What biomechanical principals are associated with an efficient open water freestyle technique?

Athletes rely on equipment and technique when swimming in open water. The movement of swimming has two forces, a resisting motion of drag and an assisting motion of propulsion. The forward motion and speed of a swimmer is determined by forces resisting motion (drag) and forces causing motion, propulsion. There are three main types of drag: form, surface and wave (Blazevich, 2012). To achieve an optimal swimming velocity, the goal of the swimmer is to create optimal propulsion and minimise drag.

 

HYDRODYNAMICS-DRAG

How can we improve a swimmers efficiency in open water?

Drag occurs when molecules of a fluid (air or water) collide with an object and take energy away from it (Blazevich, 2012). The bigger the surface area of the object or the faster the molecules are moving the bigger the drag. The loss of energy from the object (swimmers body) to the fluid (water) can be viewed as laminar flow: water moving towards the swimmer in ordered layers ( refer to picture below). As the water approaches the swimmer, the layers separate. As the water passes the swimmer, the water changes direction and therefore, gains energy. The energy gained by the fluid is always equal to the energy lost from the object and is called non-laminar flow or turbulent flow (Blazevich, 2012). 

At some point of contact with the swimmer the water flow may become turbulent as the fluid towards area of low pressure and takes away energy from the swimmer.

FORM DRAG- relates to the surface area and shape of the swimmer.

The only way to limit the affect of drag on performance is to reduce the amount of surface area coming into contact with the molecules (water). The swimmers surface area is affected by their position in the water. Form drag or frontal resistance occurs at its greatest when the swimmers body is too low in the water and therefore, producing a larger surface area ( refer to picture below). The water pushes against the body in the opposite direction, causing the swimmer to lose velocity (Blazevich, 2012). If the swimmer continues to swim in a low body position, they then need to use more energy in order to maintain velocity in comparison to a higher body position.

Whereas, if the swimmer is positioned flat across the surface of the water, the surface area of the body in which the water is hitting from the front is greatly reduced. Swimmers avoid Frontal resistance by rolling their body from side to side with the stroke and by kicking their legs. This keeps the swimmer up and moves the water past the swimmer more effectively.

 

Another factor influencing form drag is the shape of the object. This will affect how much laminar flow turns into turbulent flow (Blazevich, 2012). This refers back to the entry of the swimmers hand into the water. If the hand enters the water on an angle, the direction of the water hitting the hand will be changed more slowly than if the hand hits the water abruptly ( refer to picture below). Furthermore, the entry of the hand position relates back to Newton’s third law: For every action, there is an equal and opposite reaction (Blazevich, 2012). With an angled hand entry, there is less upward motion when the swimmer starts their pull as opposed to a flat entry where all the force will be going down when they commence their pull. This results in the body going up instead of forward.

 

SURFACE DRAG

Surface drag is affected by the roughness of the surface of the object. For a swimmer this relates to their skin and swimming suit/bathers. As the water makes contact with the swimmers body, small pockets of ridges in the skin or clothing catch the fluid, thus allowing a force to be applied and energy transferred. Therefore, wearing non-porous materials, allows the water to travel over the surface easily.

Wetsuits

To optimise body position, the entire body needs to be flat in the water and to ensure the forces lifting the body are equal to the forces pulling the body down. Therefore:

·         Body mass- gravity pulls the swimmer down

·         Buoyancy- is the force lifting the swimmer up in the water

This means the buoyancy to body weight ration must be equal across the entire body. As humans lungs have a large volume with a low density and the legs are dense, there is a natural tendency for the legs to drop or sink while swimming. The most effective way to minimise the sinking of the legs is to use a wetsuit.

The wetsuit does the following to aid in buoyancy:

·         The wetsuit covers both the legs and is relative to the volume of the legs, however it is very light so the buoyant forces increase but does not increase the weight forces.

·         On the upper body the wetsuit adds some volume but not as much in comparison to the effectiveness on the lower body.
 

Wetsuits assist with enhancing buoyancy by lifting the swimmer higher in the water, therefore, reducing the amount of form drag and aiding in overall performance (Cordain & Kopriva, 1991). Also by increasing buoyancy it allows the swimmer to expend more effort in the propulsive movement. Leaner swimmers, tend to benefit more from wetsuits than swimmers with a larger mass due to their natural buoyancy (Cordain & Kopriva, 1991).  Swimming equipment of goggles reduce drag reduction by 2.2% and swim caps by 3.4% as they help the swimmer become streamline (UCAerospace2012).

The extra buoyancy to help the swimmer remain streamline assists with the overall swimming time and allows the swimmer to concentrate on creating horizontal propulsion through the water instead of focusing on vertical propulsion to lift their body.

Speed Suits: Speed suits are made from a thin layer of material that mimics the skin of a shark. They compress the body circumference by up to 5%, which aids in decreasing the overall surface area of the swimmer (Phill Ligett UCAerospace2012).

Surface drag is the friction created by the fluid on the swimmer. Recent research and design of swim suits have reported to have lower drag coefficients with a small buoyancy effect (Blazevich, 2012). The idea of these suits is it increases surface drag, allowing the water to remain attached to the swimmer as a boundary layer, like a golf ball.  The swimsuits have been made so that the surface is rough not smooth (refer to picture below). The rough surface creates a small amount of turbulence in the thin layer of water flowing close to the surface of the material (Blazevich, 2012). This is the same principle behind the dimples in golf balls.

Golf balls have dimples because the majority of the drag from a golf ball comes from the separation of the flow behind the ball, which is known as the pressure drag due (Cislunar Aerospace, Inc 1998). For streamline flow past a sphere, the flow separates very early as shown in Figure 1. However, for a turbulent flow, separation is delayed as can be seen in Figure 2. The separation region in the turbulent case is much smaller than in the streamline case. So the larger separation region of the streamline case implies a larger pressure drag on the sphere. The turbulent flow has more energy than the streamline flow and thus, the flow stays attached longer and the ball travels further (Cislunar Aerospace, Inc 1998). This applies to a swimmer because air resistance follows the same principles as fluid resistance. So if a swimmer was to wear a swimsuit with a slightly rough surface a turbulent flow would be created, causing the swimmer to be further projected through the water. The idea behind this is the attached layer reduces the pressure differences around the body and minimises form and wave drag.

 

Wave Drag

Wave drag is created through the interference of water and the air as the swimmer pushes through the water (Blazevich, 2012). The water in front of the swimmer acts as a resistance and pushes back against them and in turn slows down their speed. Other water movement that also forms around the body is due to the pressure difference and can take away ‘energy’ from the swimmer. As the swimmer increases their speed, so does the length and height of the wave drag, as the waves travel at the same speed as the swimmer (Blazevich, 2012). Research on measurements of drag have indicated that total drag will continue to increase with velocity and will be smaller or equal to the drag arising from the body travelling through the water (Blazevich, 2012). Furthermore, it can be thought there is no particular speed to swim at, that will minimise wave drag, however, swimming technique of the arm action and body roll might help reduce the wave build up and in turn minimise drag.

 

Skilled swimmers have demonstrated they can create smaller waves in comparison to less-skilled swimmers by (Blazevich, 2012):

·        Increasing body length- stretching the arm in front of the body at the end of the recovery phase.

·        The arm entering the water might cause earlier separation on the oncoming flow of water. Therefore, reducing the pressure at the front of the head and reduce wave build-up.

·        Swimming with their head down, chin on their chest, reduces the up and down movement of the body. It is thought, lower head position reduces the pressure at the front of the head.

·        Body rotation reduces the effective surface area.

·        Small leg kick, reduces the pressure difference around the leg area and therefore, reduces

wave formation.

Overall to minimise wave drag the best option is to swim as much as possible underwater, only lifting the head occasionally to sight the direction is which the swimmer is aiming for i.e the buoy.

Further ways to improve open water swimming

Technique is even more important, as the body rotates; athletes use as twice as much kinetic energy on their front as opposed to on their side. Rolling from one side of their body to the other transfers power from the rotation to the propulsive arm, allowing for more efficient stroke (Phill Ligett UCAerospace2012).

The biggest advantage in open water swimming is ‘drafting’, swimming behind the person in front of you, provides energy savings of 18-25% as the water is already moving forward  (Phill Ligett UCAerospace2012).

SUMMARY:

In comparing the two types of drag, the effects of surface drag are not as significant as those of form drag. However, reductions in surface drag can have measurable effects on performance.

Hydrodynamics-propulsion

Newtons Third Law, for every action there is a reaction. For the swimmer to move forward in the water, they need to apply a backward force (Blazevich, 2012). As the aim of the swimmer is to move as quickly as possible through the water, the focus becomes on power, action power v reaction power. However, the reaction power is not equal to the action power as water is not solid and moves when the force is applied. This means some of the power is used to induce movement rather than propel. The idea is to increase the amount of reaction power from the given action power (Blazevich, 2012).

The hands should have a relaxed grip with the fingers slightly spread apart, (the thought of running them through your hair) and should be the first thing to touch the water (refer to picture below). As the water flows through it sticks to the fingers and therefore creates a larger surface area. The increased surface area creates drag and in turn increases propulsion. Furthermore, it can be thought taller swimmers with longer limbs could be able to create greater drag forces which could be of benefit to them (Blazevich, 2012).

For a swimmer to maximise the surface area to propel themselves forward, they begin with the arm outstretched, having a high elbow, using the hand and forearm to pull through the water. The elbow and wrist is flexed so the palm is facing backwards. The stroke ends with the hand leaving the water close to the hip ( refer to picture below). The high elbow is especially important when the water is choppy, to clear the water and not make contact, if contact is made, the wave will force the swimmer backwards.

 

In the 1960’s the ‘S’ pull was discovered as a sculling movement (Blazevich, 2012). Essentially the hand moves laterally through the water and is on a slight tilt towards the incoming water. This lateral movement of the hand creates lift on the palm of the hand and effectively the hand can pull. Furthermore, by using a slight ‘S’ pull, allows the swimmer to pull through still water. From pushing against still water the swimmer can exert more force and hence giving more propulsion. Furthermore, the efficiency of swimming can be improved by applying the forces in line with the direction of the buoy.

There is debate to whether the ‘S’ pull has its advantages. Research using particle image called Velocimetry which tracks the motion of water and quantifies how fast it is moving. This technology has proven the change in direction produces a jet flow that itself is propulsive. This new information suggests the greatest propulsion is when the hand is moving slowly, i.e. during the change in direction of the hand (kamata, et al 2006). These subtle changes in hand movement are very important in optimising propulsion.  At this point the researchers are not suggesting for swimmers to change their stroke movement, however, encourage them to ‘feel for the water’ and recognise that the directional changes the hands make are in fact generating propulsion (kamata et 2006).

The breathing action in swimming inflicts the swimmers streamline position and propulsion as the heads moves out of normal position to take in air (Pedersen & Kjendlie, 2006). This means the more often the swimmer breaths, the less speed they will have overall. Tests have indicated if a swimmer breaths every 3, their speed should not decrease due to the breathing action (Pedersen & Kjendlie, 2006). If the water is choppy, it is best to breath to the side the wave is not crashing, this avoids receiving a large amount of water in the mouth.

How else we can use this information:

These principals can be taken to other swimming strokes in regards of how to minimise drag and at the same time maximise propulsion.

Breaststroke swimmers propel themselves under the water and only surface to breathe, this motion does create wave drag but is minimised due to the swimmer keeping their hands out in front like a point. In butterfly the underwater arm movement is quite similar to freestyle and when turning use the underwater dolphin kick to their benefit. Furthermore, as more research is done on hydrodynamics and our understanding increases there will be more advances in the speed that water-based sports can reach. This can include speed boats, yachts and jet skis.

The Answer

For effective open-water swimming, drag needs to be minimised and at the same time increase propulsion and this is achieved through modifying the swimming technique:

·         Head position low- chin tucked onto the chest. To minimise wave drag by keeping the body under the water and keeping the streamline position.

·         Small leg kick maintained at all time- will assist keeping the hips up and reduce wave drag.

·         Use of wetsuit or speed suit with swim cap and goggles.

·         If possible swimming directly behind another swimmer.

·         Streamline position, lifting head only occasionally to sight the buoy, as lifting will increase frontal resistance and form drag.

·         Maintain alignment of the direction of the buoy.

·          Angled hand position when entering the water.

·         Transferring the power from the rotation to the propulsive arm, this allows for a more efficient technique.

·         Fingers slightly spaced apart to increase the effective surface area and in turn increase the drag and lift forces of propulsion.

·         High elbow, wrist slightly flexed and ‘S’ pull to grip the water and propel forward.

Combining these technique principles will give  a swimmer the best opportunity to achieving optimal results in open water swimming.

References

Blazevich, A.J. (2012). Fluid Dynamics- Drag. David Pearson (Eds.), Sports Biomechanics. The    Basics. Optimising Human Performance (pp.136-153). London: A&C Black publishers Ltd.

Blazevich, A.J. (2012). Hydrodynamics- Drag. David Pearson (Eds.), Sports Biomechanics. The Basics. Optimising Human Performance (pp.154-166). London: A&C Black publishers Ltd.

Blazevich, A.J. (2012). Hydrodynamics- Propulsion. David Pearson (Eds.), Sports Biomechanics. The Basics. Optimising Human Performance (pp.167-186). London: A&C Black publishers Ltd.

Cislunar Aerospace, Inc (1998). Why does a gold ball have dimples? Retrieved from http://www.fi.edu/wright/again/wings.avkids.com/wings.avkids.com/Book/Sports/instructor/golf-01.html

Cordain, L, & Kopriva, R. (1991). Wetsuits, body density and swimming performance. Department of Exercise and Sport Science, Colorado University.

Kamata, E., Miwa, T., Matsuuchi, K., Shintani, H., and Nomura, T. (2006). Analysis of sculling propulsion mechanism using two components particle image velocimetry. International Symposium on Biomechanics and Medicine in Swimming, 50-52.

 

Pedersen, T., Kjendlie, P. (2006). The effect of the breathing action on velocity in front crawl sprinting. International Symposium on Biomechanics and Medicine in Swimming, 75-77.

UCAerospace2012. (2012, October, 29^th). 2012 Ironman World Championship Kona. Retrieved from  http://www.youtube.com/watch?v=gge7Ag__Pm8