Dive starts

The grab start and the track start, with variations in which body weight is positioned forward or backward, are the most commonly used start techniques. The major differences between the grab start and track start are how the feet are placed on the block and how the athlete's body weight is distributed with regard to the base of support. The technique employed by a given swimmer is selected in part based on personal preference, but the design of the starting block can also have an influence (Pearson et al. 1998). FINA, the international governing body of swimming, requires that starting blocks be constructed with a 0- to 10-degreeslope and a height between 0.5 and 0.75 meters above the water (www.fina.org/rules/rules_index.htm). Thus, the swimmer can encounter considerable variability at a competition. Additionally, FINA recently approved the Omega OSB11 starting block for use in international competitions, which has the potential to alter optimal start technique considerably. This block has an inclined kick plate at the rear and side handles, which will further affect the type of start that athletes use. The addition of the FINA-approved backstroke feet wedges is likely to see further modifications in the backstroke starting technique and times.

The basic techniques for the block starts are presented in the sections that follow. The backstroke start will be addressed separately, later in the chapter.

Grab Start

The recent decision by FINA to allow starting block configurations that have an adjustable slanted rear footrest or the addition of side handles has the potential to have a substantial influence on the start performance of swimmers. The adjustable footrest (commonly termed kick plate) on the Omega blocks can be moved forward and backward at set positions along the block to allow swimmers to use a crouch start and have the rear-positioned leg achieve a 90-degree knee angle (figure 6.5). The kick plate conceivably allows the rear leg to produce more force and generate higher horizontal velocities than can be developed with a track start on a traditional block. Further research is required to determine whether the swimmer's dominant leg would be better positioned at the front or rear of the block with this new configuration.

Several studies suggest that the new block configuration can have an effect on start performance. Honda et al. 2010 indicated that when compared with starts performed on a traditional block, starts that use the kick plate can significantly decrease block time and time to five meters, increase the force output of the rear foot, and increase horizontal takeoff velocity. In a separate study researchers found that on a custom-built instrumented block, a rear incline (at 36 degrees to horizontal) led to a less than 2 percent increase in horizontal velocity and a 3 percent decrease in the time to six meters when compared with the traditional start platform (Vint et al. 2009). This same study reported more significant benefits from the use of handles at the side of the block compared with the kick plate. These block modifications appear to favor the track start more than the grab start, so we may see a gradual phasing out of the grab start in international competition as these new block designs are used.

As a swimmer pushesoff the block, force is generated and applied against the starting block, which in turn pushes back against the swimmer according to Newton's third law - for every action, there is an equal and opposite reaction. The applied force can be broken down into vertical, horizontal (antero-posterior), and lateral (side-to-side) components and produce the swimmer's takeoff velocity. Downward force application into the blocks accelerates the body vertically (increased height), and the component of the force directly backward generates propulsion in the forward direction. Any lateral force is essentially wasted and should be minimized. In the track start, however, some lateral force is unavoidable because the legs contribute to force generation at different times (Benjanuvatra et al. 2004).

The way that the three components of force are generated dictates the takeoff velocity of the swimmer and the resultant momentum that the swimmer carries through the air. The interplay of the horizontal and vertical forces also determines the angle at which the swimmer's CG leaves the block. Generating more vertical force makes the angle of takeoff steeper; if a swimmer generates more horizontal force, the angle of takeoff will be flatter. Other information that can be derived from the force profiles includes the swimmer's reaction time, defined as the time from the starting signal to the first movement. Note that electronic displays of swimmers' reaction times at various competitions actually display the swimmers' block times - the combination of both reaction time and movement time on the starting block - which can vary considerably depending on the start used.

A number of researchers have examined how force is developed for the different start types (Arellano et al. 2000; Kruger et al. 2003; Vilas-Boas et al. 2003; Benjanuvatra et al. 2004; Honda et al. 2010). Sample force profiles for the grab and standard (forward-weighted) track starts are shown in figures 6.6 and 6.7.

Although the initial movement of swimmers pulling against the starting block with the arms is similar for both grab and track starts, subtle differences can be identified from the force-time curves. In the grab start, this effort is applied mainly in the vertical direction, reflecting the action of the arms pulling the body toward the starting block (represented by first elevation of the vertical force curves, region 1 on figure 6.6a and b). Conversely, the arm action in the track start appears to generate impulse in both the horizontal and vertical directions (region 1 on figure 6.6c and d).

In the horizontal direction, the grab start is characterized by the gradual development of force, reaching a peak just before the swimmer leaves the block. In contrast, the horizontal force for the track start develops earlier and is followed by two separate peaks. The first peak corresponds to the push-off from the rear foot, and the second peak corresponds to the push-off from the front foot (figure 6.6). Aggressive arm action and a strong rear-leg drive are used to generate force and forward momentum in the early part of the dive, but the front leg typically generates the major propulsive force on a traditional starting block (figure 6.7). The greater contribution of the front leg is likely because of the forward position of the swimmer's CG at takeoff. Greater vertical force is developed by the front leg at the beginning of the start, and both legs contribute considerably during the middle and later parts of the front-weighted track start. Although Honda et al. (2010) and Vint et al. (2009) have measured the total horizontal force using a back kick plate compared with traditional starting blocks, neither group of researchers has reported on the relative contribution of the front and rear feet.