Abstract
INTRODUCTION: Observing the velocity decay after swimmers push off from the wall has previously been used as an indirect method for estimating passive drag (Kjendlie & Stallman, 2008). This method is based on the assumption that when a swimmer glides passively through the water, the water resistance is the only force retarding the swimmer and causing the velocity decay. The aim of this study was to test the reliability of the velocity decay method by measuring the consistency of the data from repeated tests under the same conditions. METHODS: 21 trained swimmers aged 16.4(±2.0) performed the velocity decay test twice on the same day. They were all familiar with the method due to previous testing. In prone position with arms above their head, the subjects pushed off from the wall before passively gliding to a stop. The gliding velocity decay was measured by a swim-meter with a fishing line secured to the swimmer passing through a rotating wheel. The swim-meter counts the rotations and thereby calculates the instantaneous velocity over the decay period. The added mass effect was taken into account by adding 27 % to the body weight in the calculations. A The added mass percent was the result of an oscillation test of 32 subjects (Caspersen et al. 2008). A Matlab routine seeking the least sum of squares was run through the velocity data, v(t), and estimated the highest velocity immediately after push off (v0) and passive drag coefficient (D) from: v(t) = v0 / ((Dv0t/mv) +1), where mv is the virtual mass, which is the sum of the mass of the subject and the added mass. The estimated D values from the subjects¿ two tests were compared. RESULTS: The mean(±SD) passive drag coefficient values from the first and second push off were 23.9(±4.9) and 23.0(±2.2), respectively. The reliability analysis revealed a Cronbach`s alpha value of 0.973, and a one sample t-test t=2.74 (p=0.013). The average difference between the two test expressed as a present of the mean value was 6.1 %. DISCUSSION: The velocity decay method seems to give somewhat the same result from test to test under the same conditions. The statistical analyses also revealed high reliability between subsequent tests. This does not mean that the correct passive drag values are measured. More research is needed to validate the method. The results from this study only indicate the reliability of the method carried through with experienced swimmers. Results from an unpublished study with students (non-swimmers) performing two velocity decay tests showed less reliability (cronbach`s alpha=0.889, t=-2.1(p=0.089) and average difference from the mean =14.1%).
