PROPULSIVE AND RESISTIVE FORCES IN CRAWL STROKE HAVE BEEN QUANTIFIED BUT ARE LIMITED TO ONE MODEL

Keys, M., Lyttle, A., Blanksby, B. A., & Cheng, L. (2010). A full body computational fluid dynamic analysis of the freestyle stroke of a previous sprint freestyle world record holder. A paper presented at the XIth International Symposium for Biomechanics and Medicine in Swimming, Oslo, June 16–19, 2010.

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This study undertook a case-study approach to examine the propulsion and drag forces across the body experienced during full-body freestyle swimming using Computational Fluid Dynamics (CFD). The swimmer used held the 50m and 100m freestyle World Record at the time of the testing and a full 3D surface scan of the swimmer was used for the CFD simulations. Manual 3D digitizing provided the 3D kinematics to animate the model. A realisable k-epsilon turbulence CFD model was used in the analysis.

The overall changes in forces throughout the stroke were characterized by six clear cycles, containing four small peaks and two large peaks. These peaks represented the six-beat kick pattern that was adopted, with the two large peaks correlating with the peak propulsion of the left and right arm strokes; which occurred simultaneously with two of the kick cycles. These peaks, and in particular the peaks associated with the arm stroke propulsion, were reflected in increases in the swimmer’s instantaneous velocity. An examination of the breakdown in the distribution of forces revealed that the arms and legs create a significant amount of the total propulsion, with the trunk contributing to the majority of the drag force. The hands provided a total propulsive momentum of 23.8Ns while the combined contribution of the wrist, forearm, and elbow was 27.6Ns. This highlights that the forearm position during the underwater arm stroke is as critical as that of the hands. Likewise the thighs, knees and shanks also contributed a greater percentage of the propulsion than the feet.

Implication. The current theoretical study estimated how propulsion and drag forces are generated throughout a full freestyle swimming stroke through the use of Computational Fluid Dynamics. [The findings of this theoretical study are very different to one that actually took measurements of the hand and forearm forces created in a swimming flume (http://coachsci.sdsu.edu/swim/bullets/icar9192/icar311.htm). In that study, the contribution of the forearm increased to more than three times that of the hand as velocity increased.]

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