Physical fitness during childhood and adolescence has been identified as an important determinant of current and future health status. While research has traditionally focused on the association between cardio-respiratory fitness and health outcomes, the association between muscular fitness (MF) and health status has recently received increased attention.

The aim of this systematic review and meta-analysis was to evaluate the potential physiological and psychological benefits associated with MF among children and adolescents.

A systematic search of six electronic databases (PubMed, SPORTDiscus, Scopus, EMBASE, PsycINFO and OVID MEDLINE) was performed on the 20th May, 2013. Cross-sectional, longitudinal and experimental studies that quantitatively examined the association between MF and potential health benefits among children and adolescents were included. The search yielded 110 eligible studies, encompassing six health outcomes (i.e., adiposity, bone health, cardiovascular disease [CVD] and metabolic risk factors, musculoskeletal pain, psychological health and cognitive ability). The percentage of studies reporting statistically significant associations between MF and the outcome of interest was used to determine the strength of the evidence for an association and additional coding was conducted to account for risk of bias. Meta-analyses were also performed to determine the pooled effect size if there were at least three studies providing standardised coefficients.

Strong evidence was found for an inverse association between MF and total and central adiposity, and CVD and metabolic risk factors. The pooled effect size for the relationship between MF and adiposity wasr = −0.25 (95 % CI −0.41 to −0.08). Strong evidence was also found for a positive association between MF and bone health and self-esteem. The pooled effect size for the relationship between MF and perceived sports competence was r = 0.39 (95 % CI 0.34–0.45). The evidence for an association between MF and musculoskeletal pain and cognitive ability was inconsistent/uncertain. Where evidence of an association was found, the associations were generally low to moderate.

The findings of this review highlight the importance of developing MF in youth for a number of health-related benefits.

Alex's notes: 

Muscle is metabolic currency. This soon to be famous quote from Carl Lanore has been proven in the science continuously. As highlighted in this review, children exhibit a gradual linear increase in muscular strength and muscular power from 3 years of age until puberty, and these changes are closely associated with changes in body size and fundamental movement skill aptitude. After this time, boys show a dramatic acceleration of muscular strength until the age of 17 and beyond, and girls show a pronounced plateauing and regression in late adolescence and beyond. Not surprisingly, a stronger and more powerful musculoskeletal system will enable children and adolescents to perform bodily movements more efficiently and effectively.

Despite the above, much of the focus on youth health has been centered on cardiorespiratory fitness, and the purpose of this review is to examine the relationship between musculoskeletal fitness and the range of health benefits in youth. No doubt cardiorespiratory fitness is an important factor in health, but muscular fitness has been directly linked to all-cause mortality, reduced adiposity, increased insulin sensitivity, bone health, and psychological health and academic performance. Many of these benefits are independent of cardiorespiratory fitness to boot.

Based on the 110 studies included in the meta-analysis, there is strong evidence for a positive association between musculoskeletal fitness and bone health and self-esteem, although the associations are low to moderate, and there is strong evidence of an inverse association between musculoskeletal fitness and total and central adiposity, and cardiovascular disease and metabolic risk factors, although the associations are also low to moderate. Youth is the time to set the stage for the rest of your life. It is when the foundation is laid. How strong is your foundation? Muscle = metabolic currency. For those of you working with children, they can improve strength by 30% to 50% after just 8 to 12 weeks of a well-designed strength training program, and need to continue to train at least 2 times per week to maintain strength.