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Hannah Guthrie 
Senior Thesis 
May 4th, 2020 
PHOTOGRAPHY BY SYDNEY EDWARDS 
 
 
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Introduction 
 Success of a dance production lies in a dancer’s ability to be otherworldly, show 
complicated physical movement effortlessly, and to take the audience on a journey. Beyond the 
physical requirements and demands of dance, other factors of a performance environment can 
help or hinder a dancer’s ability to perform at their physical best. The success of movement in 
the dance vocabulary; turning, jumping, and even simple balance actions can be susceptible to 
the introduction of theatrical elements not utilized in the dance studio space. Brighter, more 
drastic lighting can affect visibility, forcing the dancer to rely on their instinctual muscle habits 
and proprioception for success. 
 In Fall 2019, Dr. Margaret Wilson, head of the University of Wyoming Theatre and Dance 
Department, collected balance data on cast members of the fall production to explore balance 
under different conditions in the studio and theatre. The dancers each went through an exercise 
series known as the Star Excursion Balance Test (SEBT) on a force plate, a tool that measures 
how far an individual’s balance sways away from their center. In a study conducted by Doctor 
Philip Gribble, looking at the implementation of the SEBT, it was found that : “The Star 
Excursion Balance Test is a reliable measure and a valid dynamic test to predict risk of lower 
extremity injury and to identify dynamic balance deficits in patients with lower extremity 
conditions” (Gribble, 2012). In the Star Excursion Balance Test, the participant stands on a one 
leg, and reaches out the opposite leg in eight directions, shown in Figure 1. The dancers 
performed the SEBT in ambient/typical studio lighting, as well as under theatre/stage lighting. 
 
 
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Figure 1. Pathways of the Star Excursion Balance Test 
  
An individual’s goal is to be able to reach as far as possible in the eight directions of the 
test, while the goals of the SEBT is to identify individuals who may be at risk for a lower extremity 
injury (most commonly the ankle, foot, or knee) through decreased range, and to assess 
symmetry of the body. The force-plate measures balance instability as sway, and the data 
collected by Dr. Wilson couples the notion that a dancer with a high reported measurement for 
sway and decreased range of reach in the SEBT is at risk for injury. 
Population 
 The seven dancers who participated in the exploratory study are all students in the 
University of Wyoming’s Dance Program, and were cast members in the Fall 2019 production, 
“Wonderland: Alice’s Adventures in 3-Dimensional Space”. At the start of the study, none of the 
participants reported an existing injury. 
 In classical statistics, a sample of this size (7) isn’t considered substantial enough to 
assume normality for analysis. However, Bootstrapping, is an analysis tool that circumvents the 
issue of small sample size, and thus was used to continue analysis for this data set. 
Bootstrapping 
 
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 The bootstrapping method simulates repeating a study a large number of times, thus 
generating an estimate of the distribution of the statistic in consideration. This estimation 
allows for a bootstrap confidence interval to be constructed. For the data collected by Dr. 
Wilson, the bootstrapping method was utilized to estimate the average sway in the medial-
lateral, and anterior-posterior directions from the SEBT.  
Relevance for Analysis 
 In a pooled meta-analysis from 19 studies concerning the musculoskeletal injury 
prevalence in ballet dancers, it was found that “In a total of 7332 injuries reported in 2617 ballet 
dancers, the period prevalence of musculoskeletal injury was 280% (95% CI: (217%, 343%))”, and 
that the most common reported musculoskeletal injury conditions were: “hamstring strain, 
tendinopathy of muscles contributing to ankle motion, and generalized low back pain” (Smith, 
2016). Furthermore, it’s imperative for dancers to perform in a space as safe as possible, given 
the existing susceptibility to injury. It’s important to both address the position or intensity of the 
theatre lighting appropriately, and to establish proprioceptive habits in dance training that 
attempts to make the transition in awareness of balance in a theatre setting as equitable as 
possible to the studio setting. In a pilot study conducted by Hutt and Redding that also utilized 
the SEBT, it was found that “Visual conditions for a dancer vary greatly between theatrical 
performance environments and dance studios, and this variability may be detrimental to their 
dynamic balance performance, particularly under stage lighting. In order to maintain balance 
control, dancers reportedly rely heavily on visual input, yet those who rely more on 
proprioceptive strategies for balancing have been found to be more stable” (Hutt, 2014).  
 
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Given the high physical demands of dance, and conclusions from studies like the former, 
it’s imperative that statistical analysis be a tool for examining the high proportion of dancers 
that incur injuries, and that any indication of weakness or insufficient ability should be addressed 
and analyzed to be avoid future injury. A dancer’s ability to balance is integral to every aspect of 
movement they perform, and if a dancer either shows abnormal sway, or is consistently swaying 
largely, the dancer’s training should be adjusted in order to avoid injury. Statistics for dance 
related data is incredibly helpful in determining and limiting these injury risk factors, and at the 
very least, encourages exploration of why dance activity data needs to be collected, monitored, 
and analyzed to support the health of the dance community.  
 
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Question of Interest 
Does theatre lighting affect the sway of a dancer differently than ambient lighting? 
Hypotheses 
From studies discussed previously reflecting on the lack of visual aid, and the increased 
demand for proprioceptive awareness for stability in typical theatre performance environments, 
it is hypothesized that the average sway under theatre lighting will be greater than the average 
sway under ambient lighting. 
The consideration of incremental versus relative change is important for this scenario. A 
dancer who is typically more stable will consistently record lower measurements for sway, while 
a dancer who is more “wobbly” will typically consistently record higher measurements for sway. 
For the explorative purposes of this analysis, both incremental change and relative change 
conclusions are provided.  
Data Collection 
 Dancers completed the SEBT for each leg under theatre and ambient lighting on two 
separate occasions during the Fall 2019 production “Alice's Adventures in 3-Dimensional Space”. 
Participant four completed their second visit in a boot, due to possible a stress reaction on their 
left foot. The measurements recorded by the force-plate were compiled into an Excel 
spreadsheet. The average of these two recorded measurements for each participant are shown 
in the plots depicted in Figures 2-5. Plots stating “right leg” indicate that the right leg was 
gesturing, and plots stating “left leg” indicate that the left leg was gesturing, in the SEBT.  
 
 
 
 
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Figure 2: SEBT Sway, Right Leg Gesturing 
 
Figure 3: SEBT Sway, Left Leg Gesturing 
 
 
 
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Figure 4: SEBT Sway, Right Leg Gesturing 
 
 
Figure 5: SEBT Sway, Left Leg Gesturing 
 
 
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The recorded measurements for each participant shown in Figures 2-5 were then analyzed using 
a bootstrapping tool. Figure 6 shows a histogram of the averages from each participant, as well 
as the bootstrapped distribution of the average sway in the medial-lateral direction with the 
right leg gesturing, in ambient lighting. 
 
Figure 6: Histogram, Bootstrapped Distribution of Medial-Lateral Average Sway with Right Leg Gesturing, 
Ambient Lighting 
The histogram for the average sway in the medial-lateral direction with the right leg gesturing 
in ambient lighting, has a greater frequency of lower reported sway measurements. This is 
further indicated with positive skew in the bootstrapped distribution shown in Figure 6.  
 
Figure 7: Histogram, Bootstrapped Distribution of Medial-Lateral Average Sway with Left Leg Gesturing, 
Ambient Lighting 
 
 
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The bootstrapped distribution for the average medial-lateral sway with the left leg gesturing in 
ambient lighting shown in Figure 7 follows a normal distribution closely, with slight positive 
skew. For other comments on the bootstrapped distribution of averages, see Appendix figures 
10-15.  
 
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Statistical Hypotheses for Incremental Change 
H0: μ2  - μ1 = 0 
HA: μ2  - μ1 ≠ 0 
 Anterior-Posterior Medial-Lateral 
Right Leg (Bootstrap) (-23.1636, 27.1445) (3.9321, 40.3993) 
Right Leg (Normal) (-34.3205, 46.4325) (-9.5190, 50.1483) 
Left Leg (Bootstrap) (10.6940, 40.6003) (-12.7115, 43.1949) 
Left Leg (Normal) (-0.7719, 48.7122) (-28.2074, 62.7378) 
 
Figure 8: 95% Confidence Intervals for Mean of Differences 
 Only the bootstrapped confidence intervals, for anterior-posterior sway in the left 
leg, and the medial-lateral sway with the right leg show enough evidence that theatre 
lightning has a significant effect on a dancer’s sway. On average, a dancer had a force 
plate sway measurement between 10.69 and 40.60 units higher under theatre lighting 
than the force plate sway measurement under ambient lighting in the anterior-posterior 
direction. In the medial-lateral direction, a dancer had a force-plate measurement 
between 3.93 and 40.39 units higher under theatre lighting than under ambient lighting. 
Because the null value of zero is not included in these confidence intervals, there is 
sufficient evidence to reject the null hypothesis that theatre lighting has equal effect on 
sway as ambient lighting, and thus can infer that dancers experience greater sway in the 
anterior-posterior direction when gesturing with the left leg, and greater sway in the 
medial-lateral direction gesturing with the right leg under theatre lighting conditions. 
  
 
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Statistical Hypotheses for Relative Change 
H0: μ2 / μ1 = 1 
HA: μ2 / μ1 ≠ 1  
 Anterior-Posterior Medial-Lateral 
Right Leg (1.2822,1.4301) (0.9806, 1.4991) 
Left Leg (1.1859,1.5904) (0.8822,1.7186) 
 
Figure 9: 95% Confidence Intervals for Ratio of Means 
 For the right leg executing the SEBT, the sway of a dancer in theatre lighting in the 
medial lateral direction is between 1.94% less and 50% more than the sway observed under 
ambient, lighting on average. For the left leg executing the SEBT, the sway of the dancer in the 
medial lateral direction in theatre lighting is between 11.78% less and 71.86% more than the 
sway observed under ambient lighting, on average. From the results of these confidence 
intervals, there is not sufficient evidence to suggest that theatre lighting has impact on the sway 
of a dancer, as the null hypothesis ratio (1.00) is included in this interval. 
 For the right leg executing the SEBT, the sway of a dancer in theatre lighting in the 
anterior-posterior direction is between 28.22% and 43.01% more than the sway observed under 
ambient, lighting on average. For the left leg executing the SEBT, the sway of the dancer in the 
anterior-posterior direction in theatre lighting is between 18.59% and 59.04% more than the 
sway observed under ambient lighting, on average. From the results of these confidence 
intervals, there is enough evidence to reject the null hypothesis, and to conclude that theatre 
lighting impacts the sway of a dancer in the anterior-posterior direction. 
 
 
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Conclusions 
 Three out of the four confidence intervals in each direction for incremental change, and 
two out of the four confidence intervals for relative change did not indicate sufficient evidence 
for theatre lighting having an effect on the sway of a dancer, and only two confidence intervals 
for incremental change indicated sufficient evidence that theatre lighting has an effect on sway. 
This result can be interpreted ambiguously. It’s desirable for dancers to be able to maintain their 
proprioception and manage their balance on stage and under theater lighting; it’s the scenario 
that matters most in a dancer’s career and is the penultimate showcase of their abilities. Thus, 
failing to reject that theatre lighting and ambient lighting have equal effect is a favorable 
conclusion- it’s a true mark of a professional dancer to be able to minimize and be aware enough 
of their muscular habits so the transition from studio to stage is seamless and autonomous. 
However, this result also inhibits the ability to establish the force plate and the SEBT test as a 
significant test for risk of injury, but this conclusion is also related to the issue of small sample 
size. Although the bootstrap analysis is useful and valid for this scenario, collecting similar data 
on more participants in the future would only solidify and give more insight to conclusions about 
average sway. Balance data is also susceptible to high variability, and by nature could be 
introducing some contamination to the data. 
  The confidence intervals for relative change in sway in the anterior-posterior direction 
did not include the null value (a 1 to 1 ratio), and thus indicates that a hypothesis test would have 
yielded a significant p-value, and thus show that perhaps dancers lack the proprioceptive 
awareness in the anterior-posterior directions, have greater instability in these directions, and 
that theatre lighting effects average sway more significantly in these directions. If going forward 
 
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with this study or recreating this study, perhaps more focus should be given to exploring the 
actions and sway in the anterior-posterior direction as a primary indicator of instability and risk 
of injury. 
  
 
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Appendix 
 
Figure 10: Histogram and Bootstrapped Distribution for Average Sway in the Medial Lateral Direction: Right 
leg Gesturing in Theatre Lighting 
 
The bootstrapped distribution shown in Figure 10 shows some positive skew. 
 
 
Figure 11: Histogram and Bootstrapped Distribution for Average Sway in the Medial Lateral Direction: Left 
leg Gesturing in Theatre Lighting 
 
The bootstrapped distribution shown in Figure 11 follows a normal distribution closely. 
 
 
 
 
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Figure 12: Histogram and Bootstrapped Distribution for Average Sway in the Anterior-Posterior Direction: 
Right Leg Gesturing in Ambient Lighting 
 
The bootstrapped distribution in Figure 12 follows a normal distribution closely, but the 
histogram of the data doesn’t show normality.  
 
 
Figure 13: Histogram and Bootstrapped Distribution for Average Sway in the Anterior-Posterior Direction: 
Left Leg Gesturing in Ambient Lighting 
 
The bootstrapped distribution in Figure 13 follows a normal distribution quite closely, while the 
histogram of the data shows more frequency at high and low reported sway values. 
 
 
 
 
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Figure 14: Histogram and Bootstrapped Distribution for Average Sway in the Anterior-Posterior Direction: 
Right Leg Gesturing in Theatre Lighting 
 
The bootstrapped distribution in Figure 14 follows a normal distribution quite closely, while the 
histogram of the data shows more frequency at lower reported sway values. 
 
 
Figure 15: Histogram and Bootstrapped Distribution for Average Sway in the Anterior-Posterior Direction: 
Left Leg Gesturing in Theatre Lighting 
 
The bootstrapped distribution shown in Figure 15 shows negative skew, and the histogram 
shows greater frequency of higher reported sway values. 
  
 
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Works Cited (APA) 
Gribble, P. A., Hertel, J., & Plisky, P. (2012). Using the star excursion balance test to assess 
dynamic postural-control deficits and outcomes in lower extremity injury: A literature and 
systematic review. Journal of Athletic Training, 47(3), 339-357. doi:10.4085/1062-6050-47.3.08 
Hutt, K., & Redding, E. (2014). The effect of an eyes-closed dance-specific training program on 
dynamic balance in elite pre-professional ballet dancers a randomized controlled pilot study. 
Journal of Dance Medicine & Science, 18(1), 3. 
Smith, T. O., Davies, L., de Medici, A., Hakim, A., Haddad, F., & Macgregor, A. (2016). Prevalence 
and profile of musculoskeletal injuries in ballet dancers: A systematic review and meta-
analysis. Physical Therapy in Sport, 19, 50-56. doi:10.1016/j.ptsp.2015.12.007 
 
 
  
 
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Statistical Analysis Tools Used: 
Dr. Ken Gerow’s Bootstrapping Tool and Confidence Interval for Paired Data Tool using Microsoft Excel 
R, for plots (Code Below) 
dancedata=read.csv(file.choose(),header=TRUE) 
plot(dancedata$ML.Sway.R.A~dancedata$Participant,ylab="Sway",xlab="Participant", 
     main="Average Sway in Medial Lateral Direction: Right 
Leg",col="purple",pch=8,lwd=3,ylim=c(0,160)) 
points(dancedata$ML.Sway.R.T,col="turquoise",pch=8,lwd=3) 
legend(x=1.,y=130,c("Ambient Lighting","Stage 
Lighting"),pch=8,col=c("purple","turquoise"),cex=.90,lwd=3) 
 
plot(dancedata$ML.Sway.L.A~dancedata$Participant,ylab="Sway",xlab="Participant", 
     main="Average Sway in Medial Lateral Direction: Left Leg",col="purple",pch=8,lwd=3,ylim=c(0,160)) 
points(dancedata$ML.Sway.L.T,col="turquoise",pch=8,lwd=3) 
legend(x=1.,y=130,c("Ambient Lighting","Stage 
Lighting"),pch=8,col=c("purple","turquoise"),cex=.70,lwd=3) 
 
plot(dancedata$AP.Sway.R.A~dancedata$Participant,ylab="Sway",xlab="Participant", 
     main="Average Sway in Anter-Posterior Direction: Right 
Leg",col="purple",pch=8,lwd=3,ylim=c(0,170)) 
points(dancedata$AP.Sway.R.T,col="turquoise",pch=8,lwd=3) 
legend(x=1.,y=50,c("Ambient Lighting","Stage 
Lighting"),pch=8,col=c("purple","turquoise"),cex=.70,lwd=3) 
 
plot(dancedata$AP.Sway.L.A~dancedata$Participant,ylab="Sway",xlab="Participant", 
     main="Average Sway in Anter-Posterior Direction: Left 
Leg",col="purple",pch=8,lwd=3,ylim=c(0,170)) 
points(dancedata$AP.Sway.L.T,col="turquoise",pch=8,lwd=3) 
legend(x=1.,y=50,c("Ambient Lighting","Stage 
Lighting"),pch=8,col=c("purple","turquoise"),cex=.70,lwd=3)