When swimming through the water, the body will undergo a retarding force due to resistance, or drag. This force is, given the magnitude of the competitive swimming speeds, predominantly due to turbulence behind the swimmer. Furthermore, when movement occurs at the water surface, additional resistance will arise due to wave formation by the swimmer. This total drag force is depending on swimming velocity to the power of (at least) two. Drag is therefore one of the factors that may limit swimming performance.

Throughout the history of swimming research attempts have been made to measure this resistance. Amar (1920) was the first to assume that the resistance is related to the square of the swimming velocity according to:

Fd = K•v²

in which Fd denotes drag force, K is a constant incorporating the density r, the coefficient of drag CD, and the frontal area Ap, while v is the swimming velocity. The relation between resistance (N) and velocity (m•s^-1) based on Amar’s towing experiment was approximately Fd = 29•v². It was conjectured that the movements necessary to create propulsion could induce additional resistance (see for comparison active versus passive drag here). This led to attempts to determine the drag of an active swimming person.
Techniques to determine this active drag were developed by several groups in the 70’s (Clarys, Jiskoot, Rijken & Brouwer, 1974; Prampero, Pendergast, Wilson & Rennie, 1974; Clarys & Jiskoot, 1975; Holmér, 1975) all relying on extrapolation techniques; see for an overview Toussaint, Hollander, Berg & Vorontsov (2000). In the mid-80’s, a technique was developed that relies on the direct measurement of push-off forces while swimming the front crawl: the system to measure active drag (M.A.D. system, see Figure below). Using this MAD-system, mean values for K of about 30 for male top-swimmers and about 24 for female top swimmers when swimming the front crawl were found (Toussaint, Groot, Savelberg, Vervoorn, Hollander & Ingen Schenau, 1988). Kolmogorov and Duplisheva designed yet another method to determine the active drag. In their so-called velocity perturbation method subjects are asked to swim a 30 m lap twice at maximal effort: once swimming free, and once swimming with a hydrodynamic body attached that created additional resistance. For both trials the average velocity is calculated. Under the assumption that in both swims the power output is maximal and constant, active drag can be calculated since power equals force times speed. Recently, we compared both methods of active drag determination. A paper on this comparison is now 'under construction'.

It is remarkable that more recent determined K-values for top swimmers are about 10% lower than those determined for top-swimmers 18 years earlier: 22 for females and 27 for males (Toussaint, Truijens, Elzinga, Ven, Best, Snabel & Groot, 2002). Still, the total average drag force acting on the swimmer when swimming at a speed of 2 m/s is with about 110 N considerable. This makes it interesting to investigate whether drag can be reduced using a proper technique by for example reducing the frontal area and/or the drag coefficient. However, the literature does not provide a straight forward answer: On the one hand drag seems determined by anthropometric dimensions (e.g. body cross-sectional area and height) in groups of elite swimmers that are more or less homogeneous with respect to swimming performance (Huijing, Toussaint, Clarys, Groot, Hollander, Vervoorn, Mackay & Savelberg, 1988). Probably a small reduction in drag can be achieved by stretching the arm in the glide phase of the stroke, as was suggested by Holmér (1979b) (see also effect on wave drag). On the other hand, reduced velocity oscillations are observed in the more proficient swimmers (Kornecki & Bober, 1978; Holmér, 1979a; Colwin, 1992), suggesting that with a proper swimming technique drag might be reduced swimming the front crawl.