Background No previous systematic review has quantitatively compared the effects of resistance training, endurance training, or concurrent training on hormonal adaptations in children and adolescents. Objective was to examine the effects of exercise training and training type on hormonal adaptations in children and adolescents. Methods A systematic literature search was conducted in the following databases: PubMed, Web of Science, and EBSCO. Eligibility criteria were: population: healthy youth population sample (mean age < 18 years); intervention: resistance training, endurance training, or concurrent training (> 4 weeks duration); comparison: control group; outcome: pre- and post-levels of hormones and cytokines; and study design: randomized and non-randomized controlled trials. We used a random-effect model for the meta-analysis. The raw mean difference in hormones from baseline to post-intervention was presented alongside 95% confidence intervals (CI). Further, the certainty of evidence quality and the risk of bias were assessed. Results A total of 3689 records were identified, of which 14 studies were eligible for inclusion. Most studies examined adolescents with fewer studies on children (age < 12 years, N = 5 studies) and females (N = 2 studies). Nine exercise training programs used endurance training, five studies used resistance training, and no eligible study used concurrent training. The meta-analysis showed no significant effect of exercise training on testosterone (MD = 0.84 nmol/L), cortisol (MD = − 17.4 nmol/L), or SHBG (MD = − 5.58 nmol/L). Subgroup analysis showed that resistance training significantly increased testosterone levels after training (MD = 3.42 nmol/L) which was not observed after endurance training (MD = − 0.01 nmol/L). No other outcome differed between training types. Exercise training resulted in small and non-significant changes in GH (MD = 0.48 ng/mL, p = 0.06) and IGF-I (MD = − 22.90 ng/mL, p = 0.07). GH response to endurance training may be age-dependent and evident in adolescents (MD = 0.59 ng/mL, p = 0.04) but not when children and adolescents are pooled (MD = 0.48 ng/mL, p = 0.06). Limited evidence exists to conclude on IL-6 and TNF-α effects of exercise training. Assessments of GRADE domains (risk of bias, consistency, directness, or precision of the findings) revealed serious weaknesses with most of the included outcomes (hormones and cytokines). Conclusions This systematic review suggests that exercise training has small effects on hormonal concentrations in children and adolescents. Changes in testosterone concentrations with training are evident after resistance training but not endurance training. GH's response to training may be affected by maturation and evident in adolescents but not children. Further high-quality, robust training studies on the effect of resistance training, endurance training, and concurrent training are warranted to compare their training-specific effects. Registration: PROSPERO: CRD42021241130.

Purpose:In cross-country (XC) skiing, the ability to use an efficient technique is essential for performance. The study aimed to compare the effects of supplemental static or dynamic core strength training on skiing economy in elite junior XC skiers. Methods: Twenty-four elite junior XC skiers (14 women, 10 men; 17.8 ± 1.1 years; 67.8 ± 10.0 kg, 173.7 ± 6.4 cm) participated in this study. Participants were allocated either to a static core training (ST) group (n =12) or to a dynamic core training (DT) group (n = 12). Both groups continued their normal aerobic endurance and muscular strength training. Experimental groups performed a 15 minutes, 3 days/week core strength-training program for 9 weeks and in addition to their training. Submaximal and maximal roller ski testing was conducted before and after the 9-week training period. Results: Results showed no significant interaction between groups for energetic costs in any of the submaximal workloads (first, p = .33; second, p =.79; third, p = .25). Pooled data showed a significant improvement in energetic cost pre-to posttesting in the first and third workload (ES 0.40, p = .0006 and ES 0.42, p = .04 respectively). Nine weeks of static or dynamic core strength training in elite junior XC skiers had a small effect on energetic cost in submaximal roller skiing. Conclusion: The type of supplemental core strength training does not seem to affect economy in submaximal roller skiing.