|The Bicycle Slide in Water Polo: A Description|
The bicycle slide is a rapid sideways movement of a water polo player, in which she moves to the side of her current position while leading with her feet. The bicycle slide is primarily a key defensive skill in water polo when playing a zone in a gap between two players. It is used when the player has to move rapidly sideways to cover a shooter or a pass to a shooter, while still retaining visual contact with the ball. This technique allows the defender to move feet first toward the space, in order to provide defensive coverage. The defender executing the slide is horizontal on her side in the water with her head toward her primary defensive focus. In the attached photo (Figure 1), the defender is wearing a white cap and has her arm pointing upward. The center forward is in front of the net with the American (behind with white cap) holding on to her. The American defender with her arm up would use the bicycle slide to cover the area between the center forward and the player with the ball, before rising over her feet to present the blocking arm. The defender in this situation would slide toward the person she is defending with her feet leading the movement, which is a unique method of covering this distance.
This skill would be used primarily by a defender who needs to move quickly to the side to defend a potential shooter. Similar to the USA defender in Figure 1, she has to cover the shot on goal as well as the possible pass to the center forward in front of the goal. She could use the bicycle slide to move backwards towards her feet, while defending the person with the ball, and use head up front crawl to move toward and cover and the center forward. This skill allows her to play defense on the player with the ball while still enabling her to work with the other perimeter defenders to cover the pass to the center forward while moving toward her. If she had to turn and swim toward the perimeter player it would take much longer to actually turn around and swim than to pull the body toward the ball using the bicycle glide. It is a surprisingly fast movement over a short distance, and an effective tool for all good defenders.
Any player with this skill will also use it offensively to quickly slide into open space to receive a pass, but the player will be covering less water that way and likely be more vertical. The center forward in Figure 1 could use the bicycle slide to move to her right to get clear for a pass or shot.
Figure 1. The USA defender with her arm up would use the bicycle slide to cover the area between the center forward and the player with the ball, before rising over her feet to present the blocking arm.
Figure 4 . Player in a position of trunk lateral flexion to the left, with the left leg as the top leg. Note the trunk lateral flexion helps bring the left thigh up closer to the surface so the force application of the lower leg is more sideways.
Figure 5 . Front view of athlete moving sideways out of the page. Right leg has completed the power stroke and is pulling the athlete to her right. Note extreme hip flexion and knee flexion of the top leg.
Lift and Drag in Swimming
There has been considerable controversy in the world of swimming biomechanics regarding the main types of forces that produce propulsion in swimming (McCabe and Sanders 2005). The original description of propulsion in water was that the force was primarily propulsive drag as described in Newton's Third law of Action Reaction. If the swimmer is pushing water backward they will go forward (McCabe and Sanders 2005). This propulsive drag theory was replaced by the lift theory, in which the limbs were thought to produce lift forces by means of their orientation in the fluid and the high velocity flow over the leading surface. This high velocity flow produces an area of relative high velocity and low pressure on the limbs which helps pull the athlete through the water (McCabe and Sanders 2005). A more recent theory suggests that sculling is the major source of propulsion, which may be lift dominated or drag dominated (Arellano, 1999) depending on the orientation of the limbs. After numerous alterations in coaching opinion over the years, it is now generally agreed that swimming propulsion, especially in the freestyle stroke, is dominated by drag (Sanders 2000).
Drag forces are forces that are created by pushing against the water in the same way as a paddle when paddling a canoe, or when performing a swimming stroke such as the dogpaddle (Hay 1993). The water is pushed backwards by the paddle or limb segment, and the resistance force of the water propels the canoe or the person forward in the water. The movement of the swimmer is in the direction opposite to the direction of the applied forces on the water. In the canoe the paddler applies forces backward against the water, and the reaction force propels the canoe forward. In the dogpaddle the dog pushes the water back towards his tail with his four legs, and the reaction force propels him forward in the water.
Figure 7. Player performing the bicycle slide at completion of the leg drive. Note she is lying on her right side, while travelling to the left using paddling lower leg movements and sculling and paddling arm movements.
Drag Forces in the bicycle slide
The primary source of propulsion in the bicycle slide appears to be drag forces produced by both the arms and legs. These limbs push the water backwards, towards the head, and the body moves towards the feet in reaction to these forces (Figure 7). The player uses all four limbs in order to create drag forces towards the head, which is the body part trailing the movement, and reaction forces toward the feet. The limbs are oriented in order to push the maximum amount of water backwards toward the head. These paddling movements produce drag forces toward the head, and the resultant reaction forces act to propel the body forward in the direction of the feet. Skilled athletes can produce reasonable velocities using these techniques, as one of these skilled athletes produced an estimated horizontal velocity of .745 m/s .
The most effective source of force appears to be the top leg, or the leg closest to the surface of the water, which changes position from top leg to bottom leg as the player changes direction in moving from left to right. If the player is moving to her left, with her feet leading the movement, her top leg is her left leg. This leg acts to pull the body along to the left using several related joint movements. The left leg starts the skill in an extended position with the knee and hip in neutral flexion, and the foot in eversion and the hip slightly medially rotated. The most important force producer is likely knee flexion, as the left knee flexes forcefully and drives the water towards the head using the lower leg as a paddle (Figure 2). After the left lower leg completes its excursion back towards the trunk the hip is abducted slightly while the knee is extended for the next stroke. Pushing the water back to the right with the lower leg produces reaction forces to the left that help to propel the swimmer in that direction. The top leg also undergoes some hip abduction as the knee near nears complete flexion (Figure 3), which may also contribute to increasing the range of drag propulsion while pushing water back while flexing the knee of the top leg. The drag force produced by the lower leg segment flexing at the knee joint is accompanied by some horizontal abduction of the left hip that may also contribute some drag force to the movement. As the thigh moves back to the right, the drag force acts forwards to the left to help propel the swimmer.
Figure 9. Beginning of the bicycle slide stroke, in which both knees are extended prior to knee flexion, the lower legs are medially rotated and the arm action is pushing the water towards the head. Note her top leg is her left leg as she is moving toward her left.
The drag forces produced by the top leg stroke are continued by the bottom leg, or the leg closest to the bottom of the pool. Following the flexion of the knee and hip abduction of the top leg (Figure 7), the bottom leg then performs a power stroke in which the lower leg is flexed forcefully at the knee. This backward movement also produces drag forces in which the lower leg drives the water backwards in the direction of the head, which produces a reaction force forward in the direction of the feet. These two strong movements of alternating knee flexion likely contribute significantly to the propulsion in this skill (Figure 9). This knee flexion is accompanied by some hip extension that may also make a contribution to propulsion. After the bottom leg has flexed maximally, the knee then extends while the lower leg and foot perform a circular movement in the clockwise direction. The foot is slightly laterally rotated with the foot held in inversion to maximize the surface area of the foot producing backward forces on the water. The hip movements of flexion/extension and abduction/adduction likely also contribute to the drag forces produced to propel the player sideways toward her feet. Their contribution is likely related to increasing the range of motion of the lower leg in knee flexion, or paddling the athlete forward.
Figure 10. The top leg has completed the forceful knee flexion to produce drag forces backwards on the water to propel her in the direction of the feet. The left hand is also pushing the water backwards in order to move toward the feet.
Lift Forces in the Bicycle Slide
As well as the drag forces (action-reaction forces) produced by the legs in this skill, there are also some lift forces being produced. The foot of the top leg is aligned like an airfoil as it moves back towards the hips due to knee flexion, and is moved so that the high velocity flow is across the top of the foot (Figure 12). Recall that high velocity flow produces an area of relative low pressure, into which a person or object will move. This airfoil shape is produced by inversion of the foot, medial rotation of the lower leg and medial rotation of the hip joint. The toe is now pointing in towards the midline of the body and the foot is in inversion. This high velocity flow of water produces lift on the top of the foot that acts upward and helps the player to remain near the surface of the water. As Bernoulli has stated (Hay 1993) high velocity flow produces an area of relative low pressure that can produce lift forces on a swimmer or an airfoil.
The bottom leg also has the potential to produce lift forces on the swimmer while it is driving backwards in knee flexion (Figure 12). The foot of the lower leg is also cocked with the foot in inversion and the leg medially rotated, so it is possible that some lift forces could be produced as the foot moves backwards towards the hips while the knee is being flexed.
Figure 11. The bottom leg (left.) has just completed the forceful knee flexion to produce drag forces in the direction of the feet. The top leg is in mid stroke as knee flexion is partially completed.
Figure 15. Athlete moving forward out of the page by pushing water back towards her head using knee flexion. Large range of hip flexion suggests that hip flexors also play a role in propulsion in this skill.
Arm Action in the Bicycle slide
The top arm
The arms and hands also perform sculling movements, likely producing both lift and drag forces, to help propel the player in a direction across the pool toward her feet. The top arm, the arm closest to the surface of the pool, begins the stroke with the elbow flexed maximally and pointing downward toward the top hip (Figure 12), and the hand held at the height of the chest. From this starting position the hand moves across in front of the trunk and down towards hip level, so at the end of the stroke the elbow is extended and the hand is close to the thigh (Figure 7). As the hand is drawn across the front of the trunk, the palm of the hand is initially facing inward towards the trunk, and is then is rotated so the back of the hand is facing the surface of the pool (Figure 11). As the hand is drawn down in front of the trunk, lift forces may be produced by the hand and arm in the direction of desired movement, towards the left. The hand must be cupped to form an airfoil so that the high velocity flow is directed towards the feet, or in a sideways direction across the pool (Figure 9). The path of the hand then levels out parallel to the water surface as the hand moves laterally from the midline. This movement occurs with the palm down, so the lift is produced on the top of the hand and is direction towards the surface of the water. This lift force may help to keep the player suspended at the surface of the water.
As the arm is moving back towards the starting position, this arm is mainly producing propulsive drag forces on the way back toward the shoulders, as it pushes the water directly backward toward the top shoulder (Figure 16). The key power stroke of the top arm is the movement from thigh level to chest level with the hand cupped and the palm facing the head (Figure 16). The player is driving the water upwards towards the head, producing drag forces to propel her toward the feet. This top hand continues to produce a circular movement, from shoulder to thigh and back to shoulder, in a slightly different pathway in each part of the movement. The speed and vigor of this arm movement suggest that it makes a significant contribution to propelling the player towards the feet, using both the lift and drag forces produced by these movements.
The trailing arm
The trailing arm (or bottom arm) is the one that remains behind the player throughout the movement (Figure 17). This trailing arm has a relatively small range of motion in this skill, close to ninety degrees of pronation/supination with the wrist slightly extended, fingers extended and slightly abducted, close to sixty degrees of elbow flexion. The shoulder remains at approximately ninety degrees of abduction during the stroke, while the lower arm moves from ninety degrees of elbow flexion to almost full extension. The key movements of this limb are the hand and lower arm rotations performed during the stroke, which consist primarily of pronation and supination accompanied by elbow flexion and extension. The hand and lower arm are attempting to keep the palm facing in a direction behind the swimmer, pushing the water backward in order to propel the swimmer forward (Figure 19). This hand also appears to use a combination of drag forces pushing backwards on the water, and lift forces produced by the sculling of the hand side to side during the movement. It has been suggested that skilled swimmers use a series of complex sculling motions of the hands to generate especially lift forces. By changing the orientation of the hand the propulsive force acting on the hand is aimed successfully in the direction of motion (Toussaint and Beek 1992).
There is another type of propulsion that may be used by some swimmers, known as vortex propulsion. In this technique the hand and lower arm appear to act like a small motor, rotating and flexing and extending to create turbulent flow behind the arm. The production of propulsive forces due to whirlpools and eddying wakes is called vortex generation (Van Loebbecke, Mittal et al. 2009) . This recent theory has suggested that vortex generation may have a role in propulsion in swimming (Arellano, 1999; Hall, 2005). A vortex is a column of rotating water, or a series of eddies and whirlpools such as seen behind a motor boat when moving across the water. Depending on the direction of the whirlpools produced in the wake of the swimmer, this turbulence has the potential to produce a propulsive flow in the direction of movement of the swimmer (Arellano, Pardillo et al. 2002).
Propulsive flow can be created by whirlpools if the direction of alternate whirlpools is different. If the first whirlpool is moving in a clockwise direction, then the next whirlpool produced should be moving in a counterclockwise direction. If both of these whirlpools act together to push the water back away from the direction of motion, the reaction to the flow produced is to drive the player in the direction of her feet. This mode of propulsion has been clearly explained in the propulsion of fish by their tails moving side to side through the water and producing whirlpools moving in opposite directions. This has been shown to produce a reaction force in the direction of the fish movement.
It is unclear if vortex generation is being used for propulsion in the bicycle slide. Since the whirlpools appear to be moving in opposite directions as they are formed behind the trailing arm, they are driving a column of water backwards behind the swimmer. The action of this column of water projected behind the player is to drive the player forward. Vortex generation has been found to play a role in both flying and swimming vertebrates and insects, as well as in the thrust in canoeing and kayak paddling. Video taken for this paper from a superior view of the player did not reveal extensive whirlpools or eddies behind the trailing hand. However, the view was not very clear, and there could have been undetected vortices occurring from the motion of this hand.
Figure 20. The trailing arm is the one that remains behind the player and helps to propel the player in the direction of the feet by using sculling movements and creating lift and drag forces toward the feet.
Timing of Arm and Leg Movements in the Bicycle Slide
The arms and legs produce force alternately in the bicycle slide, unlike the butterfly stroke in which both arms, and then both legs produce force simultaneously. In the bicycle slide, the top leg produces the initial propulsive force by performing knee flexion and hip flexion. This lower limb segment acts as a paddle pushing the water back towards the head and driving the player in the direction of the feet. As the top leg is applying this force to the water, the trailing hand is also sculling and pushing the water backward away from the head. The orientation of the trailing arm is constantly changing in order to maximize the amount of water that can be pushed backward (Sanders 2000). The trailing arm also assumes an airfoil shape as it moves from side to side- this airfoil shape is likely producing lift forces on the top of the hand that can assist in keeping the athlete on the surface of the water. The lift forces are created on the top of the hand and are produced due to high velocity flow over the top of the hand, much like an airfoil in the moving air. The trailing arm appears to have a propulsive power stroke along with each stroke of the legs, so as the top leg pushes back on the water the trailing hand also pushes back. The lower legs alternate in their power stroke, so as each leg alternately pushes back on the water the trailing arm also pushes back.
The top arm appears to act primarily as a paddle, in which the hand is cupped and the water is pushed backward toward the head in order to move the player toward her feet. The action of the top arm starts just as the bottom leg is completing the power stroke, so the power strokes do not occur simultaneously but alternate between the legs and arm. The sequence of the propulsive strokes appears to be: top leg, trailing arm, bottom leg, top arm, trailing arm and so on.
Figure 21. The path of the trailing right hand is undulating as the hand moves sideways and backwards to produce some drag forces against the water. The arm has to push the water away from the body to create the required drag forces.
Figure 22. The trailing (right) arm remains at close to 90º of shoulder abduction and slight elbow flexion. This trailing arm performs circular sculling movements due to pronation and supination and elbow flexion and extension occurring in the right arm.
Eggbeater kick in the bicycle slide?
The movements of the bicycle slide in water polo resemble the eggbeater kick, except the body is tilted sideways in the water instead of upright, and the range of motion of the legs is smaller. This sideways tilt of the player allows her to propel herself sideways across the pool using arm and leg motions, while moving in a backwards (feet first) direction instead of a forwards direction. However, the eggbeater kick in water polo produces upward lift forces on the athlete by using the feet primarily as airfoils. The feet are moved in circular patterns in the eggbeater kick, with one leg moving clockwise and the other leg moving counterclockwise (Alexander and Taylor 2009). These eggbeater lift forces act upwards on the player and allow her to remain upright in the water while shooting the ball or playing defense (Sanders 2005).
The legs in the bicycle slide also move in circular patterns, but in the bicycle slide the direction of movement of both legs is the same, both legs are moving clockwise during the kick. The bicycle slide produces propulsive forces in the sideways direction, and in a direction towards the player's feet instead of the head. Therefore, the player is moving sideways in the water in a direction away from the head along the surface of the water. Instead of the lift forces being produced at the top of the feet to hold the player upright, these forces are produced in a sideways direction, in order to pull the player through the water in the direction of the feet.
Summary of Bicycle Slide
The bicycle slide in water polo is a difficult skill to master, and one that has been mastered in only a limited number of players. No previous descriptions of this skill were located, so this paper may be one of the first attempts to describe the skill in detail. Since it is a first attempt at describing the skill, it may contain some minor inaccuracies. However, it does give water polo coaches a basis from which to teach the skill, and to start their athletes in learning the movements required to perform the skill. It is important that water polo coaches understand the key movements of the bicycle slide, and attempt to develop logical progressions to teach the skill to their players. We are in the process of developing an instructional video for coaches describing the skill and some logical progressions in teaching the skill.
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