# A second look at growth measurements affecting young athletes

### We can't do much about how children grow, but knowing when it's happening can shape coaching and development strategies that keep kids involved in sport long enough to make a difference.

This article was one of the first I published on Substack. I had only a handful of subscribers at the time so the articles distribution was small compared to later efforts. So here it is again to illustrate the framework of measurements supporting ideas around physical growth, maturity, and young athletes.

Rate of growth can have significant physical effects on childhood athletic performance. But growth rates affect psychological and social aspects of youth sport as well. When considered together, the physical, psychological, and social elements that make up the sometimes troubling brew of childhood experience are at the root of everything good and bad about youth sport.

Practitioners can’t do much about how children grow, but they can help alleviate some of the darker consequences of unusual growth rates at young ages if they are aware of what’s happening *while it’s happening*. That’s why a measurement like *peak height velocity* and a few others mentioned below can be useful.

As with any measurement of human growth the calculations produce estimates. The input measurements are easy to do, and for the scientifically minded they may become important parts of club training programs.

##### This article was originally published in March 2021:

**Athlete development measurements and the lingo that goes with them**

The language we use in coaching is often specific to the sport itself but the language of sport and athlete *development* uses terms from other fields of expertise and are often only vaguely familiar to coaches and administrators. In this letter I want to define some of those terms and discuss how they are used in sport.

**Growth calculations**

Even though we may not know exactly where a young athlete is in their growth process we can, through the use of a few measurements and some fancy calculations, predict certain things about when they will reach the important growth milestone called peak height velocity.

**Peak height velocity (PHV)** refers to a span of months where the greatest *rate of change* in body stature occurs. It is the last significant period of growth before humans reach adult height. It is commonly known as the teenage growth spurt although in some children it may occur prior to the teen years. Once it begins it lasts for approximately 12 to 18 months but this, like almost everything else related to human growth, can vary.

## Resources

Calculate your own team's growth metrics. Here's a link to a spreadsheet that does the heavy lifting with the maturity offset calculations and a few other metrics discussed in the article below:

Starting at birth the human body grows until the mid- to late-teen years with the fastest growth rate occurring during the first year of life (about 20 cm/year) and then again during PHV where growth rates can be as high as 8 to 10 cm/year. Outside of these two periods growth rate is around 5 cm/year.

**Being able to identify when PHV will occur in young athletes can help coaches design effective and time-appropriate training plans.**

Coaches in almost every sport are familiar with the concept of periodization where various aspects of performance are scheduled or emphasized in the training calendar so metabolic systems, skill acquisition, game strategy, competition, and whatever else the sport requires are part of an athlete's overall preparation. One of the key aspects of periodization is once the cycle is complete it starts all over again for a new season or new year.

We can use the periodization concept and apply it to the early years of a young athlete's life creating important periods for learning skills, training metabolic systems, learning how to train and compete etc. The primary difference between this kind of periodization and what we typically recognize as periodization later in the athlete's career is none of the specific early periods reoccur for youngsters. So if it’s possible to actually know where young athletes are in the development process then it's worthwhile to use that information.

In the LTAD model, for example, there are ideal periods when certain components can be learned, trained, and enhanced. Some of these phases are simply based on age. Others such as PHV occur in age *ranges* and knowing when those ranges begin can inform preparation and strengthen the foundation a young athlete brings to their training later on.

Growth calculations produce estimates. The maturity offset is used to predict the onset of peak height velocity (APHV). Periodic measurement — we recommend measuring athletes every four months — yields more precise results the closer the athlete gets to the onset of PHV.

**Maturity offset** calculations are used to estimate how far away a child is from peak height velocity. Sportkid Metrics uses both the Moore and Mirwald equations to calculate the offsets. You can read about these equations and why we use both of them here. As the term implies maturity offset is a measure of how far away, in years, an athlete is from the beginning of peak height velocity.

A *negative maturity offset* indicates the athlete has not yet reached PHV. A *positive offset *means PHV has already begun (or has completed depending on the size of the offset). For example, an offset greater than 1.5 (years) might indicate the athlete is no longer in the PHV period, since it usually lasts for less than 18 months.

Figure 1 shows the offset calculations for a 10-year-old boy and an 11-year-old girl. The boy's maturity offset is -2.2, suggesting he will reach PHV in a little over two years. His predicted age at peak height velocity (APHV) is 12.6, which is simply his current age minus the offset (10.4 - (-2.257) = 12.69. The offset for the 11-year-old girl is 0.08 which indicates she has just started PHV.

The measurements needed for the Moore equation are age and height for females, and age and sitting height for males. The Mirwald equation requires the same five measurements for both sexes: Age, height, weight, sitting height, and leg length. There is no need to actually measure leg length, it can be derived by subtracting sitting height from height. Like the Moore equation, the Mirwald is also slightly different for each sex.

Both equations produce an estimated maturity offset. I am not recommending one equation over the other but if you download the spreadsheet linked above you will see each calculation produces a slightly different result. The differences are not significant, however if you are interested in validity here are some studies that tested the equations:

Koziel, S.M. and Malina, R.M. (2017), Modified Maturity Offset Prediction Equations: Validation in Independent Longitudinal Samples of Boys and Girls, Sports Med. 2018; 48(1): 221–236.

Mirwald, R.L., Baxter-Jones, A.D.G, Bailey, D.A. & Buenen, G.P. (2002). An assessment of maturity from anthropometric measurements,

*Med. Sci. Sports Exerc.*, Vol. 34, No. 4, pp. 689–694.

**Age at peak height velocity (APHV)** predicts how old an athlete will be at the onset of PHV. APHV is the difference between the athletes current age and the maturity offset.

**Non-growth related calculations**

**Arm-span height ratio **measures an athlete's reach. In some sports such as basketball (longer reach) and weightlifting (shorter reach) it can be an important performance factor. It's a simple calculation that divides arm span by height.

Typical arm span ratios are in the range 0.97 to 1.03, which indicate that height and arm span are almost the same. A ratio of 1.09 or higher is considered an outlier and indicates a longer than normal reach. Doctors also use higher than normal ratios to aid in spotting certain health problems.

The ratio changes often as children age so early results, while not entirely irrelevant, aren't really significant. Meaningful arm span ratios only emerge once an athlete has reached their adult height.

**Body mass index (BMI)** is the relationship between height and weight. It's best used as a population metric. Using it for individuals, especially older athletes, can be misleading in activities requiring strength or higher levels of fitness, or when all of those tested are in a unique, self-selecting group. Athletes, for example, are more likely to possess more muscle mass or other physical characteristics that directly influence the BMI result.

Not all athletes have higher amounts of muscle though. The majority of athletes at the developmental or grassroots level are physically no different than typical non-athletes when it comes to muscle mass. Additionally BMI for self-selected groups can be useful when in-group comparisons are made since the data is relative to the group itself.

In the spreadsheet linked above only the standard BMI equation is used, the actual children's BMI is lookup based and not tied to a specific formula, so the BMIs produced on the linked sheet for younger athletes are not very useful.

**What are these calculations for?**

Calculations like these can produce data that benefits coaches, clubs, and national governing bodies. Depending on the number of athletes you work with the spreadsheet above may be enough of a tool, however NGBs may have to look into coding their own tools if they measure and track thousands of athletes nationwide.

But why bother measuring and predicting at all since the athlete's growth is easily observable while PHV is underway? Physical growth is only one part, albeit the most obvious, of what occurs during PHV. Other maturational changes are also taking place within the athlete's body that are not observable and which occur before and after any obvious growth has taken place. These changes, whether they are visible or not, have an effect on young athletes. The point of measuring is to zero in on when PHV actually starts before any changes can be seen. The benefits available during PHV, if indeed there are any, are not dependent on growth *per se*, we're just using growth to help identify a unique, non-recurring process.

PHV lasts between 12 to 18 months and although significant changes can take place throughout the entire period, increases in stature are not continuous and may occur over a much shorter period while other PHV changes are still ongoing.

### Don’t over-science it!

Finally, we need to be aware that anytime we start using statistics or math in an area not dependent on statistics or math we run the risk of being overwhelmed with the numbers and hypnotized by the imaginary power these numbers seem to provide. The calculations discussed here are coaching tools or demographic information for clubs and NGBs, other than that they're not magic.

I'm speaking from experience here. When I was coaching swimming, at the first long course meets each year swimmers (but probably really their parents) wanted to know what their meter times converted to in yards. If you're a swimming fan you know what I mean. There are dozens of ways to do a 'conversion' like this and the more technical you can make it the more scientific gravitas the result seems to have. But let's face it, there is no time conversion for a human swimming 100 meters to 100 yards. Don't waste your time. The point is if you decide not to calculate maturity offsets for your team the sun will still rise tomorrow, albeit a little bit later.