Toussaint, H. M., Roos, P. E., & Kolmogorov, S. (2004). The determination of drag in front crawl swimming. Journal of Biomechanics, 37(11), 1655-1663.

This study compared two active drag measurement systems in elite swimmers (N = 6) to assess whether both measure the same retarding force during swimming. In method 1, push-off forces were measured directly using the system to measure active drag (MAD-system). In method 2 (the velocity perturbation method (VPM)), drag was estimated from the difference in swimming speed when Ss swam twice at maximal effort (assuming equal power output and assuming a quadratic drag-speed relationship): once swimming free, and once swimming with a hydrodynamic body attached that created a known additional resistance.

The average drag for the VPM tests (53.2N) was statistically significant and different from the active drag for the MAD-test (66.9N). A post hoc analysis was performed to assess whether the two methods measured a different phenomenon. Based on the drag speed-curve obtained with the MAD-system, the VPM-data were re-examined. For diverging drag determinations the assumption of equal power output of the ‘free’ trial (swimming free) vs. the towing trial (swimming with hydrodynamic buoy) was not supported. The major part of the difference in active drag values was due to a non-equal power output in the ‘free’ relative to the towing trial during the VPM-test.

Implication. The MAD system is simpler to interpret than the VPM assessment of active drag. It is possible with the VPM method to correct the findings with the error estimate from the regression equation from the towing part of the test.

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