ADMINISTRADORES Y OPERADORES

The resistive or drag forces during swimming act opposite to the direction of the motion. These forces are related to the water flow around the body and how the body interacts with that flow. There are three main forms of drag; form, friction (or skin), and wave (Bixler, 2005; Lyttle et al., 1998; Naemi et al., 2010; Naemi & Sanders, 2008; Sheehan & Laughrin, 1992; Vennell et al., 2006). Frictional drag represents the drag produced as a result of friction between the water and the surface of the swimmer when water passes over the body surface, and is reported to increase linearly with an increase in swimming velocity (Lyttle et al., 1998; Rushall, Holt, et al., 1994; Sheehan & Laughrin, 1992). The main two factors that affect frictional drag are body surface area and type of surface. These two factors would change the flow of the boundary layer around the swimmer. Any irregularities in the surface would result in more turbulent flow where small eddies are formed next to the surface and absorb energy resulting in more frictional resistance (Rushall, Holt, Sprigings et al., 1994). While on the other hand, having a completely smooth or oiled surface may not necessarily be advantageous to performance, as the resistance of oiled skin repelling the water is greater that the friction of water on skin (Bixler, 2005). The ideal situation for reducing friction drag is a smooth granulated surface such as shaved skin

24 that creates a thin layer of water that adheres to the skin, essentially resulting in friction between water and water which is much less than water and skin. Larsen, Yancher, and Baer (1981) estimated the frictional drag component to comprise 18 to 20% of total drag in boats. However, the relative contribution of friction drag to total drag in humans is much harder to determine due to the non-streamlined nature of the human body (Gadd, 1963).

Form drag is the result of the differences between pressure at the leading and trailing edges of the body (Naemi et al., 2010). Form drag is named so because the shape and form of an object or swimmer can play a major role in determining when the boundary layer separates and the severity of what the form drag would be (Bixler, 2005). Specifically, form drag is related to the cross sectional body surface area of the swimmer causing boundary layer separation. Boundary layer separation behind the swimmer is a main cause of the differences in pressure (Bixler, 2005). This separation in flow leads to the formation of large and small eddies which result in the creation of form drag because the eddies exert less pressure on the body than the water in the upstream section that have not yet separated from the body (Naemi et al., 2010; Rushall, Holt, et al., 1994). Certain technique changes such as lifting the head to breathe, dropping the legs and the angle of attack would affect the amount of drag due to an increase in projected area exposed to the water (Naemi et al., 2010). Angle of attack is the angle between the reference line on a body and the vector representing the relative motion between the body and the fluid it is moving through. Hence, the greater deviation from the streamlined position while travelling though the water would result in earlier boundary later separation and greater form drag acting on the swimmer (Rushall, Holt, et al., 1994). Form drag is reported to increase by the square of velocity and becomes increasingly more important the faster the swimmer travels (Bixler, 2005; Rushall, Holt, et al., 1994).

25 Wave drag occurs when swimming at or near the surface of the water when the swimmer and the movement of body segments create waves. Wave drag is said to be the most deleterious of all the types of drag because it increases to the sixth power of the swimmer’s velocity (Lyttle et al., 1998; Vennell et al., 2006) and is mainly due to energy lost in creating wave systems around the vessel or swimmer (Vennell et al., 2006). If a body is swimming below the surface deep enough and at a sufficient enough distance from the bottom the flow of water around the body is the same as if the body were flying in free air space (Hertel, 1969). When the swimming body moves closer to the surface the resistance increases and waves are generated on the surface of the water. Early towing research found that the minimum resistance was achieved when the body was submerged at a depth equal to approximately three times the diameter of the body of revolution (Hertel, 1969). The maximum resistance occurred when the body was lying directly under the undisturbed water level. The amount of resistance measured here was about five times the minimum resistance (Hertel, 1969). With this in mind it is necessary to determine how the influence of wave drag changes and affects the swimmer as they rise to the surface following a dive start.

Towing swimmers through the water at various speeds in a streamlined position is a common way to measure passive drag and conduct detailed analysis on the relationship between drag and speed (Clarys, 1979; Lyttle & Blanksby, 2000; Lyttle, Blanksby, Elliott et al., 1998)There are currently conflicting results between the few studies which have investigated drag forces underwater. Clarys (1979) reported 20% higher drag values when being towed underwater compared to the surface whereas Maiello et al. (1998) found higher drag force at the water surface. A limitation of these two studies was towing speed not being fast enough to translate into the speeds which swimmers would be travelling at during the start and turn phase of a swimming race. In another study using higher velocities, Lyttle et al. (1998) sought to establish

26 the optimal gliding depth and velocity using faster velocities which mimicked the speeds travelled by elite and club swimmers during the turn phase. The results from this study found that at velocities higher than 1.9 m·s-1 swimmers should aim to perform their glides approximately 0.4 m underwater to gain maximum drag reduction methods. A 15-18% decrease in drag was found at this depth when compared to gliding at the surface (Lyttle et al., 1998). In the same study by Lyttle et al. (1998), total body drag force reduced at the velocities of 1.6 m·s-1 _{and 1.9 m·s}-1 _{when swimmers were kicking while being towed.}

Additionally, there is one study that disagrees with other similar passive drag studies. Jiskoot and Clarys (1975) reported that swimmers experienced 20% higher drag 0.6 m below the surface compared to swimming at the surface. Jiskoot and Clarys (1975) used velocities that were slower than speeds produced by swimmers in competition (1.5-1.9 m·s-1_{), particularly} during the start phase and did not describe how depth was controlled. The results from Lyttle et al. (1998) differ from Clarys (1979) because wave drag is expected to contribute more to total resistance as velocities increase. Hence, at the low velocities of 1.5 – 1.9 m·s-1 used by Clarys (1979) and Jiskoot and Clarys (1975) would not have been high enough to produce a substantial amount of wave drag. More research which isolates the contribution of wave drag on human swimmers would be beneficial in determining the optimal depth swimmers should travel at, particularly during the underwater phase of a swimming start.

Even though towing is a common method to estimate drag, there are limitations associated with previous towing systems. These include the inability to sufficiently control towing velocity and depth, unnatural streamline towing positions and the use of towing at speeds which are less than that used during starts and turns. To overcome this problem Vennell et al. (2006) used mannequins and a flume to conduct their drag research and Bixler et al. (2007) used a CFD

27 model. However, a disadvantage of these methods is that they do not take into account positional changes that naturally occur during swimming. Further, these methods can be time consuming and as a measurement of drag CFD, has an ability to model the complete surface of the body using software designed to model flow over solid bodies (Naemi et al., 2010).