Encuesta Aplicada A Los Habitantes De Las Parroquias San Francisco, El Sagrario

The measure used to assess postural stability depends on the setting in which testing occurs. They range from highly technologically advanced laboratory methods, to fairly subjective, but clinically friendly tools. Laboratory methods for measurements are preferred when the purpose of the research is to most accurately and precisely describe balance and postural stability. They have higher sensitivity for the detection of movement and small changes and can also help make inference to performance of the underlying sensory motor systems responsible for maintaining balance and postural stability.60,216 These methods are preferred for the current project because the purpose of the study is to describe the balance ability of different types of dancers, as well as determine the relationship of postural stability with the other variables being measured. Therefore, force plate measures of postural stability will be used.

Postural stability is a specific component of the ability to maintain balance. Balance is the ability to maintain the center of mass over the base of support.28 Postural stability is the ability to maintain the body in equilibrium by keeping the center of mass within the base of support.181 Maintaining postural stability requires the integration of sensory information and execution of appropriate motor responses.26,27 Postural stability is most accurately assessed using force plate

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technology which quantifies the movement ground reaction forces about the base of support. This provides an objective measurement of the ability to stay as motionless as possible, or steadiness.217 When steadiness is maintained on a fixed, firm, unmoving base of support, static postural stability is achieved.179 Dynamic postural stability is the ability to achieve steadiness after performing a movement requiring a change in position of the stance leg or change in location.217,218

Postural stability will be quantified using ground reaction forces rather than center of pressure measurements because the former have been found to be more sensitive in discriminating changes in steadiness.217 Dynamic postural stability measures the ability to stabilize when transitioning from a dynamic, or moving, to static state.217 Time to stabilization measures have been used to assess dynamic postural stability, with longer times indicating worse dynamic postural stability.219 However, it has been proposed that time to stabilization calculations can be influenced by individual differences in range of variation of the ground reaction force measurement.219 A recommendation for correcting this potential error was made by Ross, et al., using a vibration magnitude of curve fit.219 Furthermore, time to stabilization is reported in each of the three force directions (x, y, and z). Newer dynamic postural stability methods control for original calculation errors and report a score that incorporates movement in all directions.87,220 The dynamic postural stability index (DPSI) was calculated using the x, y and z ground reaction forces, using the equation by Wikstrom et al.87 Larger values indicate less postural stability. This equation for calculating DPSI has been found to be reliable with an ICC of 0.96.87 This equation is beneficial because it incorporates the ground reaction forces in all axes for a total balance score, rather than providing multiple scores for each direction.

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The dynamic task used for dynamic postural stability measurement was an anterior single leg jump from a distance of 40% of the subject’s height over a hurdle. Sell et al.,221 have demonstrated this specific task, and calculation of the DPSI score, to be reliable in physically active adults (ICC = 0.86). This task was chosen, as opposed to a dance jump, because there has been little research in dynamic postural stability of dancers to date. As a preliminary study, a task that is reliable, standardized and easily repeated will allow for a basic description of the dynamic postural stability of dancers, as well as allow for comparison to other types of athletes. There is no reason to believe the dancers will not be able to perform the task as they are familiar with jumping from two feet and landing on one foot.187

2.6.4 Biomechanics

Kinematic variables were used to describe lower extremity biomechanics of dancers performing a dance specific jump task. Three dimensional motion analyses using an infrared camera system tracking reflective markers was used. Three dimensional motion analyses are preferred compared to two dimensional methods because they more accurately describe joint position.185 The ankle and foot joints are especially important in dance, and motion at these joints involves complex movements about all three axes. The marker set chosen for this study uses multiple markers about the lower leg, ankle and foot to adequately describe motion of the segments in each plane. A 3D motion capture system using multiple cameras is more appropriate for this type of study because it allows for a larger capture volume than an electromagnetic tracking system. This is necessary for the dancers to perform the dance jump task. Additionally, an electromagnetic tracking system was not appropriate for this study because it involves wires being attached to the

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subject which would limit their motion. Even though electromagnetic tracking does not have line of sight issues which can occur with 3D motion analysis, the ability to create a larger capture volume and no use of wires outweighs this limitation.185

A dance specific task was chosen for this study because previous work has been able to describe dancers in comparison to other athletes performing traditional tasks; drop landings. However, less work has been done to analyze how dancers jump during performance of dance tasks. Moving forward these tasks are important to investigate, as dancers are injured performing movements specific to dance technique. The dance task chosen is a forward grad jeté. This task is appropriate to use in the study population because dancers regularly perform this kind of jump. It is similar to the saut de chat jump used by Kulig and her colleagues in their studies on dancers.101 Both are forward moving leaps where the dancer takes off from one foot, moves with the legs in a split position through the air, and lands on the opposite foot. The grad jeté was selected for analysis as it is a well-practiced movement and considered easier for the dancer to perform consistently and requires less room. The difference between the two jumps is the movement of the front leg and height as the dancer moves into the air. The saut de chat uses a

developé (or kick moving from a bent knee to straight knee). With the “brush” approach the

dancer keeps the knee straight as is moves forward into the air. The grad jeté was easier to perform with a two-step approach, which more appropriately fits on the platform surrounding the force plate in the motion capture space, and consistently land in the center of the force plate. During pilot testing dancers performing the saut de chat often felt more comfortable using a three to four step approach for the jump and either over or under shot the force plate depending on the height they achieved during the jump.

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To standardize the dance jump task, all dancers took off from a position of 60% of their height away from the force place. This position was chosen during practice testing with a professional and a collegiate dancer. One was a female and one was a male. Both dancers performed multiple jumps landing on the force plate and their take off position was measured. The most frequently chosen take off position for both dancers was approximately 60% of their height. The pilot subjects performed leaps ranging from approximately 45% to 90% of their height. This jump distance is safe because it does not result in maximal axial or shear forces, yet still appropriately challenge the subjects.188,222 Simpson et al., investigated the axial and shear forces on dancer’s lower extremities performing jumps of varying distances (30%, 60%, and 90%) based on dancer’s maximum jump distance. Overall, as jump distance increased, so did the axial and shear forces at the ankle and knee.188,222 A dance jump distance is likely to be safe and representative of a typical movement, improving the applicability to the typical dance setting. A dance jump distance of 60% of dancer height is more reliably standardized than a jump distance based on maximum jump distance because the maximum distance a dancer jumps may be variable. The height of the jumps will not be standardized for this task so that the dancers’ natural movement will not be hindered. The jump height will be collected and reported with the results to be used for discussion and explanation of the findings.

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3.0 METHODOLOGY

3.1 EXPERIMENTAL DESIGN

This was a cross sectional study of professional ballet and collegiate dancers. Dancers were tested over two test sessions.

3.2 SUBJECT RECRUITMENT

The study was approved by the Institutional Review Board at the University of Pittsburgh prior to data collection. Subjects were recruited from professional dance companies and collegiate dance programs. The Principal Investigator (PI) has developed relationships and contacts with several institutions through clinical and academic work. The PI provided information about the study to Pittsburgh Ballet Theater (PBT), Texture Contemporary Ballet (TCB), Point Park University Dance Department (PPU), and Slippery Rock University Department of Dance (SRU). Informational presentations describing the study were provided to recruit volunteers. Interested subjects contacted the PI and completed a phone screening to determine eligibility. Eligible subjects who wished to participate scheduled a time to complete their testing sessions. On the first day of testing the inclusion and exclusion criteria were re-confirmed and they

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completed informed consent with the PI. The proportion of genders recruited were 60% female and 40% male. This was reasonable and reflective of the dance population and will help with the generalizability of the study to this community.

3.3 SUBJECT CHARACTERISTICS

3.3.1 Inclusion Criteria

Male and female dancers between the ages of 18 and 45 years were recruited for the study. Dancers had a contract to dance with a professional company or had been enrolled as a dance major at a collegiate institution, in the past twelve months. All dancers were currently dancing in class, rehearsal or performances at least three days a week at the time of the study.

3.3.2 Exclusion Criteria

Subjects were excluded from the study if they had a current musculoskeletal injury that was currently preventing the dancer from full participation in required dance activities (class, rehearsal, performance) and had been diagnosed by a licensed health care professional. Dancers were also excluded if they had any current neurological disorder, concussion, or allergy to adhesives at the time of the study.

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3.4 POWER ANALYSIS

The statistical program G Power 3.1.5(Franz Faul, Universitat Kiel, Germany) was used for power analyses based on two independent means (two groups). The power analyses were completed using a t-test for between group comparisons, with alpha of 0.05, power set at 0.80, and a large effect size of d = 0.80. The effect size is Cohen’s effect size for the expectation that there will be large differences between groups.196 Means and standard deviations of quadriceps strength (peak torque/body weight) of professional ballet and collegiate dancers were used to calculate effect size, which was found to be d = 0.93.52 To the author’s knowledge, other studies have not provided sufficient example data (means and standard deviations) for effect size calculations for other muscle groups and variables. The author expected that similar differences will be seen for other muscle groups, as well as for the kinematic variables due to increased training of the professional group. Results indicated that 52 total subjects would be needed, with 26 subjects in each group. To account for possible 15% attrition and data loss, an additional 4 subjects were added to each group. Therefore 30 subjects were screened and recruited from professional dance organizations and 30 from collegiate dance institutions, a total of 60 subjects for the study.

Power analysis for regression using an F-test linear multiple regression fixed model R2 deviation from zero, with alpha 0.05, power set at 0.80, a large effect size f2 = 0.35, and six predictor variables indicated 46 subjects were needed..196 Grouping all dancers together supported the sample size requirement for the regression analyses. Level/type of dancer and gender were used as two of the predictor variables for each regression. Four other variables were chosen from the strength variables for each regression.

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3.5 INSTRUMENTATION