“1lb of extra fat costs you 2 seconds a mile!” was the message pinned on a noticeboard in my local gym where I infrequently ran on a rickety treadmill back in the mid-1990's.

It was a memorable statement and it stuck with me for 20-odd years as a trusty ‘rule of thumb’ when it comes to the subject of bodyweight and running performance.

It was only recently that I realised I’d never critically questioned the value of this piece of gym lore in any way. It was just something I’d absorbed unquestioningly at an impressionable age and then hardwired into my brain under the header, ‘fact’.

I started thinking about the supposed relationship between weight and performance in more detail recently as I followed the explosive revelations that came out of the Mary Cain/Alberto Salazar story with great interest.

The Mary Cain-Alberto Salazar story

Just in case you've been living on Mars and missed this, one of the central issues raised by Cain in her shocking and immensely brave revelation was that Salazar was brutal in setting a highly specific target weight for her to hit in order to be deemed ready for competition as a track runner with the Nike Oregon Project team.

Ultimately, this emphasis on 'making weight' at any cost (and the questionable psychological tactics employed to hammer the point home) pushed Cain to obsess about eating and burning calories to the point that her health, performance and athletic career were all damaged, and she eventually dropped out of the sport.

Cain’s individual story is both disturbing and incredibly sad (although it appears to be slowly working towards a happier chapter now as she’s rightly received a lot of positive attention and started competing again since speaking out), and when reading her story I couldn’t help wonder about the actual numbers involved in weight and running performance.

At a superficial level, it’s not controversial to observe that the vast majority of elite endurance runners (and endurance athletes in general) conform to a certain body stereotype (i.e. extremely skinny and light) and, in the world of engineering and physics, it's a simple law of motion that lighter objects move faster, when all other inputs are equal.

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The science behind the weight-performance relationship

So, in a bid to start to understand the figures involved I decided to try to investigate some of the scientific facts involving mass and running performance to see if the old ‘1lb = 2 seconds per mile lost’ rule of thumb was actually in the right ballpark.

An initial bit of internet-based research led me to an article by the ever-reliable Alex Hutchinson. He highlights a couple of decent studies that have looked at the question from two different angles:

  • In 1978, researchers in the USA added 5, 10 and 15% bodyweight to runners via a shoulder/waist harness system and measured how much this impacted upon their performance over a 12-minute track run. The bottom line was that the addition of ~1lb seemed to add an average of about 1.4 seconds to runner’s mile times in this study. These were people running at around 6 minutes per mile pace.
  • More recently in 2016, scientists in Greece tried the opposite approach. They used pullies to effectively ‘reduce’ bodyweight and found around a 2.4 second per mile benefit for every lb they took off. Again, these runners were going at around 6 minutes per mile pace.

It's probably obvious, but worth stating, that these studies were looking at running on a track or treadmill - so effectively on flat terrain - and at a relatively narrow speed range.

These limitations notwithstanding, it does appear that the ‘1lb per 2 seconds’ rule won't be horribly wrong (so props to the New Parks Leisure Centre employee who scribbled it on the noticeboard circa 1994) and I would imagine you could start to extrapolate the results outwards a little bit from that point too.

For example, if 1lb is going to add ~2 seconds per mile on the flat then it may not be outrageous to suggest that +/- 5 lbs might be worth about 10 seconds per mile one way or the other too, and likely considerably more on a hilly course where the energy cost of carrying extra mass uphill is disproportionately higher.

The danger is that getting overly enamoured with this particular line of thinking is perhaps how problems, like those that beset Alberto Salazar and Mary Cain, begin.

Now, I don’t think that many people would seriously argue with the fact that, as a gross generalisation, being lighter is often advantageous for performance, particularly as the numbers cited in the aforementioned studies support what we see and perceive anecdotally when watching elite athletes in action.

It certainly chimes with my personal experience in that most of my own PB's were set when I was around my lowest adult bodyweight, so I have always seen being lean as an important personal indicator of being ‘in shape’.

There are problems associated with embracing this generalisation wholeheartedly though as it's an oversimplification of the relationship between weight and performance, while many athletes are very target-driven, motivated people and love taking things to extremes. 

The danger of focusing on weight loss

We thrive on shooting for pretty much any measurable metric that will tell us that we are progressing towards a goal (just look at the success of Garmin, Strava and TrainingPeaks) and this is often what drives us and allows us to improve, hit targets and PB's. It's a fundamental part of the attraction of endurance sports.

The problems can come though when certain metrics are overemphasised, either inappropriately, in isolation, or without checks and balances in place to make sure that there aren't any unintended negative ‘side-effects’ when they are pursued particularly vigorously.

One example of this would be the idea of hitting weekly mileage targets as a runner and it's often thrown around in conversation that top marathon runners will do ‘100 miles per week’ in training. 

There will always be some runners who can (and do) log more miles than that and there will be some who can perform brilliantly on considerably less.

Yes, all elite marathoners will run ‘lots of miles’ - there is no getting away from that fact - but by setting an arbitrary and specific bar at a high level there will often be a number of determined and maybe even talented, but more physiologically fragile, athletes for whom a 100-mile week is likely to simply end in injury or overtraining syndrome.

For these athletes, shooting for higher quality 60-mile weeks might actually be the recipe for better performance (the ultimate goal of course), but if ‘100 miles’ is held up as the target then they may still feel inferior for not being able to hit that marker.

In turn, they could keep running into problems if they're not confident enough or don’t have external support to reject that particular ‘norm’ and pursue a more individualised approach that works optimally for them.

Essentially the same is true when it comes to bodyweight and performance.

Bodyweight targets, if they are employed, tend to be more individualised than mileage targets. It's pretty easy to acknowledge the fact that a 5ft 3in woman is unlikely to have the same optimal bodyweight as a 6ft 6in man.

But the danger with bodyweight targets is still that they're still pretty arbitrary and fail to take into account the factors that mean all weight, when it comes to bodyweight at least, is not created equally.

The targets are almost always lower than an athlete’s current weight, so the underlying message with them tends to reinforce the ‘lower is better’ train of thought too, and this can be very dangerous from a psychological point of view.

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As an illustration, if you give a runner a backpack with 5lb in it, the resulting performance decrease is likely to be totally different to a situation where the same runner was to suddenly gain either 5lb of muscle mass or 5lb of body fat (or some combination of the two for that matter).

This might be because the loss of body fat is a loss of relatively ‘inert’ mass, whereas muscle gained could result in an increase in power output which counteracts some or all of the potential negative effects of the extra mass.

It could also be complicated by the fact that muscle and fat might be lost (or gained) in different areas of the body, which can also make a huge difference to performance outcomes (for example, it’s well-documented that extra weight in the lower leg is more detrimental to running performance than the same amount of mass added around the midriff).

The effect of weight loss on energy balance

Furthermore, the focus on bodyweight as a singular target misses the glaring fact that in order to lose weight athletes must exist, for some of the time at least, in a state of negative energy balance (i.e. by consuming fewer calories than are being burned).

A level of temporary negative energy balance can be tolerated by the body (and is necessary if bodyweight loss is going to happen), but it also exerts a stress on nearly all of the body’s finely balanced systems of homeostasis which, if the stress is too great or becomes too chronic, can set off a chain of reactions that can be extremely detrimental to health and performance.

Ongoing weight loss can also simply mean that there's less energy available for directly fuelling activities; something that clearly impacts performance in a negative way as well.

When highly motivated athletes start to strive for weight loss it can be, in some cases at least, the start of a potentially slippery slope towards chronic under-fuelling and/or over-exercising with the desire to attain (or retain) low body mass at the centre of it all. In extreme cases, this can turn into a diagnosable eating disorder if left unchecked.

The ‘syndrome’ of chronic, detrimentally low bodyweight in athletes used to be best known as ‘The Female Athlete Triad’ as it was, for many years, only considered to be something that affected women. The Triad part of the name referred to three main characteristics underpinning the 'syndrome:

1. Menstrual dysfunction
2. Low energy availability
3. Low bone mineral density 

More recently it has been recognized that a very similar set of issues, minus menstrual problems but with other hormonal disturbances, can affect male athletes too.

So, a new term - ‘RED-S’: Relative Energy Deficiency in Sport - has become predominant and more widely studied.

This excellent article on TrainingPeaks explains RED-S in a very easy to understand manner so is probably a good place to start if you want to learn more. If you want to take an even deeper dive into it then the IOC consensus statement on RED-S is a more comprehensive technical overview.

One of the most important takeaways from the research on RED-S are the tell-tale signs which might indicate that an athlete is suffering with the condition. Some commons ones (taken from the TrainingPeaks article linked above) are:

  • Recurring injury (soft tissue and/or bone)
  • Recurring illness
  • Unexpected fatigue
  • Disrupted sleep patterns
  • Change in mood 
  • Reduced social interaction
  • Problematic relationship with food and/or training
  • Stagnation and deterioration in performance 

If you notice a number of these in yourself, or in anyone close to you, it's a good idea to seek some further information, assistance and to talk to someone with experience in diagnosing and working specifically with RED-S. There are a number of great online resources, including Health4Performance, that can help.  

What is an optimal racing weight? 

Because of the potential pitfalls of aiming for a set bodyweight target, it's widely recommended that optimal racing weight for an athlete is, as Alex Hutchinson puts it in his aforementioned Runners’ World article, “a consequence that flows naturally from training hard and eating well, rather than a distinct goal that you have to obsess over separately.”

When I look back at my own racing experiences, I can definitely recall times when I was close to my lightest and leanest (for me this was around 68kg / ~150lbs). I was racing really well and approaching food with no greater plan than to try to eat healthy foods and as much of them as I felt I needed to fuel up before and after training sessions.

I felt lean and strong under these conditions and I would say that, at that point in time, 68kg seemed to be pretty optimal as I naturally hovered around it without specifically working hard to keep it at that level.

Years later, when I returned to racing from a bit of an extended break, I had initially gained a few kilos so had set myself a target to get back down to (or below) 68 kg as a sign that I was fully ready to race again.

I wanted results quickly so I did it by periodically eating very little whilst training hard and, although the weight did come off, I failed to consistently sustain the intensity and volume of training to get back to my PB times again.

During that period, I remember cuts, grazes or blisters would take ages to heal and that I seemed far more prone to injuries, but at the time I didn’t entirely recognise (or didn’t want to recognise) that not eating enough might have been the reason.

Reflecting on all of this, I do wonder whether my deeply held belief in the simplicity of the ‘1lb will cost you 2 seconds’ rule was part of what pushed me to focus too much on my bodyweight. The influence may have been subtle but I'm pretty sure it was there and, with the benefit of hindsight, I don’t think it was an especially healthy mindset to have.

The bottom line with bodyweight and performance is that, whilst it's true that being light and lean can be advantageous for certain types of athletes, it's definitely not the be all and end all.

Optimal racing weight is a side-effect of doing a number of other things right and something that ideally needs to be defined in a very functional way (i.e. by training hard, eating well and seeing where that gets you).

Setting firm, arbitrary bodyweight targets to hit in isolation from any other factors is unlikely to be helpful or healthy in the long run.