Friday, September 14, 2012

A Little Science Behind Propulsion

As you swim, have you ever wondered how much of your arm stroke effort is wasted? What percentage really goes into pushing you forwards through the water and what is lost as water movement down, to the side and into eddies?

The technical term for this is ‘propelling efficiency’ and it makes a direct contribution to your overall efficiency as a swimmer. If more of your effort creates forwards propulsion and less is lost as water movement then you become a faster and more efficient swimmer.

When you work on your catch and pull technique to press the water backwards with good timing you are working on improving this propelling efficiency.

Swimmers vs. Triathletes

Last year we designed the exciting new range of HUUB wetsuits with Dean Jackson and top sports science Professor Huub Toussaint (we liked his name so much Deano named the company after him!). As well as a guru on minimising drag in wetsuits, Huub is also the world leading sports scientist looking at propulsion in swimming.

In 1990 he conducted a study comparing high level swimmers and triathletes in the water [1]. The results were fascinating. At the same swimming speed, the triathletes converted 44% of their work output into propulsive power whilst the swimmers converted 61%. A very large difference in propelling efficiency highlighting the superior stroke technique of the swimmers over the triathletes.

Huub Toussaint and Dean Jackson with Swim Smooth's
Adam Young and Paul Newsome
This study looked at national and international standard athletes. However if you are a little lower down the pecking order yourself then you can expect your propelling efficiency to be lower with the majority of your effort lost into the water. Your figure might be in the 10-40% range. At the other end of the spectrum, Huub has measured propelling efficiencies as high as 80% in Olympic swimmers at racing speed.

Propelling Efficiency And Effort

Huub's study compared swimmers all moving at the same speed. What happens to your propelling efficiency when you vary your effort from slow to fast swimming? You may find the answer quite surprising: As you work harder, your propelling efficiency improves and as you slow down it gets worse. In other words, if you increase your effort (e.g. by 10%) you get a greater return in propulsion (e.g. great than 10%). Conversely, drop your effort by 10% and you lose more than 10% in propulsion.

Although this change in efficiency with effort is well recognised by sports scientists, there is very little high quality data out there to quantify it for human swimmers. However, we can get a rough idea by looking at studies of fish. Webb studied trout swimming over their full speed range [2]. The results show a very strong relationship :

We have to be careful with this data as it relates to fish not humans but the strength of the relationship is striking. At slow speeds over 80% of their effort is lost into the water whilst when moving quickly they become much more efficient, losing only 20%. The hydrodynamics of why this happens is complex and beyond the scope of this blog but the key point to appreciate is how been overly gentle with the water is actually extremely inefficient.

(In an interesting aside, Webb noted that the fish in the study were reluctant to swim very gently, perhaps innately recognising the inefficiency of doing so!)

Efficiency Folklore

Much of the information out there in swimming books and on the internet purely focuses on decreasing your drag to make you a more efficient swimmer. For that reason it’s hardly surprising if in your mind you equate efficiency with reducing drag but really that’s only half the story. Your overall efficiency is a combination of your propelling efficiency and your drag profile.

You could say:
overall efficiency = propelling efficiency x drag efficiency

It’s important to appreciate that both elements are of equal importance, not one over the other. If your propelling efficiency is poor then you can work all you like on reducing drag but you will only make small improvements in efficiency.


If you are a bit of a swim geek (like us!) then hopefully you found that interesting. But what practical conclusions can we draw? Here are our thoughts:

Eddies in the space time continuum
1) To become a more efficient swimmer, work on both your propulsive technique (e.g. catch and pull) and reducing drag (e.g. balance, alignment and kick). Don’t be sold on the idea that one is more important that the other or more of a priority to improve your stroke efficiency.

2) As we discussed last week on the blog be careful with the notion of swimming ‘effortlessly’ to become more efficient. As we have seen above, swimming too gently actually reduces your overall efficiency. This is one of the main reasons why swimmers trying to glide effortlessly down the pool become very frustrated with their lack of speed.

3) Don’t become a ‘technique hermit’, shunning all swim specific fitness training and solely focusing on swimming drills. The more swim fit you are, the higher effort you can sustain and therefore the more efficient you will be. Swim fitness is a vital ingredient of an efficient swimmer!

4) A key area of development for the Bambino Swim Type is better rhythm in the stroke. Although this involves turning the arms over quicker, the result is normally a reduction in effort, not an increase. This happens because the Bambino has such a poor feel for the water that their propelling efficiency is very low indeed and by lifting their stroke rhythm this improves.

5) Don’t use this as an excuse to muscle the water, that’s not what we’re talking about here. Efficient propulsion is generated through pressing the water in the right direction with good rhythm and timing. Whilst it certainly does not feel effortless, the catch should not be a forceful movement either.

Swim Smooth!

[1] Toussaint HM. Differences in propelling efficiency between competitive and triathlon swimmers. Med Sci Sports Exerc 1990;22:409-415.

[2] Webb PW. The swimming energetics of trout II: Oxygen consumption and swimming efficiency. J Exp Biol 1971;55:521-540


Anonymous said...

excellent post! the subject itself is very interesting! :D

Anonymous said...

I agree, very interesting subject.

But I heard elsewhere that elites convert 10% max of energy into propulsion, and the average swimmer around 3%. Which info is correct?

gear jane said...

Great tips on the implications. I've been working on my technique for this year until I finally saw the video of myself swimming and notice my balance was off. Now I know what I should concentrate on. Thank you for reassuring us.

Do you recommend a timer?

Adam Young said...

Thanks guys!

In terms of efficiency, I think what you're referring to there is figures that include the metabolic efficiency of the body too. i.e. converting the carbohydrate in your muscles into body movements and then in turn into propulsion. That's an overall efficiency figure but with our propelling efficiency we're only looking at the second part - body movements into propulsion. This is of more interest as it isolates the technique aspect of things.

In Huub's study, he measured that metabolic conversion to be around 8% efficient. This ties up I think - the 8% of the metabolic conversion * 80% propelling efficiency of a top elite swimmer gives an overall of ~7% which is much closer to what you are quoting.

The metabolic efficiency is low for all sports and not particularly trainable which is another reason why it's not particularly of interest. The propulsive efficiency varies to a much greater extent and is very much changeable with technique and fitness levels.

Hope that answers your question!


Adam Young said...

Hi Jane, I'm not sure what you mean by timer?

zackme said...

I think the person quoting different numbers may have mixed up the data I probably heard in the same readings/interviews:
- 10% of the propulsion effort would translate into speed vs 30% of penetration improvement translate into speed (the difference being explained by a square root in a formula used to derive this).
- that a normal swimmer peneration efficiency (or rate) is 3%, vs 9% for M. Phelps and 90% for Dolphins.
And indeed, I am struggling to reconcile these data with those in your interesting article. Any clue?

Anonymous said...

as a competitive swimmer in my early years and a open water/pool swimmer now, at 57, I have worked at being very balanced and smooth for 8 years now. I love knowing that a study has shown propulsion efficiency is vital, too. I had let that go by the way side as I have been so into the b&s. I'm excited to add that piece prominently to the mix again.

thank you,

gear jane said...

Sorry about that Adam. I am referring to the tempo trainer.

Every time I see them, it reminds me of a watch without the wristband. lol

sheila sekhar said...

Please tell me if training with the Finis freestyle paddle will help.

Adam Young said...

Hi Zack, I think you're going to have to link to the studies that produced those figures. They seem to me to be mixed. 9% for Phelps would have to be gross efficiency (including metabolic) and the 90% for dolphins would have to be propulsive efficiency only. There's no way a mammal can convert 90% of their calories to energy output.

Hi Jane, Tempo Trainers are great for adjusting your stroke rate up or down and pacing out your training sets accurately. If you feel like your balance is off (low sinking legs?) then it won't help you with that directly. First thing top focus on there is exhalation technique into the water to reduce buoyancy in the chest and second thing your leg kick technique. By all means post more info on your swimming!

Hi Sheila, the Finis Freestyler is a great paddle to help you remove a crossover of the centre line with the lead hand in front of the head - which will cause you to snake down the pool and also harm your catch and so propulsion. It can also help you setup for a better catch with the fingertips lower than the wrist and the wrist lower than the elbow. In that way yes it does help you develop your propulsion and so improve your propelling efficient!



Anonymous said...

The reason behind "As you work harder, your propelling efficiency improves" is that at low speeds, the traditional drag force (or form frag, F ~ v^2) is a very small portion of the total drag, but the skin friction (stokes law) force (F ~ v) is "bigger" compared to the total drag.

And in addition to this, the reynolds number of the human bode in water changes as a function of speed, and at attainable swimming speeds the interference drag forces might make it so that the reunolds number goes down with speed (& and so the drag forces of the body won't increase as much as you would think).

Adam Young said...

Hi anonymous, when you say form drag is lower than skin friction, are you talking predominantly about the head/body/legs of the swimmer?

Anonymous said...

What would be far more interesting is the cost of locomotion. This is expressed as energy per metre travelled (equivalent to miles/gallon).

The drag forces on a swimmer are approximately quadratic (but with some cubic/wave too), so whilst ones' propulsion efficiency may increase with speed so to do the losses.

Look at COL WRT running. Walking has a minima at about 3.1mph, above 4mph it's more efficient to run. Walking is about 200J/m (normalised), whereas running is about 230J/m, but it gets slowly more efficient as one goes faster. If one has an efficient running style you can get to 215J/m, so about the same cost over a distance as if you walked. So you can run a marathon without eating. But if you have poor technique you could be consuming 300-400J/m, so you will bonk quite early.

Similarly with COL for swimming. Applied well that will leave you fresher off the swim to tackle the bike.

Adam Young said...

Hi Anonymous,

Here are my thoughts on your thoughts :)

- A 'miles per gallon' type figure would be interesting but since drag increases with speed, going slower would lower your energy per metre travelled significantly - which you could do whatever your stroke technique. So really the ideal measure would be rate of energy use at your target race speed.

- Obviously lowering your drag profile is very important but the point of this post is that you also need to be concerned with your propelling efficiency (PE) too in order to be an efficient swimmer. A low PE is a HUGE waste of effort - and as the data shows it gets lower if you deliberately lower your effort level. An example in numbers: there's no point focusing on lowering your drag profile by the last 10% when your propelling efficiency is only 15% and it should be nearer 50%...

- If you properly train for swimming 3x per week there's no reason why you can't swim near your lactate threshold and then get off and ride and run to your maximum. Sure for Ironman distances you might swim 10-20% easier but there's no need to deliberately slow right down and try and swim effortlessly. You're just giving away lots of time (and efficiency) and sports scientists are very clear - you can't get that energy back.

Hope that helps!