Three optical joint angle monitoring devices with the ability of real-time assessment of the knee, elbow and ankle movements are presented. All the devices were tested in different selected tasks and its performance evaluated. Results have shown that these side-polished plastic optical fiber based devices are a valid technique and suitable to aid physicians in the diagnosis and rehabilitation of joint injuries, to monitor the performance of high competition athletes or even to quantify daily activities. The wearable sensor systems have properties such as low-cost, friendly operation and less effect to human which are important topics in biomechanics and clinical applications.