Isotonic exercise occurs when the tension or torque generated by a muscle is constant throughout the movement. In practice it is very difficult to maintain the tension constant. Evaluation of isotonic exercise should include the works of concentric and eccentric contraction and the changes in tension and torque during joint motion. However, optimal evaluation tools for isotonic exercise has yet to be developed. We authors, used virtual reality motion analysis system(VRMAS) which was codeveloped by us and EMG system(Cardwell Excel). Healthy male adult subjects(n=10) were recruited from hospital personnel. After dominant upper arm and trunk of each subject was fixed with velcro, he was instructed to repeat flexion and extension of his dominant elbow grasping dumbell (9 kg) in hand with maximal velocity as possible as can until the point of exhaustion. We measured the elbow angle, the angular velocity, torque, power, total work and the work of concentric and eccentric contraction during exercise. The results were as follows: there were four distinct elbow flexor muscle contractions during flexion and extension of the elbow with the first and the second contractions during concentric contraction and the third and the fourth contractions during eccentric contraction. Between the peaks of contraction, motion was maintained due to the momentum that was generated. Of the ten subjects, seven had higher concentric work than eccentric work with the eccentric work being higher in the remaining three. But there was no statistic significance between the works of concentric contraction and eccentric contraction(p>0.05). The biggest work was observed in the fourth contraction, while the third contraction showed the least work(p<.05). In conclusion, there were four distinct muscle contractions during elbow isotonic exercise and the works of each muscle contraction were different. This result is contrary to the definition of isotonic exercise. This study also shows that the VRMAS could be a very useful evaluation tool for several types of isotonic exercise.