Sensitivity of Shoulder Musculoskeletal Model Predictions to Muscle-Tendon Properties.

Area of Science:

• Biomechanics
• Musculoskeletal modeling
• Human upper limb function

Background:

• Sensitivity of estimated muscle forces to muscle-tendon properties is established for lower limbs.
• Limited understanding exists regarding this sensitivity in the shoulder and upper limbs.

Purpose of the Study:

• To assess the sensitivity of estimated shoulder muscle forces to muscle-tendon properties.
• To evaluate the impact on scapulohumeral joint force.

Main Methods:

• A single healthy participant performed arm flexion and throwing motions.
• Kinematics were captured using intra-cortical pins.
• Muscle forces were estimated via static optimization in OpenSim, with variations applied to tendon slack length, optimal fiber length, maximal isometric force, and pennation angle.

Main Results:

• Muscle-tendon property variations caused up to 174% changes in nominal muscle forces.
• Rotator cuff and prime mover muscles showed the highest sensitivity.
• Maximal isometric force and optimal fiber length most influenced muscle force variability.
• Glenohumeral force showed minimal sensitivity (<8%) to these variations.

Conclusions:

• Generic musculoskeletal models without personalized muscle-tendon properties risk misinterpreting shoulder muscle forces.
• Focusing on maximal isometric force and optimal fiber length of key shoulder muscles is crucial for accurate modeling.