Arm load magnitude affects selective shoulder muscle activation

Area of Science:

• Biomechanics
• Human Movement Science
• Musculoskeletal Modeling

Background:

• Isometric tasks often assume linear scaling of shoulder muscle forces with external load.
• Inverse dynamic modeling typically predicts this linear behavior based on cost functions.

Purpose of the Study:

• To investigate the linearity between external arm load magnitude and shoulder muscle activation.
• To compare experimental findings with simulations using different cost functions in the Delft Shoulder and Elbow Model (DSEM).

Main Methods:

• Experimentally determined the Principal Action (PA) of shoulder muscles across varying isometric force levels and directions in 12 healthy subjects.
• Simulated the experiment using DSEM with two cost functions: minimizing muscle stress and an energy-related cost function.

Main Results:

• Experimental PA significantly changed with external force magnitude, indicating load dependency.
• DSEM simulations showed minor non-linearities with the stress cost function and more pronounced non-linearities with the energy cost function.
• Simulated PAs did not match experimental PAs.

Conclusions:

• Shoulder muscle activation exhibits non-linear relationships with external load magnitude, contradicting simple linear assumptions.
• Current DSEM cost functions do not fully replicate experimental muscle activation patterns, suggesting limitations in modeling shoulder complex biomechanics.