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Biomechanics of the foot in dance a literature review.

Abstract

In any form of dance, great strain is placed on the lower extremity and the strong but sensitive foot. A large percentage of injuries to dancers involve the foot and ankle. Understanding the structure, biomechanics, and physics of the lower extremity helps to diagnose and evaluate the mechanics behind these injuries.

The lower extremity function is complicated and needs to be studied carefully to understand its laws and principles. What often happens when injury occurs is that the dancer is not satisfying the natural requirements of movement through the joints. For example, her releve is causing sickling and unstable foot positions because the forefoot is not strong enough and the leg external rotation and hip joint muscular support are not simultaneous with the heel raise. In the opposite direction, when the dancer is executing plie, the outcome of poor control of a weakened foot is strain on the passive supporting structures, such as the plantar ligaments, joint capsules, and plantar fascia. This leads to faulty bone alignment, increased bone load, and risk of overuse syndromes in various tissues.

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This article explains how the normal biomechanics of the foot and lower extremity affect dancers. Failing to use normal biomechanics in dance may lead to acute and overuse injuries of the foot and ankle. (1-4) Ankle injuries represent 20% to 25% of all injuries sustained by dancers. (5-7) In this article three seemingly simple dance movements--demi-pointe, full pointe, and demi-plie--are broken down into their biomechanical principles. These movements are very familiar to dancers, but exactly how they should be executed and what kind of benefit is gained by following the laws of physics and human biomechanics needs to be clarified. Three different axes of pronation and supination will be used as a base for describing related movements in closed kinetic chain function. Also, the talocrural joint (TC) axis is discussed as the axis of demi-pointe, full pointe, and demi-plie. Dysfunctions of the chain, and solutions for correcting the functions, will be discussed.

One of the goals is to compare the terms used in dance with the language of anatomy and research in biomechanics. There is a clear understanding of normal foot function in the movements that are called "flex" and "pointe" in dance terms and dorsiflexion and plantar flexion in anatomical terms. This understanding is based on numerous studies conducted by researchers in the biomechanics of human gait and podiatry. (8-10) In biomechanical and sports medicine literature the terms "pronation" and "supination" are discussed widely. This information may be applied to dance movements.

Preventive exercises are important to combat foot injuries. They should be based on a knowledge of lower extremity biomechanics.

Open and Closed Kinetic Chain

Dancers execute movement in both the open and closed kinetic chain. When the foot is not weight bearing but moving freely in the air, movement is in open kinetic chain. (11) The ankle joint is not supporting it; rather, stability is dependent on passive and active supporting mechanics (ligaments and muscles). Any joint in the lower extremity is able to move independent of the talocrural joint (TC); that is, the other joints are not necessarily affected by ankle movements. However, when the foot is weight bearing, any movement in any joint is related to the other joints, creating a closed kinetic chain that follows the biomechanical rules. (12) The basic rule of the closed kinetic chain is that the distal segment is bearing weight while the proximal segment is moving relative to it. If the movement desired does not follow the rules dictated by joint formation and normal muscular activity, there are consequences in the form of injury, either acute or stress-related, somewhere along the chain. The bone structures, joint capsules and ligaments, as well as the muscles and their tendons, are under stress. It is a matter of safety and ergonomics in dance to evaluate the freedom of movement of the joints regularly, but especially when an injury or pain is present. (13)

Pronation and Supination in the Open Kinetic Chain

In an open kinetic chain, pronation and supination are possible in the foot around the axis that lies along the center line of the foot. The whole foot may pronate in (eversion, valgus position), or supinate out (inversion, varus position). The foot and ankle may be twisted to very extreme positions in air without causing any problem. In dance this often happens for artistic or aesthetic reasons.

Architectural Foot Structure and Function in Releve

The foot has been described as a triple-arch structure and compared to architectural solutions in buildings like stone bridges or the vaulting in churches. (14) Hiss used the term "flexible arch" for the medial arch, and the lateral arch he called "rigid arch," or "weight bearing arch." (14) The medial longitudinal arch is supported by the plantar fascia and a ligamentous group comprising the long plantar, short plantar, and plantar calcaneonavicular (spring) ligaments, and when weight is placed on the foot it works as a spring. However, the lateral longitudinal arch is supported by the proximal tuberosity of the fifth metatarsal bone, thus giving the structure full stability. As long as weight is balanced on the lateral arch the supporting plantar structures are able to keep the medial arch elevated, as a taught spring. (15) The third arch crosses the foot transversally, from the medial side to the lateral. This arch extends all the way from the metatarsal heads back to the area between the cuboid and the cuneiforms. (16,17) Shifting of body weight makes this arch structure change form, according to whether the weight is on the medial or lateral side, or moving from the heel toward the toes.

Architecture is transformed into function when the heel is lifted off the ground in releve. The arches must become more rigid to stabilize the foot and to create a good lever to push against. Thus there must be normal supination of the subtalar joint in heel raise. Simultaneously there is supination in the oblique midtarsal joints as well as pronation in the longitudinal midtarsal joint axis, moving the first ray into plantar flexion. The medial arch will become rigid due to the formation of the bones and tightening of the plantar structures. This is called the "windlass-effect" in the biomechanical literature. (18,19) The combination of these movements is needed for the normal demi-pointe, and it helps the dancer to balance on one foot without too much effort (Fig. 1).

Anatomy and Biomechanics of the Foot and Ankle

Anatomically the foot consists of three sections: tarsus, metatarsus, and phalanges. There are seven bones in the tarsus: calcaneus, talus, cuboid, navicular, and internal, middle, and external cuneiform bones. (20) Donatelli has described the triplanar movement of the foot and ankle, meaning that talocrural, subtalar, and midtarsal joints, as well as the first and fifth rays, have axes of motion that are oblique to the body planes. The axes of motion are at an angle to three body planes. They work as pivoting points for any movement in the feet. It is important to recognize these axes in order to be able to evaluate biomechanical movements. All of these axes need to be used correctly to execute movements in dance. They also need to work in unison with each other, executing movements at the right time. Understanding these basics is also helpful in assessing why certain movements cannot be executed properly when there is a dysfunction involved. When movement occurs in all three body planes simultaneously, it is referred to as triplanar motion. The triplanar movements of the foot and ankle are supination and pronation. (21,22) In turn, the triplanar motions in pronation are abduction (transverse plane), dorsiflexion (sagittal plane), and eversion (frontal plane). Conversely, supination is a combined movement of adduction, plantar flexion, and inversion.

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Terminology in Dance

Plie means "fold" or "bend." In ballet it refers to bending the knee or knees of the standing leg or legs. The barre exercises in ballet class usually begin with demi-plies and continue to grand plies. Their purpose is to warm up the muscles and joints of the legs, as well as the crucial muscles that control turnout. They help establish correct placement and are the foundation of every turn, every jump, and every safe landing. Demi-pointe, or releve, means "rise." Many schools distinguish between a rise, in which the dancer presses up with straight knees, and a releve, in which a tiny demi-plie precedes the movement to provide a little spring. (23)

Kirstein and Stuart defined the muscular control of the movement in releve as follows: "Executing releve, simultaneously tighten buttocks and straighten knees. Turn thighs outward, bring heels forward and together in first position." (24) This suggests that when the heel is lifted off the ground it must be turned forward (i.e., in supination of the subtalar joint, which is the base for normal heel raise). When lowering the heels back to the ground there still needs to be eccentric muscle activity in the external rotator muscles of the hip.

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Grieg explained the importance of a slow progression in order to gain good control of demi-pointe, three-quarter pointe, and finally full pointe. The muscles of the foot must be strengthened to create strong arches before pointe work can be recommended. In all foot positions the action of the foot and ankle should not be compromised, even if turn out of the hip is not fully satisfactory. (25)

As with any human structure, the lower extremity is extremely complicated. It is a combination of bones, joints, muscles, nerves, and their supporting systems. (26) When used in dance, it must provide a perfect platform for any movement, be it simple or complex. Unfortunately, when there is a dysfunction this wonderful system can stop working, and may cause injury or pain.

Talocrural Joint and Its Axis

The talocrural joint is the junction connecting the distal parts of the tibia and fibula with the dome of talus. It is made up of three joints, the tibiotalar, fibulotalar, and tibiofibular joints. (27,28) Ankle joint stability in weight bearing depends on several factors, including the congruity of articular surfaces, the orientation of ligaments, and the position of the ankle at the time of stress. The muscles crossing the talocrural joint are also responsible for stability. (29,30) The functional axis of the joint is diagonal to all three body planes. In open kinetic chain movement, this orients the foot in slight abduction during dorsiflexion and slight adduction during plantar flexion. However, in closed kinetic chain movement, the foot stays in place on the ground while the leg internally rotates on the foot during dorsiflexion (demi-plie), or externally rotates on the foot during plantar flexion (demi-pointe). In teaching dance, it is a great challenge to apply this fact in such a way as to achieve proper alignment of the whole extremity and not allow the knee to pass the foot into too much of a medial alignment in plie. (31-33)

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Anatomically the joint is not a simple hinge joint; rather, the talus travels forward in a gliding movement on the dome of talus in closed kinetic chain dorsiflexion (i.e., demi-plie) (Fig. 2). The dome of talus is wider in the anterior section than in the rear. The tibio-fibular mortise is narrower in plantar flexion and wider in dorsiflexion, stretching and straining the anterior tibio-fibular ligament in demi-plie (34) (Fig. 3). A free range of motion is needed to execute demi-plie deep enough to accomplish shock absorption in the landing of jumps. In limited range of movement in dorsiflexion the motion is transferred as compensation to the subtalar joint, leading to hyperpronation and compromised medial knee alignment. The distal tibiofibular splaying also has an effect at the proximal tibiofibular joint, requiring good stability with normal joint play there as well.

As noted earlier, "flex" and "pointe" (i.e., dorsiflexion and plantar flexion, respectively) are dance terms for movements where the ankle joint or talocrural joint is acting in opposite directions. The axis of this joint is oblique, running from medial to lateral, downward and posterior. This orientation of the axis suggests that when flexing the ankle into dorsiflexion the foot tends to abduct, and when pointing into plantar flexion the foot moves into adduction (Fig. 4).

When the dancer is told to point the foot in the air (open kinetic chain), there must be other muscle activity to prevent the foot in plantar flexion from deviating medially, causing an aesthetically "ugly foot." The muscles needed in open chain plantar flexion other than m. gastrocnemius are the assisting plantar flexors, such as m. peroneus longus and brevis, as well as m. tibialis and m. flexor hallucis longus and brevis. They are all important in improving the alignment and guiding the ankle and foot in the creation of a good looking pointe.

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As the dancer is executing releve she is performing demi-pointe in closed kinetic chain. This movement is seemingly a simple heel raise, but actually it is a very complicated series of many simultaneous movements in the foot, ankle, and whole lower extremity (Fig. 5). To train it well requires seeing the movement as a whole, rather than just one simple action of the foot-ankle unit. The dance teacher may give the image "Turn your heels forward," or "Turn the back of the knees forward." To be able to execute a correct heel raise all the assisting parts must work in harmony. In gait analysis the normal heel raise and calcaneus inversion are always connected with mid-foot and forefoot activities. Therefore, the dancer's foot function can be related to the research that has been done in gait laboratories.

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Limitation in the ability to stand en pointe is often related to tightness of the dorsal ligaments of the foot, anterior joint capsule in the talocrural joint, or posterior ankle problems such as os trigonum, retrocalcaneal bursitis, and m. peroneus longus or m. flexor hallucis longus (FHL) tenosynovitis. If plantar flexion is limited, dancers cannot execute a full releve or full pointe. This often leads to hypertrophy and hypertension of the m. soleus-gastrocnemius unit.

Subtalar Joint

Although the ankle joint is capable of some movement in the transverse plane, this movement is not sufficient to translate the rotational relationships of the leg and foot into smooth motion in the different directions of demi-plie and releve. The subtalar joint is a complex junction between the calcaneus and talus bones just below the ankle joint. It is uniquely designed to allow the leg to undergo additional rotational movements in response to different closed chain foot positions in dance. The subtalar joint is a functional joint of the human foot that acts as a mechanical link between the foot and the rest of the lower extremity. Transverse plane rotations of the leg are converted into frontal plane rotations of the foot, and vice versa, by the oblique triplanar orientation of the subtalar joint axis. Two of the more important functions of the subtalar joint are: 1. to allow the foot to pronate and act as a mobile adapter when bearing weight on irregular surfaces; and 2. to allow the foot to supinate into a position of increased sagittal plane stability during the propulsive phase of gait, or during dancers' releve. (35)

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An interesting fact is that there are no muscles attached to the talus, so the bone is totally dependent on the movements of tibia and calcaneus and the stability and congruity of the joint capsule and ligaments. The mechanics of the subtalar joint dictate the movements of the midtarsal joint and forefoot. In weightbearing gait the subtalar joint is the first joint to absorb the shock caused by gravity. Movement between the talus and calcaneus occurs around an oblique axis. The axis of the subtalar joint extends anteromedially from the neck of the talus to the posterolateral portion of the calcaneus. The average inclination of the axis is 42[degrees] from the horizontal and 23[degrees] from the midline (Fig. 6).

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Donatelli explained that there must be a free movement of 20[degrees] inversion in calcaneus and 10[degrees] eversion, measured in non-weightbearing position. (36) This can easily be tested manually with a goniometer. In normal weight bearing and ankle dorsiflexion there should not be more than 7[degrees] of calcaneus eversion as a result of shock absorption. If this movement is exceeded, in compensation the tibia rotates internally, drawing the knee medially out of alignment. This leads to internal rotation (adduction) and loss of control at the hip joint as well. (37)

On the other hand, complications are to be expected if calcaneus eversion is limited. As with any hypomobile joint, it causes compensation at the next functional unit. (38) In the foot, the compensation occurs in the oblique midtarsal joint axis, and simultaneously in the longitudinal midtarsal joint axis. This leads to first ray dorsiflexion and poor balance control of the forefoot while losing the important supporting point of the first metatarsal head. Thus the demi-pointe and full pointe are very weak due to late (prolonged) pronation, (39) and other dance movements are not executed with precision. If dance is continued with a "locked" subtalar joint limiting normal eversion, it leads to problems elsewhere. Many stress fractures, tendonitis problems higher up in the lower extremity, and compartment syndromes may be results of the impaired movement. (40,41)

Rear Foot Joint Activities in Dancers' Demi-Plie and Releve

Rear foot action is based on subtalar joint movements. The common term pronation relates to calcaneus eversion in demi-plie, and supination to calcaneus inversion in releve. These two movements are part of the normal shock absorption system of the foot, and thus are very important. In heel raise subtalar supination is needed to force closure of the medial midtarsal joints and to provide strong leverage to push against in releve. In closed kinetic chain movements, supination is always related to external rotation of the whole lower extremity. Thus, it is important to encourage dancers to strengthen their deep external rotators of the hip joint and to teach them to use those deep muscles actively in demi-pointe.

If the leg is turned out with no proper control of the movement, it may lead to sickling of the foot-ankle unit, and hence to inversion ankle injuries. For this reason, the hip adductors also need to be well-trained and "connected" in releve.

In demi-plie the foot tends to abduct due to the orientation of the talocrural axis. Therefore, the muscles of the lower extremity must be trained to counteract the tendency for the tibia to rotate internally on the talus when doing plie (this is especially important in landings jumps). Those muscles are found behind the hip joint (mm. piriformis, quadratus femoris, gemellii, and obturatorii). The calcaneus must evert with every dorsiflexion movement in the closed kinetic chain. This tiny movement of the heel bone (approximately 6[degrees]) is needed to open the mid-foot joints so that they become flexible and are able to participate in shock absorption. (43) If the foot is frequently forced into too much turnout, the supporting ligaments and muscle tendons on the plantar and medial side of the foot-ankle lengthen and lose their ability to support the medial side of the ankle and the medial arch of the foot (Figs. 8 and 9). The same thing happens if the calcaneus is not free to evert in the subtalar joint. The lack of normal movement will be compensated for in the next available unit, in this case the midtarsal joints. Forced turnout and dropping of the medial arch are considered to be responsible for the development of stress fractures of the feet, as well as the plantar fasciitis and excessive hallux valgus that are common in classical ballet dancers. (44)

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Calcaneo-Cuboid Joint

In 1949 John Martin Hiss introduced a theory of the calcaneo-cuboid joint and its relevance to weight bearing in gait function. (45) This is a three-dimensional joint with very limited movement. Still, it is the key element in foot biomechanics. It is bound with strong ligaments that do not allow much movement. When weight is distributed over the foot in a balanced way, slightly more weight is placed on the lateral arch (also known, as previously noted, as the rigid arch or weightbearing arch). The proximal tuberosity of the fifth metatarsal bone supports the lateral arch on the ground, thus providing a stable position for the cuboid as well. Hence, there is no need for the plantar fascia under the lateral arch, but it mainly supports the medial arch. The joint between calcaneus and cuboid is vulnerable to inversion ankle injuries. Often, if a dancer has twisted her ankle, the calcaneo-cuboid joint is also affected and can be subluxated. After the injury, the cuboid's movement is not as supple as normal, which can lead to compensations such as limited subtalar joint function transferring more weight to the medial arch. The calcaneo-cuboid joint is fairly easy to mobilize and adjust back to its normal position. (46,47)

Also, if the dancer's foot is forced into too much turnout (abduction) and the hip external rotation is limited, the stress of movement is transferred onto the foot joints. This causes twisting and loosening in various joints, including the calcaneocuboid joint. (48) Therefore, it is very important that teachers respect the individual structure of each dancer's lower extremities.

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Mid-Tarsal Joint Motions in Dancers' Releve and Demi-Plie

To be able to avoid sickling in releve (the uncontrolled medial-lateral movement of the foot when on demi-pointe), the dancer must shift the weight to the first and second metatarsal heads while simultaneously turning out in the tibia and femur using the deep external rotators of the hip joint. This means that the muscles controlling the mid-tarsal joints must act at the same time to elevate the medial arch and evert the forefoot. The movements of the forefoot are executed around the midtarsal joint axes and the first ray axis. The oblique mid-tarsal joint axis is the base for lifting the medial arch, elevating the navicular bone as well as the cuneiform and proximal first metatarsal. In union with subtalar supination, this movement is also called supination (Figs. 10 and 11). To push the first ray into plantar flexion and to stabilize the metatarsal heads of the first and second rays, the forefoot must evert--in other words, move into pronation as a counter action to rearfoot and oblique mid-foot supination. The first ray plantar flexion is executed with the help of m. peroneus longus, m. abductor hallucis, and m. flexor hallucis longus. (49,50) The m. tibialis posterior assists heel raise and navicular bone lift at the same time.

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First Ray Plantar Flexion in Releve

When the dancer is raising her heel in releve, there must be strong support on the medial side of the forefoot. The targeted area is between the first and second metatarsal heads, and the main focus is on the first ray plantar flexion. The second metatarsal head is tightly connected to the first metatarsal head by way of a strong interosseus ligament, and these two metatarsals act together as long as this ligament is intact. (51) First ray plantar flexion is a joint effort of m. peroneus longus and m. flexor hallucis longus, and is associated with abduction and eversion. At the same time, the m. tibialis posterior is supporting the medial arch, lifting up the inside of the talus head and bringing the navicular and sustentaculum tali closer together, thereby taking tension off the plantar calcaneo-navicular ligament. All these muscles are activated by heel raise since they serve as assisting plantar flexors to the primary plantar flexors m. gastrocnemius and m. soleus. Good support of the hip adductors is needed to direct the load onto the medial forefoot instead of letting the weight shift too much laterally, causing sickling (Fig. 12).

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Fifth Ray Function in Releve

When the dancer raises his or her heel and places weight on the first and second metatarsal heads, the head of the fifth metatarsal stays in the air. Thus, the fifth metatarsal is more important in demi-plie than in demi-pointe because it remains in contact with the ground during demi-plie. However, when the dancer is on full pointe in pointe shoes even the fifth metatarsal is part of the support system assisting in balance. As the cuboid bone is part of the fifth ray joining the forefoot to the calcaneus, the limited movement of the calcaneo-cuboid joint or joints between the fourth and fifth metatarsals and the cuboid may lead to decreased ability to rise onto demi-pointe or full pointe. If so, these joints may need to be adjusted or mobilized to reach full mobility, thus avoiding the need for compensatory movements elsewhere in the foot. (52)

Failing the normal function of releve, the dancer is prone to many stress-related injuries in the lower extremity because the alignment is compromised and the normal action of the closed kinetic chain is sacrificed. The ergonomics of the bones, joints, muscles, and tendons are jeopardized, and this is considered a risk factor in dance. (53)

Forefoot Stability in Releve

Numerous investigators have demonstrated that in a normal foot the longitudinal arches are not supported by muscles. (54-57) Lapidus was one of the first to describe the concept of the foot functioning as a truss. A truss is a triangular structure with the beams connected by a tie rod. The vertical forces transmitted to the foot in weight bearing are attenuated by the truss mechanism. (58) Therefore, intrinsic and extrinsic muscles are not used in static arch support if the foot is in neutral position. Trying to increase the hip-leg turnout over the limits of normal range will impair the passive static support system of the foot. Mann reported activity of the intrinsic muscles, adductor hallucis brevis, flexor hallucis brevis, flexor digitorum brevis, and abductor digiti minimi in stabilizing the transverse tarsal joint during the stance phase of gait. In normal feet, the onset of intrinsic muscle activity is initiated at approximately mid-stance. In standing positions, shifting the body weight posteriorly tends to shut off the intrinsic activity, while shifting forward turns the intrinsic muscles on. A fallen and flattened arch has poor intrinsic muscle activity, and some studies suggest that it cannot be raised by exercises. (54)

As previously mentioned, the transverse arch is formed by the cuneiforms and the cuboid. (17) Support of the forefoot is by means of the heads of the metatarsals. The metatarsals are held together by ligaments and muscles. The six elements that prevent splaying of the forefoot include: Lisfranc's ligament, the transverse metatarsal ligament, the interosseus muscles, the peroneus longus muscle, the plantar extension of the posterior tibial tendon, and the adductor hallucis muscle.

When standing, the second and third metatarsal heads bear the greatest forefoot pressures. (59)The fact that the first ray does not bear the greatest pressure in standing indicates that it has a more dynamic function during push-off in gait and releve in dance. The first metatarsal bone is twice as wide as the second metatarsal, and four times as strong. (60) Furthermore, the peroneus longus, posterior tibialis, and anterior tibialis muscles attach to the first ray and function to stabilize it in the propulsive phase of gait and in releve in dance.

Toe Activity in Dance

The toes are fairly passive and peak pressure on them is minimal when in standing position with the heels on the ground.61 However, among dancers, hyperactivity of the toe muscles is common in order to improve balance and foot stability (especially when leg turnout is increased beyond the normal hip-leg rotation of the individual). In releve, as in the propulsive phase of gait, the long tendons of the toe flexors maintain floor contact and help to stabilize the longitudinal arch. (62)

Demi-Plie in Dance

Demi-plie is executed in all five ballet foot positions, and also in parallel (the sixth position). (63) This basic exercise turns out the legs; develops the tendons and muscles of the thighs, calves, ankles and feet; and increases flexibility and strength in the Achilles tendon. The spring-like action of demi-plie is essential to all jumping movements as preparation for jumping upward and shock absorption when returning the feet to the floor. (64)

Dancers must have enough dorsiflexion (flex) in the talocrural joint to facilitate deep plies in landings. Otherwise there is the risk of developing a hypermobile medial arch, which may lead to a forefoot varus with compensatory hyperpronation, or, in dance terms, "rolling in of the foot." (65) Limitation to plie may be caused by a short calf muscle or Achilles tendon; short assisting plantar flexors such as flexor hallucis longus (FHL), flexor digitorum longus, peroneals, and tibialis posterior; or tightness in the talocrural joint. Limited subtalar joint (STJ) eversion-pronation also restricts dorsiflexion.

In demi-plie, the ankle and foot act as a shock absorption system. The subtalar joint is located anatomically between the talus and the calcaneus. It is a fairly complicated joint, with three surfaces connecting two bones tightly together, yet allowing some transition between them. However, the two bones positioned on top of each other have two parallel loading lines, causing them to share normal movement. (42) Newton's third law of motion (which describes force and counter force) and the orientation of the talocrural joint axis together create subtalar pronation.

Midtarsal Joints as Part of the Supportive and Shock Absorbing Systems of the Foot

The midtarsal joints are supported by both passive and active players in foot function. The passive structures are all the plantar and medial ligaments, joint capsules, and fascias of the foot. Part of the support is also derived from the form of the joints, which when loaded in good alignment, does support the bones under and over each other. Therefore, the dancer's ability to control shifting of weight and alignment of the leg is vitally important. The active support of the foot is dependent on the intrinsic muscles of the foot, the short and long toe flexors, as well as m. peroneus longus. The most important of the medial ankle supports is m. tibialis posterior.

First Ray and Its Relevance in Shock Absorption and Support

Dancers need strong plantar flexion of the first ray. If the ray is weak and limited for full plantar flexion, the result is hypersupination of the forefoot and hyperpronation in the subtalar and midtarsal joints. Forced turnout and "rolling" of the foot into hyperpronation often gradually leads to a loose interosseus ligament between the distal heads of the first and second metatarsals. This is also one of the causes of hallux valgus, which is very common among dancers who have been forced to use turnout beyond their natural limits. It also makes the skin thicker on the inside of the ball of the foot and the big toe, an indication that even the skin is overloaded.

Strength of Peroneus Longus

Peroneus longus is one of the strongest muscles for pushing the head of the first metatarsal down, thus cushioning the gravitational forces in landing and demi-plie. Weakness of this muscle leads to poor control of balance in demi-plie and overload of the flexor hallucis longus muscle. An overly tight tendon of the peroneus longus muscle is typical of the rigid pes cavus foot type, and easily locks the first ray down into plantar flexion, making the foot even more rigid and shifting weight onto the lateral arch.65 In dance, this leads to poor technique, especially landings in demi-plie that lack shock absorption.

Flexibility and Strength of the Flexor Hallucis Longus

The flexor hallucis longus muscle assists m. peroneus longus in bringing the first ray into plantar flexion. If too tight, the tendon of the flexor hallucis longus muscle easily locks the subtalar joint because it passes behind the talus and under the sustentaculum tali of the calcaneus. It forces those two bones together. (66) When a dancer is doing demi-plie the tendon of flexor hallucis longus is tightened even more because the muscle is one of the assisting plantar flexors and gets more taut in dorsiflexion. It may also limit the big toe from extension and could cause tenosynovitis behind the medial malleoli, where its tendon often thickens. It may need to be under a continuous stretching program for as long as the dancer is active in her career.

Fifth Ray as Part of the Shock Absorption System in Demi-Plie

The lateral part of the foot forms a fairly rigid arch, which supports the foot in a loaded position. It works well if the dancer is able to direct her weight to the mid-foot, dividing the load evenly onto the medial and lateral parts. If the calcaneus is not moving freely into eversion in the subtalar joint, or if the calcaneo-cuboid joint is not free in motion, there will be consequences in other parts of the foot. The weight will be transferred more medially, leading to excessive internal rotation of the tibia as well as the femur; thus, the weight will not be supported by the lateral arch and the fifth ray. The interosseus ligament between the first and second metatarsal heads becomes stretched, loading too much weight onto the second metatarsal, which explains why dancers with this failure are prone to suffer from bone edema or stress fractures of the second metatarsal bone. This same dysfunction also stretches the plantar fascia and other plantar structures excessively, and may cause strain-related problems even there.

However, full shock absorption is possible with proper eccentric work of all the body's spring action muscles. These muscles are: back extensors; hip extensors such as m. gluteus maximus, hamstring group, and m.adductor magnus; knee extensors m. quadriceps femoris; and ankle plantar flexors m. triceps surae. In addition, medio-lateral support of the hip is important to eliminate faulty alignment. A well-trained and properly controlled dancer's foot is able to give full support in landing and work at the same time as a perfect shock absorption unit.

Conclusion

There is overwhelming evidence in gait research, sports science, podiatry, and dance medicine that proves the importance of control and proper technique in dance training. The questions raised by dance medicine as to why so many dancers become injured may find answers in a biomechanical understanding of lower extremity functions. The function-related problems need to be solved with correction of the failures rather than with medications or surgical interventions. (67) Preventive exercises are important in combating the increasing number of foot injuries. They need to be well designed and performed, based on knowledge of lower extremity biomechanics. Even the structural differences among dancers can be mediated with knowledge of the laws and principles of physics, biomechanics, and physiology as they relate to how the body can adapt through training.

This review is fairly superficial compared to the complexity of foot biomechanics as a whole. Nonetheless, it is hoped that it will help dance professionals and medical professionals in their search for answers to the questions surrounding dance injuries in biomechanics, fatigue, exhaustion, (68) and failed technique. Much of the research represented here has been done in podiatry, sports medicine (on runners and jumpers), or in gait laboratories (on walking). While it should be applied in dance training and dance conditioning or rehabilitation, more research that specifically investigates these mechanisms in dancers needs to be performed.

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Jarmo Ahonen, P.T.

Jarmo Ahonen, P.T., is at the Finnish National Ballet Company, Finnish National Opera Ballet School, Helsinki, Finland.

Correspondence: Jarmo Ahonen, P.T., Finnish National Ballet Company, Finnish National Opera Ballet School, Helsinginkatu 58, 00250 Helsinki, Finland; jarmo. ahonen@ftk.fi.
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Publication:Journal of Dance Medicine & Science
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Date:Jul 1, 2008
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