Water polo is one of the largest collective sport which has been considered an Olympic sport since the early beginnings of the modern Olympic Games. Throughout the course of years, its rules have changed in such a way that led to a change in the training process. From a kinesiology point of view, water polo is a complex sport with a large presence of cyclic and acyclic movements for which the shoulder joint is most prominent. Due to large load, the shoulder joint is frequently pained. The purpose of this research was to determine whether there’s a connection between the internal and external rotators (UVR) and the frequency of shoulder injury in water polo. In addition, the study was to establish whether there is a strong reliability on isokinetic tests of the shoulder joint as well as whether there are differences in the UVR ratio when comparing water polo players with swimmers and healthy individuals. With said intent, a standardised two-speed testing was conducted on an isokinetic device Cybex 300. There were 100 participants in the test, all of which were divided into three groups. The first group was constituted of professional water polo players of the Croatian National First League. The second group consisted of 25 individuals, swimmers of the Croatian National League, while the third group was constituted of 25 healthy individuals who weren’t professional athletes and were of gender and age as the athletes. On the basis of the concluded study, a conclusion can be made that the isokinetic dynamometer is a reliable instrument in measuring the force and endurance of different muscle groups. While preforming tests on such devices, much caution is required due to the fact that the wrong posture of the participant as well as not following protocol can lead to wrong results. This is especially expressed when testing the shoulder joint which is submissive to such errors because of its structure. The obtained results show that there exists a statistically significant difference in the muscle power of the internal and external muscle rotators in water polo players compared to the other two research groups. While observing the muscle force of the external rotators of the dominant hand, no statistically significant difference was found when comparing the produced strength of the group of water polo players and the group of swimmers. However, the control group which consisted of healthy individuals did show a significant difference in the produced strength compared to the two aforementioned groups, which leads us to the conclusion that the difference is caused by intensive swimming present in both swimmer’s training and water polo player’s training. As far as internal rotator muscles are concerned, a statistically significant difference was found when comparing the swimmers’ group and water polo players’ group. The values for the external rotators of the subdominant hand obtained in a test with an angular velocity of 60 units of an angle per second were much higher for the water polo players compared with the control group, but were not much different from the values of the swimmers. The values for the internal rotators of the subdominant hand obtained in a test with an angular velocity of 60 units of an angle per second were different for the water polo players’ group and the control group, but there wasn’t a significant difference with the water polo players’ values in comparison with the swimmers’ values. The results have shown that there is a difference in the ratio of the antagonistic muscle groups; that is the internal and external rotators of the dominant hand of water polo players in comparison to other tested groups. A direct connection between the isokinetic parameters and injury was not found, except in the case of ‘beka’ and ‘centra’ where a statistically significant relationship between the injury and disrupted ratio was found, but taking age into consideration.