Shoulder muscle moment arms during horizontal flexion and elevation

Area of Science:

• Biomechanics
• Human Anatomy
• Kinesiology

Background:

• Understanding shoulder muscle function is crucial for diagnosing and treating shoulder injuries.
• Moment arms, which dictate muscle torque, are key to analyzing shoulder joint mechanics.

Purpose of the Study:

• To calculate the instantaneous muscle moment arms for 10 shoulder muscles.
• To analyze these moment arms across four distinct glenohumeral motions.

Main Methods:

• Moment arms were derived from tendon excursion versus glenohumeral joint rotation angle plots.
• Calculations were performed for horizontal flexion, and elevation in sagittal, scapular, and coronal planes.

Main Results:

• The deltoid muscle demonstrated the largest moment arm during elevation.
• Specific deltoid portions showed varied moment arm strengths across different planes.
• Pectoralis major, latissimus dorsi, and teres major exhibited the largest depressor moment arms.
• Supraspinatus and infraspinatus showed smaller elevation torque in the scapular plane than previously reported.
• Subscapularis may be a more significant elevator in the scapular plane than supraspinatus or infraspinatus.

Conclusions:

• Muscle moment arm analysis provides detailed insights into shoulder muscle contributions during various movements.
• Findings on the supraspinatus, infraspinatus, and subscapularis challenge previous understandings of their roles in elevation.
• This data is vital for understanding shoulder biomechanics and informing clinical interventions.