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Can weight loss affect power?

Michael Donlevy
20 Apr 2021

Bigger equals stronger, right? Not necessarily...

Average power is quite a simple concept: it’s your total physical output divided by the time you rode for. Your output is measured in kilojoules and your time in seconds, so if you expend 2,000kJ on a three-hour ride then 2,000 divided by 10,800 (seconds) equals 0.185. You then multiply that by 1,000 to convert the figure to watts, and get 185W.

And in terms of our question there’s good news here, because your average power won’t necessarily go down if you lose weight.

OK, if you lose weight too quickly you may lose muscle or you may not have enough energy to sustain your normal performance because the glycogen in your muscles and liver – your body’s fuel – is low. But if you fuel yourself well and look for a gradual reduction in weight (or more specifically fat) you’ll be fine.

You also need to understand that during a weight-loss phase you might struggle to sustain hard efforts so may need to change the focus of your training. The easiest way to lose weight is to cut down the energy – in the form of calories – you take in so you burn more than you consume. But once you’ve reached your ideal weight you can start to eat more and then sustain those harder efforts.

The even better news is that once you’ve lost weight you may well find you need less power to ride at a certain speed, especially uphill.

This is where power-to-weight ratio comes in. It’s a key performance metric because your speed uphill is determined by the power you’re producing and your weight. So if you ride uphill at 300W and you weigh 100kg your power-to-weight ratio is 3W/kg.

Illustration: Clear as Mud

If you ‘only’ put out 250W but you weigh 65kg your ratio is 3.85W/kg and you’ll be a lot faster (uphill at least) than the heavier but more powerful 300W rider.

It’s unlikely that if you weigh 100kg you’ll get down to 65kg. A more realistic target may be 90kg. If your power stays at 300W your ratio will be 3.33W/kg and you’ll be faster. Even if your power drops to 290W your ratio at 90kg would be 3.22W/kg and you’d still be faster.

In other words, it can be worth losing a little average power if your power-to-weight ratio is going up. As you lose fat mass you’ll be more thermally efficient and will have better cooling (fat is an insulator), so you probably won’t fatigue as rapidly because you’ll be cooler.

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There are things you can do to help make sure you don’t lose power. If you’re close to your target weight, you may be able to hit it simply by riding more and not altering your diet.

If you have a larger amount of weight to lose you’ll need to alter your diet in some way and this may have a temporary negative effect on your power. But by including some high-intensity efforts in your training – even if it’s once every 10 days – such as some hard three to five-minute intervals you should maintain or even increase your power output.

Weight training can also help. Key exercises are squats and deadlifts with heavy weights, although I wouldn’t suggest using a heavy weight if you’re new to it or out of practice – if you injure yourself you’ll definitely lose power.

Finally, remember that losing weight for too long or too rapidly isn’t good for you. If you look at GC riders on the Tour they’re extremely lean but they can only maintain this for a short time and have a team of nutritionists helping them. You don’t, so be sensible – and keep on riding.

The expert: Ric Stern is a road racer, sports scientist and cycling and triathlon coach. He has competed in the UCI Gran Fondo World Championships and has coached elite riders, Paralympians and beginners alike. Visit cyclecoach.com.

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