Three-dimensional continuous muscle moment arm maps for the anatomical shoulder

Area of Science:

• Biomechanics
• Human Anatomy
• Musculoskeletal System

Background:

• Muscle moment arms dictate mechanical advantage and joint stability.
• Understanding shoulder muscle function is vital for diagnosing and treating injuries.
• Previous research has provided limited insight into the 3D moment arms of major shoulder muscles across a full range of motion.

Purpose of the Study:

• To map the three-dimensional moment arms of eight major shoulder muscles.
• To analyze the functional roles of these muscles across a continuous range of shoulder motion.
• To elucidate the complex and orientation-dependent actions of shoulder muscles.

Main Methods:

• Utilized an ex-vivo simulator to measure moment arms.
• Employed the tendon excursion method for data acquisition.
• Investigated eight major shoulder muscles (deltoid, subscapularis, supraspinatus, infraspinatus, teres minor) in eight human specimens.

Main Results:

• Demonstrated multifaceted functions for most shoulder muscles, varying significantly with arm orientation.
• Identified specific roles, such as the anterior deltoid's larger elevation moment arm in anterior planes and the lateral deltoid's in posterior planes.
• Revealed the supraspinatus's biphasic function and the distinct rotator cuff muscle actions (infraspinatus, superior/inferior subscapularis, teres minor).

Conclusions:

• Each major shoulder muscle exhibits complex, dynamic functions that are significantly influenced by arm position.
• The generated muscle moment arm maps provide a detailed understanding of shoulder muscle contributions to joint actuation and stability.
• This research advances the comprehension of shoulder biomechanics and has implications for clinical applications and rehabilitation strategies.