BMI stands for Body Mass Index. It is a correlation of person’s weight to his/her height. BMI is an attempt to standardize and to define deviations from “normality” and to put the person whether adult or child in one of the categories. The categories are “underweight”, “overweight”, “obese” and “normal”.
BMI was developed to help to work with statistical data for large groups, and for fast and easy identification of people in one of the risk-for-health groups.
While it might work to collect data and to determine general trends, help insurance companies to justify reimbursement for treatment etc. BMI is being less helpful on individual basis especially while measuring athletes and to-be athlete population.
Carl Lavie has written that, "The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals."
The problem of BMI is that tool is too generalized, and it is based on “normal”, “average” population data. It is especially critical for kids, because kids grow and develop with various speeds and averaging measurements and BMI standards would be somewhat non-informative in terms of children development. Expecting average, normal BMI values in children that are being selected to do competitive sports, which is most of the times is based on abilities, assumed genetics (by looking at parents to determine how the future athlete will develop) and other specific sport related factors is somewhat impractical and most of the times will show way different finding from expected. The competitive sports are not for “average”, people become champions in because of various factors including genetics.
As an example, elite body builders and elite basketball players would be on opposite ends of the BMI spectrum. Both will be athletic, healthy and with no more risk factors than the average person of their age. But bodybuilder most likely would be in “overweight”, “obese” and basketball player most likely would be “underweight” category. Only because math will show that bodybuilder and his body weight is too significant compared to his height and the same math will show the opposite in basketball player’ weight to height ratios.
In conclusion, every case is individual. More specifically, a child’s development depends on many factors like genetics (how tall, what body composition the parents carry in their genes), nutrition (what someone eats), activities (what someone does physically, how active the child is) and many other aspects. Parents, healthcare practitioners, teachers and other should not blame physical activities, including competitive sports for child not looking like his/her peers, being taller or shorter, heavier or skinnier than children in their peer group, this judgement is too premature. Every child is an Individual and that the main factor in determining if the child is developing the “right” way.
P.S.
While discussing that children are too tiny and to skinny when they see kids in gymnastics pre-teams and team, everyone should understand that kids are undergoing natural and special selection before and during team competitive gymnastics. It is easier to progress and learn skills in gymnastics if your body is shorter and lighter. That fact allows kids to stay more involved during practices and help them to have motivation to stay with the sport despite hard work, that fact also decreases the rates of injuries in the future. Average gymnast will always be shorter and skinnier that average person of the same age because he/she would be, most likely, shorter and skinnier even if he/she would not do gymnastics competitively… it’s called Genetics.