Power to weight ratio: is it really so important?

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SPORTS NUTRITION SUNDAYS BLOG

As a Sports Dietitian I am often asked about maximising power to weight. Power (the amount of work completed in a certain time) to weight or watts per kilogram is a measure that allows us to compare individuals or improvements following training particularly in cycling. Though we need to know what power we are talking about: is it the maximum power you can sprint at, your functional threshold power (FTP) or power for the duration of an IM bike leg? Andy Coggan has a nice article on Power Profiling on TrainingPeaks if you want more information.

Why is it important?

Bragging rights between you and your training buddies of course… Power to weight in cycling has traditionally been used to compare the abilities of climbers. When the gradient increases, power to weight becomes more important as we are working harder against gravity. Thus heavier riders need to produce more power to keep up. It is also beneficial to have a higher power to weight ratio if your sport depends on accelerating, decelerating or changing direction quickly (I know engineers, they are all forms of acceleration).

Why isn’t it important?

I hear you; you don’t participate in hilly road cycling and you aren’t planning on doing Embrun Man anytime soon. While power to weight is important up a hill, on a flat course it makes much less difference. For two riders: Mike (80kg and FTP 300W or 3.75W/kg) can average 37.3kph* on the flat, while Amanda (60kg and FTP 250W or 4.2W/kg) can only manage 36.9kph* (*theoretically). Even though Amanda’s power to weight is greater, it is Mike’s greater power (300W) that is more important.

But what about aerodynamics, doesn’t a bigger body push more air?

Yes that is true, in fact overcoming aerodynamic resistance is responsible for 90% of the energy used in a time trial over 40kph. Though Amanda’s lower drag isn’t enough to counteract her lower power compared to Mike.  A more recent study showed just this in the real world: the best predictor of time trial performance was average power during a time trial normalised to a cyclist’s drag area (Peterman et al, 2015). Or more basically: better performances were those individuals that could produce more power in an aerodynamic position.

Considerations when maximising power to weight.

For most athletes it is more important to maximise training gains (power), focus on fuelling during training and competition and healthy eating for recovery. For those who have ticked all the boxes, then you may benefit from some fat loss (improved power to weight). Though we then need to make sure that you actually have some body fat to lose and can do so safely. I would always recommend discussing any weight loss goals with your coach to make sure it fits in with your training plan. A sports dietitian can work with you to map out a realistic, individually structured plan for you to maximise training adaptions and potentially power to weight.

Peterman, J. E., Lim, A. C., Ignatz, R. I., Edwards, A. G., & Byrnes, W. C. (2015). Field-measured drag area is a key correlate of level cycling time trial performance. PeerJ, 3, e1144. doi: 10.7717/peerj.1144

Peter Herzig (AccSD, APD)