Quantifying the Multidimensional Impedance of the Shoulder During Volitional Contractions

Area of Science:

• Biomechanics
• Neuromuscular control
• Human joint mechanics

Background:

• Understanding shoulder neuromuscular control is crucial for regulating complex 3D joint mechanics.
• How shoulder mechanics change with varying directional loading remains largely unknown.

Purpose of the Study:

• To quantify the changes in three-dimensional (3D) shoulder mechanics during voluntary torque production.
• To investigate the relationship between volitional torque and shoulder joint impedance.

Main Methods:

• Eleven participants produced isometric torques in six different directions.
• Shoulder impedance was measured using pseudorandom angular perturbations.
• Nonparametric impedance frequency response functions were modeled using second-order linear systems.

Main Results:

• Shoulder stiffness scaled linearly with torque, and viscosity increased nonlinearly.
• Maximal stiffness and viscosity aligned with the direction of torque production.
• The shoulder exhibited lowest stiffness and viscosity during internal/external rotation.

Conclusions:

• The neuromuscular system actively regulates 3D shoulder mechanics based on volitional muscle activation.
• Findings suggest increased injury susceptibility along the internal/external rotation axis.
• A mathematical model was developed to describe these 3D shoulder mechanics.