A biomechanical model to quantify shoulder load at the work place.

Area of Science:

• Biomechanics
• Occupational Health
• Ergonomics

Background:

• Workplace activities often involve elevated arm postures, leading to significant shoulder muscle and joint loading.
• Quantifying these loads and predicting fatigue is crucial for preventing musculoskeletal disorders.

Purpose of the Study:

• To develop and validate a simple two-dimensional biomechanical model of the shoulder.
• To quantify shoulder muscle load, joint load, and endurance time in occupational settings with elevated arm positions.

Main Methods:

• Developed a 2D biomechanical model analyzing shoulder muscle and joint forces.
• Input variables include upper arm elevation angle, total body weight, and upper arm length.
• Model predicts deltoid, supraspinatus muscle forces, glenohumeral joint reaction force, and endurance time.

Main Results:

• Maximum muscle forces observed at 80° arm elevation (deltoid ~22%, supraspinatus ~25% of max).
• Glenohumeral joint force reached ~43% of total body weight at 80° elevation.
• Endurance time at 80° elevation was approximately 5 minutes.

Conclusions:

• The model effectively quantifies shoulder biomechanics in elevated arm postures.
• To prevent fatigue during >1 hour of work, arm elevation should be kept below 15°.