Table of Contents:
The general and individual growth pattern
Children and adolescents do not grow at a continuous rate. School-aged children grow at a rate of more or less 6cm/year until they reach puberty, after which they experience a ‘growth spurt’. After reaching 'Peak Height Velocity' (PHV, the moment in their growth spurt when they are growing fastest), growth velocity drops rapidly and the he/she eventually stop growing. This is what is called the general pattern of physical growth (see figure 1).
Although all people follow the same general growth pattern, when individual growth patterns are compared, significant differences in timing and magnitude of the growth spurt can be noted (See figure 2). In other words, some youth athletes might experience a large increase in stature around 12 years old, while others will only experience a smaller growth spurt around the age of 15.
Consequently, between the ages of 11 and 16 years old, there can be considerable differences in maturity between athletes of the same age, resulting in a large difference in height, weight, and muscle mass. Athletes who have their growth spurt significantly earlier or later than average are referred to as early or late maturers, respectively. Having these early and late maturers train and compete in the same age category can lead to competitive inequity, selection bias, increased injury risk, diverging needs during practice, and differences in psychological and emotional development.
When talking about the growth spurt of youth athletes, it is important to know the difference between growth and maturation, between maturation status and timing, and between chronological and biological age. :
- Growth refers to the increase in size of the body whereas maturation refers to the progress toward the adult or mature state. In sports, the term maturation is often used to refer to the maturation of musculoskeletal system, and is therefore similar to growth. However, note that different systems mature at different rates. For example, a 14 year old early mature athlete might the physical appearance of a 16 year old, but might still have the same psychological maturation state as other 14 year olds.
- Maturity status refers to the current state of maturation of the athlete. This is typically measured in pre-PHV, Approaching-PHV, Circa-PHV, and Post-PHV. Maturity timing refers to when the athlete reaches certain developmental milestones (such as PHV), relative to their peers. For example, athletes who reach PHV significantly earlier than their peers are referred to as early maturers.
- Chronological age is the age based on calendar date on which the athlete is born (i.e. the ‘actual’ age), whereas biological age reflects how far an individual has progressed through the stages of growth and maturation compared to typical age-related norms. A 14 year old athlete with a biological age of 16 therefore has a body that is at the developmental stage for a typical 16 year old, although he/she has a chronological age of 14. In other words, he/she looks more like a 16 year old rather than a 14 year old.
How to measure maturation
To take into account the maturation status of youth athletes, you need an accurate and reliable way to measure or estimate it. Below, we briefly describe four methods and discuss their (dis)advantages to apply them on large scale in youth sports.
Perhaps the oldest way to estimate maturation, is by tracking the sexual maturation. As the onset of puberty does not only kickstart the growth spurt but also the development of secondary sex characteristics, the development of pubic hair and breast/testicular tissue can be used to estimate maturation. Evidently this is not an appropriate method to use in an athletic context.
Perhaps the most objective way of measuring maturation is by using skeletal age. Based on a radiograph from the hand and wrist, the development of bones and joint tissues can be compared to reference standards (e.g. Greulich-Pyle or Tanner-Whitehouse) to determine ‘skeletal age’ as a proxy for biological age. Unfortunately, the fact that specialized equipment is needed to make a radiograph makes this method expensive and rather difficult to apply on a larger population. Furthermore, this method only provides insights in maturation, not in growth speed.
In search of a method that does not require expensive equipment, the Mirwald equation (maturity offset) was proposed. Knowing that growth takes place in a distal to proximal order (i.e. first hands & feet, followed by legs & arm, and finally the trunk), a formula was proposed to estimate the moment of Peak Height Velocity (PHV) using age, leg length, and stature. Although this method is still being used fairly often, research has shown that there can be considerable errors in the estimation.
Finally, maturation can be estimated by calculating the Percentage of the Predicted Adult Height (%PAH). This percentage is calculated by comparing the current height of an athlete to that of their predicted adult height (for example calculated using the Khamis-Roche equation). As key events in maturation tend to happen around the same percentage of adult height (e.g. PHV at ±92%PAH), the %PAH can be used as a proxy for biological age. This has been the preferred method in large scale interventions such as in the Elite Player Performance Plan of the English Premier League, and is also the method that is applied in the Hylyght Growth Tracker.
Gender differences in maturity
Until puberty, the physical development of boys and girls is rather similar. However, the timing and effects of the growth spurt are vastly different
The timing of the growth spurt is about 1,5 year earlier in girls than in boys, and the PHV is - on average - lower. This is why 13 year old girls are often taller than boys of the same age, but most boys will surpass girls in stature by the time they are 14.
Puberty also has a different effect on the body composition of male and female youth athletes. Whereas the increase in mass is mostly due to the increase of lean body mass in boys, this is accompanied by gains in fat mass in girls. Both boys and girls become taller and stronger after the growth spurt, but in contrast to boys, this does not result in improved speed, agility, and relative strength for girls.
Changes in body size and composition due to puberty also have a different psychological effect on boys and girls. Whereas being taller and bigger tends to lead to higher self-esteem in boys, the opposite is often the case in girls - although these effects are highly dependent on individual and cultural expectations.
As a result of the different effects of puberty, selection bias also tends to differ based on gender. Whereas early maturing boys are overrepresented in most sports, early maturing girls are only overrepresented in sports where size and strength play a bigger role (e.g. basketball), and are underrepresented in sports that emphasize aesthetic qualities or agility (e.g. figure skating).
| Age at Onset Growth Spurt (y) | Age at Peak Height Velocity (years) | Age at Peak Weight Velocity (years) | Peak Height Velocity (#cm/y) | Peak Weight Velocity (#kg/y) | Age at adult Stature (years) | |
|---|---|---|---|---|---|---|
| Boys | ±12 | 13.4 ± 1.0 | 13.8 ± 1.1 | 10.4 ± 1.2 | 10.3 ± 1.9 | ±18 |
| Girls | ±10 | 11.8 ± 0.9 | 12.3 ± 1.2 | 8.6 ± 1.1 | 8.7 ± 1.4 | ±16 |