Children sport shoes – A systematic review of current literature

Markus Walther MD PhDa,*, Dirk Herold b, Angela Sinderhauf c, Robert Morrison c
a Department of Foot and Ankle Surgery, Orthopaedic Hospital Munich-Harlaching, Harlachinger Straße 51, 81547 Munich, Germany b Klinik für Unfall- und Wiederherstellungschirurgie, Caritas-Krankenhaus, Uhlandstraße 7, 97980 Bad Mergentheim, Germany c Department of Orthopaedic Surgery, University of Wuerzburg, Brettreichstrasse 11, 97074 Wuerzburg, Germany
Received 31 December 2007; accepted 9 April 2008

Abstract

Objective: The child’s foot is clearly distinct from the adult foot in its functional anatomy and ability to cope with pressure. This requires special considerations in the development of a children’s sport shoe.

Methods: Medical and sport science databases were thoroughly searched for studies pertaining to the anatomy and biomechanics of children’s feet during their development. With the data found, a list of requirements for the children’s shoe was compiled.

Results: Small children should have a sports shoe, which is as flexible as their own foot. The small impact forces during their sports activities make extra cushioning superfluous. During school age the connective tissue gains stability. The growing amount of sports activities, much of which is performed on hard indoor surfaces, enhances the need for cushioning. At the same time there is a growing necessity for adequate mechanical stimuli to help the muscles and bones develop. The strength of the connective tissue and the flexibility of the joints reach adult levels by the age of 15. In small shoes, the displacement of proportions can lead to improper positioning of the flex zone and thereby causing harmful stress on the foot. Cushioning elements are often oversized. Considering the wide range of anatomy in the child’s foot, it is advisable to produce children’s shoes in different widths.

Conclusion: The child’s foot differs in anatomy and function from the adult foot. Children sport shoes should meet the child specific requirement.

# 2008 European Foot and Ankle Society. Published by Elsevier Ltd. All rights reserved.

1. Introduction

There have been hundreds of studies made covering every aspect of sport shoes for adults. The reason for this intensive research is a constant rate of sports related injuries over the past decades [1,2], combined with the desire to reduce them, by optimizing the sport shoes. There have been far less studies pertaining to children’s sport shoes. This could be due to a number of reasons. Foremost the range of tolerance in the immature bones and tendons is very wide, so that problems caused by the shoes are seldom expressed. Parents will acknowledge that their children often wear shoes which are too small for them, simply because it is their favourite shoe. Continued wear of shoes, which are too small or bio mechanically incorrect, will have negative affects on the children’s feet, even though there might not be acute signs of distress.

Another reason for the lack of research in this field is the relatively small market, compared to the demand for adult sport shoes. The effort required in the production of children’s shoes is almost equivalent to that needed in the adult product. But the slightly lesser amount of material needed to produce a children’s sport shoe, is more than offset by the much lower sales price. The adult product benefits from high-profile athletes, demanding to have their material improved so as to give them a split-second advantage over their opponents. Children, who have not reached puberty, do not have the fitness required to compete in international competition, because their muscles and tendons are not strong enough to take that kind of pressure. For the sport-shoe-industry, the lack of pressure from international competition means a lack of incentive for investment. There is no demand for high cost innovations to close in on a ‘‘perfect’’ sports shoe.

These three issues are the main cause, why research for children’s sport shoes has been seemingly non-existent. Since children usually adjust their taste to mimic that of their adult sports idols, the shoes designed for these stars are often simply broken down into smaller sizes without giving consideration to the specific needs of children’s feet. The goal of this bibliography is, to show the specific requirements of children’s feet during different stages of their development.

2. The evolutionary stages of the children’s foot

2.1. Newborns

The feet of newborns are characterized by their prominent supination and forefoot adduction. The soles
are facing each other. The big toe often stands away from the other toes (sandal gap). The dorsal extension has a wide range of mobility, permitting the back of the foot to be brought up to touch the shin. On the other hand the plantar flexion is very limited. The arch is high with newborns, and even higher in prematurely born children. Within the first few weeks of their life, the arch diminishes and is filled with a fat pad (called Spitzy’s fat pad) which remains until late childhood [3]. Anatomical studies on newborn feet [4] prove that their feet show no differences when compared to feet of children during the last months of carriage. The feet are in a supination position and the ankle joint does not lay fully over the calcaneus, though overriding it on the medial side. During the first months of their life the distinct dorsal extension gets less, and the plantar flexion is more freely possible. The plantar fat pad grows thicker and an existing supination persists throughout the first steps of the baby [5]. The feet of newborns are, like the whole body of the babies, always in motion, as long as the baby is awake. When the toes 2–5 are flexed, the big toe is automatically extended [5]. This movement pattern shows aspects needed for walking later on. During the swing phase the toes are extended, during tip-off the toes 2–5 are flexed and thereby help with the push off, while the big toe is pressed onto the surface. Starting in the seventh month of life the child begins to attempt an erect posture. By the age of 18 months almost all children can walk with assurance and have good control of their balance [6,7].

2.2. Age 2–6

As soon as the child starts walking at the age of 2, the feet are still in a slight supination and the soles are still turned a bit towards each other. Toddlers walk rather like a primate with the hips and knees flexed, the knees bow legged and the legs externally roatated [5]. Caused by the bow legs, the line of weight does not go through the central part of the knee, but rather passes the joint medially.

This causes an increase pressure on the medial, and an increased tension on the lateral part of the growth-zone [8]. This makes the medial femoral condyle and tibial plateau grow faster than the lateral parts. Thereby the leg axis straightens out. In many cases excessive medial growth results in a valgic leg axis, which reaches its maximum at the age of 3. The axis can exceed a valgus angle of 108. The neutralization of the leg axis is normally finalized by the age of 6; however with overweight children it can be prolonged or never reached. If an valgus angle of 5–78 persists through the age of 14, it is pathological [9]. The reduction of the anteversion of the femoral neck in toddlers brings the foot into an outside rotation of about 208. An excessive anteversion can be functionally negated through inside rotation, which brings the heel into a stronger eversion and therefore is jointly responsible for a flatfoot in children. By the age of 16, the anteversion of the femoral neck has regressed to about 168 [10]. A sign for an incomplete retroversion of the thigh is the inability of a child to sit cross-legged. Before the retroversion is completed children prefer sitting on their knees with their lower legs rotated outward (Fig. 1). As soon as children begin to walk, the rear foot starts moving into its upright position. This evolutionary step is made possible through bone growth, outward rotation of the malleoles, a denser tissue and strengthened muscles [11]. Similar to the knee, the vertical stress does not come into contact with the epiphysis at a 908-angle, but rather at a crooked angle which results in asymmetric growth. The parts of the bones, which have the highest tension, also show the strongest growth until the epiphysis plane is at a 908-angle to the direction of the load. The talus bone and the calcaneal bone grow faster on their medial side than on the lateral part during this phase. This brings the upper ankle joint into a vertical position, and the same time the bones of the midfoot (cuboid bone, navicular bone, and cuneiforme bone) move from their supination position to a more pronated location. This also brings the talus around, as it is free of muscular tensins [5]. As children begin to walk, the muscles holding the foot in an upright position start to strengthen and the surrounding tissue grows denser and the collagen fibers interconnect stronger. An interesting observation in this context is the fact that many children at the age of 3 or 4 use a forefoot stride.

It looks like the clinical picture of a shortened Achilles tendon when the calf muscles are flexed. This leads to torsion of the foot by bringing the rear foot into supination and the forefoot into pronation, thus causing the plantar fascia to tighten, and together with the small muscles within the foot leads to a higher arch [12,13]. This development stage lasts for a couple of weeks or even months. There are no studies though, that show how many children go through this facultative phase or how long it lasts. The typical inward rotated walking style performed by many children is a normal step in their development [14]. One can distinguish between ‘‘kneeing in’’ and ‘‘toeing in’’. The retroversion of the femur and the torsion of the tibia, which both lead to a turning of the maleolar fork, are not finished at the same time, because the tibial torsion normally takes longer. So it is quite normal for children to walk inwardly for a fairly long time. The spread of the standard values is quite big [7,8,15]. In addition, with toddlers beginning to walk a remaining supination of the forefoot, together with a tendency of the foot to roll over the outside of the sole, can fortify the clinical picture of an inward rotated gait [16]. The calcification of the foot starts at the outward side of the rear foot and ends at the inner part of the forefoot. The first bone nucleus within the tarsus emerges in the heel bone during the fifth or sixth month of pregnancy. During the seventh or eighth month two more nucleuses emerge in the talus and an additional one develops in the cuboid bone at the time of birth. The nucleus leading to the navicular bone emerges quite late, around the age of 2 or 3. The time spread again is quite large, girls’ feet being calcified by the age of 22 months and those of boys 8 months later [5]. Another fundamental difference between the feet of adults and those of children is the connective tissue. The mechanical stability of the tendons and ligaments develop during school age and continue strengthening until puberty. The high elasticity of the connective tissue in small children is due to the high percentage of polyglycans and elastic fibers and only a small partition of undeveloped collagen. The few and undeveloped cross-connections of the ripe collagen are also part of the cause. This very soft and plastically workable tissue is of fundamental importance to the development of children [3,5]. The missing or poorly developed arch in small children and those of school age often slight supination may mislead the clinician. The soft foot of a child flattens inwardly during stance. Clinically the inside edge of the foot can lie on the ground and appear to be a flatfoot. This impression can be reinforced by an increased valgic position of the rear foot. The arch will show regularly when standing on the toes or when forcefully overextending the big toe [5,12,17].
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The often wrongfully diagnosed flatfoot in children is almost always a normal foot. Because of the soft connective tissue, the children’s foot is not yet adapted to the high static pressures forced on it and therefore simulates a flatfoot [18,19]. Another reason is increased anteversion of the femoral neck and physiological valgus of the knees. Furthermore the arch is filled by the Spitzy’s fat pad [3]. For these reasons the condition is referred to as ‘‘Developmental flat-foot’’, which is a normal transitional phase leading to the adult foot and does not require treatment [21–23]. Different studies have been performed to analyze the development of the foot [24–26]. The results confirm a fast rate of growth during the first yearwith boys and girls having a growth of almost 1 in. Between the ages of 4 and 12 the girls’ feet growabout 0.1 in. less than those of boys. After the age of 14 the feet of girls almost cease growing, while the feet of boys continue to grow until the age of 16. During these 2 years the main size difference develops. The opinion that feet grow intermittently has not been scientifically proven [3,5,27,26]. Slight differences between the two feet can be found in more than 50% of all children, but most of them are slight. The frequently expressed opinion that the left foot is longer cannot be seen in the data. However during development it is always the same foot, which is longer. With many adults there is no perfect match between the sizes of their feet, so that one stays longer for the rest of their lives [28].

3. Demands towards a children’s sport shoe depending upon the age

3.1. Infancy (0–1 year)

During infancy there is no need for a shoe, let alone a sports shoe. During the first few months a pair of socks provides satisfactory protection from cold and weather factors.

3.2. Young infant (1–2 years)

During the phase of learning to walk the main purpose of a shoe is to protect from weather and environment. A shoe should be limited to these basic needs at this age.We always should keep in mind, that the foot needs no shoe for its proper development. Over a period of hundreds of thousands of years our genetic program has adapted to a ‘‘shoe-less’’ foot. So at this age, shoes should be extremely soft and flexible to allow the foot a freedom to move as if it were barefooted [29]. If a sole, which is too thick, it would serve as a lever and thereby add strain on the foot und the ankle joint, this could lead to a negative growth impulse towards the development of the upright foot position [20]. Whether these shoes should be produced by the sports shoe industry or children’s shoe companies will be left open at this point. The functional requirements regarding the shoes are the same.

3.3. Middle aged infants (2–4 years)

At this age children start getting more active. They begin kindergarten and start attending their first ‘‘sporting events’’ such as children gymnastics. Seen from the standpoint of their evolution, this is the time where their short foot bones ossificate. As mentioned before, the load induced on the bones is a major reason for the regular growth of the bones. Fundamental changes to the loads upon the foot, compared to walking bare foot, can be an unfavorable stimulus to the growth of the foot [30]. Strain-stimuli from the outside are necessary and should remain unchanged by the shoes [31]. During this phase, the physiological flatfoot is the most evident [16]. Sports shoes for the activities in kinder garden and children gymnastics should therefore have a flat and soft sole. Gym shoes nearly meet these demands. The integration of an arch support, as it is called for by a group of Australian orthopedists, is seen by us as not only being unnecessary but even dangerous for the normal development of the foot [12,32,18]. A group led by R. Baumgartner recommends rear foot support for the phase of learning to walk. This leaves the main axis of movement in the ankle joint and the metatarso-phalangeal articulations free [33]. In the construction of children shoes, stability is not referred to the same as in ski boots, but rather describes a ‘‘dynamic stability’’ in the sense of functionality. In this manner the foot can be supported in its natural motions and protected against unphysiological movements [34]. Stability is called for, because today static strains from extremely hard surfaces affect the still untrained young foot [35]. A stable guide such as a high strap over the Achilles tendon is essential for the rear foot. When constructing such a stable heel cap it is of great importance, that the edges are not made too stiff [36]. To guarantee a natural movement of the base of the toes one must take into consideration that the medial edge of the foot grows significantly faster than the lateral part. Therefore the axis going through the metatarsophalangeal articulations moves toward the rear of the foot and its angle decreases from 798 to 728 [37].

3.4. Late infant age (4–6 years)

At this age most of the children attend kindergarten and start participating in sports activities. The coordination of their body improves greatly at this age and walking, running,  and climbing steps can be easily accomplished. Most children are able to ride a scooter and begin riding a bicycle with support wheels. Their increasing activity and endurance brings new forms of stress for the feet. During this phase of their evolution, the inside of the feet grow much faster than the rest, resulting in a reduced width and height compared to the length. Due to the increased stress, Staheli [38] recommend a shoe, which comes up over the ankle.  This shoe should also have a stable rear foot. Most of the sports activities for children at this age take place indoors. Leather soles are too slippery on those M. Walther et al. / Foot and Ankle Surgery 14 (2008) 180–189 183 surfaces and a lot of the rubber soles cause so much friction, that the risk of a sprained ankle is drastically increased [42]. Rubber soles, as they are also used with adults have prevailed. Caused by the Spitzy fat pad, the foot of a child has a larger contact area, compared to adults [13]. Therefore there is no necessity for a special absorption element or more arch support. Shoes should ideally have a flat sole and good forefoot flexibility. The shoe should naturally flex at the height of the metatarso-phalangeal articulations when it is pressed onto the floor at a 458-angle (Fig. 2a). A flexion between the midfoot segment and the rearfoot results in an unphysiological stress in the midfoot and should therefore be avoided [39] (Fig. 2b).

3.5. Elementary school age (6–10 years)

At this age the connective tissue provides increased stability [22,27], causing the mobility of the foot to decrease compared to that of younger children [40]. They start going to school and joining sports clubs. This growing intensity of sports activities and the fact, that indoor floors are made for the biomechanics and the weight of adults, make a good absorption even more important [41]. Furthermore it must be taken into account, that children during this phase of their development put more pressure on their heel then adults do [42,43]. Ground reaction force, in relation to the body weight reaches the adult level by the age of 8 [7,44,45]. The idea of putting additional cushioning in the children’s athletic shoes is not so easy. Along with the stress, the need for stimuli for the organism grows. These stimuli are needed, to ensure growth of the muscles and bones [37]. All in all the higher stress can be taken into account by adding stability to the midfoot and rearfoot segments. At this age differences in the foot shape between boys and girls start to evolve. While girls have a slender, delicate foot, boys have a much more voluminous midfoot. Differences in circumference in relation to the foot length are common between the sexes
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3.6. Late school age (12–15 years)

Within this age group the foot reaches its final size. The connective tissue of a 15-year old is almost as stable as that of an adult. Girls’ feet stop growing at about 12–13 years of age and those of boys at about 14–15 years [27,26]. In some cases growth has been observed past the age of 20 [5]. The physical and athletic fitness is getting close to that of adults [48]. The stresses on the foot increase along with these activities, and are well tolerated by the strengthened bones,muscles and connective tissue [20,49]. So for children at this age, sport shoes can be constructed in the same manner as those for adults. According to the kind of sport and the foot shape, a certain amount of cushioning is necessary. The cushion is not actually needed for the foot, but is necessary due to the construction of the surfaces, which are very hard. Running on these hard surfaces without cushioned shoes will lead to injuries and strains [50,51]. On the other hand too much cushioning can lead to problems in the neuromuscular transmission process and therefore cause sports related injuries [34]. The amount of cushioning required is dependant upon the kind of sport, the surface and the individual preferences, which makes a general recommendation difficult. For example, streetball requires a lot of cushioning, as it is mostly played on hard surfaces like concrete and involves a lot of jumping and therefore a lot of impulses to the foot. Track and field on the other hand requires very little cushioning, as surfaces like grass and tartan are softer. So when selecting a sport shoe, it is important to decide ahead of time, what the shoe will be used for. Excessive pronation can be decreased by using a medial arch support [52]. But the pronation itself is a natural movement of the foot, which has great individual deviations [53]. Athletes with excessive pronation, but without complaints do not necessarily need a medial arch support.

4. Requirements for the children’s shoe, independent of the age

4.1. Fit

The critical point in selecting sport shoes for children is the evaluation of the correct size and the perfect fit. When determining the size of the foot, the length and the width should be measured and the shoe should then be selected by using the WMS-System. The WMS-System was invented (in Germany) in 1974 by the ‘‘Arbeitskreis Kinderschuh’’, a consolidation of several children shoe brands [5,35,54]. This system uses length andwidth to determine the shoe size and thereby respects the great variance of the foot shape. Every product is sold in widths of narrow (s), medium (m) and wide (w). Furthermore this classification describes the relation of rear- and midfoot to the forefoot part. This is ofgreat importance to make the toebox sufficiently large and  place the flex lines under the base joints of the toes. The WMS-System recommends a rearfoot-forefoot-ratio of 63– 37, with a deviation of less than 1%.At children’s shoe size US 4 a deviation of 1% would be 1.2 mm off, and at children’s size US 12 it would already be 2 mm. Renowned specialists of shoe- and last-design have agreed that a forefoot proportion of 37% should not be exceeded and
under no circumstance fall short of that in sport shoes [5]. Children’s sport shoes, which are not produced according to the WMS-System often show substantial deviations from these values. This result in shoeswith a short forefoot and an optically pleasant, long and lean rearfoot. These shoes cramp the forefoot, and the toes do not have enough room, thus encouraging malformations [55]. The length measurements used in central Europe aremeasured on the insole and not on the sole of the foot. This results in a difference of foot length and shoe size (Fig. 4). In the sport shoe industry the British length measurement has taken a dominant position, but in most cases the French measurements are given along with it. A French unit is 6 2/3 mm. Shoes are manufactured starting with children’s US 3, which has a sole length of 4 1/ 8 in. The British system is basically measured in inches, so a British unit is about 8.46 mm, which is 1/3 in.
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4.2. Clasp

The clasp should be easy to use and close the shoe evenly throughout the whole length. To prevent the foot from slipping forward, the shoe should be fit tightly around the instep and arch. It is also beneficial for stability, when the upper eyelets are placed at a 35–468- angle to the floor (Fig. 5) [33,35]. The shoe laces should be the correct length.

4.3. Material

A children’s foot loses about 20 g of moisture during an 8-h time span [27]. During phases of intensive sports activity the amount of moisture can increase drastically. Therefore the upper material should be capable of absorbing moisture and conduct it towards the outside. In recent years tremendous improvements have been made by using new materials.

4.4. Weight

Especially with children’s sport shoes a light weight is important, so that good energetic requirements for the sports activities are met. The usage of newly developed materials throughout the past 10–20 years has led to a noticeable decrease in the weight of shoes.

4.5. Price

Orthopedic and biomechanically correct shoes don’t necessarily have to be expensive. The fact that many technical elements like improved forefoot- and rearfootcushioning systems, foot bed and anti pronation systems as they are used in adult running shoes are not required in the production of children sport shoes. This can help maintain production costs at a low level. The amount of material needed to produce the shoes is also less. On the other hand, production costs are increased due to the small number of shoes produced.
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5. Discussion

Above all considerations concerning children’s sport shoes stands the statement of Ernie Maier, who has devoted all of his scientific work towards the children’s foot: ‘‘No shoe is capable of improving the maturation process. A shoe concerning all aspects of a children’s foot is distinguished by the fact, that it impairs the maturation of the foot less than others do.’’ The market for children’s sport shoes is not led by orthopedic and biomechanical considerations. The consumer behavior, shaped by trends and sports idols has a much bigger influence. Which kid does not want to wear the same shoes as Michael Jordan or Kobe Bryant? This often leads to a simple downsizing of the hyped-up adult shoes for the children, and the maker of the shoe is confident the marketing will be successful. Functional aspects of the shoe are often neglected. Differences in the maximal forces and leverages in the children’s foot lead to a sole, which may be right for the cushioning and stability of adults, but feels hard as a plank for children.

Even intensive campaigns cannot keep children from wearing shoes which are much too small for them. The WMS-Event 2001 showed, that only 46.7% of the tested children wore shoes with a proper fit. Children wearing shoes which were too big were found in 9.6% of the cases, 33.4% wore shoes, which were one size too large and 10.3% wore shoes which were two or even three sizes too big for them. This shows even clearer how important further education is especially for orthopedists treating the children. Children often do not complain about shoes which are too small [5]. A lot of parents underestimate the growth rate of children’s feet, which can be up to three sizes in only 1 year in the case of infants. The feet of children in elementary school can still grow up to two sizes each year. Another problem is that not all sport shoe makers actually manufacture their children shoes according to the WMSSystem. Not only do they offer just one width, but considerable discrepancies could be found between the length of the insole and the actual shoe length [58]. Most of the shoes, that were not produced according to the WMSSystem, were produced smaller than the size indicated, others were made too large [57]. The shoe stores are very reluctant to sell shoes according to the WMS-System. Their main argument is the high storage costs, since they must stock three pairs of every size. The understandable wish of the children, to get the best product available, often turns out to be quite the contrary with children sport shoes. Sometimes it is better to have less. A lot of the technical elements like a specific cushioning system, certain insoles or added medial support are often not needed in children shoes [59,60]. In daily use however a shoe, which is too stiff is the problem encountered most often.  In summary it can be said that the demands made for an ideal children’s sport shoe are essentially dependant upon the current stage of development of the children’s foot. An attractive appearance, which can also be made to look like the shoes of the children’s idols, can still be used. The orthopedists play an important role in schooling the children and their parents and thereby preventing the use of the wrong shoes. The possibilities of education are numerous. Parent– teacher-meetings in kindergartens and schools as well as sporting events or the opening of a new shoe store are just a few of the opportunities.

6. Ten tips for buying children sport shoes

(1) Check the fit of children’s shoes once a month. A child can outgrow up to three sizes in only 1 year. When taking the shoes off after sports, check for red marks on the feet. Red marks on the balls of the toes are a sign, that the shoes are too short whereas red marks on the top of the joints of the toes show that the shoes are cut too low. When the inside or the outside of the feet are red, the shoes are too narrow and if the toenails are bent upward or worn down, it is a clear sign, that the shoes are too short.

(2) The shoe should be about 2/3 in. longer than the foot. This extra space ensures that the toes have enough room and prevents malformations of the toes.

(3) A shoe should rather be bought a little too long and narrow, than too short.

(4) Make sure, the shoe has a flexible sole. When pressing the shoe onto the floor diagonally the sole should flex where the balls of the toes would normally be and not flex within the midfoot segment.

(5) Sports shoes for children do not need heels. The less the better.

(6) A little cushioning of the sole is useful beginning at school age. While children are in kindergarten
cushioning is superflous because the forces are so small within the foot.

(7) The flattening of the arch is a normal phase during the development of the foot. So the normal childrens foot needs no support of the arch and there is no indication for an orthopaedic inlay.

(8) Make sure there is enough ventilation in the upper material, as the childrens feet sweat more than adult feet.

(9) The best time to try on sport shoes is the late afternoon, as feet are swollen after walking and standing all day. This ensures that shoes are not bought too small.

(10) The edges of the shoes should be well padded and not cut into the Achilles tendon or the ankle. Acknowledgements

We thank Frank Kleindienst, Koerger Harald, and Krabbe Berthold of the Biomechanical Laboratory of the adidas AG (Biomechanical Lab, Adi Dasler Strasse 24-26, 91443
M. Walther et al. / Foot and Ankle Surgery 14 (2008) 180–189 187
Scheinfeld, Germany) for their support and all helpful discussions.

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