The aim of this study was to verify the linear and nonlinear (quadratic) relationships between BMI and lower limb strength in children and adolescents of both sexes in a region of Chile.
The results of this study have shown a greater non-linear (quadratic) relationship between BMI and HJ test in children and adolescents of both sexes. In addition, we identified that Z-score categorized schoolchildren with underweight and overweight reflected poor performance compared to schoolchildren categorized with normal BMI.
These findings are consistent with other studies conducted in various geographic regions of the world, which have shown quadratic (non-linear) relationships, clearly reflecting that schoolchildren categorized as underweight and overweight according to BMI induce negative effects on physical fitness levels in children and adolescents [17, 19, 20, 24] On the contrary, in the normal BMI category, the best results in HJ were observed in schoolchildren of both sexes.
In fact, it is widely known that subjects with excess weight (body fat) have their physical performance impaired due to the additional load or dead weight that acts negatively on physical performance [25]. On the contrary, in individuals categorized as underweight, it is possible that physical performance is affected due to a disproportion between muscle mass and total body weight [10]. Thus, both categories may have a detrimental impact on physical fitness, particularly lower limb strength among children and adolescents.
Therefore, screening for thinness, normality, overweight, and obesity are indicators of weight status that are often managed as key components of health in school physical education. Therefore, from our results, we emphasize that schoolchildren should reflect a normal BMI for their age and sex. This can guarantee, not only a better performance in lower limb explosive strength in sports practice [22, 26], but also in relation to health. For example, in cardiovascular diseases, metabolic profiles, skeletal health, body adiposity and in simple tasks of daily life [27–29].
In general, muscular fitness during childhood and adolescence has been identified as an important determinant of current and future health status [30]. Thus, a stronger, more resilient and powerful musculoskeletal system will enable children and adolescents to perform body movements more efficiently [31], especially during the performance of activities of daily living, in sports practice and in unforeseen emergencies [29, 32].
Consequently, lower extremity explosive strength has been widely used to assess muscular fitness and general health during physical education classes. Therefore, the functions and capacities of the neuromuscular and musculoskeletal systems play an important role in defining the physical fitness of individuals and populations [33] and for this, it is necessary to achieve an optimal nutritional status (BMI) according to their age and sex as observed in this study.
In general, in school populations where children and adolescents report low levels of physical performance regardless of their nutritional status, it is necessary to promote intervention programs among young people to encourage the maintenance of physical fitness, adaptation of healthy lifestyles, counteracting the stigma of excess body weight among young people. The WHO [34] has also recommended the practice of muscle and bone strengthening in young people, at least three times a week.
This study has some limitations. These have to do firstly with the cross-sectional design used in this study, since future research should develop longitudinal designs, through which, it is possible to verify the causal relationships, with which it is possible to confirm our findings. Secondly, it was not possible to evaluate other lower limb explosive strength tests (e.g., vertical jump), as this information would have allowed us to analyze our results in greater depth. Thirdly, there is a need for a Gold standard method to analyze body composition, as BMI does not have sufficient capacity to distinguish fat mass and/or fat-free mass.
We also emphasize that this research has some strengths, for example, it is one of the first studies carried out on a sample of Chilean schoolchildren. In addition, the type of sample selection allows us to generalize the results to other populations with similar characteristics.