Resistance exercise training (RET) in children and adolescents has become a popular area of research, with a growing body of evidence supporting its use. Position and consensus statements about RET for children indicate that it is safe and effective at increasing muscular strength, improving sport performance, and mitigating injury risk. Neural and muscular mechanisms can improve muscle strength following RET. Neural factors include improved recruitment and firing of an individual's motor units, and muscular factors primarily include an increase in the size of the muscle (hypertrophy).

In children, little is known about how these mechanisms relate to muscle strength. There is very little evidence of morphological changes following RET in children. Therefore, conventional wisdom is that children rely only on neural factors to improve strength following RET. Nevertheless, some studies have suggested RET-induced muscle hypertrophy in children and adolescents, indicating that with certain training protocols, children may achieve muscle growth.

Hypertrophy of muscle fibres occurs when the rate of muscle protein synthesis (MPS) is greater than the rate of protein breakdown, and is enhanced with the ingestion of dietary amino acids. Due to ethical concerns with obtaining muscle samples (i.e., from muscle biopsies) in pediatric populations, MPS rates have not been previously assessed following RET in children. Recent advancements in stable-isotope methodology (specifically, leucine) allow for the estimation of MPS in a non-invasive breath test.

The objective of the proposed research is to examine the effects of an acute bout of RET on leucine retention (a proxy for MPS) in children, adolescents, and adults using a non-invasive breath test.

Resistance exercise training (RET) in children and adolescents has become a popular area of research, with a growing body of evidence supporting its use. Position and consensus statements about RET for children indicate that it is safe and effective at increasing muscular strength, improving sport performance, and mitigating injury risk. Neural and muscular mechanisms can improve muscle strength following RET. Neural factors include improved recruitment and firing of an individual's motor units, and muscular factors primarily include an increase in the size of the muscle (hypertrophy).

In children, little is known about how these mechanisms relate to muscle strength. There is very little evidence of morphological changes following RET in children. Therefore, conventional wisdom is that children rely only on neural factors to improve strength following RET, possibly due to their lower levels of circulating androgens. Nevertheless, some studies have suggested RET-induced muscle hypertrophy in children and adolescents, indicating that with certain training protocols, children may achieve muscle growth.

Hypertrophy of muscle fibres occurs when the rate of muscle protein synthesis (MPS) is greater than the rate of protein breakdown, and is enhanced with the ingestion of dietary amino acids. Due to ethical concerns with obtaining muscle samples (i.e., from muscle biopsies) in pediatric populations, MPS rates have not been previously assessed following RET in children. Recent advancements in stable-isotope methodology allow for the estimation of MPS in a non-invasive breath test, which is based on the retention of an essential amino acid (i.e., leucine) that is preferentially metabolized within skeletal muscle. Given that amino acids can only be 'stored' in functional body proteins with any excess being converted to energy (i.e., oxidized), this non-invasive technique is ideal to safely estimate the anabolic (i.e., growth) potential of RET in children. Indeed, similar techniques using ingested stable isotopes have been safely and successfully used in children and adolescents.

The objective of the proposed research is to examine the effects of an acute bout of RET on leucine retention (a proxy for MPS) in children, adolescents, and adults using a non-invasive breath test. It is hypothesized that following a bout of RET, protein retention will be augmented in all groups compared to a non-exercised condition. However, due to a greater basal rate of leucine retention (i.e., for growth processes), the RET-induced increase will be relatively lower in the children and adolescents.

Design: The proposed study will use a cross-sectional design that will compare leucine retention at rest, as well as following a bout of resistance exercise in children, adolescents, and adults.

Procedures: Participants will be required to make three visits to the Applied Physiology Laboratory at Brock University for the completion of all procedures.

During visit 1, the participants and their guardian(s) (if under the age of 18) will be informed of all tests and procedures that will take place over the duration of the study. Written and verbal consent will be obtained from the participants and their guardians (in the case of children and adolescents) before proceeding with further questionnaires. Questionnaires screening for medical concerns, pubertal stage, athletic training practices, menarche and oral contraceptive use (for adolescent and adult females), habitual eating habits, and habitual physical activity will then be completed. Anthropometric measurements will also be performed (i.e., height, seated height, body fat composition, skinfold thickness, limb lengths, limb circumferences, muscle cross sectional area). Following questionnaires and anthropometric measurements, participants will be familiarized with the exercises to be performed in their RET session. Each participant's strength will also be tested for these exercises.

Visits 2 and 3 will serve as the experimental sessions and will consist of a RET and a rest session, done in a counterbalanced order. For these sessions, participants will arrive fasted in the morning, and will be provided with a standardized breakfast and beverage. At the beginning of the experimental sessions, participants will have their breath collected. This will be followed by RET in one of the sessions, or continued rest. A dose of L-[1-13C]leucine isotope will then be consumed based on each participant's body mass (within a protein and carbohydrate beverage) and the breath of each participant will continue to be collected for the next 5 hours (total of 11 breath samples). A small snack will be given to each participant during this breath collection period.

Resistance Exercise: Strength testing and the training protocol will involve the following exercises which will all be performed using strength training machines. The use of such equipment is considered safer than the use of free weights as there are less degrees of freedom during limb movements, preventing technical failure prior to muscular fatigue. The following exercises will be performed: seated leg press, seated overhead press, seated chest press, seated horizontal row. Exercises will be performed with techniques described by the Canadian Society for Exercise Physiology (CSEP - PATH, 2021).

The RET protocol will comprise of 3-5 sets of each exercise at 70-80% of 1RM. Each exercise will be performed until failure (i.e., until the participant can no longer complete the movement with safe technique), as this has been shown to be optimal for stimulating MPS and eliciting muscle hypertrophy. Approximately 8-15 repetitions are expected to be completed per set and ~90s of rest will be given between sets. A similar acute RET protocol has previously been shown to be a sufficient stimulus to elicit differences in leucine retention compared to rest (tested using the non-invasive breath test that will be used in the proposed study) in adults.

The participants in the proposed study will be male and female prepubertal children (~7-12 years), adolescents (~13-16 years), and adults (18-35 years).

Inclusion Criteria:

• healthy
• free of injury that would prevent resistance exercise

Exclusion Criteria:

• consumed any medications in the past year which may affect muscle function
• had an injury in the past 6 months that would limit the movements required for the protocols
• been told that has diabetes
• been told that had a heart problem
• been told that have a breathing problem (e.g., asthma)
• been told that sometimes experience seizures
• had joint instability or ongoing join chronic pain
• been told that had kidney problems
• had stomach problems such as ulcers
• experience prolonged bleeding after a cut

Data will only be available to researchers directed involved in the study