Shoulder position sense and kinesthetically guided reaching accuracy in individuals with anterior shoulder instability

Altered neuromuscular control due to compromised position sense may contribute to shoulder instability. The purpose of this study was to investigate whether unstable shoulder subjects exhibit larger errors than intact shoulder subjects in kinesthetically guided active positioning and reaching that are of greater functional significance than passive testing of shoulder position sense. Ten subjects with a history of anterior shoulder dislocation and 15 intact shoulder subjects participated in the study. Shoulder position sense was examined with three different protocols (imposed motion to remembered shoulder rotation angles and active shoulder abduction/rotation to verbally specified positions) with targets located in both the mid- and end-range of rotation. Three dimensional end-point accuracy of kinesthetically guided reaches to visually specified targets, along with the shoulder rotation angle and scapula orientations at the end-point, were also analyzed. In agreement with previous studies, unstable shoulder subjects exhibited significantly larger errors in perception of shoulder joint angles than healthy controls in a protocol involving imposed motion to remembered shoulder rotation angles. However, the clinical significance of the observed deficit is questionable because the averaged rms error differences between unstable and intact shoulders were relatively small (average: 1.8°). During tests of active positioning, unstable shoulder subjects were able to move the shoulder to verbally defined angles as accurately as healthy controls in both shoulder abduction and rotation. Unstable and intact shoulder subjects exhibited similar reaching accuracy and scapular orientations in the kinesthetically guided reaching test, but unstable shoulder subjects consistently used less shoulder rotation angle than healthy controls. However, they were able to point to a remembered target as accurately as intact shoulder subjects, suggesting that a different reaching strategy was adopted by unstable shoulder subjects to minimize shoulder rotation. Results of this study show that unstable shoulder subjects can perceive shoulder angles and reach to visually specified targets in space as accurately as healthy controls in functional activities with voluntarily arm movements. The results suggest that less sensitive joint receptors due to over-stretched shoulder stabilizers following shoulder injury have little impact on the neuromuscular control of the shoulder joint.