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How Much Does Weight Affect Triathlon Performance?

One of the biggest misconceptions in the sport is about how much weight affects triathlon performance. Whether athletes obsess over the weight of their bike, embark on extreme diets, or spend hundreds on a pair of running shoes which are 20g lighter, they believe that the key to unlocking performance is by racing with less weight. I’m going to delve into a bit of the science and mathematics behind the relation between athlete weight and performance.

Trigger warning:
I will be using the word weight a lot, and talking about the effect it has on performance. Usually I will tie myself in knots when writing to try to avoid using that word in relation to an athlete, but there’s only so much I can lean on a thesaurus before the article starts to sound unnatural, and weight is the correct clinical term. Usually I would recommend those who are at risk of, or recovering from an eating disorder to move onto the next article, but as the theme of the piece is that weight doesn’t matter as much as people think, I will instead recommend you scroll down to the conclusion at the bottom, have a read, and start the article again from the top if you feel comfortable.

As a bit of context I am a level 3 triathlon coach and nutrition coach. I weigh in at around 53-55KG most of the time, but have clocked in at 49 the day after a race in the past. This is important for points I will raise within the article. I’m also going to work off of the assumption that you are an athlete currently at a healthy weight who is looking to improve your race times, not someone who has recently taken up the sport with weight loss goals.

Swim

A very muscular swimmer stands on the shoreline in front of a sunset
Ross Edgley became the first person to swim around the UK. Photo credit Red Bull

The swim is an anomaly, in that nobody serious about their performance will ever make weight loss a major part of a swim programme. In fact, weighing less may have a negative impact on your swim. I have got hypothermic in open water on more that one occasion as my small frame struggles to cope with spending prolonged periods in cold water, and my noodle arms just can’t develop as much force as someone 20KG heavier. This is something I am addressing with strength training, but as any extra weight from muscle mass becomes essentially weightless in the water, it’s not going to slow me down in the swim. It may mean a bit extra to carry on the bike and run, but in isolation, we don’t need to worry too much about our weight for swimming. In fact, carrying some extra timber can very much help us. Just look at your average long distance swimmer.

Bike

Now we’re talking, this is what most of you have come for. You want to see how much time you can save by buying a lighter bike, or losing some weight.

To work out just how much time we can save, I’m going to use a piece of software called Best Bike Split, which is a maths engine that can help us predict our cycling performance using data we input on ourselves and our bike. Let’s look at a race plan for an athlete I coached to qualification at the Duathlon World Championships.

A power plan for a bike race, which denotes a finishing time of 1:04 over a very flat course, calculated using elevation, power, weight and wind data.

The course really is flat as a pancake, but so are many triathlon courses, especially in North America. For the 40K bike ride around a motor racing circuit (lots of corners), we’re looking at 1:04. Pretty handy by anyone’s reckoning. His power target is 272W, and his weight is 86KG. Let’s draw up a second plan, where he theoretically loses 4KG. This would take a large amount of dedication as he’s already pretty trim, and take him to around 4-5% body fat.

A second race plan showing an improvement of only 10 seconds by lowering weight from 86 to 81.8KG

Yes, you are reading that correctly. He gained 10 seconds over 40KM by losing 4KG. When you consider the sacrifice this would involve, and the impact it would have on his training as well as other areas of his life, it’s not a lot of time to gain. It’s the kind of time you could easily lose in transition. If he’s focused on losing weight, he’s not going to be putting on or maintaining muscle mass in the same way, and will likely end up losing leg strength, resulting in a drop in power, so he could actually end up going slower.

For the next plan we’re going to have a look at what happens if he were to focus on hit fitness rather than weight. He eats a higher calorie diet to fuel his hard workouts, with lots of protein to facilitate the muscle development required for recovery and muscle growth, putting on another kilo, and his power target jumps up by 30W as a result of his hard training. Let’s see what that does to our time prediction.

A third race plan which shows an improvement of 1:29 over the same course with a power target of 291W, instead of 272W.

That’s an extra 1:29 we’ve found. The extra kilo only cost us two seconds (not shown). One and a half minutes may not sound like a lot, but imagine dismounting your bike and having to wait all that time while competitors you overtook on course stream past you to start their run. That’s going to feel like the longest 89 seconds of your life.

Now, I can feel the counter arguments coming already. What about on a hillier course? Well, let’s take that same athlete to the Ironman World Championships in Nice. The intensity factor is reduced to 0.73 due to the increased distance, but we’re keeping everything else the same from the first plan for the sake of illustrating the point.

Another race plan, this time looking at a much longer, hillier triathlon. The finish time is estimated at 5:25

We’re looking at a very respectable 5:25. Now let’s see what happens when we lose 4KG again.

A second race plan from the same race, where losing 4KG is illustrated to save 4 minutes and 32 seconds

4KG has found us four and a half minutes. That’s quite a lot of time. The green bars show us where time is gained (climbs), with red denoting where time is lost (flats and descents). This is also assuming that we managed to maintain the same amount of power after losing the weight, which is far from a given. Now let’s try what we did for the duathlon, and add an extra 30W to our power target with an extra kilo in weight.

A third race plan showing that increasing weight by 1KG, and power by 30W results in an improvement of 11 minutes and 58 seconds

We’ve found the best part of 12 minutes by focusing on power rather than weight loss. That’s not far from three times as much of a gain as focusing on weight loss alone.

Now, these are very extreme examples. It will take a lot of time and dedication to improve your power by 30W or lose 4KG in a healthy way, but they illustrate a point. If you have to make a choice between focusing on power or on weight loss, you’re better off focusing on power development. For most new athletes, you will be able to achieve both power development and weight loss simultaneously as your fitness increases rapidly, but for an experienced athlete who feels they’re on a plateau they will likely need to choose one or the other.

Finally, I want to talk about myself a bit. Selfish, I know. As a very lightweight rider I wanted to offer a bit of perspective. I’m very good at cycling up long hills, I can drop people without really trying. On the flats though, I struggle. My FTP is normally around 230W, which puts me at 4.2W/KG, a number many riders would love to hit. However, when I’m trying to keep up with athletes whose FTPs are closer to 320W on the flat, even taking into account the draft effect, I’m going to be working an awful lot harder than they are. Take away most of the draft effect in a race situation, and I’m at even more of a disadvantage. Even on short, steep hills I suffer as the guys I’m riding/against with can put out an explosive 700-800W for the duration of the short hill. I’m just not that strong or explosive, and I’ll be hitting 500-600W on the same hill. I’m just not getting up it as fast as they can.

Long, steep hills are my hunting ground however, which means I should do really well at the worlds in Nice, right? Let’s see how I go head to head against my client in the mountains of Nice, the only chance I’ll realistically ever have of beating him. The same intensity factor of 0.73, with our normal rigs. Remember, my client would get round in 5:13.

Another race plan using the author's data who is much lighter (53KG), but much less powerful (only 126W target), and much slower as a result, with an estimated finish time of six hours and 41 minutes

Despite the fact I weight 53KG to his 86, he would still beat me by well over an hour. Sure, I’d be ahead of him until the top of the first climb (assuming we came out of the water together), but he’d catch me rapidly over the next 120KM.

There are of course other factors. The eagle eyed among you may have spotted that my drag is significantly less than his (which will be influenced by my small frame) and there’s a difference in the rolling resistance of our tyres, but these could be improved, and aren’t the focus of this article. The take home message is that being lightweight doesn’t trump being more powerful. something to consider before you spend a month’s wages on lighter wheels to save 250g.

Remember, we’re triathletes, not cyclists. We generally ride on flatter terrain, and still have a run to do at the end. We’re not launching attacks up the Alpe d’Huez in a bid to win the Tour de France where 50g really might make a difference.

Running

Now, you may be expecting some equally data heavy analysis here. Many of you will know I’m a fan of running power, but I’m afraid I’m going to disappoint you here, as there’s nothing I can use as a direct comparison. We have ascertained so far that weight doesn’t really have an effect in the swim, and that losing weight won’t have as much of an effect on the bike as we may think, but running is where you really do benefit from having a lighter frame. Triathlon is a combination of all three sports, so focusing our efforts on two disciplines and just accepting our mediocrity on the third is a recipe for disaster.

However, what really matters in running is our power to weight ratio. That is, how much force we can develop as we push off of the ground with each step, relative to our weight. If you’re 20KG heavier than me, but you have much stronger, more elastic leg muscles and a huge aerobic system to boot, you’re going to be outrunning me. Remember my client who is over 20KG heavier than me? You may think that despite the fact he’d wipe the floor with me on the bike, I’d have him on the run. Well, you may be surprised to hear that after he rode a 1:04 at the duathlon we looked at earlier, he went onto run a 37 minute 10K. My PB is 39 minutes.

Carrying some extra weight does increase the risk of some injuries as more force is going through your joints with each step, but if you’re a real flyweight runner, you’re more prone to other injuries from a lack of stability. Looking at the Olympics, we can see that the shorter the distance the larger the runner, so as distances get longer size will become more of a factor. However, after we go over the marathon distance and towards ultra marathon, things start to swing the other way. At 53KG, I’m going to be at a much higher risk of hypothermia than other athletes when running through a forest in the middle of the night, and my metabolism combined with low body fat percentage means nutrition is even harder for me to manage over a long race.

Apologies for the lack of hard evidence on the run, but the same principles apply as for cycling. Weighing less will help, but being fitter and stronger is more important every single time.

Conclusion

If weight was everything in triathlon, at 53KG I would be winning everything, but I’m not. There is a tendency to use the weight of ourselves or our kit to explain poor performances, but this is very rarely our primary limiter. If an athlete is a Clydesdale or Athena then they will see a notable improvement in their performance as they start to lose some body fat, but if you’re a 70KG athlete looking to take your racing to the next level, then cutting down on your weight shouldn’t be seen as a silver bullet. Save the money you were planning to spend on that weight saving bike part, and invest it in a coach who will help you take far more time off of your finish time, or spend it on more race entries!

Kristian Bulmmenfelt, with visible arm and legs mucsles, breaks the tape at Tokyo 2020 to win olympic gold in the individual triathlon
While he isn’t going to be selling tickets to the gun show any time soon, we can see that Kristian hasn’t been afraid to put on muscle in the pursuit of performance. Photo credit Olympics

Look at the top athletes in triathlon at the moment, the Kristian Bulmmenfelts, Lucy Charles-Barclays and Lionel Sanders of the world. They’re all very lean, but they’re also very strong. Kristian is 75KG but ran 10K in 29 minutes at Tokyo to take Olympic gold. Lionel Sanders decided to forego the Ironman world championships in 2023 due to the mountainous nature of the course, but this was more to do with the technical nature of the bike course than a belief he wouldn’t be able to keep up on the climbs, as he is actually lighter than the eventual champion Sam Laidlow based on publicly available data.

In years gone by it was thought that weighing as little as possible was key to triathlon success, and athletic success in general, but our understanding of human performance has moved on. Coaches and athletes focusing on weight loss has lead to widespread injury, malnutrition, burnout and ultimately athletes leaving the sport. Thankfully most of us have moved on in the last ten years, but there’s still a lot of bad advice doing the rounds. Often simply telling an athlete to lose weight is a smokescreen to make up for a lack of knowledge of the athlete’s real performance limiters.

 

 

Your body is yours, and it’s not for me or anyone else to tell you what to do with it, but I hope this article has helped you understand that simply getting as light as possible isn’t the answer to all your triathlon problems. If you’ve had a penny dropping moment and want the same results as the athlete in my case study, check out my bespoke triathlon coaching.