Plyometric training comprises one of the most widely used training methods in both individual and team sports, and is widely used by coaches as one of the main training in both adults, and children. Plyometric training highly includes the component of eccentric contraction. However, eccentric muscle action, especially when unaccustomed, can lead to exercise-induced muscle damage (EIMD), which is accompanied by increased delayed onset of muscle soreness (DOMS), inflammatory responses, increased levels of muscle proteins into the circulation, oxidative stress, and reduction of muscle function and performance the following days. Although plyometric training is widely used in children and may lead to EIMD, there is limited data regarding the acute effects of plyometric exercise training in children. Additionally, the effect of the biological maturation status of children on EIMD after acute plyometric exercise training has not been investigated. The aim of this study is to examine the effect of biological maturation on EIMD after acute plyometric exercise training in children.

Plyometric training comprises one of the most widely used training methods in both individual and team sports. Plyometric training has been shown to improve neuromuscular stimulation, jumping ability, muscle strength, flexibility, muscle mass and muscle performance, running speed and muscle power. Therefore, it is widely used by coaches as one of the main training methods, in both adults, and children).

Plyometric training consists of exercises performed through the stretch-shortening cycle of the muscle where the pre-activated muscle is first stretched (eccentric action) followed by the shortening (concentric) action. Therefore, plyometric training highly includes the component of eccentric contraction. However, eccentric muscle action, especially when unaccustomed, can lead to exercise-induced muscle damage (EIMD). EIMD, amongst others, is accompanied by increased delayed onset of muscle soreness (DOMS), inflammatory responses, oxidative stress, increased levels of muscle proteins and collagen into the circulation, and reduction of muscle function and performance.

The effect of acute plyometric training on EIMD, neuromuscular fatigue and performance has been adequately investigated in adults. Indicatively, an acute protocol of plyometric training increased DOMS, blood inflammatory markers, creatine kinase (CK) and lactate dehydrogenase (LDH) activity, while decreased jumping performance for up to 72 hours after the end of the training. In contrast, data regarding the effect of acute plyometric training on the above indices in children are scarce. Additionally, although some data exist on children versus adults, as far as we know, there is no relative data between children with different stages of biological maturation, regardless the common use of plyometric exercises in youth training. However, such data is crucial for both coaches and young athletes to effectively design the training microcycles and incorporate the training components, but also to reduce the risk of injury.

The aim of the present study is to examine the effect of different biological maturation on EIMD, metabolism, neuromuscular fatigue, oxidative stress, and muscle performance after acute plyometric exercise training in children.

According to a preliminary power analysis (probability error: 0.05, power: 0.80, effect size: 0.30), a total sample of 9 participants per group was considered appropriate in order to detect statistically meaningful changes between groups. Thus, twenty healthy male children, aged 8-15 years old, will participate to the study. Written informed consent will be provided by the parents or legal guardians of children after they will be informed about all risks, discomforts, and benefits involved in the study. The procedures will be in accordance with the 1975 Declaration of Helsinki, as revised in 2013. Approval has been received from the bioethics committee of the Department of Physical Education and Sport Science, University of Thessaly.

The study will be performed in a parallel, repeated measures design. The participants will visit the Department's Exercise Physiology laboratory 9 times in total. During the 1st visit, the participants will be examined by a pediatric endocrinologist in order to estimate their biological maturation and be assigned το a condition of either pre-adolescent or adolescent. A familiarization period with the plyometric training protocol and the evaluation procedures with low intensity will be induced (1st, 2nd, 3rd visit). During the next week (4th and 5th visit), fasting blood samples will be collected in order to estimate testosterone levels, CBC, muscle damage, and oxidative stress markers. Participants will be instructed by a dietitian how to record a 7-days diet recalls to ensure that they do not consume to some greater extent nutrients that may affect EIMD, oxidative stress, and fatigue (e.g. antioxidants, amino acids, etc.). Additionally, assessment of body mass, body height, BMI, body composition, sprint performance, jumping performance, isokinetic strength (concentric, isometric, eccentric), aerobic capacity, EIMD (CK) will be performed. After at least 3 days (6th visit), participants will perform the acute bout of plyometric exercise training, that is 8 sets of 10 maximal squat-jumps with a 2-min rest between sets. DOMS will be estimated prior to and post-training, and lactate concentration prior to, and 4 min after the 4th set and 4 min after the 8th set. Neuromuscular fatigue (maximal voluntary isometric contraction, MVIC) estimation will be performed prior to, and 1 hour, 2 hours, and 3 hours post-training. Additionally, DOMS will be estimated at post-, 24 hours, 48 hours, and 72 hours (7th, 8th, and 9th visit) post-training. Sprint and muscle performance, CBC, CK, and oxidative stress indices will also be estimated at 24 hours, 48 hours, and 72 hours post-training.

Inclusion Criteria:

• Normal BMI
• No history of growth irregularities
• No musculoskeletal injuries for at least six months prior to the study
• No use of drugs or ergogenic supplements for at least one month prior to the study
• No participation at exercise with eccentric component for at least three days prior to the study
• No energy drinks consumption before each experimental trial

Exclusion Criteria:

• Abnormal BMI
• History of growth irregularities
• Musculoskeletal injuries in the last six months prior to the study
• Use of drugs or ergogenic supplements in the last month prior to the study
• Participation at exercise with eccentric component in the last three days prior to the study
• Energy drinks consumption before each experimental trial