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Lateral bias, functional asymmetry, dance training and dance injuries.

Some of the major technical requirements for a dancer are: static and dynamic stability, most often on one leg; a good range of motion in specific joints to create an aesthetic line in the gesturing leg and in the upper body; leg strength for take off and landing; and the ability to turn efficiently. In an ideal world dancers would be able to perform any of these dance tasks equally on either leg and to either side, and thus provide a "neutral" instrument for the choreographer. Realistically, it is more likely that a trained dancer has an asymmetrical body structure, a dance technique that is functionally asymmetrical, as well as a preference for learning and performing specific skills on one leg or one side. Exploring the potential contributing factors to this asymmetry--the degree of lateral preferences in the dancer, the actual extent of structural and functional asymmetry, as well as evaluating the existence of laterally biased dance experiences--may reveal interrelationships among these factors. Furthermore, the role that asymmetry may play in contributing to injury is not yet known. Assuming that a more symmetrical dancer is desirable, this topic of inquiry is surely of much interest to the dance educator, as is the extent to which asymmetry may be preventable. The following questions need to be answered if we are to improve a dancer's training:

* How can lateral preference of the dancer, and possibly of the teacher, be assessed most effectively?

* Does a lateral preference affect how well the dancer can learn and perform dance skills on either leg?

* Is some asymmetry in the body "natural" and if so, to what ex tent is it increased or decreased through training? Is asymmetry necessarily a negative factor? How, and to what extent, does it precipitate injuries in dancers?

Some information relating to these issues is currently available in the dance medicine and science literature, but to date exists only as individual pieces of the puzzle; it is not yet combined within a holistic picture that would allow for the exploration of potential relationships between lateral biases, functional asymmetry, dance training, and dance injuries. What is lacking in the literature is a framework within which to put the pieces of the puzzle together. A second difficulty is that anatomical and functional data and injury reports often do not supply sufficient separate data for the right and left sides nor provide lateral preference information.

As a representative sample of current data in the field, a review was undertaken of articles published in the Journal of Dance Medicine & Science from 2001 to 2006, as well as abstracts published in the Proceedings of the International Association of Dance Medicine and Science (IADMS) 2003-2007. Studies included in the review were those providing ankle, knee, or hip data from anthropometric measures, screening parameters of dancers or non-dancers, biomechanical data of dance skills, dance injury reports, or risk assessments.

Laterality, Lateral Preference, and a Division of Labor

The term laterality (or lateralization) is a "catch all" used in the literature for a broad area of study that encompasses a number of different concepts. It may be used to describe a lateral preference, a structural or functional asymmetry, a difference in brain organization, or a cognitive concept. In its most basic sense laterality refers specifically to a process in a child's cognitive and perceptual development that involves an awareness of the two distinct sides of the body. Correctly labeling and being able to move the right or left limb on command follows that body awareness, and develops in the typical child as early as 5 or as late as 8 years of age. (1)

Awareness of laterality, however, is different from a lateral preference. The terms of interest in this article are lateral preference, or lateral bias. Research suggests that some lateral biases are evident as early as a head turning preference at birth, soon followed by reaching and grasping and a bimanual manipulation preference by the end of the first year of life. (2,3) At the time of school entry a preference for handwriting, throwing, hopping, and ball kicking has been established in most children. (1,4)

It is also true that children live in a right-biased world, where play, sport, and activities of daily life tend to be spatially orientated to accommodate a cultural right preference. Many motor skills are practiced more on one side than the other, and it is not surprising therefore that this promotes more proficiency on one side; that is, there is a functional asymmetry in many motor skills. Consensus in the literature suggests that the mobilizing limb--i.e., the limb used to manipulate an object, as in kicking a ball or starting up stairs--is considered the dominant or preferred leg, and the posture stabilizing leg is the non-dominant. (1,2)

There is an extensive body of laterality research in psychology, motor learning, and motor development documenting the acquisition of lateralized hand and foot skills, identifying the degree of lateral preference, and comparing the functional asymmetry in skilled performance. A key finding relevant to dance is that lateral preference does not necessarily equate to proficiency. The degree of lateral preference is task dependent, and a division of labor that produces an asymmetry in hand and leg usage may be functionally efficient. (5,6)

Lateral preference in either hand or foot is not uni-dimensional, however, and identifying the "dominant" side is not as straightforward as it may appear. Preference differs depending on the action, so it is necessary to examine the type of action being performed. Additionally, it is important to look at hand or foot use bilaterally, as a collaboration. The majority of daily living tasks are bi-manual. Each hand has a different role: one hand holds an object, while the other manipulates it. Thus, one hand holds the nail, sandwich, or coffee cup, while the other hand hammers, slices, stirs, and so on. We have a similar role differentiation in the stabilizing and mobilizing functions of our legs.

This bi-manual role differentiation serves us well. However, it has been demonstrated that our preferences can switch, depending on the complexity and neurological demands of the task. (6) This switching of roles creates a problem in researching laterality in dance if one only wants to use one specific skill to classify the lateral preferences of the participants. Which skill is appropriate? Which leg do we label as "preferred," the support leg (thereby focusing on strength and balance) or the gesture leg (range of motion)? Bilateral collaboration needs to be considered, rather than simply dealing with single leg preference.

A second challenge is to identify the appropriate difficulty level of the skill(s) in question. The hand a child uses to reach and grasp for an object is a good indicator of manual preference at 6 months, when these skills are still being learned and are difficult. However, by 12 months the child will reach with his non-preferred hand, so he can do the more challenging object manipulation with his preferred hand. Similarly, while the question "On which leg do you prefer to stand" is one of several standard child and adult foot laterality tests, it is not a particularly relevant laterality question for a ballet student who daily performs a lengthy barre alternately balancing on the right and left leg.

There is support in the literature for using dynamic rather than static balance tasks to evaluate a dancer's balance ability. (7) In a study of lateral preference in university dancers, standing on one leg did not produce a strong right bias. (8) A second study using a more difficult task, pique releve, found a stronger right bias. (9) This supports Stadler's findings of a stable right preference for take off and landing in temps leve, but variable support leg preferences in other dance skills, and some preferences changing after strength and stability training. (10) "Preferred balance leg," therefore, is a slippery concept, and reinforces the results of the footedness and task complexity studies of Hart and Gabbard cited earlier. (6)

The implication of the existing hand and foot research in dance is that identification of dancers' foot or side preference is complex, and must either be limited to a task-specific skill or be much broader in its testing approach. For example, in a biomechanical study of a skill such as passe, the appropriate inquiry is to determine the preferred standing leg for that specific skill. Conversely, in a screening assessment or an injury history investigation, a larger variety of dance skills should be included in order to take into account the different task demands performed by one or the other leg. It is important to include data on take off and landing legs in jumps, range of motion and flexibility of the gesturing leg, and strength of the supporting leg when a balance task is difficult or the gesturing leg has to be held in the air for a long time. In addition to a preference for performing a particular skill with one leg, the preferred spatial direction of turning, for example in pirouette or chaine, should be considered. While research studies to identify footedness in the general population typically employ ball kicking or foot stamping, (1) when examining a dance population it is more relevant to administer a dance specific questionnaire that generates a laterality profile, representing a variable preference that takes into account the different demands of particular dance tasks and the skill level of the dancer. Kimmerle and Wilson proposed such a standardized laterality questionnaire; however, this needs further validation. (9)

Lateral Preference and Structural and Functional Asymmetry Data

What type of data is available to determine the extent of lateral preference in dancers, and to support or refute the task-specific view of laterality? In the range of studies reviewed only three focused specifically on dancers' laterality preferences. (8-10) A leg preference seemed to be assumed for some skills, as many studies measured only one leg or side; for example, the right leg in a vertical jump (11) and strength of plantar flexion, (12), a left support and right gesture leg in developpe (13) and rond de jambe. (14) Strong right turning preferences were identified in two questionnaires, (8,9) but this right bias did not necessarily carry over to other skills, which varied between right and left. A right preference for balance was evident when balance was challenged (picque). (9) When range of motion (ROM) was the issue, however (in battement a la seconde and ronde de jambe), (9) the balance preference switched to the left leg. Range of motion, therefore, apparently overrides balance demands. This reiterates a bilateral collaboration view of lateral preference. The right ROM-left balance pairing is perhaps viewed as the most common, as this is the one typically tested. However, at the individual level it is likely that each dancer has to negotiate her own trade-off between balance, strength, and flexibility, depending on body structure and injury history.

The lack of preference data, combined with the issues involved in pinning a right or left label on a dancer, makes it difficult to determine if a dancer's preference also dictates performance differences on the two legs or sides. Preference may not necessarily be reflected in performance. In a study of passe kinematics, for example, Bronner and Ojofeitimi found no difference between the preferred and other leg. (15) Leg dominance was also not a significant factor in a study by Thomas of functional releve performance. (16) It is difficult to determine whether preference results from preexisting structural asymmetries on the one hand, or causes structural asymmetries or performance differences due to unbalanced usage on the other. This is ultimately a "chicken and egg" question.

Not only is preference data limited, but the majority of studies provide no right-left comparisons of musculoskeletal parameters for the hip, knee, and ankle joints. Of the articles and abstracts reviewed, only seven studies reported quantitative right and left measures (11,17-22) (Table 1). Specific parameters were selected from these reports that are most relevant to the present discussion. Of the remaining studies, some do not record which leg was measured or measure only one, while others measure both limbs but report only the mean of the two. This is not surprising, as lateral differences were not the main focus of these papers, and most screening studies or risk assessments already contain a large number of parameters that have to be analyzed and summarized. Even when right-left data is reported, however, it is seldom discussed in relation to a preference for performing specific skills, to dance training, or to the effect that asymmetries may have on skill performance. Herein lies the problem again of having only some elements of the whole picture.

Are dancers' bodies asymmetrical? From data reported in Table 1 the answer would hesitantly be yes, but the differences are small and inconclusive. It is not yet possible to comment on whether anatomical differences, such as strength or lack of flexibility, produce an individual's biases or differences in skilled performance. The lack of meaningful differences explains why data is often collapsed or not recorded for both limbs. Measurement issues further complicate questions related to asymmetry; for example, the best method of establishing passive and active external rotation in turnout, (23) which may explain the range of findings seen in Table 1. A recent major review of turn-out research also found minimal right-left differences (0[degrees] to 2[degrees]) in hip external rotation and total turnout. 24 However, anatomical differences are often masked in group data; right-left differences may be larger and more critical for the individual dancers.

If dancers' bodies are in fact asymmetrical, does asymmetry exist before dance training or does dance training combined with the dancer's preference for practicing skills on one side produce physical changes (again, the "chicken and egg" question)? This is difficult to answer, due to the lack of longitudinal data. It would be necessary to examine children at the start of dance training and follow them from pre-adolescence through adolescence, comparing dancers to non-dancers, to look for an initial symmetry on both sides of the body that becomes more asymmetrical with training, or perhaps the reverse.

In the absence of longitudinal studies, cross sectional data might yield some answers. Crookshank reported on 10 to 16 year olds. (18) Both passive and active external rotation (ER) increased slightly in her subjects each year to a maximum at age 13, after which it appeared to plateau. With the exception of a slightly greater ER on the left in the 10 year olds, there was minimal or no difference between right and left across all ages tested. Steinberg and Siev-Ner documented passive ER in 8 to 16 year olds, and found a similar plateau at 12 years, with no right-left differences at any age. (20) The extent to which more serious daily training in adult dancers would start to produce differences is difficult to predict. However, the limited data on pre-professional and professional dancers seen in Table 1 similarly showed minimal differences.

One could look at the other extreme and examine older dancers. Solomon reported on degenerative injuries in older dancers. (25) Do such injuries occur more on one side than the other after a long career? Given the lack of published right and left measurements across the age span it is not possible to determine the effect of training in this regard.

Functional Assessment of Dance Skills

How do we determine the relationship between dancers' preferences and their mechanical efficiency in performing dance skills? Perhaps there is no relationship. (15,16) There is a similar lack of reported right-left data examining dancers while they perform specific dance skills. We now have a well established biomechanical data base (EMG, force platform vectors, kinematic analysis, and similar data) on a variety of skills: static and dynamic one foot balance, developpe or battement a la seconde, rond de jambe, vertical jumps and pirouettes. However, these data are labor intensive to collect and analyze. Furthermore, lateral differences are not the focus of the mechanical analysis, so measurements are usually taken only on the typically used leg side. (7,12-14) No lateral comparisons can therefore be made, nor any relationship determined, between the participants' leg preferences and their ability to perform that specific skill.

A few studies have offered some functional comparison of skills. Active and passive external rotation was greater on the right than the left in developpe a la seconde in a university sample (131[degrees] vs. 126[degrees] in active ROM and 160[degrees] vs. 157[degrees] in passive ROM). (26) Another study of the same skill found similarly small differences in active ROM. (27) The small differences favoring the right side are parallel to the slight right advantage ROM seen in Table 1. Right-left differences have been found in postural control in a one-legged balance on a moving platform, with the right leg showing less stability. (28) The only other study examined hop control in 11 to 14 year olds. (17) Good hop control was found on the right foot in 38% versus 30% on the left foot. Although dance instructors can no doubt identify asymmetries in their students' skills, without reliable data it is difficult to know if there is a standard profile of functional asymmetry in performing dance skills.

It is also difficult to answer the preference versus proficiency question. Are actual performance parameters on the right and left leg identical, or do they perhaps show only small differences despite the fact that the dancer tested might indicate a strong preference? Can the largely right preference for the gesturing leg in rond de jambe, for example, be explained by a greater external rotation or greater hamstring stretch on the right? The data reported in Table 1 show only small differences in ROM. Is the preferred take off leg or one-leg balance based on a strength differential? Although several papers on leg strength were examined, (11,12,16,29,30) there is little right-left comparative data, and what is reported does not help answer the leg preference question. Leg dominance was not a significant factor in functional plantar flexor strength, (16) nor was there a significant bilateral difference found in leg girth measurements in a study of vertical jumping. (30) So the role that measurable differences in flexibility and strength might play in determining preference or proficiency is yet another missing piece of the puzzle. Perhaps it is the motor control of the limbs, or the dancer's perception of control, that may provide some answers.

Teaching and Learning Biases

Perhaps preferences and asymmetries are not due to differences in a dancer's body, but in the brain; that is, in the ability to learn and control movements. A quick glance at a sample of dance texts on any reader's shelves will likely show a model performing a dance skill while gesturing with the right leg, in battement, developpe or arabesque, with the accompanying description of the steps starting with the right foot or the right side. The traditional format in a technique class is that the skill is demonstrated, marked, and practiced on the right side before the left. Two observational studies of dance classes demonstrated that this right bias is common. (31,32) Kimmerle and Bowes-Sewell found that university dance majors' expectations of class format were that the barre and center portion of the class would start right. (8) The number of repetitions also favors the right, with up to a 26% higher prevalence of right over left. (32) Although anecdotal reports from teachers suggest that some are experimenting with starting on the left or alternating sides from day to day, there is also reluctance to deviate from students' expectation of starting on their comfortable and preferred right side. (31) A self-evaluation format might be useful for instructors to quantify their own bias, and as a tool in pedagogy courses to generate more data on this subject. (33)

Given a population preference for performing motor skills on the right, it is not yet known whether this means that one also learns better on this preferred side. To explore that question one could examine the large number of existing studies on learning right or left hand skills using a transfer paradigm. A novel skill is learned first on one side and then transferred to the other. Magill has identified some of the arguments proposed for either side. (34) For teaching right-transfer-left, it could be argued that it is easier to learn a new skill with one's well skilled, highly refined dominant hand. One should learn faster on the right, and the motor program would more easily be transferred to the left hand. The counter argument, however, is that while it may initially be more difficult to perform a novel skill with the non-dominant hand, it is processed at a deeper level, and therefore when the newly learned skill is transferred to the right, the more "competent" right hand will achieve a higher transfer performance. The results of numerous hand transfer studies were inconclusive. (34) There was no consistent right hand learning preference. Transfer differences again appear to be task dependent.

It is not surprising that some dance instructors and researchers have been intrigued by the transfer question, as learning to transfer skills and combinations from one side of the body to the other is such an integral part of the dance class. While it is difficult to draw comparisons from discrete fine motor hand skills described in the literature to gross motor dance sequences, the results to date in dance studies have also been inconclusive. Puretz found better transfer from the non-dominant to dominant side in learning a sequence. (35) Lavington found that transfer of single novel skills was better from left to right. (36) In a recent study Kimmerle and colleagues found no difference in performance of a sequence first learned and then transferring to the other side between the group that first learned it right versus the group that first learned it left, although the left learning group took more time. (37) It is difficult to compare studies, as learning discrete skills provides different challenges than learning sequences, and the nature of the sequence to be learned is also a factor. In this study the demands were largely cognitive; that is, participants had to accurately reproduce a series of basic dance steps with the correct foot in the correct spatial direction. Learning in this instance represented a memory challenge of learning a pattern, and not a technical challenge of mastering a novel, physically difficult skill, such as a new turn, fall, or jump. Becoming competent in these specific skills would obviously involve adjustments of strength, balance, and range of motion of one limb compared to the other, which would influence the actual level of skill demonstrated. Any difference found would not necessarily be due to one side "learning" the skill better.

How might biased practice influence learning? The performance of most skills is dependent on neuromuscular control, not simply on physical parameters. The height of a leg in seconde, or the balance on releve, is certainly dependent on more than muscular strength or range of motion. Our ability to control the limb or limbs in question is clearly a factor, for example control of balance in the supporting leg (38) as well as a strategy of reorientation of the pelvis to maximize rond de jambe. (39) The degree of motor control we have in either limb would certainly be affected by how often we practiced skills on one side or the other. It is worth speculating that there may be a differential response to training. Roberts and associates examined the effect of core training on passe developpe and found more improvement in stability on the left leg. (40) Stalder trained dancers for strength and stability of the standing leg and found some shifts in their preferences. (10) Learning is no doubt also affected by our perceived competence of performing skills on one side or the other. No studies were found that addressed the psychology of learning right-left. However, anecdotal reports in the transfer experiment mentioned previously suggested that many students exhibited or reported some lack of confidence in their ability to learn the sequence when it was presented to the left side, as well as surprise at how effectively they were eventually able to learn it. (37) This issue of perceived competence on one leg or the other may also be a developmental issue. It has been suggested that children are less right lateralized than adults, with a greater percentage being mixed footed. (1) Could this mean that if extra effort were made to offer more laterally balanced experiences to young dancers a more balanced older dancer would emerge?

Much more data is needed from different dance forms, levels of experience, and type of dance skills or sequences before we can reach any conclusions about learning skills on the right or left, or the potential impact that biased dance training might have. However, the proposition has to be entertained that dance training can either exaggerate an already existing bias or help reduce it. Dancers' preferences and the amount they practice will interact with their ability to learn lateralized skills.

Lateral Preference, Functional Asymmetry, and Injuries

The numerous studies of dance student screenings, assessments, injury histories, injury reports, and surgical procedures could be a rich source of laterality data if they included the subjects' leg preferences for performing different dance skills and injured leg occurrences. However, data on dance screenings typically refer to age, gender, dance form, and past dance experience. Injury occurrence is largely reported by frequency, type of injury, and body part injured or treated, but not by side. While injury history questionnaires sometimes ask about the specific dance skill the dancer was performing when an injury occurred, or which leg the dancer typically uses for a given skill that might result in an overuse injury, that individual laterality information is generally lost by the time group data is collated and reported in a risk factor or epidemiological study. It is hoped that the recommendations for standardized injury reporting formats currently being developed will include the documentation of lateral injury information. (41,42)

There are many in-depth analyses of anatomical abnormalities or incorrect techniques suggested as precursors to specific injuries at the hip, knee, ankle, or foot. With a lack of right versus left injury data, however, one can only assume these occur randomly on one limb or the other, or perhaps on both limbs. Of the injury reports examined for this study only one included any right versus left data. Purnell and coworkers' data were based on injuries in 11-14 year olds and may not represent the injury patterns of older professional dancers. They reported overall frequencies of 35.3% on the right side versus 29.4% on the left, and comparisons of right and left knee (15.7% vs. 17.6%) and ankle injuries (22% vs. 16%). (17)

This leaves only speculation as to what the relationship might be between dancers' lateral preferences for specific skills, their structural and functional asymmetries, potentially biased training, and the occurrence of injuries. The studies reviewed suggest that there are relatively few traumatic injuries in dance, and the majority of lower limb injuries are due to overuse or microtrauma. (24,43,44) Without right-left injury data it is impossible to explore a number of questions, such as the following:

* Do trauma injuries occur in a purely random pattern, or are they more frequent in the non-preferred leg?

* Do overuse injuries occur with the same frequency in the preferred gesture leg versus the weight-bearing leg?

* Do different types of injuries occur in different joints on each limb? If so, this might suggest that the dancer either repeatedly practiced or was asked to perform certain skills on one leg rather than the other--e.g., exaggerated extensions or forceful landings on a particular leg.

* Are injuries the result of being asked to repeatedly perform particularly difficult skills on one leg or side in a choreography, or do they occur more frequently on the non-dominant side when a dancer is asked to reverse the choreography by coming in on the opposite side of the stage?

Limited amounts of data were found to answer these questions. General injury risk factors include the following: structural abnormalities, muscle imbalance, hypermobility, lack of strength or neuromuscular control, incorrect technique, floor surfaces and footwear. Any of these factors could presumably affect both limbs. Other potential causes, however, are quite specific, and could potentially affect one side more than the other. These include: lack of external rotation leading to medial knee or foot injuries; (17,45) lack of dynamic control of turn out beyond the external rotation issue; (45) overuse, muscle tightness, or imbalance resulting from a snapping hip or iliopsoas tendinites; (25,43,44) repetitive plantar flexion and resultant calf tightness leading to lower leg and ankle injuries. (43,45) This is where right-left injury reports would be helpful. Do these risk factors occur with similar frequency on both sides? Which side has more foot and ankle injuries? Is it the weaker side or the side that is most often the landing leg from jumps? Do more knee injuries occur on the preferred, stable leg, and more hip injuries on the gesture leg? From the limited measurements seen in Table 1 it is evident that anatomical differences are small and not likely the clue to any injury differences that may have been recorded. Perhaps the answer lies in strength differences or neuromuscular control in the respective limbs.

[FIGURE 1 OMITTED]

Without right versus left limb injury data that could be compared to right versus left preferred limb usage, it is impossible to identify how a dancer's preference and biased leg usage might be a potential injury risk factor. With the explosion of ergonomic research has come an awareness of the need to rotate workers on the line, alternate jobs, and find other ways to reduce the overuse of one limb in an industrial setting. Dance scientists and dance educators should be equally ready to evaluate the potential lateral bias in dance training and consider a conscious attempt to modify that bias.

A Laterality Model

The pieces of the puzzle gathered in this article can be represented in a model (Fig. 1) that shows an interrelated matrix of factors that may guide future practices in dance with regard to laterality. Studies in the neurosciences, motor development, and motor learning suggest that there is an initial lateralized bias in the brain for certain perceptual-motor functions, and this bias is reinforced by functional usage--i.e., repeated practice in daily life activities. (4-6) It appears that these preferences are well established by school age. This relationship is depicted in the top half of the model, the "pre-dance experience."

The novice dancer, therefore, brings to her first dance class some structural asymmetries and differential proficiency in lateralized fundamental motor skills, along with a distinct preference for learning and performing motor skills on one side. The bottom half of the model, "The Dance Experience," illustrates how she may then be exposed to teachers who also have asymmetries and preferences, and class content and traditional teaching methods that may potentially provide further laterally biased learning experiences. In the worst-case scenario, this model could produce a strongly biased, potentially injury-prone dancer, who is laterally limited as a performer. Conversely, an ideal outcome of a well considered training program that attempts to balance any asymmetries would be a functionally balanced, healthy, and choreographically versatile dancer.

Potential Research Topics and Recommendations

A number of potential areas for future research have been indicated. Baseline data before children begin their dance training is needed to establish what anatomical asymmetries may exist in the average child and adolescent. These data appear to be available from the entry screening of dance schools, but need to be combined across studies. In order to do this a standard method for identifying children's lateral preferences is needed. For a young child or a dance novice at any age this could involve simple criteria such as which foot kicks a ball or initiates stair climbing. This identifies the mobile or gesture leg. For an experienced dancer, however, a variety of dance skills need to be examined that are appropriate to the technical skills of that dancer. (9) Without a consensus on how to describe a dancer's lateral preferences it is not possible to relate preference data to body structure or to skill performance.

Comparisons of right and left musculoskeletal parameters need to be documented for a larger variety of dance skills. To date these are available primarily for static and dynamic balances, skills involving range of motion at the hip, or for vertical jumps (typically recorded for one side only, or reported with collapsed data). While sophisticated technological tools can provide detailed quantitative comparisons, these tools lend themselves largely to simple skills, and are labor intensive. Even an apparently "simple" skill such as a leap would require force measurements on the takeoff and landing leg, EMG on numerous muscles, amplitude and ER for both legs, as well as height and distance traveled to adequately compare the preferred and non-preferred leg. It is, therefore, not surprising that measurements have often been limited to the "preferred" leg. Furthermore, in addition to complex biomechanical measurement tools, practical criteria and coding systems are needed for teachers (and students) to assess lateralized skill differences in dance classes if we wish to assess and educate students about their own asymmetries.

Although teacher behavior data is growing, there is a need for more extensive data across different dance forms, involving both novice and experienced dancers and teachers. We need more motor learning studies to examine students' ability to learn skills on their preferred versus their non-preferred side, and to examine effective strategies used in transferring skills and combinations from one side to the other.

There is now an extensive database on the frequency and types of injury in professional and non-professional dancers. However, until now this data has not generally included lateral information. While it may not be possible to collect and combine information on dancers' dominant foot for difference skills in a large retrospective study, it should be possible to document frequency of right versus left injuries to different body parts. This would begin the detective work of trying to correlate structural and functional asymmetries with biased usage as risk factors for specific injuries.

It is time for the dance medicine and science community to explore the extent to which dancers arrive at the studio with strong lateralized preferences and bodies that are already asymmetrical on the one hand, or how our dance training may potentially produce a functionally asymmetrical dancer on the other. Is asymmetry a characteristic of the human body, or do we create the asymmetry through biased experiences? What are the implications of this asymmetry for the dancer's professional life? Is asymmetry an issue for the choreographer?

There is much research to be done by the dance medicine and science community to establish the relationship between preference, skill proficiency, training, and injury. It would be very helpful if authors of lower limb studies with right versus left limb data would consider adding these parameters to a data bank, thus making it available to researchers. An equally important component, however, is to hear from the dance professionals who train dancers about what actually goes on in the studio and on the stage with regards to laterality, and how choreographic demands factor into the injury equation.

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Marliese Kimmerle, Ph.D.

Marliese Kimmerle, Ph.D., is Professor Emeritus, Department of Kinesiology, University of Windsor, Windsor, Ontario, Canada.

Correspondence: Marliese Kimmerle, Ph.D., Department of Kinesiology, University of Windsor, Windsor, Ontario N9B 3P4, Canada; kimmerl@uwindsor.ca.
Table 1 Right and Left Side Comparisons *

Anthropometric
Data                Right            Left

External             34.1[degrees]    34.9[degrees]
  rotation           22.5[degrees]    20[degrees]
  active             26[degrees]      26[degrees]
                     56[degrees]      56[degrees]
                     50[degrees]      49[degrees]

External             56.3[degrees]    56.3[degrees]
  rotation           59.6[degrees]    60.3[degrees]
  passive            48.9[degrees]    48.6[degrees]
                     48.7[degrees]    48.5[degrees]
                     31[degrees]      31[degrees]
                     39[degrees]      39[degrees]
                     62[degrees]      63[degrees]
                     53[degrees]      43[degrees]

Hamstring            86[degrees]      87[degrees]
  flexibility       107.6[degrees]   110[degrees]
                    117.5[degrees]   111.9[degrees]
                    111[degrees]     112[degrees]
                    158[degrees]     156[degrees]
                    155.7[degrees]   148.6[degrees]

Plantar flexion      56.6[degrees]    58.9[degrees]
                    177[degrees]     180[degrees]
                    180[degrees]     180[degrees]
                    182[degrees]     182[degrees]
                     71.1[degrees]    69.5[degrees]

Ankle plantar-       97.3[degrees]    93.5[degrees]
 dorsiflexion

Ankle                 8.5[degrees]     7.9[degrees]
  dorsiflexion       15.4[degrees]    14.9[degrees]

Soleus (cm)           8 cm             7 cm
                     11 cm            11.3 cm
                     13 cm            13 cm
                     12 cm            13 cm

Anthropometric
Data                Subjects                       Study

External            11-14 yr old dance students    Purnell [17]
  rotation          <10 yr old dance students      Crookshank [18]
  active            First year university          Crookshank [18]
                    Pre-professional               Crookshank [18]
                    University dancers             Nemeck [19]

External            8 yr old ballet students       Steinberg [20]
  rotation          12 yr old ballet students      Steinberg [20]
  passive           8-13 yr old dance students     Kadel [21]
                    11-14 yr old dance students    Purnell [17]
                    <10 year old dance students    Crookshank [18]
                    First year university          Crookshank [18]
                    Pre-professional               Crookshank [18]
                    University dancers             Nemecek [19]

Hamstring           <10 year old dance students    Crookshank [18]
  flexibility       8-13 yr old dance students     Kadel [21]
                    11-14 yr old dance students    Purnell [17]
                    First year university          Crookshank [18]
                    Pre-professional               Crookshank [18]
                    Semi-professional              Harley [11]

Plantar flexion     10 yr old dance students       Kadel [21]
                    <10 year old dance students    Crookshank [18]
                    First year university          Crookshank [18]
                    Pre-professional               Crookshank [18]
                    Professionals                  Kuno-Mizumora [22]

Ankle plantar-      Semi Professional              Harley [11]
 dorsiflexion

Ankle               10 yr old dance students       Kadel [21]
  dorsiflexion      Professionals                  Kuno-Mizumora [22]

Soleus (cm)         <10 yr old dance students      Crookshank [18]
                    11-14 yr old dance students    Purnell [17]
                    First year university          Crookshank [18]
                    Pre-professional               Crookshank [18]

* Note, since different subjects; different tasks, and different
measurement techniques are involved, the intention here is not to
compare absolute numbers between studies, but simply to look for
relative right-left differences specific to the particular studies.
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Title Annotation:Original Article
Author:Kimmerle, Marliese
Publication:Journal of Dance Medicine & Science
Article Type:Report
Geographic Code:8AUST
Date:Apr 1, 2010
Words:7751
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