Purpose: To examine serum creatine kinase (CK) activity after resistance exercise bouts with different rest intervals between sets and exercises in high responding (HR) and normal responding (NR) subjects. Methods: During each resistance exercise bout, three sets with 10-repetition maximum (10RM) loads were completed for the chest press, cable pulldown, biceps curl, triceps extension, leg extension, and prone leg curl. Each bout differed in the length of the rest interval between sets and exercises, specifically either 1 or 3 min. After blood analysis, subjects were separated into NR or HR on the basis of the peak serum CK activity being in the 90th percentile. Results: The volume completed (load × sets × repetitions) was significantly greater for the 3-min bout versus the 1-min bout, with no significant differences between the HR and the NR groups. For the NR group, serum CK was significantly elevated from 24 to 72 h after each bout, with no significant differences between bouts. Conversely, for the HR group, the 1-min bout resulted in serum CK activity levels that were approximately 70% greater than the 3-min bout at the 48- and 72-h time points. Conclusions: The key finding from the current study was that the HR group experienced significantly greater CK responses when using shorter rest intervals between sets. Conversely, for the NR group, CK responses were not significantly different between bouts. These findings may have implications for resistance exercise prescription in that some individuals might be less tolerant of shorter rest intervals between sets with greater skeletal muscle microtrauma.

Creatine represents a natural supplement and ergogenic aid for sport performance, but there are several concerns regarding its safety for health. The present double-blind placebo-controlled study evaluated the effect of creatine monohydrate supplementation on a panel of blood and urine health indicators in resistance training practitioners. Eighteen males performing resistance training three times per week were supplemented with 0.3 g/kg per day creatine monohydrate for 7 days and compared with matched controls supplemented with dextrosol. Blood and urine samples were collected pre- and 30 days post-supplementation to evaluate 41 biochemical parameters and renal function. Creatine monohydrate supplementation did not cause adverse events and, as expected, promoted an increase of the performance and body weight. No modification of red blood cells parameters, white blood cells profile, blood lipid profile, metabolic and urine markers, hepatic and renal function were observed in the supplemented group. Despite the expected weight increase, the creatine monohydrate supplementation is safe for health and no detrimental effects on different organs and physiological systems were observed in our cohort of volunteers.