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Background: Behaviour-based physical intensity evaluation requires rigorous calibration before application in long-term recordings of children's sleep/activity patterns. This study aimed at (i) calibrating activity counts of motor behaviour measured simultaneously with MotionWatch 8 (MW8) and ActiGraph (GT3X) in 3-year-old children, (ii) documenting movement intensities in 30s-epochs at wrist/hip positions, and (iii) evaluating the accuracy of cut-off agreements between different behavioural activities.

Methods: Thirty 3-year-old children of the NorthPop cohort performed six directed behavioural activities individually, each for 8-10 minutes while wearing two pairs of devices at hip and wrist position. These naturally-occurring behaviours were aligned to movement intensities from 'motionless' (watching cartoons) and 'sedentary' (recumbent story listening, sit and handcraft) to 'light activity' (floor play with toys), 'moderate activity' (engaging in a brisk walk) and 'vigorous activity (a sprinting game). Time-keeping was ensured using direct observation by an observer. Receiver-Operating-Curve classification was applied to determine activity thresholds and to assign two composite movement classes.

Results: Activity counts of MW8 and GT3X pairs of wrist-worn (rho = 0.94) and hip-worn (rho = 0.90) devices correlated significantly (p < 0.001). Activity counts at hip position were significantly lower compared to those at the wrist position (p < 0.001), irrespective of device type. Sprinting, floorball/walk and floorplay assigned as 'physically mobile' classes achieved outstanding accuracy (AUC > 0.9) and two sedentary and a motionless activities assigned into 'physically stationary' classes achieved excellent accuracy (AUC > 0.8).

Conclusion: This calibration provides useful cut-offs for physical activity levels of preschool children. Contextual information of behaviour is advantageous over intensity classifications only, because interventions will focus on behaviour-allocated time to reduce a sedentary lifestyle. Our comparative calibration is one step forward to behaviour-based movement guidelines for 3-year-old children.

Background: This study investigated the associations of muscular strength measures with anthropometry, chronological age, biological maturation, and training experience in trained prepubertal and pubertal males. Another aim was to investigate if handgrip strength can predict general or overall muscle strength in the same population.

Method: Forty-one (n = 41) trained male children and adolescents aged 10–16 participated in the study. The 10-repetition maximum (RM) leg press and bench press were used to assess upper- and lower-body muscular strength, handgrip strength was used as an overall strength assessment, and a countermovement jump with arm swing (CMJa) was used to estimate extensor muscle power of the lower extremity. The maturity status was determined using the Tanner scale. Anthropometric factors included height, body mass, two skinfolds, limb length, and lean leg volume. Multivariable linear regressions were performed on absolute strength values to explore predictors of muscular strength and power.

Results: Body mass explained 81% of the variance in leg press strength (p < 0.001), whereas bench press was associated with body mass and chronological age, explaining 83% of the variance (p < 0.001). The countermovement jump (CMJa) height was positively associated with lean leg volume, which explained 52% of the variance (p < 0.001). Chronological age and fat-free mass explained 87% of the variance in handgrip strength (p < 0.001). Biological maturity (Tanner) did not contribute to the final models. Handgrip strength was strongly associated with total muscle strength (r = 0.89–0.91, p < 0.001).

Conclusion: The results indicate that anthropometrical factors, rather than biological maturity, are associated with muscular strength in trained male children and adolescents. Our findings suggest that handgrip strength may be a quick and effective screening tool for assessing total muscle strength in youth.

Purpose: Examine the acute hormonal and cytokine responses to free-weight resistance training in trained prepubertal and pubertal male children.

Methods: Prepubertal (n = 21; age 11.4 ± 1.1 years; Tanner I–II) and pubertal male children (n = 20; age 15.8 ± 0.7 years; Tanner III–V) conducted a moderate-intensity free-weight resistance training program to failure with venous blood sampling before (pre), immediately after (post) and during the recovery phase of the program (post-15,-30 min). Growth hormone (GH), insulin-like growth factor-I (IGF-I), cortisol, testosterone, IL-6, and TNF-α were analyzed in serum samples. Biological maturation was assessed according to the stages of the Tanner scale.

Results: There was a significant time-by-group interaction in IGF-I response (p = 0.044; η2 = 0.209) and testosterone (p < 0.001; η2 = 0.508), indicating a greater change in the pubertal group compared to the prepubertal group. Both groups significantly increased post-exercise GH levels (p < 0.05). Only the prepuberal group significantly increased levels of IL-6 at all post-exercise time points (p < 0.05). Both groups showed a significant (p < 0.05) increase in TNF-α levels compared to resting levels.

Conclusion: These data suggest that acute testosterone and IGF-I response following resistance training differ between trained prepubertal and pubertal male children. Moderate-intensity resistance training performed to failure may thus have different effects in trained prepubertal and pubertal male children, which should be considered when giving training advice. Trial registration: Clinical trials number: NCT05022992.

Background: Regular physical activity during childhood and adolescence can positively affect overall cardiometabolic health, fundamental motor skill development, bone density, quality of life, and psychological well-being. Research on physical activity in children is growing continuously, and one area is focused on assessing children's physical activity. Advances in wearable technology have provided more reliable tools for assessing physical activity, particularly in young children. These wearables must be calibrated to age-specific groups, body positions, and epoch times. Furthermore, muscle strength is an important health indicator in children; however, little is known about how muscle strength is influenced by age, maturity, hormones, and cytokines in pediatric populations. This thesis aimed to examine methods to estimate physical activity in children, understand which factors are associated with muscular strength in trained male children, and increase our understanding of how exercise-related hormones and proinflammatory cytokines (IL-6 and TNF-α) adapt to acute and long-term training. 

Methods: This thesis included four studies. Paper I aimed to calibrate two accelerometer devices, MotionWatch 8 (MW8) and ActiGraph GT3X ( GT3X), worn on the hip and wrist (n = 30), and to develop age-specific cut-offs for physical activity intensities in 3-year-old children. Paper II was a cross-sectional study that examined the associations of muscular strength measures with anthropometric factors, chronological age, maturation, and training experience in trained prepubertal and pubertal males (n = 41). Another aim of Paper II was to examine whether a handgrip strength test can predict the total muscle strength assessed with whole-body free-weight exercises. Paper III was an intervention study that examined acute hormonal and cytokine responses to free-weight resistance training in trained prepubertal and pubertal male children (n = 41). Paper IV was a systematic review and meta-analysis that assessed the evidence of the effects of exercise training and training type on hormone and cytokine adaptations in children and adolescents. 

Results: There was a strong correlation between MW8 and the GT3X device (counts/30 s) at both hip and wrist levels (Paper I). The devices' cut-off scores for physical activity levels were classified with outstanding and excellent accuracy (Paper I). The cross-sectional study showed that muscular strength tests in trained male children are mostly associated with anthropometric factors, which differ depending on the exercise test chosen (Paper II). Furthermore, the handgrip strength test was strongly associated with total muscle strength in trained male children (Paper II). A single resistance training session induced greater acute post-exercise testosterone and IGF-I levels in pubertal children than in prepubertal male children (Paper III). Post-exercise IL-6 levels were significantly increased only in the prepubertal group. Lastly, the systematic review and meta-analysis showed that long-term exercise training had a small effect on resting hormonal concentrations. Resistance training, but not endurance training, increased resting testosterone levels in healthy children and adolescents (Paper IV). 

Conclusions: Measuring and classifying physical activity levels in preschoolers can be achieved accurately using MW8 or the GT3X device (Paper I). Another finding was that anthropometric measures such as body mass and fat-free mass are important factors associated with muscle strength, and they may be used to scale muscle strength scores to provide a fair interpretation across children of different body sizes (Paper II). A simple handgrip strength test could be a quick and effective screening tool for practitioners and researchers to estimate total muscle strength in trained male children (Paper II). Furthermore, pubertal children were stronger than prepubertal children and had greater post-exercise IGF-I and testosterone responses following a single resistance training session (Paper III). Finally, the systematic review and meta-analysis suggested that exercise training had a small effect on hormonal concentrations in healthy children and adolescents (Paper IV). 

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.

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.

Children recover faster than adults in repeated sprints, but it is unclear if their aerobic responses differ. Purpose: This study tested the hypothesis that aerobic response (VO2) during repeated sprints is greater in preadolescent boys than in men. Further, this study compared normalization with conventional ratio-scaling and scaling with the use of body mass (BM) as a covariate. Methods: Nine boys (age: 11.8 ± 0.6 years, swimmers) and 11 men (age: 21.7 ± 0.6 years, recreational athletes) performed 10 maximal 6-s cycling sprints separated by 24-s of passive recovery, against two loads (optimum and 50% of optimum). Oxygen uptake (VO2) was measured continuously. Results: Men’s mean power output (MPO) was greater than boys in each sprint, both in absolute (unscaled) values (p < 0.05) and when adjusted for lean leg volume (p < 0.05). Children had lower absolute VO2 (p < 0.05) than men, but when it was adjusted for BM or power-output, VO2 was comparable between men and boys. Thus, most of the difference in VO2 between men and boys was due to body size and power-output differences. Conclusion: Our results suggest that men and boys have similar VO2 during repeated sprints when appropriately adjusted to body mass or power output. Results highlight the importance of using appropriate scaling methods to compare adults’ and children’s aerobic responses to high-intensity exercise.