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Introduction

The eggbeater kick is used in synchronized swimming and water polo to raise the upper body. At the FINA Conference in Hong Kong I presented the results of a three-dimensional study of the eggbeater kick and started to draw implications for coaches and athletes. The purpose of this paper is to look more deeply into the implications of the findings with particular emphasis on identifying muscle-specific requirements of strength, flexibility and timing of movements that will guide dry-land and water based training.

The eggbeater kick comprises cyclical actions of the lower limbs. The right and left sides rotate in opposite directions and opposite phase. The action is a complex combination of hip, knee, and ankle motions. Essentially, the action is designed to 'scull' the feet to produce forces to raise the body.

A Model of Factors Contributing to Maintaining Height Using the Eggbeater Kick

Before looking at the implications for training we need to revise the important factors that contribute to maintaining height using the eggbeater kick. To elevate the body, an impulse in the upward direction to counteract the gravitational impulse is required. This impulse is provided by buoyancy and the impulse generated by the eggbeater kick. The greater the mass above the water, the less the buoyancy force and the greater the impulse that must be supplied by the kick to maintain that position.

In accordance with hydrodynamic principles, the force generated by the kick at any specific time is dependent on the cross sectional area of the feet, shanks and thighs, the speed of motion of those limbs, and the coefficients of lift and drag.

In turn, the speed of the limbs is dependent on the joint actions. The coefficients of lift and drag depend on the orientation of the limbs. Orientation of the limbs depends on joint angles.

In studies of the eggbeater kick in water polo players I have focused on the aspects of foot motion over which the athlete has control. These are the speed of the limbs, the joint actions, and the orientation of limbs resulting from the joint angles used. Particular emphasis has been accorded the speed and orientation of the feet because the feet are deliberately oriented to produce an upward force and are the fastest moving of the lower limbs.

Foot Velocity

As would be expected from hydrodynamic principles, height maintained in the eggbeater kick was strongly related to foot speed (r=0.85). However, it was also found that the direction of foot movement was important. In particular, better performers used more anteroposterior (forward and backward) motion and less vertical (up and down) motion of the feet than poorer performers. In the figure below the better player's foot paths are shown on the left.

An important consideration in maintaining the forces produced by the feet is that the motion of the feet is as continuous as possible. That is, the feet need to move quickly throughout the whole cycle rather than having large fluctuations in foot speed.

Pitch Angles

The kick is cyclical and the movements are 'rounded out' with a combination of hip flexion and extension, hip adduction and abduction, internal and external rotation, and knee flexion and extension. The more effective kickers were able to generate upward forces with the feet oriented so that pitch angles were small and positive. That is, the flow of water hit the underside of the foot at an acute angle, thereby generating upward forces primarily by lift rather than drag. In the figure below the better player's pitch angles over two cycles are shown in the upper graph.

The time of negative pitch, that is when the flow is hitting the upper side of the foot, needs to be minimised as the forces generated are likely to have a downward component. This tends to occur as the foot is being raised. For this reason better players minimise the periods of negative pitch by having motions that are more horizontal than up and down, and by clever timing of actions to orient the foot appropriately.

When the kick is executed properly, with continuous 'rounded' motions and with appropriate timing of medial and lateral rotation, the flow is across the medial (inside) of the foot for more than half of the duration of the cycle. The short period of flow across the outside of the foot occurs late in the period of knee flexion and early knee extension as the foot moves outward. To achieve favourable pitch angles during the period of outward foot movement, the ankle is strongly dorsi flexed, laterally rotated and everted. During the forward and inward motion the ankle is dorsi flexed and rotates medially. During the backward movement the ankle is plantar flexed with inversion. Obviously, to maintain optimal pitch angles, the timing of the transitions between the ankle actions is critical.

Implications for Training

Training to improve the eggbeater kick should be directed towards development of:

• Speed and power in flexors and extensors of the hips and knees.
• Flexibility, strength and power in hip abductors and adductors.
• Flexibility, strength and power in hip and leg internal and external rotators.
• Flexibility of the ankles in eversion and inversion.

Specific Implications for Training

The eggbeater kick is a complex and unusual action that does not resemble usual activities. This means that some muscles are used more than usual. As a prelude to designing suitable training programs, we need to identify the muscles that would be used in the eggbeater action. By combining our knowledge of the actions required in the eggbeater kick with knowledge of the muscles that can produce those actions we have a basis for developing a training program to optimise performance.

Psoas Major and Iliacus

The conjoined Psoas Major and iliacus flex the hip and assist in medial and lateral rotation of the hip. Thus, it plays an important role during the part of the cycle in which the knee is being elevated through hip flexion.

Gluteus Medius

The gluteus medius abducts the hip and medially rotates the hip. Thus, it probably plays a role during the part of the cycle in which the foot is moving outwards. The assistance in hip abduction with an influence towards medial rotation may be beneficial at this time. As this is moving into quite an awkward and weak position good strength of the gluteus medius will help to maintain foot speed during this part of the cycle.

Gluteus Maximus

The gluteus maximus extends and laterally rotates the hip and assists in the abduction of the hip. Thus, it probably combines with other muscles to play an important role during the part of the cycle in which the hip is abducted and the foot is starting to move inward due to lateral rotation of the hip. As this is moving into quite an awkward and weak position good strength of the gluteus maximus will help to maintain foot speed during this part of the cycle.

Quadriceps

Collectively, the quadriceps strongly extend the knee. The rectus femoris also assists in hip flexion. Therefore, these muscles play an important role in the knee extension that generates much of the speed of the foot.

Unlike many of the other muscles involved in the eggbeater kick, the quadriceps may already be strongly developed through general training programs and activities. It is probably necessary to work on the endurance aspect of these large muscles rather than strength per se. However, explosive strength and power of these muscles is desirable when the standard eggbeater kick is adapted for a sudden upward 'boost' that is necessary at particular times in water polo and may be required for particular skills in synchronised swimming.

Adductor Longus

The adductor longus adducts, medially rotates, and assists in flexion of the hip joint. Thus, it probably combines with other muscles to produce continuous motion during the transition from outward to inward motion of the foot. This is one of the most effective parts of the kick because the foot is moved and oriented so that the flow is across the foot from the medial to the lateral side with a favourable positive pitch angle. The action of the adductor longus when timed appropriately with the knee extension may contribute to achieving high foot speeds.

To achieve the high foot speeds the adductors such as the adductor longus need to be very strong. Given that this action is somewhat unusual in everyday activities, the muscles may not naturally be well developed. The action of adducting the hip while the knee is extending means that inertia is great. Also, there is a large fluid resistance to rapid movement of the thigh, shank and foot against the water.

Flexibility is important. The medial rotation and adduction commence from a very laterally rotated and abducted position. Thus a long adductor longus will ensure that it is strong in this position as well as minimising resistance to the position being attained. Working on the flexibility of this muscle will help in the prevention of groin injuries. Stretching and warm up of the adductor muscles prior to performing the eggbeater kick is essential.

Pectineus

The pectineus adducts and flexes the hip joint. Thus, it plays an important role during the part of the cycle in which the foot is moving forward and inward.

Similar to the other adductors, strength and flexibility are important.

Adductor Magnus

The adductor magnus adducts, medially rotates, and assists in extension of the hip joint. Thus, it plays an important role during the part of the cycle in which the foot is still moving inward but changing direction from forward to backward motion. The timing of muscle action to maintain fast motion while economically transferring loads from one muscle to the other is important for continuous kicking. While no EMG data exist to verify the timing of the contribution of specific muscles, it may be expected that skilled technique involves appropriate sequencing of muscle action. Given that the adductor longus assists flexion and adductor magnus assists extension we could hypothesise that that adductor magnus 'takes over' from the adductor longus and pectineus at the phase of the cycle where an extension influence rather than a flexion influence is required.

The need for strength and flexibility in the adductor magnus is similar to that of the adductor longus and pectineus. However, in training for strength and endurance of this group, it may be beneficial to design exercises that mimic the eggbeater kick. This would develop the ability to change the relative contributions of muscles in the adductor group to develop force as the hip action changes from flexion to extension. Certainly, it would be advantageous to develop exercises which incorporate some flexion with adduction, as well as exercise that incorporate some extension with adduction, rather than merely exercises that adduct without flexion or extension.

Semimembranosus and Semitendinosus

Both the semimembranosus and semitendinosus flex the knee and medially rotate the leg as well as having a hip extension influence. Thus, they play an important role in moving the foot backwards and continuing the inward rotation following the extension of the knee. Thus the semitendinosus and semimembranosus play an important role in maintaining foot speed during this phase of the cycle in which favourable pitch angles ensure the generation of upward forces.

The inward rotation of the lower leg is an action that is not common and therefore not strongly developed through everyday activities. Further, the action in water invokes resistance additional to inertial resistance. Thus, attention to developing the strength of the action of the semimembranosus and semitendinosus would be beneficial. Knee flexion exercises that include internal rotation should be designed to ensure that the semimembranosus and semitendinosus are being developed independently of the other knee flexors.

Gracilis

The gracilis flexes the knee and medially rotates the leg. Thus, together with semimembranosus and semitendinosus plays an important role in moving the foot backwards and continuing the inward rotation following the extension of the knee. This assists in maintaining foot speed.

Like the semimembranosus and semitendinosus development of the strength of the gracilis would contribute to strong medial rotation.

Biceps Femoris

Both the short and long heads of the biceps femoris flex the knee and laterally rotate the leg. Thus, they play an important role in moving the foot backwards and rotating it outwards. During the outward movement the lateral rotation of the leg is accompanied by eversion and the flow of water has a positive or close to positive pitch angle for as long as possible during this period. The lateral rotation of the leg positions the foot in a favourable position to generate lift forces in the subsequent forward motion that is accompanied by internal rotation and adduction. Thus, biceps femoris plays an important role in preparing the foot for its most effective action. Given that inward rotation is required at the beginning of knee flexion, we would expect increasing activity and involvement of the biceps femoris towards the end of the period of knee flexion. Thus, we would anticipate that efficient performance would be linked to timing the biceps femoris action to 'take over' from the other flexors of the hamstring group, particularly the semitendinosus and semimembranosus which have an inward rotation influence on the lower leg.

The outward rotation of the lower leg is an action that is not common and therefore not strongly developed through everyday activities. Further, the action in water invokes resistance additional to inertial resistance. Thus, attention to developing the strength of the action of the biceps femoris would be beneficial. Knee flexion exercises that include lateral rotation should be designed to ensure that the long and short heads of the biceps femoris are being developed independently of the other knee flexors.

Tibialis Anterior; Extensor Digitorum Longus; Peroneus Tertius; Extensor Hallucis Longus

This group of muscles dorsi flexes the foot. The foot is dorsi flexed as it is moved upward, backward and outward towards the end of the period of knee flexion. Concurrently, the foot is everted to optimise the flow characteristics at a time when there is potential for the flow to hit the top of the foot to produce undesirable downward rather than upward forces. The foots is then in a favourable position to optimise speed and lift forces when the period of knee extension, adduction, inward rotation, and plantar flexion commences. Correct timing of dorsi flexion is an important part of skilled performance in the eggbeater kick.

The strength and endurance training of these muscles should be accompanied by ankle flexibility work to ensure that obtaining the optimal orientation of the feet is not constrained by a lack of flexibility.

Peroneus

The peroneus group of muscles, comprising Peroneus longus, tertius, and brevis, evert the foot. This is an important action to establish flow characterstics that promote upward rather than downward forces during the backward, upward and outward movement of the foot. Unfortunately, the peroneus longus also has a plantar flexion influence. Therefore, if peroneus longus is used strongly to evert the foot then the plantar flexion effect must be overcome via the action of the dorsi flexors.

Given that the muscles that evert the foot are not generally considered in standard training programs, and are not likely to be well developed through everyday activities, some attention to training this group independently may be beneficial in reducing decrements in performance that might occur through early fatigue of these relatively small muscles.

It is not clear from the available research whether the ability to obtain optimal foot positions is limited by anatomical constraints. It may be that increasing the range of eversion might be achieved by increasing the flexibility of ligaments on the medial side of the ankle such as the deltoid ligaments.

Gastrocemius; Soleus; Flexor Digitorum Longus; Flexor Hallucus Longus

This group of muscles plantar flexes the foot. The foot is plantar flexed as it is moved downward, and forward and during the first part of backward movement. Concurrently, the foot rotates as a result of medial rotation of the lower leg. These actions contribute greatly to ensuring a favourable pitch angle for the generation of upward forces. Correct timing of plantar flexion is an important part of skilled performance in the eggbeater kick.

Given the combined muscle mass of the plantar flexion muscles, and their regular use in daily activities where there are large forces to be overcome due to plantar flexing against body weight, the strength and endurance of the plantar flexes is unlikely to limit performance of the eggbeater kick to the same extent as smaller muscles involved in the unusual elements of the eggbeater kick action. Therefore, greater benefits to performance may accrue by devoting a greater percentage of time to other muscle groups than to the plantar flexors.

Tibialis Posterior

Tibialis posterior and tibialis anterior are the principal invertors of the foot. Inversion places the foot at a favourable angle to the flow to generate upward forces during most of the eggbeater kick cycle. Inversion is concurrent with plantar flexion due to the natural anatomical structure of the foot and ankle. It is not clear at this time whether additional inversion at any time in the cycle optimises performance. Therefore, it is not clear whether action of the tibialis muscles is necessary. If additional inversion is required, then it is logical that tibialis posterior would be the main contributor as it has a plantar flexion influence whereas tibialis anterior has a dorsi flexion influence.

If action of the tibialis posterior is beneficial to performance then exercises which encourage increased inversion while plantar flexing should be used.

Other Muscle Groups

To perform the eggbeater kick, a strong and stable 'platform' is required. Thus, in addition to the muscles used to move the lower limbs, many 'stabilisers' are brought into action. These need to be strong and fatigue resistant. Important muscle include the abdominal group and the stabilisers of the pelvis and spine.

Conclusion

To optimise performance in the eggbeater kick, attention should be given to strength, endurance, and flexibility training of specific muscles involved in the action. This will allow maintenance of speed throughout the motion, favourable angles of orientation of the feet, and will assist in the development of efficient timing.