Purpose : The purpose of this study was to evaluate the activity of the biceps brachii muscle in the vulnerable abduction and external rotation position of the shoulder in patients with anterior instability. Materials and Methods: This experimental study include a prospective analysis of the electromyographic(EMG) data on a group of patients with traumatic unilateral anterior instability of the shoulder. The EMG data of unstable shoulders was compared with those of opposite shoulders as control. The optimal sample size for the case-control study was calculated using an nQuery Advisor program(nQuery Adviser 3.0, Statisticl solutions Ltd., Ireland). The EMG analyses were conducted in 76 shoulders in 38 patients who had a traumatic anterior instability in one shoulder. The EMG records were obtained at different position of shoulder, which included 0° , 45° , 90° and 120° of shoulder abduction. In each angle of shoulder abduction, the arms were placed in an external rotation as tolerated by the anterior apprehension. The paired-sample T test was used to compare the difference of the root mean square(RMS) voltages between the stable and unstable shoulders in each degree of arm position. Results : The RMS voltage of the biceps muscle was significantly greater in the unstable shoulder than opposite stable shoulder in all position of the arm(p<0.001). The RMS voltage of the biceps was maximal at 90° and 120° of external rotation in the unstable shoulder(p<0.05). The RMS voltage of the supraspinatus muscle revealed no differences in any of the test conditions(p=0.904, 0.506, 0.119 and 0.781 in 0° , 45° , 90° and 120° , respectively) Conclusion: In the vulnerable abduction and external rotation position, the biceps muscle plays an active compensatory role in the unstable shoulder while not in the stable shoulder.