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Eyes-closed single-limb balance is not related to hypermobility status in dancers.

Abstract

Hypermobility may be associated with decreased lower extremity proprioception, which in turn may increase injury risk. The prevalence of hypermobility in dancers varies across studies, but joint hypermobility appears to be more common in dancers than in the general population. The purpose of this study was to determine how hypermobility affects eyes-closed single-limb balance as an indirect measure of proprioception in dancers. The secondary aim was to compare hypermobility and balance across dancer affiliation groups. Data were collected from 45 professional dancers, 11 collegiate modern dancers, 227 student dancers, and 15 pre-professional dancers during routine dance screens. Dancer hypermobility status was assessed via an eight-point Beighton-Horan Laxity test. Single-limb balance time, in seconds, was assessed in parallel position with the eyes closed. Hypermobile (HM) and non-hypermobile (NHM) dancers showed very similar balance times (HM median: 36.5 seconds; NHM median: 33.0 seconds; p = 0.982). Hypermobility was not significantly different between dancer affiliation groups (p = 0.154): 47% in ballet academy students, 27% in collegiate modern dancers, 62% in pre-professional dancers, and 36% in professional dancers. The student, pre-professional, and professional ballet dancers all demonstrated longer balance times than the collegiate modern dancers; however, this difference was only significant between the professional ballet dancers and collegiate modern dancers (p = 0.026). Dancers demonstrated a higher prevalence of hypermobility than what has been reported for the general population. Joint hypermobility did not affect eyes-closed single-limb balance time. Future studies are needed to determine if joint hypermobility affects more sensitive measures of proprioception and risk of injury.

Joint hypermobility is considered by many to be a desired characteristic of dancers as it allows for the achievement of aesthetically pleasing positions and lines. (1-3) However, there remains some debate as to whether hypermobility is associated with increased risk of injury. (1,2,4,5)

It is often believed that dancers demonstrate an increased prevalence of hypermobility compared to the general population, (1) but its reported frequency in dancers is inconsistent across studies. (1-3) In 1972, Grahame and Jenkins found a greater prevalence of joint hypermobility in student ballet dancers compared to controls, even in joints not commonly affected by dance training, such as the wrist and elbow. (2) This finding may indicate that hypermobility is an inherent trait in dancers rather than the product of dance training. (2) Similar findings were observed in another study in which student and professional ballet dancers demonstrated a significantly higher incidence of hypermobility than non-dancer controls. (3) Interestingly, the prevalence of hypermobility was higher in dance students than in professional dancers. (3) On the other hand, some studies have reported that dancers are no more hypermobile than controls, especially if hip flexion is excluded as a contributing factor (as excessive hip flexion is believed to be acquired through dance training). (5,6) Because hypermobility appears to be more prevalent in student dancers than in professionals, it has been suggested that it may be correlated with a relatively short dancer career. (3,6) Evidence for the prevalence of hypermobility among dancers remains inconsistent, (1-3,5-7) and it is unclear if hypermobility affects the longevity of a dancer's career. (3,6,8)

Injuries are reported to be more common among hypermobile dancers compared with non-hypermobile dancers. (7) In addition, hypermobility is associated with increased time off from dancing due to injury. (4) McCormack and colleagues reported that while hypermobility was not associated with injuries in female dancers, it was associated with a higher rate of injury in male dancers. (3) Scheper and colleagues found a correlation between hypermobility and decreased performance on a 6-minute walk and square-hop test, both in the general population and in dancers. (9)

While hypermobility may facilitate achievement of desirable dance positions, it has potential disadvantages. For instance, hypermobility requires increased strength, proprioception, and neuromuscular control to stabilize hypermobile joints with increased ranges of motion. (10,11) Joint hypermobility is associated with deficient physical performance, including decreased balance and proprioception compared to non-hypermobile individuals. (12-14) Importantly, a systematic review reported proprioceptive deficits in individuals with benign joint hyper-mobility syndrome. (15) Multiple studies have reported impaired knee joint position sense and joint kinesthesia in hypermobile individuals compared to non-hypermobile subjects. (12,16-18) In addition, deficient finger proprioception has been observed in hypermobile individuals. (19) Impaired joint proprioception has also been suggested as a risk factor for injury since proprioception is an essential component of maintenance of joint stability. (20,21) Therefore, deficient proprioception may be one mechanism that contributes to increased injury risk and shorter dance careers in hypermobile dancers.

Proprioception, the ability to sense intrinsically where one's body segments are in space, can be measured with a variety of methods. No gold standard method exists, but previous studies have assessed joint position sense, vibratory perception sense, postural sway, stabilometry, movement threshold detection, and balance. (12,16,17,19) Since proprioception is one of the main sensory systems involved in maintenance of balance, single-limb balance is often used clinically as a representative measure of proprioception. (22) Especially when input from the visual system is removed, increased reliance on the proprioceptive system is required to maintain balance. This is easily assessed as no equipment other than a timer is needed.

Dancers have demonstrated greater upper and lower extremity proprioception than controls, (23,24) but no studies to date have compared proprioception between hypermobile and non-hypermobile dancers. Therefore, the primary aim of this study was to assess how hypermobility affects eyes-closed single-limb stance balance as an indirect measure of proprioception in dancers. Its secondary aim was to compare hypermobility and balance across dancer groups, including student, collegiate, pre-professional, and professional dancers. Based on previous literature, (3,15,17) the hypotheses tested were that hypermobile dancers would show reduced balance times and that younger dance students would be more hypermobile than professional dancers.

Methods Participants

Two hundred ninety-nine dancers volunteered to participate in this study. Since the collected data were extracted from routine dance wellness screens for different dance organizations, dancers were grouped for comparison based on the dance organization with which they were affiliated. These affiliation groups included ballet academy students (N = 227), collegiate modern dancers (N = 11), pre-professional ballet dancers (N = 16), and professional ballet dancers (N = 45).

This study was approved by the Institutional Review Board, and all subjects age 18 or over signed informed consent forms. Dancers under 18 years of age signed assent forms, and their parents signed parental permission forms. All subjects who consented to participate in these screens were included in the study. No exclusion criteria were identified; all dancers who volunteered for the study were allowed to participate, regardless of age, dance type, or history of previous injury, unless injury status was self-reported and determined by the testing physical therapists to be a contraindication to single-limb stance.

Data Collection

Data were collected during routine dance screens by practitioners who specialize in dance medicine. Dancer hypermobility status was assessed via the Beighton-Horan Laxity test, which has good to excellent reliability and is used clinically to measure generalized joint hypermobility. (25,26) Positive tests for hypermobility included the ability of the participants to passively dorsiflex their fifth metacarpophalangeal joint beyond 90[degrees], passively appose their thumb to their forearm, hyperextend their elbows beyond 10[degrees], and hyperextend their knees beyond 10[degrees] (Fig. 1). Each test was performed bilaterally, resulting in eight tests per dancer. Dancers were deemed hypermobile if at least four of eight Beighton-Horan Laxity tests were positive. Increased hip flexion range of motion is common to most dancers and is believed to be acquired through dance training as longer dance training is positively associated with the ability to complete this task. (5-7) Consistent with previous research, (6) forward flexion of the trunk, which is traditionally included in the Beighton-Horan Laxity test, was excluded from data analysis as it is not considered to be a sensitive measure of hypermobility in dancers due to the adaptive hip joint mobility in this population.

Single-limb balance time was assessed once on each limb with dancers standing in parallel position, with their eyes closed, and the non-weightbearing foot not touching the weightbearing leg (Fig. 2). Balance time was measured in seconds with a stopwatch, up to a maximum balance time of 60 seconds. (27) Non-weight-bearing leg touch down, hopping, or torso or hip bending was considered loss of balance, and the test was ended when any of these compensations were made. Dancers performed this balance task on their left and right legs separately.

Data Analysis and Statistics

Data were analyzed using IBM SPSS Statistic 21. A Shapiro-Wilk test revealed the results of this study were not normally distributed. Therefore, a Wilcoxon signed-rank test was used to compare mean balance times between right and left limbs, and a Mann-Whitney U test was used to compare the mean balance times between hypermobile and non-hypermobile dancers. The prevalence of hypermobility between dancer groups was compared using a chi-square test. A Kruskal-Wallis test was used to compare balance times across dancer affiliation groups. Median values and interquartile ranges were reported due to the non-normal distribution of the data. The significance level was set a priori to alpha = 0.05.

Results

The overall prevalence of hypermobility was 45% (N = 135) in this sample of dancers. Based on the Beighton-Horan Laxity test, hypermobility was observed in 106 of the 227 (47%) ballet academy students, 3 of the 11 (27%) collegiate modern dancers, 10 of the 16 (62%) pre-professional ballet dancers, and 16 of the 45 (36%) professional ballet dancers (Table 1).

Median balance times for dancers with and without joint hypermobility and for different affiliation groups are reported in Tables 2 and 3, respectively. Hypermobile and non-hypermobile dancers demonstrated similar balance times (p = 0.982). In addition, a chi-square test revealed no correlation between the prevalence of hypermobility and affiliation group (p = 0.154), which indicated that the prevalence of hypermobility was similar across all dancer affiliation groups.

Since mean balance times were similar between left and right limbs (p = 0.850) and the dominant limb of each dancer was unknown, the mean of each dancer's right and left limb balance times was used for further analyses. A significant main effect of dancer affiliation group was observed for eyes-closed single-limb balance (p = 0.026). Kruskal-Wallis pairwise comparisons indicated that collegiate modern dancers had significantly lower balance times than professional ballet dancers (p = 0.017). There was no significant difference between balance times of professional ballet dancers and pre-professional ballet dancers (p = 1.00), professional ballet dancers and ballet academy students (p = 0.541), pre-professional ballet dancers and collegiate modern dancers (p = 0.268), ballet academy students and collegiate modern dancers (p = 0.113), and pre-professional ballet dancers and ballet academy students (p = 1.00).

Discussion

As single-limb balance time did not differ between hypermobile and non-hypermobile dancers, based on these data, hypermobile dancers do not seem to have proprioceptive deficits that can be detected during eyes-closed single-limb static balance. These findings are in contrast to previous studies that reported a negative relationship between hypermobility status and proprioception. (15,17) However, those studies measured proprioception through joint position sense and vibratory perception sense rather than via a single-limb balance test.

These findings also may indicate that eyes-closed single-limb balance time is not an appropriate measure of dancers' proprioception levels and lead to speculation that static balance testing may not be a sufficiently sensitive measure of proprioception to distinguish differences between hypermobile and non-hypermobile groups. This is consistent with previous research that has reported increased lower extremity proprioception in dancers during joint-position matching tasks that was not observed during static eyes-closed standing. (23) Hypermobile dancers may be able to compensate for proprioceptive deficits through dance and balance training, which may result in similar static balance times in hypermobile and non-hypermobile dancers.

More pronounced differences between hypermobility status and affiliation group may be observed with assessment of dynamic balance. Recently, the Star Excursion Balance Test (SEBT) and the Y Balance Test have been used to examine dynamic balance. (28-31) SEBT performance in the dance population was positively correlated with functional turnout angle. (30) In addition, dancers demonstrated significantly improved SEBT performance after completing 4 weeks of eyes-closed balance interventions. (31) No known studies to date have compared hypermobility status to dynamic balance performance using the SEBT or Y Balance Test. Therefore, it is recommended that future studies compare static and dynamic balance test performance in dancers as well as dynamic balance tests between hypermobile and non-hypermobile dancers.

In the present study, 45% of all dancers were hypermobile, even with the forward flexion item discarded from analysis. Previous estimates of the prevalence of generalized joint hypermobility in the general population range from 2% to 57% with the majority of studies reporting between 20% and 30%. (32-33) Thus, this study provides further evidence that dancers tend to be a more hypermobile population than the general public. (1-3)

Furthermore, the prevalence of hypermobility did not differ between ballet academy students and professional ballet dancers, which does not support a prior study that reported a greater number of hyper-mobile dancers in student populations compared to professional dancers. (3) This discrepancy may be a result of numerous factors, including variations in age, sex, and dance training. Finally, the highest proportion of hypermobile dancers was observed in the pre-professional group (62%). This difference approached statistical significance relative to the professional dancers (p = 0.058) and collegiate modern dancers (p = 0.079), which may be the result of age, sex, and dance training variations. Selection bias for training programs may be another factor. Since our sample only included 16 pre-professional dancers, further study with larger sample sizes is warranted to further examine these group differences. It may also be beneficial to monitor pre-professional dancers over time to determine if their hypermobility status has an impact on injury rate.

There was a significant difference in balance times between the collegiate modern dancers and professional ballet dancers with the former demonstrating a median single-limb balance time that was about 15 seconds lower than the latter. One potential explanation for this finding is that dancers with better single-limb balance are self-selected or company-selected for professional ballet careers. It is also possible that dancers who train primarily in ballet and en pointe develop better single-limb balance over time through training and choreography. However, the design of this study did not allow for inferences about these between-group differences. Further studies would be beneficial to determine how pointe training affects single-limb stance.

Limitations

Comparisons across dancer groups in this study should be interpreted with caution as the investigators acknowledge that the results are from screening data that were not initially intended to investigate the main aim; thus, there were several potential confounding variables. For example, dancers in the collegiate group trained mainly in modern dance, whereas the other dancers trained mainly in ballet. Therefore, the observed differences between these groups could be due to differences in dance exposure, training level, primary dance genre, or other innate characteristics that led to selection into their respective dance programs. In addition, potentially confounding demographic and anthropometric data were not available for each group, preventing further analysis of between-group differences. The cross-sectional design of the current study did not allow us to examine the effects of these potentially confounding variables. Also, the collegiate modern and pre-professional ballet groups were small compared to the ballet students and professionals, which limited the power of some specific between-group comparisons. Future studies with larger samples may observe more significant differences in single-limb balance time and the prevalence of hypermobility between groups.

Another limitation of this study is the use of the Beighton-Horan Laxity Scale, excluding the forward bend item, to determine hypermobility. A score of four or greater is most consistently used to determine hypermobility; however, this score is arbitrary and may not be appropriate for dance populations.

This study only examined static balance once on each limb as a measure of proprioception and did not consider dynamic balance or joint position sense, which could be more valid indicators of dancers' proprioception. While joint position sense and vibratory perception sense may be more "isolated" measures of proprioception, they are more challenging to assess accurately during routine musculoskeletal screens. Eyes-closed single-limb balance is an easy to assess and clinically relevant measure that requires multiple skills, including, but definitely not limited to, adequate proprioception. Comparison between dancers' static and dynamic balance would aid understanding of which tests are most sensitive to balance deficits in dancers.

Conclusion

Hypermobility did not affect eyes-closed single-limb balance in our sample of professional, pre-professional, collegiate modern, and student dancers. The proportion of hypermobile dancers was not significantly different between dance affiliation groups. Collegiate modern dancers balanced about 21 seconds compared to balance times ranging from 34 to 39 seconds in the other groups. As the other groups trained primarily in ballet, this may be an effect of training or selection.

Future studies should validate the results of this study through comparison of eyes-closed single-limb stance to other measures of joint proprioception. These studies should also compare average static and dynamic balance tests, validate estimations of proprioception in dancers, and compare measures of proprioception and dynamic balance of dancers to other sport populations. If future studies find that dancers tend to be more hypermobile with fewer proprioceptive deficits compared to other populations, further study on how dance training affects proprioception or compensatory mechanisms may be warranted. This would help determine if dance training might profitably be used to improve proprioception deficits in other sport or high risk for injury populations.

References

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Tiffany A. Marulli, P.T., D.P.T., Lindsay E. Harmon-Matthews, P.T., D.P.T., O.C.S., M.P.H., J. Hope Davis-Coen, M.S., A.T., Nienke W. Willigenburg, Ph.D., and Timothy E. Hewett, Ph.D.

Tiffany A. Marulli, P.T., D.P.T., Wexner Medical Center, and Sports Medicine Research Institute, The Ohio State University, Columbus, Ohio. Lindsay E. Harmon-Matthews, P.T., D.P.T., O.C.S., M.P.H., and J. Hope Davis-Coen, M.S., A.T., Wexner Medical Center, The Ohio State University, Columbus, Ohio. Nienke W. Willigenburg, Ph.D., Joint Research, Department of Orthopaedic Surgery, OLVG, Amsterdam, The Netherlands. Timothy E. Hewett, Ph.D., Mayo Clinic, Biomechanics Laboratories and Mayo Clinic Sports Medicine Center, Rochester, Minnesota.

Correspondence: Tiffany A. Marulli, P.T., D.P.T., 920 North Hamilton Road, Suite 600, Gahanna, Ohio 43230; tiffany.marulli@osumc.edu.

Caption: Figure 1 Beighton-Horan Laxity Test. Items 1 to 4 are performed bilaterally.

Caption: Figure 2 Single-limb balance test.
Table 1 Prevalence of Hypermobility by Dancer Affiliation Group

                                            Number of
                         Number of       Non-Hypermobile
Dancer Affiliation  Hypermobile Dancers      Dancers

Student                    106                121
Collegiate                   3                  8
Pre-Professional            10                  6
Professional                16                 29

                           Prevalence of
Dancer Affiliation  Total  Hypermobility

Student             227        47%
Collegiate           11        27%
Pre-Professional     16        62%
Professional         45        36%

Table 2 Median Balance Times and Beighton-Horan Scores by Hypermobility
Status

                      Median Balance Time  Median Beighton-Horan
Hypermobility Status        (seconds)              Score

Hypermobile           36.50 [+ or -] 18.8   5.50 [+ or -] 1.46
Non-hypermobile       33.00 [+ or -] 18.9   1.00 [+ or -] 1.14

Table 3 Median Balance Times and Beighton-Horan Scores by Dancer
Affiliation Group

                    Median Balance Time   Median Beighton-Horan
Dancer Affiliation       (seconds)               Score

Student             34.00 [+ or -] 18.92   3.00 [+ or -] 2.63
Collegiate          17.75 [+ or -] 11.85   2.00 [+ or -] 2.06
Pre-Professional    37.75 [+ or -] 19.24   4.00 [+ or -] 1.75
Professional        42.50 [+ or -] 17.33   2.00 [+ or -] 2.34
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Author:Marulli, Tiffany A.; Harmon-Matthews, Lindsay E.; Davis-Coen, J. Hope; Willigenburg, Nienke W.; Hewe
Publication:Journal of Dance Medicine & Science
Article Type:Report
Date:Apr 1, 2017
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