Blog

The Case for Lowering BMI to Increase Swimming Speed

The general conclusions from my first post on the “The True Effect of BMI on Swimming Speed and Efficiency” based on the limited results of the Rio 2016 Olympic games appear to still stand.

I have collected performance data on 2,600 swimming athletes competing in 28,000 lanes at international competitions since 2014.

The charts below show some of the conclusions. The rest of the post explains the rationale. (In all cases efficiency is measured as speed / Kg or m/Kgs)

The fastest swimmers are not the most efficient.

Women are more efficient swimmers than men at lower BMIs.

You don’t have to be muscular to be fast. Low BMIs can be competitive.

In summary, here are the findings:

  • There is a negative linear relationship between BMI and efficiency measured as speed / Kg. In other words, the lower your BMI, the more efficient you are. This slanted line is what I call the Swimming Event Efficiency Line (SEEL)
  • The fastest swimmers tend to have a BMI close to the average of all competitors.
  • The most efficient swimmers are not the fastest.
  • Though the speed per Kg (efficiency) is less for the fastest swimmers, they have more Kg of body mass to power through the race to win.
  • Women with generally lower BMI values than men are noticeably more efficient swimmers.

Based on this data it would appear that swimmers would become faster if they lowered their BMI while maintaining their weight. The degree of improvement from the drop in BMI would follow the SEEL.

I can understand why people would be hesitant to do this as lowering their BMI means swapping muscle mass for fat, which is counter intuitive to success. I suspect the paradigm of muscularity for speed which applies to sports in the air, is less so for water sports, possibly altogether wrong.

The counter argument could be that the faster you are in water, the more drag you endure, so you’ll naturally be less efficient as you speed up. More muscle is required to succeed is the wisdom.

This is not always true. There are cases of men with low BMIs whose overall speeds come close to those of the fastest men. Some of these low BMI men have BMIs that closely match those of the fastest women. Lower BMIs can work, there seems to be specific evidence of this in men. There are also cases of women with lower BMIs whose speed comes close to that of the fastest women.

Bulking up to be a faster swimmer, may not be the best strategy. This is very noticeable in the following events where the fastest swimmers have below average or very low BMIs compared with the rest:

  • 200m breaststroke for both men and women (Short and long course)
  • 1500m men’s freestyle (Short and long course)
  • 800m women’s freestyle (Short course)

It would be interesting to know the relative percentage body fat values of the high speed, low BMI swimmers versus the high speed, high BMI swimmers. To see whether my theory of an alternative strategy is True.  Unfortunately the data I have cannot prove this.

The view that physical factors such as long torso length, large lung capacity and foot size contributes to speed is true, but there is something more.

The assertion that a lower BMI will lead to better performance for competitive swimmers in all events still stands.

BMIs may need to be tracked to the nearest 100th. just like swimming times.

About the charts

The charts below only plot the best performance of each of the 2800 athletes over the years in the event. This was done to reduce the clutter of poor performance days for an athlete, which would show up as multiple adjacent dots at the same BMI for each swimmer. There are not 2800 dots in each chart as not all athletes participate in all events.

Charts include those of the 200m women’s freestyle as a typical example along with the low BMI speeders mentioned previously

200m Freestyle – Men and Women

 

 

200m Freestyle – Women

 

200m Breaststroke (SCM) – Men and Women

200m Breaststroke (SCM) – Men

200m Breaststroke (SCM) – Women

200m Breaststroke (LCM) – Men and Women

200m Breaststroke (LCM) – Men

200m Breaststroke (LCM) – Women

1500m Freestyle (LCM) – Men

1500m Freestyle (SCM) – Men

800m Freestyle (SCM) – Women

 

 

The True Effect of BMI on Swimming Speed and Efficiency

Have you ever noticed that though men tend to swim faster than women, the women seem to use less effort? Well I have as I swim regularly.

This lead me to do some research and I found some unexpected results in a number of areas. The data appears unique to swimming. I haven’t seen any similar results on the web. This encouraged me to create this post.

Introduction

There are accredited Internet references to women having higher average body fat percentages than men. There are also theories that male muscularity makes their bodies denser than womens’. This makes men float lower in the water, increasing their hydrodynamic drag. The additional effort used to overcome this ‘water resistance’ probably causes the visibly greater swimming effort I’ve seen men expend.

To prove this I decided to see whether women swimming record breakers were faster per Kg of body mass than men. Comparing swimming world record holders (Wikipedia as of 01-January-2018) I found that ‘pound per pound’ women were on average 16% faster than men in all events.

The difference is especially striking when comparing this fact to track world records where across all events there is no consistent per Kg advantage of males over females or vice versa.

Men seem to be faster in swimming where their larger size and muscle strength per Kg of body mass outweigh their apparent drag disadvantage. However competing in water makes a huge difference to their efficiency.

Fortunately there are ample data online to get details of this efficiency relationship. I decided to chart the body mass index (BMI) values of all 700+ racing swimmers in the 2016 Olympic Games versus their average speed per Kg across all individual, single stroke events. I also charted their BMI versus their average speed. (All the results can be viewed below. My data can be viewed here.)

Discoveries

There are some noticeable insights.

  • For each event, there seems to be a straight line relationship between BMI and average speed per Kg. The higher your BMI, the slower you are per Kg. Let’s call this the swimming event efficiency line (SEEL). The charts below use the best fit trend line for the event finals and semifinals to create the SEEL as these are the top athletes.
  • Men and women share similar SEELs per event. (For simplicity I’ve only charted SEELs for the combined populations for each event.) Male values dominate the higher BMI end of the line with lower speeds per Kg (efficiencies) and women being seen at the lower end with lower BMIs and higher per Kg values.
  • The more turbulent events such as shorter distance freestyle and butterfly have much steeper SEELs. In other words, reducing your BMI provides a smaller speed per Kg advantage in those events than in smoother events such as freestyle and backstroke.
  • The standard deviation of the observations around SEELs is greater for turbulent events, and so will be less predictive of individual performance. You can see this where the smoother events have points clustered closer to the SEEL when compared with the more turbulent ones. In other words the SEELs for more turbulent events are less predictive of performance, but are still worth considering as the relationship still stands.
  • Each event provides one or two desirable general BMI goals to achieve maximum speed. This applies to both men and women. The relationship between BMI and average speed in less turbulent events tends to be a single curve peaking at a single BMI value. More turbulent events have two (or more) ‘BMI curves’ peaking at the BMI for maximum muscle power or that of minimum drag. I’ll admit that the number and shape of the curves is subjective. People are known to see patterns even when there are none, so more data will need to be assessed to see what clear boundaries exist. At a minimum,  clustering of the data points around BMI / speed values can be seen.
  • The BMI curves show that most men are fastest with a BMI of around 22, with women attaining maximum speed at a BMI closer to 19. In the butterfly events these values are higher. In the men’s butterfly the majority cluster closer to a BMI of 24, even though the fastest swimmers have BMIs closer to 21.
  • The speed per Kg of athletes decreases for longer events. This is to be expected.
  • Persons with the lowest BMIs are not the fastest, they are just the fastest per kG.

This lead me to consider what the implications of this discovery could be.

Possible Conclusions

This is by no means a comprehensive study, I am not a statistician by training and the sample size is very small. I suspect that larger studies will confirm some of the following very preliminary conclusions.

  • Speed per Kg should be a performance metric for coaches.
  • Persons with lower BMIs generally try harder per kG of body mass to keep up with the competition. They are not the fastest overall swimmers.
  • In some events younger swimmers will have the advantage of increased BMI over older peers. They should consider lowering their BMI, stroke turbulence and drag as they age to stay competitive. If the BMI curves are different across age groups, then the coaching strategy could be appropriately adjusted.
  • Athletes of slighter and shorter builds could become more competitive in many events if they add a little body fat without increasing drag. More muscular swimmers may benefit from less muscle and more fat.
  • Swimmers new to competition can get a general idea of their potential best events by seeing how close their BMIs are to the SEEL for that event. They can then select the events where the distance from their BMI to the BMI of the speediest swimmers is shortest on the relevant chart.
  • Swimmers in the latter stages of injury recovery, with lower muscle mass, may benefit more from adding a little fat to their bodies to lower their BMI and the stresses of drag until they are ready to get back to their competition BMI.
  • Triathletes may want to consider matching their BMIs to that of that of the fastest (and most efficient) long distance freestyle swimmers to reduce fatigue. Water is 800 times denser than air, and the energy savings from such a strategy could be significant.
  • Conventional wisdom for triathlons is to swim using as little leg effort as possible to save them for the cycling and running portions. Based on this information it may be better to use a light flutter kick during swimming to create better buoyancy. By swimming more efficiently at the same speed, you’d save energy for more energy efficient events, like cycling, where you’d benefit much more in terms of speed per Kg than ‘draggy’ swimming.
  • Athletes should consider adjustments to their strokes to ride higher in the water, reducing drag, and get closer to the SEELs of their age group or that of competitive athletes. It will help them know how much better they could improve through technique without changing their physique. This could be important for swimmers who compete in multiple events.
  • An easy to use method of measuring comparative swimmer buoyancy per event will need to be developed. This will help with overcoming BMI disadvantages.
  • Coaches could consider finding new techniques to make the individual SEELs of their trainees less steep so that relatively small changes in BMI provide a bigger gain in performance. Techniques to reduce drag could be more easily compared across large groups of swimmers by comparing before and after SEEL slopes and BMI curves.
  • Coaches should pay attention to friendly and competitive team swimmer outliers to the BMI curves to learn from their technique. (See the BMI curve for the men’s 100m breaststroke and 200m backstroke events for outlier examples). It could also be a way to more easily identify doping cheats.
  • Coaches should also observe the techniques of excellent (non outlier) competitors who have similar BMIs to that of their trainees. Strategies may need to be adjusted to match changing BMIs as swimmers age.
  • In swimming BMI is only one measure of  ‘floatiness’. More research will be required to see whether similar relationships apply to the athletes’ percentage body fat and other factors.
  • Though promising, the sample data needs to be bigger.

I’ve been surprised by the lack of quantitative data on the web related to the topic of ‘floatiness’ and swimming speed. It’s been a topic that often crosses my mind and hopefully those of others.

I hope SEEL charts will become commonly used by age group and event. This is not because I feel people should be obsessively fit, but because they will help all swimmers know how much better they could improve through technique without changing their physique. This will hopefully create reasonable goals and encourage more people to enter the sport recreationally. The BMI curves are the obsessive stuff.

If you are not convinced that improving technique to overcome drag is a major component of swimming strategy, look what simple streamlining in air can do, a much less dense medium than water.

BMI is only one factor to consider, but these results should hopefully provide a new set of tools to significantly benefit the sport that relaxes me each morning. Hopefully it may possibly inspire new world records.

Freestyle

The charts for freestyle events follow:

Freestyle – SEELs

Freestyle – 50m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Freestyle – 100m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Freestyle – 200m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Freestyle – 400m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Breaststroke

The charts for breaststroke events follow:

Breaststroke – SEELs

Where would you put the SEEL for this event?

Breaststroke – 100m

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Breaststroke – 200m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Butterfly

The charts for butterfly events follow:

Butterfly – SEELs

Butterfly – 100m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Butterfly – 200m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Backstroke

The charts for backstroke events follow:

Backstroke SEELs

Backstroke – 100m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?

Backstroke – 200m

Where would you put the SEEL for this event?

BMI vs. Speed (With and Without Curves)

Where would you draw the BMI curves for this event?