|Optimising Swim Turn Performance|
With turn times accounting for up to one third of the total race time, minor improvements in turning performance can lead to substantially improved event times. A successful swim turn results from a multitude of factors and requires a complex series of manoeuvres to optimise the total turning performance. The figure below represents an outline of some of these contributing factors in a tumble turn, as adapted from Hay's (1992) theoretical model of turning (See Fig. 1). Many components within this model work on a trade-off basis whereby there is an optimum level for each variable. By increasing one variable, there might be a detrimental effect on other factors that also affect performance and these have to be taken into account before prescribing technique changes. This paper focuses predominantly on the tumble turn (freestyle and backstroke), however many of the points can be directly related to the breaststroke and butterfly turns.
The freestyle tumble turn has evolved in its current form as a result of rule changes which no longer require a hand touch during the turn. The rotation, wall contact and wall push-off used in the current freestyle tumble turn also share many similarities with the current backstroke rollover turn. Hence, results for these aspects can be translated between the stroke types. The freestyle tumble turn can be divided into the approach, rotation, wall contact, glide and stroke preparation phases. Variations of techniques are observed within each of the phases by swimmers at all levels of competition.
Approach Phase - The approach phase is important in order to maintain momentum into the wall. This phase is defined usually as beginning at a fixed distance from the wall (eg. 5 or 7.5 m) and ending with both arms by the swimmer's side prior to the forward somersault. The movement of the arms by the side is accomplished either by stopping one arm at the end of the pull and waiting for the other arm to pull through and join it, or by stopping one arm at entry and allowing the other arm to catch up with it before executing a two-handed pull back to the hips. The distance out from the wall at the start of the forward somersault varies depending on the skill level and anthropometric considerations. Blanksby et al. (1996) found that faster age-group swimmers tended to initiate their turns further out from the wall.
Rotation Phase - The rotation phase is usually initiated by flexion of the head and spine in conjunction with a pronounced dolphin (or freestyle) kick, which drives the head and shoulder downwards and raises the hips. The increased resistance experienced by the head and shoulders as they move out of alignment with the rest of the body, together with the propulsion produced by the final kick, causes the swimmer to somersault forward. The upper body flexes about the hips and the knees are tucked close to the chest to reduce the distance from the axis of rotation and decrease the moment of inertia (which increases the speed of rotation). Generally, when the body has rotated forward (about the transverse axis) approximately 150°, the swimmer begins to twist onto their front (about the longitudinal axis). While the body is rotating, the arms are extended above the head so that the upper body is streamlined at wall contact. The degree of longitudinal rotation varies in current competition anywhere from 0° to 180°. Maglischo (1993) subjectively believed that the swimmer should initiate wall contact in a predominantly supine position (on their back) and then rotate to a prone position (on their front) throughout the wall push-off and glide. Given Counsilman's (1955) findings of higher passive drag forces when rolling about the longitudinal axis (as opposed to performing a glide on either the swimmer's front or side), this suggestion may not be the most appropriate.
Wall Contact Phase - The wall contact phase is initiated by feet contact with the pool wall and is finished at toe-off. The swimmer's feet should hit the wall at a depth of approximately 0.3 to 0.4 m (Maglischo, 1993). The degree of hip and knee flexion at wall impact varies between swimmers. Blanksby et al. (1996) found that the tuck index (the ratio of the smallest hip-to-wall distance to total leg length) was negatively correlated with turn times. This suggests that the larger the tuck index (ie. the straighter the legs), the faster the turn times will be. Ideally the angle of the knee should be in the region of 110 - 120 . A reduction in the angle of flexion at the knee (past 90 ) places the quadriceps muscle group (the prime muscle group in the wall push-off) at an inefficient muscle length and this in turn inhibits their ability to produce force quickly. Another advantage of a greater knee angle (ie. less flexion) is that the swimmer has to swim less distance before turning and this can result in significant savings on time and energy over multiple turns. Flexing the knees to any great degree after contact will result in a dissipation of any stored elastic energy and an increase in the passive wall contact phase, both of which should be discouraged.
The total time on the wall should be kept to a minimum, however should still realize sufficient impulse (amount of force produced against the wall over the whole wall contact time) so the effectiveness of the push-off is not compromised. Wall contact time can be separated into the passive and active force production phases. The passive force production phase consists of the initial wall impact and any countermovement (eccentric contraction of the quadriceps) experienced immediately following this (termed 'settling'). This settling on the wall has limited benefit to the development of the swimmer's velocity. The active force production phase consists of the concentric contraction of the quadriceps in order to create velocity away from the wall. The active force production phase is composed of a forceful extension about the knees and plantar flexion of the feet in the horizontal direction. The upper body is extended and rotated about the longitudinal axis to facilitate the body roll necessary to return the swimmer to the prone position after toe-off. To optimise the push-off, it is therefore important to minimise the passive phase and maximise the active phase while keeping the total wall contact to a minimum.
Also important when optimizing the wall push-off is the way that the force is developed on the wall. There are various schools of thoughts regarding the way that swimmers should develop force on the wall (eg. hard and fast, sink in and develop force gradually etc.). Research by Lyttle et al. (1999) show that there is optimal balance required between the amount of peak push-off force, time spent pushing off the wall and the resultant peak drag that is produced. It was found that rather than hitting the wall hard and fast, it was better to develop the force gradually so that the peak force occurred later in the push-off when the swimmer is in a more streamlined position (in that way, the peak drag force acting on the swimmer will have less of an detrimental effect on the velocity).
More detailed analyses have been performed using instrumented turning boards to measure the forces applied against the wall (see Figures 2-5 courtesy of the University of Western Australia - Department of Human Movement and Exercise Science). This type of analysis should not be taken in isolation but combined with the video footage of the turn. What could be considered an efficient push-off from a kinetics (forces) point of view may be offset by poor body position and streamlining. An optimal shaped curve would consist of the force being consistently developed throughout push-off with the peak force occurring closer to take-off when the swimmer is in a more streamlined position. Curves with multiple peaks usually mean that the segment co-ordination is not optimal or that there is a substantial settling phase on the wall (eg. there is an impact phase at contact where the force generation is not being used to propel the swimmer off the wall). The following graphs show examples of force curves from the wall contact phase of turns of elite swimmers (the white line represents the propulsive force into the wall during push-off).
The position of the feet during contact on the wall is also an important feature. It is common to see swimmers either over-rotating or under-rotating, which will cause the foot placement to be either too low or too high, respectively. This will then result in the swimmer misdirecting their push-off from the wall (by shooting to the surface or to the bottom of the pool) or adopting an appropriate compensation technique (such as arching the back). The most effective use of the push-off forces occur when ankles, hip and shoulders are aligned (see Fig. 6). Ideally, the swimmer should have the trunk horizontal with the arms out in front of the body prior to the start of push-off. This has a separate advantage in that it keeps the frontal surface area as small as possible and increases streamlining during push-off. A slight negative angle may be required to get the swimmers into an ideal depth, but the direction of force should still be running through the ankles, hips and shoulders.
Glide Phase - The glide phase incorporates maintaining a streamlined position so as to minimise the resistive forces at the higher velocities. To maximize the overall efficiency of the turn, it is important to reduce the deleterious drag experienced by the swimmer during the streamlined glide. These reductions in drag will translate directly to improved turn times. Research has shown that swimmers should aim to perform their glides at a distance of between 0.4 m and 0.5 m underwater to benefit from the reduced drag forces (Lyttle et al., 1998). It should be noted that this will also be dependent on the level of turbulence around the surface of the water. This optimum depth is gradually decreased closer to stroke resumption as the drag experienced by the swimmer is related to the swimmer's velocity as well as the depth underwater. Any increase in the glide depth will not produce any substantial reductions in drag forces and despite being a popular strategy for some international level swimmers, should be discouraged. Form drag and wave drag are important factors in determining the total drag force experienced by the swimmer and thus, it is important that the swimmer holds a good streamline throughout the glide without excess body movements. An optimal gliding technique incorporates maximising the distance achieved from the wall push-off by minimising the deceleration rate caused by the drag force. A more efficient glide depth and streamlining will result in an increased glide distance for the same time period, thereby increasing the effectiveness of the turn.
The issue of streamlining is very important, both while swimming and underwater. The velocity after push-off is the fastest that the swimmer will experience during a race (with the exception of the dive start). As such, the ability to limit the deceleration during the glide will translate directly to improved performance and/or energy savings. Small deviations in body positions will have a large impact on the drag characteristics. Common faults during push-off and glide include:
Stroke Preparation Phase - The stroke preparation phase consists of underwater kicking prior to the first stroke cycle. Various underwater kicking styles are used in current competition and include the breaststroke, freestyle (flutter) and dolphin kick (with the swimmer on their front, side or back). The kicking style selected depend on the streamlining position used as well as the stroke. For freestyle, butterfly and backstroke, the dolphin kick has become the preferred underwater kicking style in recent years. Results from Clothier et al. (2000) demonstrated that the deceleration was less during underwater dolphin kicking than flutter kicking and the velocity above that of free swimming was maintained longer when using the dolphin kick technique. Lyttle et al. (2000) found no significant difference in the net forces (propulsive minus drag forces) between the underwater dolphin and flutter kicks for elite swimmers, although there was a tendency for the dolphin kicks to produce better results. The magnitude of the kick (large, slow kicks vs small, fast kicks) is an area for debate. However, based on principles discussed in the glide phase, it could be reasoned that a small initial kick initially would be better (less of a deviation from a streamline position and therefore less drag) than a large initial kick while at the faster glide velocities.
The optimum time to initiate underwater kicking presents another area for improving turning efficiency. Observation of swimmers (of all levels) has shown that the initiation of underwater kicking can occur at any stage from immediately after wall push-off until after arm stroke resumption. Intuitively, by kicking immediately after wall push-off, the drag created by deviating from a streamline position is likely to offset any propulsive force created by kicking. Conversely, by waiting too long before initiating underwater kicking, the full benefits of the underwater kick will not be realised. This has been confirmed by Lyttle et al. (2000) while towing swimmers performing underwater kicks at velocities which are representative of those experienced during a turn. Results show that most swimmers should wait for approximately 1 sec before initiating underwater kicking (Lyttle et al., 2000; Sanders, 2003).
A similar pattern occurs in stroke resumption with Blanksby et al. (1996) reporting that a common problem associated with turns is that age-group swimmers frequently lose time by gliding and kicking too long or too little after the wall push-off. In the first case, the swimmers decelerated to less than their free swimming velocity and additional time and energy was required in order to regain race velocity. Conversely, when stroking was commenced too early in the glide phase, the swimmer's velocity was too high and any propulsive movements increased resistance before fully utilising the velocity advantage gained from the wall push-off. This was supported by Skender (1997) who found that two-thirds of male and female age-group backstrokers did not hold the streamlined position long enough to gain optimum distance from the wall and the premature initiation of stroking resulted in an increased deceleration back to the free swimming velocity. The remaining one-third maintained the streamlined glide for too long, and this was typically a result of pushing off at too great an angle. This resulted in a return to stroking at below free swimming speed and required increased energy expenditure to regain race pace.
Following are example video clips of two national caliber swimmers performing turns along with comments of their turn technique:
Backstroke Turn - The swimmer performs the rollover turn very close to the wall which leads to a poor position at wall contact. The foot position is high on the wall with a very deep knee angle. The direction of the push-off forces resulting from the position on the wall is inefficient for maximising the propulsive forces (feet, hips and shoulders out of alignment). This results in the swimmer using the extended arms to guide the depth after push-off. The large knee angle at contact leads to an increased wall push-off time which, when combined with the non-streamlined arms and trunk, will lead to high drag values being experienced during push-off. After toe-off, the angle of descent is very steep, leading to a very deep streamlining depth. The initial dolphin kick off the wall is also quite large which will mean a substantial break from the streamline position to perform the underwater kick.
Freestyle Turn - The swimmer in the footage performing a freestyle turn initiates the turn further from the wall which leads to a better position on the wall at contact. The foot position on the wall and body position at contact will lead to an efficient use of the propulsive push-off forces (alignment of feet, hips and shoulders and depth of foot contact are all in a beneficial position). The degree of knee flexion is conducive to a rapid extension of the lower limbs, while the upper body is in a more streamlined position to be able to maximise the body's acceleration without a large concurrent increase in the drag forces. This swimmer is also in a very efficient streamline position and depth after the wall push-off.