It was shown on human corpses that the glenohumeral joint may be compared to a piston surrounded by a valve. The glenoid labrum, then, should work like the valve block, sealing the joint from atmospheric pressure. In order to test this hypothesis, 18 human shoulder preparations were studied. The mean stabilizing force obtained by atmospheric pressure was 146 N. Additionally, 15 patients without any sign of instability and 17 patients with an anterior instability of the shoulder were tested under general anesthesia. In stable shoulder joints, traction at the arm caused negative intra-articular pressure that could be correlated to the amount of force exerted. In contrast, unstable shoulder joints with a tear of the glenoid labrum (Bankart lesion) did not exhibit this phenomenon. For unstable shoulder joints, the piston-and-valve model is no longer valid. This enlarges the current concept of shoulder joint stability in two ways: (a) the absence of negative intra-articular pressure disturbs joint mechanics and (b) altered pressure receptors might disturb motor coordination that dynamically protects the shoulder from dislocating forces.