Anyone hiking in the mountains with a heavy backpack will soon notice the handicap of the additional weight within a very short time. While there are many factors that ultimately determine your athletic performance capability, body weight – especially in endurance sports and thus running – is a key factor.

The underlying explanation is first and foremost purely physical in nature: carrying less kilos improves the load-strength ratio because less weight needs to be lifted and supported with every step. The force of gravity also seems more intense (like with uphill running), so your performance will also increase when you carry less weight. Whether marathon runner, tour de France winner, or mountain runner: the fastest athletes in these categories are all “pencils” and by no means “erasers”, such as sprinters, for example. 

Let’s take the marathon, for instance: successful runners such as Haile Gebrselassie (164 cm/54 kg) or Viktor Röthlin (172 cm/60 kg) are lightweight, and mountain runners and ski tourers such as Kilian Jornet (171 cm/58 kg) are also often below the 60-kg barrier. In mass sports, weight also plays a role in performance: training experts believe that when it comes to the marathon distance, every extra kilo adds around two minutes to the finishing time. Or, to put it another way: if you are battling with excess weight, for every kilo of fat that you manage to lose during your marathon preparation, you will gain a better finishing time of around two minutes. When losing weight, however, it is important that you actually lose fat and not muscle. This means changing your eating behaviour over the long-term and not putting yourself on a radical, short-term weight loss diet. And it is equally important to determine the boundary between being at your ideal weight and being overweight. 

Controversial BMI

The so-called Body Mass Index, or BMI for short, is used to measure this ratio. It makes it possible to measure the correlation between the height and weight of different people. The calculation is done using the formula: BMI = weight divided by height squared. Example: a 1.80 m tall runner with a weight of 75 kg has a BMI of 23.1. 

Using this standardized evaluation, a BMI of less than 18.5 is considered underweight, a BMI of between 18.5 and 25 is considered a normal weight and a BMI above 25 is considered overweight. When comparing the fastest runners in terms of peak performance, the BMI varies between 18 and 21. And surprisingly, this is regardless of whether they run 800 m or the marathon.

The BMI as a form of evaluation, however, has a serious drawback: this is because it is not only the ratio of height to weight, but particularly the ratio of weight to muscle mass that is decisive for the personal maximum performance in a discipline such as running. It is therefore not possible to accurately predict the personal performance capability using just the BMI. Nevertheless, you can predict that the body weight ratio or BMI will, as a rule, move in a positive direction over the years of training: while «heavy» muscle mass is built up through training, it is overcompensated by an even greater reduction in fat tissue. 

As a result, fast to very fast athletes typically have a BMI of around 20 after years of training. This value, disregarding sprinters, is normally independent of the top runners’ chosen running distance. Sprinters have a significantly higher BMI on average due to the large muscle mass required for improved speed-strength. The BMI of the five fastest sprinters of all time lies just under 22. This is around 10% more than the average value of each of the five best runners of all time in the 800 m, 5000 m and marathon distance. 

Balance is crucial

For lightweight athletes that are already well-trained, it is not the BMI but the right balance that is crucial.  Everyone has their own individual weight limit, which should not be undercut without losing too much substance. It is often difficult to determine where this lies, especially since performance often significantly increases at first with less weight. That is, until things suddenly take a dive and any additional weight loss causes more harm than a gain in time. In some individual competitive sports categories (long-distance running, cross-country skiing or even ski jumping) this can rapidly turn into anorexia with dramatic consequences.

Conclusion: to be able to run fast for as long as possible, many conditions need to be right. Body weight is just one of many components. Alongside the physical properties (height, weight, proportions), factors such as optimised metabolic processes (oxygen intake and transport capacity), psychological factors (attitude, motivation, will), motor components (condition and coordination) and their mutual interaction are responsible for peak times. Accordingly, the focus should take all components into consideration and not just a low body weight.

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