Abstract
INTRODUCTION: Passive drag can be measured by towing swimmers passively through the water. This requires expensive and often inaccessible equipment and can be a time consuming process. The velocity decay test has previously been used as an indirect method for estimating passive drag (Kjendlie & Stallman, 2008). The aim of this study was to test the validity of the velocity decay method by comparing the passive drag coefficient (D) calculated from the velocity decay test with D from towing tests. To estimate passive drag from the decelerating phase correctly, the added mass effect was taken into account. METHODS: 24 subjects (6 women, 9 boys and 9 men) carried through an underwater oscillation decay test to find the subjects added mass. The subjects were then passively towed in a towing tank at five different velocities (1.5, 1.9, 2.3, 2.7, and 3.1 m¿s-1). D values were calculated by dividing the towing force by velocity squared. Finally, in a prone position with arms above their heads the subjects pushed off from the wall before passively gliding to a stop. The gliding velocity decay was measured. A Matlab routine seeking the least sum of squares was run through the velocity data, v(t), and estimated D and the velocity immediately after push of (v0) from: v(t) = v0 / ((Dv0t/mv) +1), where mv is the virtual mass (the sum of the mass of the subject and the added mass). Mean D values from the five towing velocities and D values estimated from the velocity decay test were compared by a paired t-test. RESULTS: The oscillation decay test resulted in an average added mass of 26.3 (2.9)% of body weight. The average D values from the velocity decay test and towing tests were 24.4 and 18.1, respectively. A paired t-test revealed a statistically significant difference in D values calculated from the towing procedure and estimated from the velocity decay method (t=5.27 and p=0.00). DISCUSSION: Most of the subjects tested were non-swimmers, inexperienced with the test method. Because of the cold water at the testing facilities (15 degrees C), the subjects were also unable to practice finding the optimal gliding position. Given these factors, it is likely that the positions from the velocity decay test and towing differ in this study, causing the differing D values from the two tests. With the exception of one test subject D values from the velocity decay test were consistently higher than D values from towing. This might indicate an effect other than unstable gliding positions from inexperienced swimmers. When towed it is easier to stretch out and keep a streamlined position due to the towing force and drag force opposing each other. This effect could be causing smaller D values from the towing tests. More research with trained swimmers familiar with the testing procedures is needed.
