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

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. 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. 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. 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. 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.