What can SWOLF tell us? Interpreting Swim Data from your GPS Watch.

Swimming watches for triathletes are ubiquitous.  These tools contain an accelerometer that can count strokes, detect how many lengths of the pool we’ve swum and keep track of our pace and time for every lap and repeat.   Triathletes and swimmers are now asking questions about what they can do with the information obtained.

The two most popular sites that currently track and report this type of data are the sites of the watch makers themselves, Garmin and SwimSense.  Training peaks also can import this data but as of this writing they lack a robust reporting feature.

The most easily obtainable and somewhat interesting metric reported is the SWOLF score.   SWOLF is an abbreviation for “Swim Golf”, and is a score obtained by adding together your strokes per length, and the time for the length.

e.g.  25 yards swim at 30 seconds in 20 strokes is a SWOLF score of 50.

Below is a screenshot of such a reported metric from Garmin Connect:


SWOLF Score Garmin 910xt GPS watch


And here is a similar example from Finis, the makers of the Swim Sense.

SWOLF Score FINIS SwimSense Watch


What does SWOLF tell us?

Many coaches believe that SWOLF is a measure of efficiency…that a lower SWOLF score is a more efficient stroke and we should strive to get SWOLF as low as we can.  However, two things must be kept in mind when comparing SWOLF score and efficiency.  Let’s talk about these two reasons and I’m sure you’ll also discover new ways to use SWOLF during your workout analysis.

The first reason that the SWOLF score is not a very good measure of efficiency…if it can be considered a measure at all, is that efficiency is the relationship between the amount of work energy that goes into an activity relative to the work output, or forward movement achieved.  For example, if we could measure how much oxygen a swimmer was consuming while swimming in a flume at a specific speed, then we could measure his or her efficiency.  External measuring tools of oxygen consumption are needed.   So while we can’t get an actual efficiency number from a swim watch, what we can do is incorporate certain clues that may tell us something about the efficiency of the stroke.

But more interesting is the second reason that SWOLF can’t tell us much about the efficiency.   Even though SWOLF is derived by adding two parameters together…namely Strokes per length (SPL) and the time in seconds for the length, we still need to know both the SPL and the TIME independently to make any sense of the SWOLF number.

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Time and SPL Matrix = Swim Golf Variations

Let’s create a two by two box of SPL and Time, to keep it simple let’s just use HIGH and LOW for SPL and time and see what may happen to SWOLF and what it means.  If it seems confusing at first, take the time to read through each scenario and you’ll create new ways to think about the metrics as well. 

Here are four hypothetical scenarios which I describe in a bit more detail after each one.

If if SPL * rate = time, then we are curious about what SPL + time or SWOLF can tell us?

Four Scenarios:

     HIGH  LOW
 Time HIGH A C


A) SPL is high and time is high = high SWOLF and inefficient swimming
B) SPL is high and time is low = medium SWOLF and improved efficiency
C) SPL is low and time is high = medium SWOLF and very efficient swimming
D) SPL is low and time is low = very low SWOLF and inefficient swimming

A) high spl and high time (ie slow) suggests a lot of drag and or inefficient catch. Scenario A swimmers can learn a lot from tracking SWOLF and watching it improve because experiencing either a lower SPL or a faster speed suggests they are improving swim efficiency

B) as speed increases an expected and normal response of efficient swimming is for SPL to increase (SWOLF may stay the same). Scenario B swimmers can expect to see a lower SWOLF for their easier swimming and a higher SWOLF for their faster swimming, and should strive to find a more sustainable faster speed…and be happy with a higher SWOLF. (but to discover what it is, or more specifically what their SPL & Pace targets are for various training distances and /or race settings

C) suggests very low stroke rate with lots of glide…little energy being put in compared for forward movement. Physiologically speaking a very efficient stroke (SWOLF may be the same is in scenario B). Scenario C swimmers should strive to increase their SWOLF, more specifically by increasing their tempo as they already have a very efficient stroke.

D) The ability to swim quickly and hold a low SPL requires high power, high strength swimming, and can be seen in Jai’s 2nd video here: SWOLF will be low but true efficiency is low. However if this is a race, fastest time, not highest efficiency wins. Scenario D swimmers are very skilled, fast powerful swimmers, and can pick and choose the stroke suited for their task.

In that video Jai swims two lengths. Both have the same SPL, but in the 2nd length he adds effort and speed to reduce his time. He is working much harder to get from one end to the other.  He gets faster while still keeping the same number of strokes.  But don’t take my word for it, go ahead and count them when you watch.

Here are the numbers:

Length 1: 16 strokes 31 seconds SWOLF 47
Length 2: 16 strokes 18 seconds SWOLF 34

The slower length is actually the more efficient stroke considering the amount of effort compared to the forward movement.   it takes Jai more energy to swim the faster length. Even without having the external measuring tools to determine true efficiency, it should not be difficult to believe that Jai could sustain the first length’s effort for quite a long time…the second length for possibly 100m or less.

A graph would be interesting here… plotting combinations of Speed vs. SWOLF at different SPLS/rates and seeing if there actually is a relationship we can glean that would be useful as a self-coaching tool. My guess is that there is not…we still need to know each metric…at least two out of the three (SPL, Time or Speed) in order to analyze data and know how use it to improve our swimming practices.

I find SWOLF most as a quick measure of consistency, rather than a measure of efficiency.  When i look at my athletes watch data, the SWOLF graph can reveal if they are swimming with too much rest or too easily. Or conversely if they are improving their ability to be consistent with a given set (eg. 10 x 100 as an example).  Pace alone doesn’t tell us, SPL alone doesn’t tell us, but SWOLF alone does not tell us either.

e.g. if I see a set of 10 x 100 with a SWOLF that does into change AND an SPL that does not change, they are swimming consistently which is good. I still need either pace or SPL along with SWOLF to learn anything about their swimming, it doesn’t stand alone. If I want them to swim faster and get fitter as a triathlete (my main market). SO I can reduce the rest, increase the repeat distance, increase the pace (which is possibly going to change the SWOLF ).

Hopefully that gives you more insight as to what some of the metrics mean and what you can do with the information.

I still find that practicing SPL ranges at will, finding your SPL, changing your SPL and holding your chosen SPL, layered with the use of the tempo trainer (steady, ascending, descending, etc)  gives the coach and athlete much more control and direction than SWOLF had the possibility to do.

IN a follow up article I will use some specific watch data graphs to help walk through an analysis of a swim set’s execution and plan further practice sets for improved skill.

Please let me know in the comments section what your thoughts are and how you use the watch data you collect.


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