-data gällande rörelse-och ledbelastning modellerades axelledens stabilitet på olika sätt genom att begränsa ledkontaktkraften så att den riktas in i eller nära glenoidkaviteten. Vi fann att en mindre strikt stabilitetsbeskrivning som uppmuntrade axelledskontaktkrafter att riktas centralt i glenoidkaviteten uppskattade kraftstorlekar och kontaktkraftriktningar relativt väl, även om vissa avvikelser med in vivo-mätningarna fortfarande fanns.

Det andra syftet med avhandlingen var att validera en biomekanisk modell av axelleden med rörelse i ryggraden (SITS), för att uppskatta axelledens biomekanik vid sittande aktiviteter. Axelbelastningen beräknades  från insamlad rörelsedata av forskningspersoner som utförde enkla hantellyftaktiviteter, i både hopsjunkna och upprätta sittställningar. Vi fann att sittställningar påverkade modellernas estimerade  muskelaktivering och ledbelastning. Jämfört med en upprätt hållning innebar hantellyftet i ett hopsjunken hållning större glenohumerala ledrörelser, mer ligamentsträckning, mer muskelaktivering samt högre ledkontaktkrafter. 

Dessa resultat stöder den föreslagna modellen av axelleden och beskrivning av ledstabilitet som benchmarkmetoder för omfattande biomekanisk analys av axelleden.