How much is your weight slowing you down?

How much is your weight slowing you down?

Have you ever obsessed about the weight of your racing shoe? It’s important because weight below the knee has a big relative impact on running efficiency. Well, if you do, you are very right but do you have a clear idea of much the weight on your body matters?

Runners sometimes laugh at the cyclist who spend thousands more on a bike to shave off a few grams, yet carries around plenty of ‘extra pounds’. A cheaper and better solution to get faster for the cyclist is to lose the waist belt – but this holds true for runners too (those of you with body-fat percentages below 6 or 12% for men and women respectively can look away now. The rest of you – read on!).

 

Weight and oxygen

Consider that we have a reasonably good relationship between VO2max (the maximum oxygen you can use) and your running performance. VO2max is generally expressed in relative terms such as ’50 ml/kg/min’ – meaning that the body can take in 50 millilitres of oxygen per kilo of bodyweight per minute. When you go to ‘VDOT’ or ‘VO2max’ calculators they provide you a rough estimate of how your recent racing times relate to your estimated VO2max or ‘VDOT’ (VDOT being a laboratory value showing ‘effective VO2max’ invented by Jack Daniels).

So if you have run 17:30 for 5 km you would get a VDOT score of 58.2 meaning that your estimated VO2max is 58.2 millilitres of oxygen per minute per kilo of bodyweight. If you weigh 70 kilos then this means you are estimated to be able to take in 4074 millitres of oxygen per minute. Now if you increase your bodyweight to 75 kilos, you can do the formula in reverse to estimate the impact on your running times:

4074 millitres / 75 kilos = 54.32 VO2max or VDOT

A VDOT score of 54.32   equates to a 5 km time of roughly 18:32 – so by gaining 5 kilos of weight I can expect to lose 1 minute and 2 seconds off my running time.

 

Lightest ‘battle weight’ is not always handsome – but effective…

How much time can be gained from dropping weight?

On a more positive note – let’s say you recently ran 18:50 for 5 km off a bodyweight of 72 kilos and you know you once ran at 66 kilos. So how much faster would you be – without any extra training – if you ‘just’ dropped the 6 kilos? 18:50 is a 53.4 VDOT meaning an estimated 53.4 millilitres of oxygen per kilo of bodyweight. Since I weight 72 kilos it means I am taking in maximum of 3911 millilitres of oxygen per minute. If I drop my weight to 66 kilos, the equation will be 3911/66 and the result is 59.23. A 59.32 VDOT means effectively a running time of 17:10 for 5 km or an improvement of 1 minute 40 seconds for a drop of 6 kilos or about 17 seconds per kilo over 5 km or 3 seconds per kilometre per kilo dropped.

These formulas are obviously simplifications and because other factors influence the cost of running (such as running technique which we’ll look at in the next article), you will never get exactly such a neat return from dropping weight. But the potential for very big gains are here – so if you’re breaking your back training while you’re eating and living in a way that keeps you clearly over your ideal weight, then you are putting your priorities in the wrong place.

Generally, we see it takes about 12 weeks to achieve a gain of 2 VDOT points and this can level off as you get more experienced. We do see bursts as well in individuals for whom all the basics have fallen in place: Jason Kehoe for instance improved from below 60 VDOT early in the year to a whopping 69 nine months later – an improvement of about 3 VDOT per months or every 4-5 weeks. Well above the average response.

 

Implications

…if the heavier version happens to look healthier just remember it’s slower!

The more ‘body’we have to oxygenate the less oxygen is available on average for all the tissues. So if you decide to add 100g to your biceps to look better on the beach, you will  add 100 grams of ‘body’ more that needs to be perfused with the same oxygen. This is the reason that any additional ‘unnecessary’ fat or muscle development is simply of no benefit to runners unless they serve a direct purpose in enhancing propulsion (for instance – while developing bulk in the lower legs is not recommended, some level of muscle development may provide better elastic recoil and make up for small extra bulk).

This is not the case with a lot of muscles on the body so as a specialist runner you need to be extremely prejudiced against any training methods carrying the risk of ‘bulking you up’. Gains to the mass of the body is like creating a larger reservoir that you need to irrigate with the same amount of water.

 

Reversal

Almost every piece of advice you will ever hear comes with a ‘reversal’ attached – an interpretation or a context where the advice does not apply. When it comes to weightloss the key is ‘how’ you lose it and ‘what’ you lose. Crash-diets are not good for your health and rarely sustainable so if you plan weightloss let it be from making small sustainable and healthy changes to your daily routines and eating habits.


Also published on Medium.

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René

Director and coach at Borg Coaching Services
Rene Borg is the head coach of Glendalough AC and a passionate runner competing over all distances and terrains.

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