Shoulder Joint Stiffness in a Functional Posture at Various Levels of Muscle Activation

Objective:
In this study, we present a method to quantify the mechanics of the shoulder joint in a functional posture, in two degrees of freedom: internal/external rotation and horizontal abduction/adduction.
Methods:
We performed experiments on 15 healthy participants using a custom perturbation robot. Perturbations were applied in internal/external rotation and horizontal abduction/adduction, whilst participants applied varying levels of joint torque. System identification techniques were used to quantify the mechanical properties of the shoulder joint at various levels of muscle contraction, including; stiffness, viscous damping, and inertia parameters, natural frequency, and damping parameter. We compared the shoulder mechanical properties between dominant and non-dominant limbs.
Results:
The mean stiffness increased 4.8 times in external rotation, and 6.25 times in internal rotation as a result of contraction to 8 Nm. It increased 2.8 times in adduction and 4.6 times in abduction as a result of contraction to 16 Nm. The mean viscous damping increased 3 times in external rotation, 2.8 times in internal rotation as a result of contraction to 8 Nm. It increased 1.6 times in adduction and 2.25 times in abduction as a result of contraction to 16 Nm.
Conclusion:
Joint stiffness, viscous damping and natural frequency all increased with the level of shoulder contraction torque, whereas the damping parameter remained unchanged. No differences were observed between dominant and non-dominant limbs.
Significance:
We have presented a method to characterize the mechanical properties of the shoulder complex during various activation states, which has application as a diagnostic and assessment tool for shoulder pathology.