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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).