Muscle length influences active joint position sense at the shoulder, but conditioning contractions do not.

Active shoulder joint position sense (JPS) is more accurate at higher angles of humerothoracic elevation. During active JPS testing, the extensor/antagonist muscles of the shoulder undergo repeated passive lengthening and do not contract to bring about shoulder extension. This may alter muscle spindle sensitivity at different angles of humeral elevation leading to angle related reduction in repositioning error. The purpose of this study is to examine active shoulder JPS when shoulder extensor muscles when were exposed to either a low-level contraction or full range of motion movement in an active angle reproduction JPS protocol. Fifteen participants completed the study. Participants completed the shoulder JPS protocol using three target angles (30°, 60°, and 90°) under three conditions: standard, range of motion exposure (fROM) between trials, and antagonist contraction (aCON) between trials. There was a significant interaction between conditions and angles (p = 0.002). JPS error decreased at higher elevations in the standard condition (p = 0.014) and aCON conditions (p = 0.035). The effect was absent in the fROM condition, where error across all target angles was consistent with the higher error at 30°. The outcomes suggest that change in the antagonist's muscle length and muscle spindle slackness are significant contributors to shoulder JPS.