Muscle Adaptations in Children and Adults Following Twelve Weeks of Flywheel or Weight Stack Resistance Exercise

The goal of this trial is to compare resistance exercise using traditional techniques and flywheel training to controls in healthy children ages nine to 13 and adults ages 18 to 50. The main questions it aims to answer are:

* The degree of response to resistance exercise in children compared to adults regarding muscle hypertrophy and strength?
* Is there a negative impact on the physis of children due to resistance exercise?

Participants will be randomized to one of three groups, control, traditional resistance exercise, or flywheel. Following a baseline battery of tests, they will enter a 12-week training period, using leg press and leg extension. Additionally, post-training tests are conducted, and an eight-week detraining period commences followed by new tests. Researchers will compare children and adults and each intervention arm to see if children can achieve muscle hypertrophy, the degree and rate of changes in strength, and the potential impact on the physis.

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.

Eligibility Criteria:
Inclusion Criteria:

* Adults at the age of 18 to 50 at time of inclusion.
* Children at the age of nine to 13 (attending Swedish middle school) at time of inclusion

Exclusion Criteria:

* Neuromuscular disease.
* Previous or current musculoskeletal injury, primarily of the lower limb, preventing resistance exercise.
* Heavy resistance training of the lower limb at inclusion (i.e., more than one session per week of weight training of the lower limb or other equivalent activity)