Quantifying Flutter Kick

Kicking is hard to teach.  Many triathletes struggle with kicking so much that they actually move backwards with a kick board!  How can a movement as simple as moving your legs up and down in the water be so difficult?  Contrary to what you may think, you can learn to kick well.  I refuse to believe that some people can kick, and others can’t.  This article is the first in a series to understand the mechanics of flutter kicking.

As I have been teaching kicking, one thing I realized was that I knew very little about how far and fast my feet and knees were traveling in the water.  I have always kicked well, but couldn’t fully describe why.  I have been teaching on experience instead of numbers and data.  So I set up an experiment using myself as the guinea pig in order to quantify what the legs are really doing during sprint kicking.

Experiment setup:

First, I tied a thin string in a loop around my ankles and above my knees.  The string had to be thin in order to minimize water resistance.  I then tied my feet together and then my knees together with the same thin string, estimating the distance of travel of feet and knees while kicking.  I kicked a length, then adjusted (and re-adjusted) the length of the string until it pulled tight at the precise distance that my feet and knees would travel while kicking at speed.  Then, I was able to take a ruler and measure precisely how far my feet and knees were moving apart with each kick.

I kicked a series of 25-meter sprints with this setup, timed to 1/100th of a second.  The number of kicks I took was counted.  In order to compare sprint kicking to sprint cycling, I assumed that sprint cyclists pedal at a rate of 120 revolutions per minute with an average crank arm of 172.5 millimeters.  It may be true that optimal cycling cadence for a triathlon or road race is 90 rpm.  However, it is necessary to approximate a maximum (not optimal) cycling cadence as a fair comparison to sprint kicking over 25 meters.    The raw data and results are below.

An explanation of the table follows:

Foot and Knee Travel (inch):  The maximum distance between feet was taken as the kicking amplitude.  This distance was small at just 16 inches.  Knee travel was less than the feet but still significant at 9 inches.  Thus, good kicking is not done with straight knees.  However, good kicking is not done when an extremely bent knee either.

Number of kicks and time:  In this experiment, the number of kicks was counted as the number of splashes made with the feet.  The number of kicks taken was high, at a count of ninety over 25 meters.  The average time to kick 25meters, at 16.22 seconds, is a fair representation of sprint kicking for most triathletes, up to the elite amateur level.  By way of comparison, the fastest competitive swimmers in the world can flutter kick 25 meters between 13 and 14 seconds.

Frequency (in rev/minute & seconds per kick):  It is clear that kicking has a frequency that is far in excess of sprint cycling.  At 166.5 rpm, the legs are changing direction (seconds per kick) every 0.18 seconds.  This number is close to human reaction time.  When kicking, you are continually sending signals to your legs to change direction.  It should feel almost “twitchy”.  An analogy is trying to start and stop a stopwatch as fast as you can, over and over.  On average, you can start and stop a stopwatch using the same finger at the rate of 0.18 seconds, the same rate as fast kicking is done.  Since the kicking frequency is so high, it appears as a continuous stream of white water to a casual observer.  Conversely, maximum cycling cadence has a frequency that is 40% slower.  Thus, it looks fast, but does not look and feel as “twitchy” as flutter kicking.

Foot Speed (mph):  It is all too easy to think that just because frequency of flutter kicking is higher than cycling, that foot speed must be faster as well.  But this experiment showed that foot speed wasn’t much different whether you were flutter kicking or cycling.  This result is counter intuitive and suggests that it is an illusion that kicking is faster than cycling.  The reality is your muscles operate at the same rates.  It is just that kicking involves a more rapid rate of direction change. 

Motion Efficiency (%):  This parameter is defined as the total distance the feet travels compared to the total forward distance traveled.  The ratio of forward movement to foot movement was 34.2%.  Each foot traveled 36.6 meters over the 25 meter pool length.  Thus, the feet were moving about three times the distance that the body moved forward while kicking.    

This exercise revealed many things about kicking.  The amplitude was low at 16 inches, but the frequency was high, at over 160 revolutions per minute.  And even with the high frequency, the foot speed wasn’t much different than the speed of cycling.  It was interesting as well that knee movement was significant at 9 inches, but was less than the distance the feet traveled.  So the next time you go kicking, think of moderate knee movement, and short feet movement.  Think of speed as a rapid change of direction rather than a rapid movement through the water.  Hopefully this will improve your kicking game!  Until next time, happy training.