Biomechanical Evaluation of Glenohumeral Joint Stabilizing Muscles During Provocative Tests Designed to Diagnose Superior Labrum Anterior-Posterior Lesions
Despite considerable advances in the understanding of glenohumeral (GH) biomechanics and glenoid labral pathologies, arthroscopy remains the only definitive means of Superior Labrum Anterior-Posterior (SLAP) lesion diagnosis. Unfortunately, natural GH anatomic variants limit the reliability of radiography. Accurate clinical diagnostic techniques would be advantageous due to the invasiveness, patient risk, and financial cost associated with arthroscopy. Twenty provocative tests designed to elicit labral symptoms as a diagnostic sign have shown promising accuracy by their respective original authors, but later studies generally fail to reproduce those findings. The purpose of this study was to compare the behavior of GH joint stabilizing muscles in promising tests. Electromyography (EMG) was used to characterize the activation of GH joint stabilizing muscles, with particular interest in the Long Head Biceps Brachii (LHBB) behavior, as activation of the LHBB and subsequent tension in the biceps tendon should illicit labral symptoms in SLAP lesion patients.
Volunteers (n=21) with no history of shoulder pathology were recruited for this study. The tests analyzed were Active Compression, Speed's, Pronated Load, Biceps Load I (Bicep I), Biceps Load II (Bicep II), Resisted Supination External Rotation (RSER), and Yergason's. Test modifications that allowed the use of the Biodex System improved reproducibility. EMG was used to record activity for GH muscles: the LHBB, short head of the biceps brachii, anterior deltoid, pectoralis major, latissimus dorsi, infraspinatus, and supraspinatus. An indwelling electrode was used to monitor supraspinatus activity, and the remaining muscles utilized surface electrodes. EMG data were recorded at 1250 Hz and filtered with custom MATLAB software. Muscle activity for each test was characterized by activation and selectivity. Muscle activation was defined as the muscle's peak normalized EMG amplitude. Muscle selectivity was defined as the ratio of muscle activation for the muscle of interest over the sum of all seven muscles' peak activations.
Results indicated that Bicep I and II had the greatest potential for the clinical detection of SLAP lesions because both tests 1) elicited large LHBB activation, suggesting that during these tests more tension was applied to the biceps tendon, and also 2) remained highly selective for the LHBB, which should reduce the potential sources for confounding results. Also, tests that elicited promising LHBB behavior for either a single suite or for both activation and selectivity, shared design patterns relating to location of the applied load, forearm orientation, joint position, and line of pull. These characteristics should be further examined to determine their potential role in optimizing SLAP test design and improving clinical diagnostic techniques.