Posts

2 years ago

Power to weight ratio: is it really so important?

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 in (particularly) cycling performance. 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), particularly if riding over hilly terrain or involving acceleration. 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) … See MoreSee Less

Photo

Centred Nutrition: Peter Herzig Dietetics and Sports Nutrition

2 years ago

Ketogenic or Low Carb High Fat (LCHF): Is it appropriate for your sport?

The concept of LCHF has been explored in waves over the past 50 years and has recently jumped back into life via expert opinion, social media and anecdotal evidence. The focus of the current LCHF movement involves limiting carbs to <30g or <50g per day and providing 75-80% energy from fat. Consuming these levels of fat usually requires a diet based on a significant amount of cheese, cream, nuts, ‘grass-fed’ meat fat, nuts and oils. With the aim of this diet being to increase ketone levels to achieve “ketosis” (not to be confused with ketoacidosis). Hence the ‘keto diet’ or ‘ketogenic diet’.

There is no question that by following a LCHF diet you will be able, in as short as 5 days, to burn more energy from fat in your muscles. Sounds good so far. Within three weeks you can achieve ketosis and reduce reliance on carbohydrate for fuel. However, the ability to perform high intensity exercise is compromised by not having the stores of carbohydrate or the ability to readily access it. Furthermore, the body is more efficient at burning glucose than other energy sources. Based on recent research, your perceived effort will increase for a given high intensity workload on a LCHF diet, meaning that it will feel like you are working harder to perform at the same level.

You may be thinking that most of your exercise is at lower intensities, however anytime we call upon a higher intensity: be it running up a hill, accelerating out of a turn or performing multiple reps, we will call upon higher energy states. If you were performing ultra-distance exercise at sub-maximal intensities or where access to carbs or any food is an issue, then a LCHF approach may be of benefit to you. Though the long term health effects by following such a diet are unknown.

For most sports, there is no question that carbohydrate intake, individually prescribed will facilitate faster times, better training responses and less effort at higher intensities. While it is easy to get caught up with the hype of trying something new, sometimes we need to step back and look at the evidence. For more information or to discuss eating around training, consult with a Sports Dietitian.

Peter Herzig (AccSD, APD) … See MoreSee Less

Photo