Why some swimmers are able to swim significantly faster with smaller average force?
I tried to answer this question by looking at the acceleration of the hand stroke. We know that when the velocity of the hand increases in backward direction, it produces high force forward and increasing speed of the swimmer. If the velocity of the hand starts to decrease, the production of force diminishes and leads to decreasing speed of the swimmer. Long periods of decreasing speed leads in turn to a high-speed variation and inefficient use of force.
The assumption was: The efficiency of the handstroke increases with the fraction of time hands accelerate in backward direction.
Efficient swimmers should swim faster with a lower amount of average force and have a higher stroke length. If the assumption is correct, we could rate swimmers stroke efficiency using the duration of the acceleration.
In order to evaluate the importance of the acceleration, I compared the stroke length and average force of six swimmers against their swimming speed. The measurements consisted of 3-4 x 50 meters freestyle with increasing speed. The average values for the whole set were applied in the calculation of the results.
The definitions were as follows:
Stroke length – the average distance body travels forward during one stroke cycle
stroke length = swimming speed x (60/stroke frequency)
*Note that the effect of the push-off from the wall is not considered. Swimmers in long tract seem slower and have a shorter stroke length in comparison.
Average force – the average force of both hands combined
Average force = ((Impulse x fraction forward)right hand + (Impulse x fraction forward)left hand) x (60/str. freq.)
Duration of the acceleration
Duration = time(max hand speed) – time(v = 0)
*Time difference between the red dots below
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| Stroke with long acceleration | Stroke with short acceleration |
The fraction of acceleration – percentage of the stroke cycle either hand is accelerating in backward direction
Fraction of acceleration [%] = (Durationright hand + Durationleft hand) / (60/str. freq.)
All parameters used in the calculations are available from the web. The swimming speed has been checked and corrected if there has been significant difference between the hands.
Results
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| Fraction of time for hands in acceleration | Average force applied at different swimming speed |
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| Stroke length at different swimming speed |
Emilia has the highest fraction of acceleration at lower swimming speeds. She is also using the least amount of force and has a decent stroke length. As a sprinter, Emilia was the only swimmer exhausted already after three trials and asked for a longer recovery period before the last 50 m. At the last trial, her hands didn’t accelerate as long as in the previous measurements, which may be due to exhaustion.
Tuomas had a significantly higher fraction of acceleration especially at higher swimming velocities than anybody else. He was able to keep the mean average force very low even when sprinting. He also had the longest stroke. The fraction of the time for acceleration increased with swimming speed as the duration of the glide phase decreased substantially.
For Eemil the fraction of stroke acceleration was fairly low at low speed but rather high at sprint. As a sprinter such a result is to be expected. The stroke length was fairly high, especially at top speed. However, Eemil used the highest average force of all swimmers. It is likely that part of the measured force is due to pressure drop forming on the back of his hand.
Siri was the only swimmer, whose fraction of acceleration decreased with increasing speed. This a clear deficiency in the swimming technique that should be corrected. She was also the only swimmer measured at long track. Her swimming speed and stroke length are therefore lower than swimmers measured at 25 m pool. Siri has the strongest leg kicks of the swimmers in this comparison. She was able to increase the swimming speed substantially, even if her average force didn’t increase in the last trial. The increase in the speed can thus be attributed completely to the kicks.
Mirka’s fraction of acceleration was fairly low in comparison. At low speed, her stroke length can be attributed to a relatively long glide phase. Nevertheless, the average force remained also rather low. It may be that as a result of low fraction of acceleration, Mirka didn’t reach as high top speed as the others. It should be considered also that the average values applied in the calculation may not represent Mirka’s technique as her stroke to stroke variation was much higher than that of the other swimmers.
Nelli’s fraction of acceleration remained comparatively low in all measurements. She had also the shortest stroke and used fairly high average force second only to Eemil. Nelli’s maximum speed was the lowest of all in this measurement.
Conclusions
Mirka’s results for average force and stroke length seem much better than what would be expected if hand acceleration would explain the efficiency of the stroke. Her high stroke to stroke variation may give misleading values for average hand speed. Also, Eemil’s average force is clearly too high, which can easily be explained by his excessive hand speed. All other results seem to agree well with the assumption. So, it seems that the fraction of acceleration is a number that reflects well the efficiency of the strokes. Increasing the fraction would either lead to faster speed or decreased use of force.
Discussion
Other factors that contribute to the stroke length and average force include i.e. swimmers drag, the strength of the leg kicks, stroke impulse, balance between the left and the right side as well as the stroke to stroke variation. It was surprising that the swimmers in the comparison ended up about in the correct order without considering any of these effects.
The calculated values are sensitive to the correct swimming speed. The measured distance should always be longer that one lap in order to minimize the possible error in the timing.
It is likely that the same criteria for stroke efficiency could be applied also for butterfly and backstroke. Its applicability for breaststroke is however by no means certain.
It is quite unlikely that the measured swimmers could maintain their stroke technique for a longer distance. Therefore, the measurements should reflect the competition distance which is of interest.
SmartPaddles is a wearable device that measures force of the swimmer in real-time. Swimmers need consistent movement patterns; as soon as the propulsive force drops the body will quickly slow down. The SmartPaddle helps to visualize when and where the changes occur during the stroke cycle. The SmartPaddles help the swimmers to understand “if they are using the right amount of propulsive force in the right direction at the right time”.
By Ari Auvinen a Founder of Trainesense Ltd.
