Dive starts

This is an excerpt from Science of Swimming Faster by Scott A. RiewaldS & cott A. Rodeo.

The benefits of an effective start in competitive swimming cannot be underestimated. Evidence from race analyses conducted at major international competitions demonstrates significant correlations between faster start times and race times (Cossor and Mason 2001; Mason, Alcock, and Fowlie 2007). The start produces the fastest velocity that a swimmer will achieve during a race. When you consider that the start includes the first 15 meters of the race, it makes up a considerable proportion of the total event, especially in the shorter sprints over 50 to 100 meters (figure 6.1).

Swim starts as a percentage of the race distance (start distance is 15 meters).

Consider these examples of how start performance can affect performance:

  • A review of Olympic swimming results from 1972 to 2004 showed that a 0.1 second improvement in time, a difference that realistically can be achieved with a better start, would have resulted in 65 medals changing hands in sprint events (Hoof 2007). More recently at the 2008 Beijing Olympic Games, the top two competitors in the female sprint events (50 meters and 100 meters) were typically separated by less than 1 percent (Slawson 2010), again an amount that can be affected by a start.
  • An analysis of the 100-meter men’s butterfly final at the 1996 Olympics showed that the eventual silver medalist was 0.4 seconds slower to 15 meters than the winner, but his final time was only 0.28 seconds slower (Schnabel and Kuchler 1998); the faster swimmer placed second and essentially lost the gold medal in the first 15 meters.

The bottom line is that although less time is spent on the start than is spent swimming, starting is still a crucial skill to master at the elite level (Miller, Allen, and Pein 2003; Hay 1988).

Types of 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 grab start is similar to a two-legged jump. To begin, the swimmer places the feet about 0.15 to 0.30 meters apart and curls the toes over the front edge of the block (figure 6.2). The hands grasp the front edge of the block, either inside or outside the feet. In this position, the swimmer’s center of gravity (CG) is in a position of dynamic stability, positioned as far forward as possible within the base of support to allow for rapid movement forward. The arms are crucial in developing the initial forward momentum as they pull down and back against the block. Both arms then swing straight out toward the far end of the pool as both legs drive powerfully and simultaneously off the block (Houel et al. 2010). Kruger et al. (2003) showed that the knee and hip extensors are the main contributors to the takeoff forces generated by the legs, and the back muscles are preactivated to enable a more powerful extension of the body at the starting signal.

Stop-action image of the grab start.

Courtesy of the Western Australian Institute of Sport.

Implications of the Recently Approved Starting Block Configurations

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.

The track start with kick plate.

Courtesy of AIS Movement Science, Australian Institute of Sport.

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.

Force Development Characteristics

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.

Force Development Profiles

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.

Total vertical and horizontal force profiles for the grab start (a and b) and track start (c and d). For the track start, R marks the first peak corresponding with rear-foot propulsion and F marks the peak corresponding with front-foot propulsion.

(a) Vertical and (b) horizontal force profiles of the rear and front foot for the front-weighted track start.

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.

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