The change in creatinine concentration in the blood serum of athletes of the examined groups using free weight exercises (A) and machine exercises (B) during the study, 
muscle mass
1. compared to the indicators before exercise; 2. compared to the indicators before the study; *The results analysis shows that the initial measurements of creatinine concentration in the blood serum decreased in athletes of all groups in response to a stress stimulus. Thus, in representatives of groups 3 and 4, the concentration of the studied biochemical indicator shows an average 2-fold decrease in response to high intensity training load in the conditions of anaerobic-alactate energy supply mode, compared to the results recorded in bodybuilders of groups 1 and 2. At the same time, there was no significant difference between the effect of using machine or free weight exercises on the parameters of the studied biochemical indicator.After 12 weeks of training, the creatinine concentration demonstrated changes in athletes of all examined groups in response to a stress stimulus. Thus, its concentration increased by 7.6% (<0.05) on average in athletes of all groups. The basal creatinine level was 3.7 times higher in athletes of groups 3 and 4. These changes in the basal level of creatinine indicate an accelerated growth of muscle mass in athletes of groups 3 and 4.