Description

The KidREx-study is an intervention study aiming to investigate the effects of two alternative forms of resistance exercise on adults and children. Exercise forms include leg press and leg extension, in either traditional weight stack machines or flywheel training machines. It has been proposed and discussed that resistance exercise would cause injury to the growth plate and even premature closure of the physis in children. Multiple studies have shown that this is not the case regarding growth height and radiography of the physis. However, the direct effects of resistance exercise on physis using magnetic resonance imaging are yet to be investigated.

This study is a randomized, controlled, prospective intervention study. Two separate groups (children and adults) are included, children ages 9 to 13 and adults ages 18-50. Participants are randomly assigned to one of six arms for the twelve-week intervention period. Arms include two control groups "living as usual" (children and adults), two flywheel training groups (children and adults), and two weight stack training groups (children and adults). Throughout the intervention period, the training groups conduct a total of 30 sessions (alternating between two or three sessions per week). Following the intervention period, all participants enter an 8-week detraining phase, where no organized resistance training is conducted.

At baseline, all participants conduct a test battery including vertical jump height, horizontal jump length, balance tests, and tests of muscle activity in the specific training machines and in an isokinetic dynamometer. In conjunction with the first exercise bout, venous blood samples are obtained from both adults and children, and skeletal muscle biopsies from m. vastus lateralis are obtained from the adults. This procedure is repeated after 12 weeks of training and following the 8-week detraining period. Outcomes related to this, blood samples and muscle tissue, beyond the fiber-specific hypertrophy, will be subject to ancillary studies.

The investigators hypothesize that weight stack training and flywheel training will result in muscle adaptation in both adults and children, where the rate of adaptation is faster for the flywheel training group. Expected muscle adaptation includes an increase in muscle hypertrophy, strength, and functional strength-related outcomes (balance, vertical jump height, and standing long jump). Additionally, the investigators hypothesize that the growth plate of pre- and intra-pubertal children will not be affected negatively by resistance exercise.