Manual Therapy 19 (2014) 562e568

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Original article

Effect of a physical conditioning versus health promotion intervention in dancers: A randomized controlled trial Nathalie A. Roussel a, b, *, Dirk Vissers a, Kevin Kuppens a, b, Erik Fransen c, Steven Truijen a, Jo Nijs b, Wilfried De Backer d, e a

University of Antwerp, Faculty of Medicine and Health Sciences, Department of Physiotherapy (REVAKI), Antwerp, Belgium Pain in Motion International Research Group, Vrije Universiteit Brussel, Faculty of Physical Education & Physiotherapy, Department of Human Physiology, Brussels, Belgium c StatUa, Centre for Statistics, University of Antwerp, Belgium d University of Antwerp, Faculty of Medicine, Antwerp, Belgium e Department of Respiratory Medicine, Antwerp University Hospital, Antwerp, Belgium b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 September 2013 Received in revised form 9 April 2014 Accepted 22 May 2014

Although dancing requires extensive physical exertion, dancers do not often train their physical fitness outside dance classes. Reduced aerobic capacity, lower muscle strength and altered motor control have been suggested as contributing factors for musculoskeletal injuries in dancers. This randomized controlled trial examined whether an intervention program improves aerobic capacity and explosive strength and reduces musculoskeletal injuries in dancers. Forty-four dancers were randomly allocated to a 4-month conditioning (i.e. endurance, strength and motor control training) or health promotion program (educational sessions). Outcome assessment was conducted by blinded assessors. When accounting for differences at baseline, no significant differences were observed between the groups following the intervention, except for the subscale “Pain” of the Short Form 36 Questionnaire (p ¼ 0.03). Injury incidence rate and the proportion of injured dancers were identical in both groups, but dancers following the conditioning program had significant less low back injuries (p ¼ 0.02). Supplementing regular dance training with a 4-month conditioning program does not lead to a significant increase in aerobic capacity or explosive strength in pre-professional dancers compared to a health promotion program without conditioning training, but leads to less reported pain. Further research should explore how additional training may be organized, taking into account the demanding dance schedule of preprofessional dancers. The trial is registered at ClinicalTrials.gov, number NCT01440153. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Pain Exercise Motor control Dance

1. Introduction Professional dancing is a heavy physical exertion, requiring almost perfect control of technical skills, combined with good physical fitness (i.e. the ability of an individual to meet the demand of a specific physical task) (Koutedakis and Jamurtas, 2004). Several studies have evaluated selected parameters of physical fitness, such as aerobic and anaerobic capacity, muscular strength, flexibility and motor control in dancers and it appears that dancers are not as

* Corresponding author. Faculty of Medicine and Health Sciences, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium. Tel.: þ32 3 821 46 99; fax: þ32 3 265 25 01. E-mail address: [email protected] (N.A. Roussel). URL: http://www.paininmotion.be http://dx.doi.org/10.1016/j.math.2014.05.008 1356-689X/© 2014 Elsevier Ltd. All rights reserved.

well-conditioned as other athletes (Koutedakis and Jamurtas, 2004; Angioi et al., 2009; Koutedakis et al., 2009; Twitchett et al., 2009). Dancers, as athletes, experience a high incidence of injuries, with a cumulative injury incidence ranging from 17 to 94% (McMeeken et al., 2001; Hincapie et al., 2008). Especially soft tissue injuries in ankle/foot, knee, hip and lower back region, of mild to moderate severity, are reported (Byhring and Bo, 2002; Hincapie et al., 2008; Roussel et al., 2009; Baker et al., 2010; Echegoyen et al., 2010). The longer periods of sick leave, which may be detrimental for their career (Nilsson et al., 2001), motivated researchers to search for potential risk factors. For example, lower oxygen uptake (VO2), reflecting reduced aerobic and anaerobic capacity, has been observed in both professional, pre-professional (i.e. full-time enrolled in a Dance degree, university level) and adolescent ballet (classic) dancers and in pre-

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professional contemporary (modern) dancers (Koutedakis and Sharp, 1999; Baldari and Guidetti, 2001; Angioi et al., 2009). Routine dance classes are considered as an intermittent type of exercise and may not provide sufficient stimuli to improve aerobic/ anaerobic capacity (Cohen et al., 1982; Schantz and Astrand, 1984; Twitchett et al., 2009). Muscle strength also appears to be an important parameter when considering musculoskeletal injuries in dancers. Reduced strength levels were associated to injuries of the lower extremities and the spine: the weaker the dancer, the greater the injury risk (Koutedakis et al., 1997a, 1997b). Several studies have revealed the high prevalence of hypermobility and increased flexibility in dancers (Gannon and Bird, 1999; McCormack et al., 2004). Evaluating the quality of movement (e.g. motor control) could be more important than quantifying movement in hypermobile individuals (Simmonds and Keer, 2007). In a prospective study, generalized joint hypermobility was not associated with increased injury risk, while dancers with impaired motor control were at risk to develop injuries during the 6-month follow-up (Roussel et al., 2009). Altered motor control has furthermore been demonstrated in dancers with low back pain (LBP) (Roussel et al., 2013). Nevertheless, no study has examined the effect of motor control training in dancers. Advantages of physical fitness programs have often been suggested to meet the demands of a choreography and reduce the injury risk (Khan et al., 1995; Koutedakis and Jamurtas, 2004; Wyon et al., 2004; Wyon and Redding, 2005; Angioi et al., 2009). However, only a few studies have investigated the effects of these programs in dancers. This may be explained by the unfounded opinion that strength training would alter dancers' aesthetic appearances (Koutedakis and Jamurtas, 2004; Koutedakis and Sharp, 2004; Koutedakis et al., 2007). A twelve week aerobic and strength training program not only increased the aerobic capacity, but also improved dance-related performances in pre-professional contemporary dancers without interfering with aesthetic or artistic requirements (Koutedakis et al., 2007). Two studies evaluated the effect of additional fitness training on musculoskeletal pain in dancers, but were uncontrolled (Mistiaen et al., 2012) or performed in small samples (Ramel and Moritz, 1994). We performed a randomized controlled study to compare the effect of a conditioning program with a health promotion intervention on aerobic capacity, muscle strength and musculoskeletal injuries in pre-professional dancers. We hypothesized that offering a conditioning program, consisting of aerobic capacity, muscle strength and motor control training, in addition to dance classes would improve the aerobic capacity and muscle strength and decrease pain in dancers, compared to a health promotion program without active exercises.

2. Methods 2.1. Design A randomized controlled trial (RCT) was organized in order to determine the effect of two interventions on aerobic capacity, muscle strength and musculoskeletal injuries in dancers. Participants were randomly allocated to the conditioning (n ¼ 23) or health promotion intervention (n ¼ 21) by manual randomization in a 1:1 ratio. In order to have the same number of men and participants with equal experience in each group, stratified randomization (gender and dance experience, i.e. first, second or third year of Bachelor in Dance) was performed. The study protocol was approved by the University Institutional Review Board. Prior to participation, the participants received information

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addressing the study nature and were asked to sign an informed consent. 2.2. Participants Eligible participants (both men and women) were recruited among pre-professional dancers (n ¼ 47) full-time enrolled at a Conservatoire (Bachelor of Dance). At the time the study took place, one student was staying abroad and two students were not fulltime enrolled, therefore not meeting inclusion criteria. Forty-four dancers agreed to participate in the study. 2.3. Procedures Body weight and height were measured at baseline using a medical digital column scale (SECA 701) and a stadiometer (SECA 225; seca GmbH & Co. KG, Hamburg, Germany) to calculate body mass index (BMI). Aerobic capacity and explosive muscle strength were evaluated on separate days to avoid bias due to muscle fatigue. These assessments were conducted by examiners blinded to group allocation and to the dancers' medical history. Injury registration occurred by another investigator only blinded to group allocation. The 4-month intervention program (conditioning versus health promotion program) was followed by a final assessment. Aerobic capacity was examined using an incremental exercise test on an electronically braked bicycle ergometer (Ergoselect 100, Ergoline GmbH, Germany). Resistance started with 25 Watts and increased in 25 Watt increments until exhaustion. This test is reliable (Wallman et al., 2003) and has been used to test aerobic capacity in dancers in a previous study (Vissers et al., 2011). Electrocardiographic and ventilatory variables, including heart rate (HR) were monitored continuously (OxyconPro, Jaeger, Germany). Perceived exertion was scored every minute using Borg's 15-point ratings of perceived exertion scale (Chen et al., 2002). Explosive muscle strength of the lower limbs was evaluated using the Standing Broad Jump test (Ortega et al., 2008). From a resting standing position, participants were asked to jump as far as possible. Two submaximal trials were performed after 5 min warming-up, followed by three maximal trials. The farthest jump was used in data analysis. Reliability of this test is acceptable (Ortega et al., 2008). 2.4. Questionnaires A standardized questionnaire was used to collect demographic information (such as gender, age, sport) at baseline. The Short Form 36-questionnaire (SF-36) is a generic questionnaire that measures health status. Reliability, validity and responsiveness of the SF-36 have been demonstrated (Beaton et al., 1997). The Dance Functional Outcome Scale (DFOS) surveyed general daily activities (such as walking, stability, stairs) and dance-related functionality (such as dance specific movements, jumping, turning, kneeling, etc.) (Bronner et al., 2007). Fourteen questions are scored on a sixpoint scale. A higher score corresponds to a better functionality (Bronner et al., 2007). A preliminary study indicated high reliability, validity, and responsiveness of the DFOS when compared to the SF-36 (Bronner et al., 2003). The Baecke Questionnaire evaluates habitual physical activity (Baecke et al., 1982). Sixteen questions are scored on a five-point Likert scale, covering three domains: work, sports, and non-sports leisure activity. Reliability and validity of this questionnaire have been demonstrated (Pols et al., 1995). The visual analogue scale (VAS e 100 mm) was used for the assessment of musculoskeletal pain severity on the

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testing day and during the last 7 days (Jensen et al., 1986). Pain drawings were used to assess the anatomical location of the pain (Lacey et al., 2005).

Campbell, 1999; Herman et al., 2009). A Fisher's exact test was used to compare nominal data between the groups. Injury incidence rates per 1000 h were calculated. Significance level was set at 0.05.

2.5. Injury registration All dancers were questioned about their medical history and their previous injuries by the principal investigator. Participants were asked to answer the following questions: “Did you, at any time during the last 12 months, experience symptoms such as pain or discomfort in: neck, upper back, low back, hips/thighs, knees, ankle/feet?”(Ramel and Moritz, 1994). An injury registration form was used for the assessment of musculoskeletal symptoms and injuries (Cumps et al., 2007; Roussel et al., 2009). Injury was defined as the result of acute trauma or repetitive stress associated with dancing activities and for which the dancer missed at least one performance, rehearsal or class (Roussel et al., 2009; Liederbach et al., 2012). Injuries were reported in terms of cumulative incidence proportions (number of injured students/population) and injury incidence rate, determined by dividing the number of recorded injuries by the exposure hours (number of dancers multiplied by total physical activity), then multiplying this value by 1000 to obtain the injury incidence rate per 1000 h dance (Phillips, 2000; Hincapie et al., 2008).

3. Results Descriptive data of the 44 participants are presented in Table 1. No significant differences were observed between both groups at baseline for any of the variables (p > 0.05). Three dancers (2 dancers from A and 1 from B) discontinued their degree and stopped their study participation. Weekly physical activity varied between 20 and 25 h per week at the beginning of the intervention to 33 h per week at the end of the intervention (data not shown), but no significant differences were observed between the groups (Table 1).

3.1. Aerobic capacity Results of the maximal exercise test following the intervention are presented in Table 2. Two dancers from intervention B were not able to perform the final exercise test due to brief stay abroad (n ¼ 1) and illness not related to musculoskeletal symptoms (n ¼ 1). There were no statistically significant differences between both groups (p > 0.05).

2.6. Intervention 3.2. Explosive strength test Participants from group A received a conditioning program, consisting of aerobic endurance, strength, proprioception and motor control training. Participants from group B received a health promotion program, consisting of both theoretical and practical educational sessions, but no active exercises. The frequency of training, duration and practical organization of the sessions in small groups was identical in both groups. The description and rationale of both programs is mentioned in Appendix 1. Both interventions (2.5 h per week) were given in addition to regular dance classes, which last approximately 20e22 h per week. Their weekly schedule included 4 h of classical ballet, 7 h of contemporary dancing, 1.5 h of Pilates and 1.5 h of yoga per week. In addition choreographies and rehearsals lasted 7 h per week. Finally, several workshops were organized, but this schedule varied between dancers. All participants were asked to keep a diary concerning their weekly physical activity to account for additional activity outside their regular training schedule.

Two dancers did not participate at either baseline intervention or final assessment due to low back pain and brief stay abroad (both from intervention group B). No significant differences were observed for the results of the explosive strength measurement between the groups (Table 2).

3.3. Results of the standardized questionnaires Two dancers did not complete the questionnaires. Results of the DFOS, Baecke Questionnaire and SF-36 subscales are presented in Table 3. No significant differences were observed between groups, except for the subscale ‘pain’ of the SF-36 (p ¼ 0.031). Table 1 Descriptive data of both intervention groups.

2.7. Statistical analyses Statistical analyses were performed with Statistical Package for Social Sciences version 20.0 (SPSS Inc. Headquarters, 233s. Wacker Drive, 11th floor, Chicago, Illinois 60606. USA). An analysis of covariance (ANCOVA) was performed. The ANCOVA model included follow-up measurement as outcome, group as fixed effect, and baseline measurement as covariate. From this model, the parameter estimate for group is interpreted as the baselineadjusted difference between the two group means. That is, the expected difference in mean between group A and B in case the baseline value is kept constant. The covariate-adjusted group means represent the expected means for each group, with the value of the baseline covariate set to its overall mean estimate. The p-values are presented for the null hypothesis that the difference between the baseline-adjusted group means are zero. An intention-to-treat analysis with “the last observation carried forward method” was used to exclude bias from drop out (Hollis and

Age (years) Gender Weight (kg) Height (cm) BMI (kg/m2) Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) HR at rest (beats per minute) Physical activity (hours per week) during the intervention Exercise duration (s) Maximal workload (WATT) VO₂Max (l/min) Relative VO2Max (ml/kg/min) Peak Ventilation (l/min) Max HR (beats/min) Standing Broad Jump (m)

Intervention A (n ¼ 23)

Intervention B (n ¼ 21)

p-value

Mean ± SD

Mean ± SD

19.9 ± 2.0 20F, 3M 59.3 ± 10.6 167.7 ± 6.3 21.0 ± 2.7 113.5 ± 11.5 70.3 ± 9.2 70.1 ± 9.6 26.2 ± 3.15

19.6 ± 2.4 18F, 3M 57.2 ± 6.2 168.0 ± 6.3 20.2 ± 1.8 113.3 ± 10.5 73.9 ± 8.0 70.8 ± 9.0 25.8 ± 3.66

0.708 0.905 0.420 0.873 0.287 0.950 0.183 0.798 0.680

523.0 ± 101.5 194.6 ± 42.0 2489.1 ± 811.4 41.7 ± 9.1 78.9 ± 26.1 183.2 ± 8.9 1.84 ± 0.234

527.1 ± 118.7 197.6 ± 49.9 2434.1 ± 675.8 42.3 ± 8.4 80.3 ± 26.9 181.6 ± 8.6 1.85 ± 0.276

0.902 0.827 0.809 0.831 0.862 0.550 0.841

SD ¼ Standard Deviation, Intervention A ¼ conditioning intervention and B ¼ the health promotion program.

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Table 2 Results of the aerobic capacity and explosive strength. Variable

Measurements in rest

Maximal Exercise Test

Explosive strength

Weight (kg) HR at rest (beats per minute) Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Exercise duration (s) Maximal workload (WATT) VO2Max (l/min) Relative VO2Max (ml/kg/min) Peak ventilation (l/min) Max HR (beats/min) Max Borg score Standing broad jumpy (m)

Group A (n ¼ 23)

Group B (n ¼ 21)

Baseline-adjusted difference group A-group B

Covariate-adjusted mean (SE)*

Covariate-adjusted mean (SE)*

Estimate

95%CI

p-value

58.827 70.295 112.729 70.086 556.145 211.091 2422.741 40.897 82.538 181.886 17.955 1.832

58.809 69.725 113.474 73.910 533.745 202.138 2451.331 41.741 81.953 182.411 18.097 1.814

0.018 0.570 0.745 3.823 22.400 8.953 28.590 0.843 0.585 0.525 0.143 0.018

0.936; 0.973 4.924; 6.065 6.580; 5.090 9.182; 1.535 1.756; 46.556 1.096; 19.002 174.403; 117.224 3.093; 1.406 6.322; 7.492 5.386; 4.336 1.216; 0.930 0.057; 0.094

0.969 0.835 0.798 0.157 0.068 0.079 0.694 0.453 0.865 0.828 0.789 0.630

(0.325) (1.879) (2.018) (1.833) (8.262) (3.436) (49.862) (0.769) (2.362) (1.659) (0.359) (0.025)

(0.340) (1.966) (2.065) (1.877) (8.647) (3.597) (52.184) (0.805) (2.472) (1.737) (0.377) (0.028)

Results of the tests for dancers from intervention A (conditioning intervention) and B (health promotion program). The p-values are presented for the null hypothesis that the difference between the baseline-adjusted group means are zero. Legend: SE ¼ standard error. yTwo dancers did not participate at either baseline intervention or final assessment due to low back pain and brief stay abroad (both from intervention group B). *Expected means for each group, for a baseline value equal to the overall baseline mean.

3.4. Injury registration

4.1. Effect of the intervention program on aerobic capacity

Injury registration was not completed in four dancers, as three dancers dropped out of School and one dancer stayed abroad. A total of 26 students developed one (n ¼ 22) or two (n ¼ 4) injuries during the intervention. The localisation of the injuries, the proportion of injured students and the injury incidence rate is shown in Table 4. No differences were found between the groups, except for the injuries to the lower back (p ¼ 0.019).

Few studies have analysed the effect of a conditioning intervention on aerobic parameters in dancers. Interestingly, a 10% increase in VO2max was reported in professional ballerinas after a 6 week summer break (Koutedakis et al., 1999). An increase in (sub)maximal oxygen consumption was observed in both preprofessional and professional dancers following an additional fitness program (Ramel et al., 1997; Koutedakis et al., 2007; Angioi et al., 2012; Mistiaen et al., 2012). In the present study, no significant increase in aerobic capacity was observed in dancers following a conditioning program. Differences between literature results and the present study may be due to differences in study methodology. Another reason may be that the amount of physical activity per week due to regular dance classes was already high (more than 20 h per week). The training volume of the intervention (2.5 h per week) may not have been sufficient to significantly increase the aerobic capacity explaining the lack of differences between both groups. Finally, the health promotion intervention may have led to positive effects on physical fitness as well. Participants from the health promotion intervention were not asked

4. Discussion The results of this study suggest that a 4-month conditioning program does not lead to a significant increase of aerobic capacity or explosive strength in pre-professional dancers, compared to a health promotion program without conditioning training. Musculoskeletal injury incidence rate did not differ between both groups, but dancers following the conditioning program experienced significantly less reported pain and low back injuries compared to dancers from the health promotion intervention. Table 3 Results of the questionnaires. Variable

Dance Functional Outcome Scale (DFOS)

Baecke

SF-36

DFOS e Daily Activities* (score on 40) DFOS e Dance Related* Functionality (score on 50) DFOS Total score* (%) Baecke Work* Baecke Sport* Baecke Leisure* T1-Physical Functioningy T2-Role Physicaly T3-Bodily Painy T4-General Healthy T5-Vitalityy T6-Social Functioningy T7-Role Emotionaly T8-Mental Healthy

Group A (n ¼ 23)

Group B (n ¼ 21)

Baseline-adjusted difference group A-group B

Covariate-adjusted mean (SE)**

Covariate- adjusted mean (SE)**

Estimate

35.948 (0.610) 45.953 (0.943)

35.375 (0.656) 45.475 (1.016)

90.989 3.443 4.165 3.031 92.038 87.997 75.005 70.560 70.772 80.035 93.883 75.307

89.849 3.510 4.316 2.940 91.157 90.678 62.245 70.881 70.112 84.209 97.035 72.597

(1.402) (0.054) (0.103) (0.075) (2.055) (5.085) (3.880) (2.756) (1.951) (3.102) (2.678) (2.077)

(1.509) (0.058) (0.114) (0.080) (2.204) (5.464) (4.161) (2.961) (2.093) (3.328) (2.873) (2.228)

0.573 0.478 1.139 0.067 0.151 0.091 0.881 2.681 12.760 0.320 0.660 4.174 3.152 2.709

95%CI

p-value

1.240; 2.386 2.345; 3.302

0.526 0.734

3.036; 5.315 0.226; 0.093 0.463; 0.161 0.135; 0.317 5.216; 6.977 17.978; 12.615 1.262; 24.259 8.584; 7.944 5.126; 6.446 13.383; 5.036 11.111; 4.808 3.464; 8.882

0.584 0.404 0.333 0.419 0.772 0.725 0.031 0.938 0.819 0.365 0.428 0.380

Results of the questionnaires for dancers from intervention A (conditioning intervention) and B (health promotion program). The p-values are presented for the null hypothesis that the difference between the baseline-adjusted group means are zero. Legend : SE ¼ standard error. * Two dancers from intervention group B did not fill in the questionnaires at baseline or follow up. yOne dancer from intervention B did not fill in baseline and follow up questionnaire. **Expected means for each group, for a baseline value equal to the overall baseline mean.

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Table 4 Overview of the localisation and number of musculoskeletal injuries.

Cervicothoracic spine Lumbar spine Sacro-iliac joint Hip Knee Tibia (shin) Ankle & foot Shoulder Total injured students Total injuries Injury incidence per 1000 h dance

A

B

p-value

Total (n)

1 0 0 5 0 4 3 2 13/20 (65%) 15 1.4

0 5 1 1 3 1 4 0 13/20 (65%) 15 1.6

1.00 0.019 0.477 0.188 0.100 0.348 0.693 0.489

1 5 1 6 3 5 7 2 26/40 (65%)

0.635

1.5

Results of the injury registration during the study. Intervention A is the conditioning program and B the health promotion program.

to keep their life style outside the dance classes unchanged, but therefore it cannot be excluded that they adopted a more active lifestyle. For example, the mean hours of physical activity did not differ between both groups, although dancers from group A were scheduled 2.5 h of physical activity per week in addition to regular dance classes. Finally, professional dancers may experience a ‘burn out’ or overtraining at the end of the season, which negatively affects fitness. Nearly eighty percent of dancers experience negative stress at work (Byhring and Bo, 2002). The fact that a summer break improves the results in professional dancers may suggest they have been overtrained during the season (Koutedakis et al., 1999). There is lack of data regarding burn out or overtraining in preprofessional dancers. Nevertheless a study evaluating injuries in pre-professional dancers indicated that the majority of the injuries occurred close to the assessment periods (Baker et al., 2010). In our study, dancers reported that the end of the intervention coincided with an exhaustive period, as also objectified by the increased amount of weekly physical activity at the end of the intervention. 4.2. Effect of the intervention program on muscle strength Neither intervention influenced the explosive muscle strength, contrasting to studies performed in amateur (i.e. not full-time enrolled in a Dance degree, often only dancing a couple of hours per week) (Vetter and Dorgo, 2009) and pre-professional dancers (Koutedakis and Sharp, 2004; Annino et al., 2007; Mistiaen et al., 2012). Several reasons may explain the differences in results. Firstly, we used a field test evaluating explosive strength that may be less sensitive to detect small changes, while standardized tests in laboratory settings were used in most other studies. Secondly, we evaluated pre-professional dancers with mixed dancing styles, while professional ballerinas (Koutedakis and Sharp, 2004), ballet students (Annino et al., 2007), or amateur dancers (Vetter and Dorgo, 2009) were analysed in other studies. 4.3. Effect of the intervention program on musculoskeletal injuries Injuries occurred predominantly in the legs/spine, which is in accordance with other reports (Hincapie et al., 2008). The proportion of injured dancers and the injury incidence rate were similar in both groups. We hypothesized that dancers in the conditioning program would increase their physical fitness and that this would be accompanied by a decrease in injuries. However, all the elements of the health promotion intervention (education, stress management, self-care etc.) were also informed by research performed in athletes/dancers and in patients with musculoskeletal pain. Since

the start of the present study, there is growing evidence suggesting benefits of bio-psychosocial education, self-care and stress management in patients with musculoskeletal disorders (Chou et al., 2007; Finestone et al., 2008; Ryan et al., 2010; Louw et al., 2011; Eccleston et al., 2013). This intervention may therefore have had an effect as well on musculoskeletal injuries in dancers. Without a true control group, it is not possible to determine the benefit of either intervention. Significant differences were observed between groups with respect to the injury location. Dancers who had followed the conditioning intervention had significantly less low back injuries. No other differences were observed. To our best knowledge, this is the first randomized controlled study reporting the effect of additional intervention on musculoskeletal injuries. However, injury incidence rate was only recorded during 4 months and a longer follow up period is needed to determine the long-term effects of this intervention. 4.4. Effect of the intervention program on the results of standardized questionnaires Results of the questionnaires evaluating quality of life and physical functioning were similar between groups. No significant differences between the groups were observed, except for the subscale ‘Pain’ from the SF-36, in favour of dancers following the conditioning program. Our results are in accordance with Ramel et al. (1997), who developed a questionnaire to record the ‘selfestimated functional inability because of pain’ in professional ballet dancers (Ramel et al., 1997). As no information is provided regarding the reliability and validity of this questionnaire, it is difficult to interpret these results. No other controlled studies have reported data regarding the effect of additional interventions to musculoskeletal injuries, pain or functional inability. The results of this study should be seen in the light of some methodological concerns. An intensive baseline assessment was performed, consisting of a maximal exercise test to evaluate aerobic capacity and a field test to evaluate the explosive strength, as we had no possibility to evaluate isometric or isokinetic strength of the lower extremities in a controlled laboratory setting. Using dancespecific tests may be more functional to evaluate dancers, and adding a motor control evaluation would have been useful. Secondly, we used a sample of convenience, and included all possible subjects. Only 6 men were included in the study sample, therefore a stratified randomization procedure based on gender was used to guarantee equal allocation between men and women. As there is only one University College offering a professional Bachelor in Dance in Belgium, it was not possible to expand the sample size. Thirdly, we did not interfere with leisure time physical activity. There were no differences in amount of physical activity between both groups, suggesting that dancers from the health promotion group were more active during their leisure time. Fourthly, no true control group was used in the present study. Finally, injuries were registered by means of questionnaires and medical history during the intervention and in the 12 months preceding the intervention. Reporting or recall bias may have played a role for the assessment of injuries in the 12 months preceding the intervention. Unfortunately, we had no information regarding the injuries prior to this period to determine whether the injuries occurring during the intervention were new or recurrent. A longer follow up-period is also warranted. 5. Conclusion Supplementing regular dance training with a physical conditioning program of 2.5 h per week during 4 months does not lead to

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a significant increase in aerobic capacity and explosive strength in pre-professional dancers. However, it yields to less reported pain and low back injuries. Further research should consider including a true control group and explore additional training components taking into account the already demanding dance schedule in preprofessional dancers. Grant support Nathalie Roussel, Kevin Kuppens en Dirk Vissers were financially supported by a research grant supplied by the Department of Health Sciences, Artesis University College Antwerp, Antwerp, Belgium (G 833). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Financial disclosure/conflict of interests The authors declare no financial disclosure or conflict of interests. Acknowledgements The authors thank Anne Schutt, Caroline Van Laethem, Elise Nackaerts and Eline Marck for aiding in the supervision of the training program; Ingeborg Van Dooren for the help with the registration of injuries, and the staff of the pulmonary department for their assistance with the assessments.

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neuromuscular systems. Also dance-specific movements from the contemporary choreography class were included. Participants from group B received a health promotion program, consisting of both theoretical and practical educational sessions, but no active exercises. The frequency of training, duration and practical organization of the sessions in small groups was similar as in the conditioning program. All the elements of the health intervention program were informed by research performed in athletes/dancers and in patients with musculoskeletal pain (Brummitt, 2008; Nijs et al., 2011; Mason et al., 2012; van den Bekerom et al., 2012; Smith et al., 2013). Theoretical sessions included anatomy and pathology lessons in order to understand most common injury mechanisms and to apply correct stretching techniques, stress management, self-care and information about nutrition. Practical sessions, organized as workshops, included massage, passive stretching (i.e. not combined with muscular contractions), taping techniques of ankle and knee, assessment of energy intake and expenditure, and stress diaries. Participants were instructed to reflect on a healthy energy balance as it has been shown that dancers and in particular dance students may experience eating disorders and are very preoccupied with thoughts of eating and body weight (Abraham, 1996a, b). Energy consumption below the recommended daily allowance of energy intake has been reported in dancers (Koutedakis and Jamurtas, 2004). Therefore dancers were asked to compare caloric values of different foods and had to put this into perspective taking into account their estimated energy expenditure.

References Appendix 1. Description of the intervention programs Participants from group A received a conditioning program, supervised by a certified physiotherapist and a dance teacher. The program lasted 75 min (45 min of strength & endurance training and 30 min of proprioception & motor control training) and was given twice a week. The rationale to include exercises improving strength & endurance, proprioception and motor control, the frequency of training, and the duration of the sessions were based on previous research performed in dancers and on recent literature regarding conservative treatments performed in patients with pathologies similar to those observed in dancers (Koutedakis and Jamurtas, 2004; Koutedakis et al., 2007; Roussel et al., 2008; Angioi et al., 2009; Twitchett et al., 2009; Collins et al., 2012; Mason et al., 2012; Rickman et al., 2012; Rodriguez-Merchan, 2012). For practical reasons, the group of dancers was divided into smaller groups, so that 10-12 dancers could participate in each session. Bicycles, steps, rowing machines, strength training equipment and free weights were used in a circuit program to improve aerobic capacity and muscular strength. The age-predicted maximal HR was calculated as 208 minus 0.7 times the participant's age in years (Tanaka et al., 2001). The training program started with an intensity of 70% of the maximal HR and was increased every 4 weeks by 5%, ending at 85% (Karvonen and Vuorimaa, 1988; Fletcher et al., 1996; Pollock et al., 1998). Random HR evaluation demonstrated that dancers trained at a correct intensity (data not shown). Variables such as the number of repetitions or the duration of the exercise, the number of sets, the load, etc. were used to determine the individual progression. In the second part, proprioceptive and motor control exercises were included with increasing complexity (Comerford and Mottram, 2001; Sahrmann, 2002; Richardson et al., 2004). Finally, the training program evolved to dance-specific exercises, such as ‘arabesque’ or ‘rond de jambe’, but were performed on unstable platforms or with elastic bands to challenge proprioceptive and

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Effect of a physical conditioning versus health promotion intervention in dancers: a randomized controlled trial.

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