Material and method
The project has been approved by the Regional Committee for Medical and Health Research Ethics (case number 2016/639). We invited medical students at NTNU to participate in the study through emails and information given after a lecture. The participants were required to have no known heart, lung or kidney disease, and to not be using statins. They were instructed to abstain from strength training for one week prior to the workout, and to abstain from all exercise for the three days leading up to the experiment. Thirty volunteers signed up, six of whom withdrew before the experiment. Twenty-four participants (14 women) performed the exercise intervention, completed questionnaires, and provided blood and urine samples. Written consent was obtained from all participants.
One high-intensity 'Tabata'-type workout was performed, lasting approximately 50 minutes. The workout began with 10 minutes of general warm-up. Strength and cardiovascular training were then performed on many different muscle groups with high intensity and short breaks (20 seconds of activity, 10 seconds rest). Each exercise was repeated six times before a one-minute break and change of exercise, with eight exercises in total. Finally, a competition was held to stand against a wall for as long as possible with 90-degree hip and knee flexion ('wall sit'). The participants were asked to drink plenty of fluids after the workout.
The day before and four days after the workout, blood and urine samples were taken to obtain baseline values and maximum values (9). Sampling and analysis of CK and creatinine in blood, as well as urine dipstick testing, were performed by the Department of Medical Biochemistry, St. Olavs Hospital, Trondheim University Hospital. Urine dipsticks positive for blood were used as an indirect index of myoglobinuria.
Before the workout, the participants completed a questionnaire on how often they engaged in strength training and on their regular medication use. On day four, the participants completed a questionnaire on muscle pain after the workout.
Statistical analyses were performed in SPSS Statistics version 25 (IBM SPSS Inc., Chicago, IL). A p-value < 0.05 was considered significant. Normally distributed data are presented as mean (standard deviation), non-normally distributed data as median (interquartile range). A student's t-test for repeated measures was used for comparison of creatinine before and after exercise, and a Wilcoxon signed rank test for comparison of CK levels before and after exercise. Spearman's rank correlation was used to analyse the association between previous exercise history, pain and CK increase.
All participants with CK levels > 5 000 IU/l after exercise were offered a follow-up measurement. Eight of 14 completed the follow-up, all of whom showed a decrease in CK levels.