Muscle activities during shoulder internal rotation differ in arm position: a preliminary quantitative analysis using...

Area of Science:

• Orthopedics
• Sports Medicine
• Medical Imaging

Background:

• Shoulder internal rotation is crucial for many upper limb activities.
• Understanding muscle activation patterns during rotation is key for injury prevention and rehabilitation.
• Positron emission tomography (PET) and MRI offer advanced insights into muscle function.

Purpose of the Study:

• To map muscle activity in the glenohumeral joint during internal rotation.
• To compare muscle engagement at 0° and 90° arm abduction.
• To utilize 2-deoxy-2-[18F] fluoro-D-glucose (FDG) PET and MRI for detailed analysis.

Main Methods:

• Six healthy males performed active internal rotation exercises with elastic bands at 0° and 90° abduction.
• FDG-PET scans were acquired at rest and post-exercise.
• PET images were fused with MRI for precise muscle identification and standardized uptake value (SUV) comparison.

Main Results:

• At 0° abduction, middle and inferior subscapularis showed significantly higher SUV post-exercise compared to the superior part.
• At 90° abduction, the inferior subscapularis exhibited significantly greater SUV than at 0° abduction and the superior subscapularis.
• Increased SUV was also observed in the inferior infraspinatus and teres minor post-exercise.

Conclusions:

• The middle and inferior subscapularis are primary internal rotators at 0° abduction.
• The inferior subscapularis plays a dominant role in internal rotation at 90° abduction.
• These findings provide detailed muscle activation data for shoulder biomechanics.