The Influence of External Loads on Movement Precision During Active Shoulder Internal Rotation Movements as Measured by 3 Indices of Accuracy

CONTEXT: Using constant, variable, and absolute error to measure movement accuracy might provide a more complete description of joint position sense than any of these values alone. OBJECTIVE: To determine the effect of loaded movements and type of feedback on shoulder joint position sense and movement velocity. DESIGN: Applied study with repeated measures comparing type of feedback and the presence of a load. SETTING: Laboratory. PATIENTS OR OTHER PARTICIPANTS: Twenty healthy subjects (age = 27.2 +/- 3.3 years, height = 173.2 +/- 18.1 cm, mass = 70.8 +/- 14.5 kg) were seated with their arms in a custom shoulder wheel. INTERVENTION(S): Subjects internally rotated 27 degrees in the plane of the scapula, with either visual feedback provided by a video monitor or proprioceptive feedback provided by prior passive positioning, to a target at 48 degrees of external rotation. Subjects performed the internal rotation movements with video feedback and proprioceptive feedback and with and without load (5% of body weight). MAIN OUTCOME MEASURE(S): High-speed motion analysis recorded peak rotational velocity and accuracy. Constant, variable, and absolute error for joint position sense was calculated from the final position. RESULTS: Unloaded movements demonstrated significantly greater variable error than for loaded movements (2.0 +/- 0.7 degrees and 1.5 +/- 0.4 degrees, respectively) (P \u3c .05), but there were no differences in constant or absolute error. Peak velocity was greater for movements with proprioceptive feedback (45.6 +/- 2.9 degrees/s) than visual feedback (39.1 +/- 2.1 degrees/s) and for unloaded (47.8 +/- 3.6 degrees/s) than loaded (36.9 +/- 1.0 degrees/s) movements (P \u3c .05). CONCLUSIONS: Shoulder joint position sense demonstrated greater variable error unloaded versus loaded movements. Both visual feedback and additional loads decreased peak rotational velocity