Isokinetic testing of the musculoskeletal system is an important diagnostic method in the course of rehabilitation because it is adapted to individual and objective in measuring biomechanical parameters. Isokinetic dynamometer serves as a measurement and/or exercising device and helps user to achieve maximum force in through a range of motion, accommodating for potential pain or fatigue. Isokinetic dynamometer measures strength, torque, and generated power as well as the range of motion of the joint and distribution of strength in the range of motion. These results, as well as the relation between agonists and antagonists, acceleration time, velocity of motion reciprocity, maximum torque angle, and the fatigue/endurance index are computed and compared with standard values using software. Repeated testing shows changes in parameters in later stages, ensuring an accurate insight in the progress of rehabilitation. This paper describes the treatment of a 14-year-old boy engaged in rowing who suffered from suprapatellar pain and instability of the knee. After isokinetic testing and rehabilitation the boy resumed his sport activities.