Background. In most sports disciplines, the level of training and the capacity for exertion is dependent on the anabolism of proteins. For this reason there has been growing interest in supplements, especially hydroxymethylobutyrate (HMB). The purpose of our research was to test the impact of isometric training and HMB supplementation on muscle strength and altered body composition in young men. Material and methods. Our experiment involved 30 physical education students who had not previously been in training for high-exertion sports. The subjects were divided into two groups: the experimental group, with isometric training and HMB supplementation, and the control group (K), with similar training and a placebo. After baseline measurement of strength and speed parameters, isometric muscle strength training was implemented. The measurements were repeated after the completion of training. Results. After 4 weeks of isometric training both groups had attained significant improvements in the results of the training exercises, but the improvements were consistently and significantly higher in the experimental group. In respect to changes in body composition there was significantly greater loss of fat gain of active body mass (LBM) in the experimental group. Conclusions. The use of HMB supplementation significantly improves the effectiveness of isometric training in the process of increasing muscle strength. Combining isometric training with HMB support has a significant favorable impact on changes in body composition. These changes should be regarded as favorable and desirable not only for athletes, but also for persons engaging in recreational exercising or under rehabilitation.