Intraarticular hydraulic distension (IHD) has been utilized to treat painful stiff shoulders by distending and then rupturing the joint capsules. However, no attempts have been made to optimize the capsule distension, which might give a better clinical outcome. To set up a prerequisite technique for a maximal distension without rupturing the capsule, real-time pressure monitoring of IHD procedures was performed in 16 patients, which revealed triphasic pressure-volume profiles in 10 patients and biphasic in 6 patients. The IHD procedures were terminated without rupturing the capsule by observing the real-time pressure-volume curves (N = 10) or by pain (N = 3). The pressure profiles of the 13 patients who had the infusion stopped without rupture demonstrated stress-relaxation curves of the capsules. The biphasic group showed a steeper slope of the second phase of linearly increasing pressure (p = 0.017) and higher pressure (p = 0.005) with slightly larger volume (p = 0.095) at the termination of fluid infusion than did the triphasic group. The stiffness of the glenohumeral joint capsule had a close correlation with the limitation in range of motion. Real-time pressure monitoring would be helpful to develop a novel IHD technique that could distend the glenohumeral joint as much as possible without rupturing it. The biomechanical parameters derived from the pressure-volume profiles would also be useful to characterize the properties of the joints of painful stiff shoulder patients.