Analysis of the hip joint moments in grand rond de jambe en l'air.
The purpose of this study was to investigate hip net joint moments in the gesture and support legs in grand rond de jambe en l'air en dehors. Two groups of dancers, skilled (N = 8) and novice (N = 6), performed grand rond de jambe en l'air at two different vertical leg angles: 90[degrees] and 105[degrees]. Three-dimensional motion data and ground reaction force data were collected. Hip net joint moments were computed through standard inverse dynamics procedures and were normalized to each participant's mass. Normalized peak hip joint moments in both the gesture and support leg were used as the dependent variables; the independent variables were the skill level and the demand (vertical angle for the rond de jambe at 90[degrees] and 105[degrees]). A two-way mixed-design ANOVA (2 groups x 2 conditions) was used (p < .05) with the group and demand as the between and within factors, respectively. The hip joint moments of the support leg were consistently larger than the matching moments of the gesture leg. However, as the demand level increased the peak hip joint moments of the gesture leg hip showed minor changes, the support leg moments decreased. It was concluded that muscular strength is not a limiting factor for the novice dancers and that an increased demand (vertical leg angle) actually puts smaller burden on the support leg hip. Increased pelvis movement (demonstrated by the skilled dancers) facilitated the range of motion of the gesture leg without increasing joint moment cost. The hip abductor moments were identified as important components in performing grand rond de jambe en l'air.
When a skilled dancer performs complex movement involving weight shifts and extreme ranges of motion, the action appears effortless and balanced. However, within this elegant performance, a detailed choreography is taking place within the dancer's body. For example, movement of the pelvis complements the height of the gesturing leg, while at the same time, movement of the trunk and head complements action in the lower body, helping the dancer maintain balance and achieve a specific line or aesthetic requirement. Understanding the dynamic relationships among these accessory movements in the body, the corresponding proportions of these actions, and the amount of muscular efforts invested during the execution is of interest to dancers, dance teachers, and researchers. (1-3)
Grand rond de jambe belongs to a family of movements in which one leg performs the required gesture (devant, a la seconde, and arabesque) while the other leg supports the body (Fig. 1). In this process, the pelvis serves as the convergence point of the other body parts: gesture leg, support leg, and the upper body. (3) While interacting with each other to produce necessary motions of the gesture leg and the body in grand rond de jambe, the four body parts form a kinetic chain in which the hip joints serve as the link between the pelvis and legs. As the gesture leg leaves the ground in grand rond de jambe en l'air, a dancer's body becomes mechanically unstable due to the changing Y-shaped body configuration over a small base of support (support leg foot). The center of mass (CM) of the dancer's body must fall and remain within the boundary of this base of support to maintain equilibrium while performing the required motion sequence. Although the gesture leg and the upper body are not constrained, the motions of these body parts are determined by aesthetic requirements (upright upper trunk) and the desired vertical gesture leg angle. Therefore, equilibrium is achieved through motion (rotation and translation) of the pelvis and the lumbar spine while maintaining the target trunk and gesture leg position. (1-4)
[FIGURE 1 OMITTED]
The primary role of the gesture leg hip joint at the pelvis is to produce discernable motion of the gesture leg with the help of the pelvic motion, whereas the support leg hip joint is responsible for weight-bearing and maintenance of the attitude of the pelvis. As the body configuration deviates from a pure Y-shape, the burden on the support leg hip joint can increase. Failure to produce sufficient muscular moments in the hip joints may limit gesture leg range of motion (ROM) or result in an aesthetically inferior body configuration. In a three-dimensional kinematic study, Wilson and colleagues (4) reported that skilled dancers showed more pelvic motion and greater vertical angle for the gesture leg position in executing grand rond de jambe en l'air en dehors, at 90[degrees], than their novice counterparts. In a follow-up study involving a group of skilled ballet dancers performing grand rond de jambe en l'air at three different vertical angles, Wilson and associates (3) concluded that the pelvis was the primary contributor to the increased gesture leg ROM as the vertical angle demand increased. In addition, the difference in the pelvic motion between the subject groups seen in the earlier study (4) was explained solely by the difference in the gesture leg ROM.
One key question in understanding the differences between the skilled and novice dancers remained unanswered. If a commensurate amount of pelvis motion was beneficial in terms of the gesture leg ROM, what kept the novice dancers from using this strategy? A possible explanation offered by Wilson and colleages (4) was that novice dancers may use less pelvic motion because increasing the pelvic motion would require more muscular effort. Estimation of the net joint moment at the hip joints would provide an answer to this question of strength or "cost" of the movement for the dancer. Therefore, the purpose of this study was to quantify the hip net joint moments in both the gesture and the support leg in grand rond de jambe en l'air en dehors and to investigate the effects of the demand (leg angle) and skill level on the magnitude of the hip net joint moments. It was hypothesized that 1. the skilled dancers would show larger hip joint moments than their novice counterparts, and 2. the hip joint moments would increase as the demand (leg height) increased.
A total of eight skilled and six novice female ballet dancers participated in this study (Table 1). Two skilled dancers were from a professional dance company while six skilled and six novice dancers were enrolled in a collegiate dance program. Group classification was based on dancer's overall dancing ability as recommended by their professors. This study was approved by the University Institutional Review Board and written consent was obtained from each dancer prior to participation.
Each participant performed three trials of grand rond de jambe en l'air in center floor at two different nominal vertical leg angles: 90[degrees] and 105[degrees]. In the 105[degrees] condition, a visual guide was placed in front of the dancer to show the target foot height at the end of the ascending motion of the gesture leg. (3) Leg length was measured to compute the target foot height in this condition. The right leg was used as the gesture leg in all participants and trials.
Dancers were asked to warm up for a minimum of 5 minutes prior to testing and encouraged to keep moving between trials. Sufficient practice trials were allowed prior to data collection for familiarization, and the dancers rested for 1 to 2 minutes between trials.
For standardization purposes, all trials were set to music (Lisa Harris, Etudes for Ballet Class, 4/4 Adagio). Each trial was completed in twelve counts, eight counts for the full grand rond de jambe en l'air followed by four counts for a rise to demi-pointe (releve) and returning to the first position (abaisse--returning the heels to the floor). (3,4) To discourage counter movements with the arms and to minimize blocking of the markers placed on the body, dancers were required to hold arms at the shoulder level and bring the hands in to touch the sternum.
For the three-dimension motion analysis, spherical reflective markers were placed on the pelvis and legs of the dancer's body: right and left anterior superior iliac spines (ASIS), mid-point of the posterior superior iliac spines (mid-PSIS), lateral and medial epicondyles, lateral and medial malleoli, tips of the second toe, and the heels. Motion video was captured by six digital camcorders (Panasonic AG-DVC15; picture rate = 60 Hz; shutter speed = 1/1000 second). The ground reaction force acting on the support leg was collected by a force plate (OR-6-5, AMTI, Watertown, MA; sampling rate = 100 Hz). Kwon3D (Visol, Inc., Seoul, Korea; version 3.1) and KwonGRF (Visol, Inc., Seoul, Korea; version 2.0) were used in capturing video and acquiring ground reaction force data, respectively.
Prior to data collection, cameras were calibrated with a calibration frame (2-m W x 2-m L x 1.5-m H) containing 36 control points. The Direct Solution Method (5) was used in camera calibration. The calibration errors of the cameras ranged from 0.53 to 0.72 pixels while the overall reconstruction error was 0.25 cm. (6)
Data Reduction and Processing
Image-plane coordinates of the reflective markers were obtained through automatic tracking using Kwon3D software. The three-dimensional marker coordinates were computed through the reconstruction procedure based on the image plane coordinates of the markers and the camera parameters. The reconstructed marker coordinates were subject to digital filtering by a Butterworth low-pass filter (fourth order zero phase-lag filter) with the cut-off frequency being 6 Hz. (7)
Hip joint centers were calculated using the method outlined by Tylkowski and coworkers. (8) It was assumed in this process that the hip joint center was medial to the ASIS by 14%, inferior by 30%, and posterior by 19% of the inter-ASIS width. (9) The knee and ankle joints were defined as the mid-points of the epicondyles and malleoli, respectively.
Local reference frames were defined for the pelvis as well as thigh, shank, and foot on each leg. The pelvis reference frame was defined by three pelvic markers (right and left ASIS and mid-PSIS) with the line drawn from the left ASIS to the right being the mediolateral (ML) axis. The longitudinal (L) axis was perpendicular to the pelvis plane defined by the three markers. The thigh reference frame was defined by the hip and knee joints and the epicondyle makers. The line drawn from the knee joint to the hip was used as the L axis and the anteroposterior (AP) axis was perpendicular to the plane formed by the four points. A similar method was used for the shank using the ankle and knee joints and the malleolus markers. The foot reference frame was defined by three points: toe, heel, and ankle joint. The line drawn from the toe to heel was used as the L axis while the ML axis was perpendicular to the plane defined by these three points.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The orientation matrices of the local reference frames were obtained from the axis vectors. The orientation angles and angular velocities of the leg and pelvis segments were computed from the orientation matrices (10) as shown in Equations 1 and 2. A and B in Equations 1 and 2 are the local reference frames (segments), G is the global reference frame, [T.sub.B/A] is the relative orientation matrix of segment B to segment A, [t.sub.11] ~ [t.sub.33] are the elements of the relative orientation matrix obtained from the axis vectors, [[[??].sub.1], [[??].sub.2], [[??].sub.3]] are the relative orientation angles of segment B to A, [c.sub.i] = cos [[theta].sub.i], [s.sub.i] = sin [[theta].sub.i], [[[??].sub.1], [[??].sub.2], [[??].sub.3]] are the first time-derivatives of the relative orientation angles, [[omega].sub.A] and [[omega].sub.B] are the angular velocities of segments A and B, respectively, and [T.sub.A/G.sup.t] is the transpose of [T.sub.A/G]. The XYZ rotation sequence (ML-AP-L sequence) was assumed in Equation 1.
The inertial properties, such as center of mass (CM) location, mass, and principal moments of inertia, of the leg and pelvis segments were computed using the ratio values reported by Chandler and colleagues. (11) The inertia tensor of each segment was built using the principal moments of inertia of the segment shown in Equation 3, where [I.sub.i] = the inertia tensor of a segment, and [[I.sub.ML], [I.sub.AP], [I.sub.L]] are three principal moments of inertia of the segment about the ML, AP, and L axis, respectively.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The local angular momentum of a segment was computed from the inertia tensor and the angular velocity of the segment (10) as shown in Equation 4, where [L.sub.i] is the local angular momentum of a segment about its CM.
The net joint moments were computed through a standard inverse dynamics procedure based on the motion and ground reaction force data as shown in Equation 5, where [N.sub.j] is the net joint moment of joint j, [r.sub.ji] is the relative position of the CM of a distal segment i to joint j, [L.sub.i] and [[??].sub.i] are the first time-derivatives of the angular and linear momentums of segment i, respectively, [W.sub.i] is the weight of segment i, C is the center of pressure of the ground reaction force, and [F.sub.G] and [N.sub.G] are the ground reaction force and torque, respectively. For the gesture leg, the ground reaction force and torque ([F.sub.G] and [N.sub.G]) were set equal to 0 in Equation 4 since no ground reaction force was acting on the gesture leg.
With the assumption that the legs (gesture and support) move as single units, only the hip joint moments of the gesture and support leg were subject to analysis in this study. The hip joint moments were normalized to each participant's mass (in Nm/kg) for group comparison purposes. Five events were identified (Fig. 1): end of the ascending motion of the gesture leg (EAM), maximum vertical gesture-leg angle (MXV), maximum left pelvic tilt (MLT), minimum vertical gesture-leg angle (MNV), and initiation of the descending motion of the gesture leg (IDM). The generalized hip joint moment patterns were derived through phase-by-phase ensemble averaging through the horizontal leg motion phase (EAM to MXV, MXV to MLT, MLT to MNV, and MNV to IDM). From the generalized patterns, notable peaks were identified for further statistical analysis.
The dependent variables in this study were the normalized peak hip joint moments of the gesture and support leg during the grand rond de jambe en l'air; the independent variables were the group (skilled and novice) and demand (vertical leg angle; 90[degrees] and 105[degrees]). Two-way mixed-design ANOVA (2 groups x 2 conditions) was used in the statistical analysis (p < .05) with the group and demand (vertical leg angle) the between and within factors, respectively. Post-hoc comparison of the group means were performed with the Sidak adjustment.
The generalized hip joint moment patterns of the gesture leg are presented in Figure 2. The ML component of the gesture leg hip joint moment was characterized by the initial flexor-dominant phase followed by the extensor-dominant phase as the gesture leg passes the a la seconde position. Two peaks were identified: flexor peak (GF) and extensor peak (GE). The AP component consistently showed abductor-dominant moment throughout the entire period (devant to arabesque) with one peak (GB). The L component revealed a horizontal abductor-dominant phase followed by a horizontal adductor-dominant phase. Two abductor peaks were identified (GHB1 and GHB2).
[FIGURE 2 OMITTED]
On the other hand, the ML component of the generalized hip joint moment pattern of the support leg (Fig. 3) was characterized by an initial extensor-dominant phase with one extensor peak (SE). The AP moment showed dominance of the abductors throughout the entire phase with one peak (SB). The L component was characterized by a lateral rotator-dominant phase in the second half of the horizontal motion cycle (a la seconde to arabesque) with one peak (SL).
[FIGURE 3 OMITTED]
Significant inter-group difference was observed only in GHB2 (Fig. 2) for the 105[degrees] condition with the novice group showing a larger mean value (Table 2). All other peak joint moments showed no significant inter-group difference at both angle conditions with a trend of smaller mean values in the skilled group. Significant inter-angle difference was observed in GB and GHB2 (Fig. 2) for the novice group only with the 105[degrees] condition showing larger values.
No significant inter-group difference was seen between groups at 90[degrees] in all peak support leg hip joint moments (Table 3). Significant differences, however, were observed in the peak extensor and abductor moments (SB and SL) between the 90[degrees] and 105[degrees] conditions in both groups with the 90[degrees] condition showing larger mean values. In general, the 105[degrees] condition revealed a trend of smaller support leg peak hip joint moments than the 90[degrees] condition. Although not significant, the skilled group showed a trend of larger extensor and abductor peaks but smaller lateral rotator peak than the novice group. The novice group also showed larger standard deviations than the skilled group in general.
It was hypothesized in this study that the skilled dancers would exhibit larger hip net joint moments than their novice counterparts making it more difficult for the novice dancers to use the strategy employed by the skilled dancers. However, only one inter-group difference was observed in the gesture leg hip horizontal abductor moment (GHB2 in Fig. 2) for the 105[degrees] condition. Moreover, the novice group showed a larger peak value than the skilled. These findings suggest that the technique used by the skilled group does not particularly require more muscular effort. Therefore, hip muscular strength, especially the gesture leg, is not a limiting factor that prevents the novice dancer from using the motor strategy that the skilled dancers use.
The second hypothesis was that the hip joint moments would increase as the demand (leg height) increased, thus making it more difficult to perform grand rond de jambe en l'air at a higher vertical leg angle. Increase in the peak moment was observed only in the gesture leg for the hip abductor peak (GB in Fig. 2) and the second horizontal abductor peak (GHB2), which are related to the transition from the a la seconde position to arabesque. In other cases, either there was no difference or the 105[degrees] condition showed smaller mean values than the 90[degrees] condition. These findings suggest that the demand level is not directly related to the muscular effort in the gesture leg hip joint and increased demand (vertical leg angle) actually puts smaller burden on the support leg hip.
The support leg moments were consistently larger than their matching gesture leg moments in performing grand rond de jambe en l'air. This is mainly due to the weightbearing function of the support leg hip and the ground reaction force acting on the support leg. It is, however, important to understand in comparing the net joint moments between the gesture leg and the support leg that the contraction conditions are vastly different. The gesture leg hip muscles contract concentrically near the end of the joint ROM at significantly shortened lengths, whereas the support leg muscles work eccentrically well within the ROM. Among the individual components, the support leg hip abductor moment showed substantially larger mean values (> 0.83 Nm/kg) than the others (< 0.48 Nm/kg). This appeared to be the result of lateral translation of the pelvis to achieve ML equilibrium as the gesture leg moved into the a la seconde position. Although statistically not significant due to large standard deviations, the skilled group revealed a trend of larger mean values than their novice counterparts in this particular moment component. The gesture leg hip abductor also showed the largest peak moment values among the gesture leg muscle groups. The hip abductors in general serve vital function in performing grand rond de jambe en l'air.
The support leg hip joint moment patterns (Fig. 3) were characterized by larger fluctuations and standard deviations. In movements on one leg, with large range of motion of the gesture leg, the CM must be maintained over a small base of support; balance is maintained by movement in the support foot. Small movements in the foot and ankle of the support foot cause fluctuations in the ground reaction force and the center of pressure location, which in turn create fluctuation in the support leg hip joint moments.
The results of this study demonstrate that pelvic movement allows the dancer to access the necessary ROM without increased muscular effort. Allowing movement in the pelvis facilitates the ROM of the gesture leg without increasing the joint moment cost. Thus the initial question of why the novice dancers did not use a commensurate amount of pelvis motion has not been answered decisively. Two observations can be made: 1. the skilled dancers are stronger than the novice group and perform grand rond de jambe en l'air more comfortably; and 2. strength is not the critical factor. The first observation can be confirmed by intentionally increasing the burden on the hip joints with a weight on the gesture leg for a given vertical leg angle. In the second scenario, the difficulty in performing grand rond de jambe is determined not by hip strength but by other factors, such as balance or flexibility. In maneuvers such as grand rond de jambe en l'air, a change in the pelvis orientation alters mass distribution of the body about the hip joint of the support leg making it more difficult to control equilibrium due to increased pelvis rotation. This adaptation for the gesture leg would increase the burden on the support leg. In fact, when considering balance and the overall execution of the movement in grand rond de jambe en l'air, a more comprehensive picture of the complexity of the movement is revealed wherein pelvic motion is simply a part of the movement whole. In addition, range of motion or ease of pelvic motion for the novice dancers may have been limited by their flexibility, muscle extensibility, or their ability to use their dynamic range of motion. Finally, the novice dancers may have purposely limited their pelvis motion in the execution of the movement during the testing due to preconceived notions of proper technique.
Summary and Conclusion
The purpose of this study was to investigate whether skill level and demand (vertical leg angle) affect the magnitude of hip net joint moments in both the gesture leg and the support leg while performing grand rond de jambe en l'air en dehors. Two groups of dancers were recruited: skilled (N = 8) and novice (N = 6). Both groups performed grand rond de jambe en l'air at two different vertical leg angles: 90[degrees] and 105[degrees]. The hip net joint moments were computed through standard inverse dynamics approach.
It was concluded from the analysis that:
1. The technique used by the skilled group did not require more muscular effort. Hip strength (especially in the gesturing leg) was not a limiting factor for the novice dancers' use of the pelvis and leg vertical angle.
2. As the demand level (vertical leg angle) increased, the peak hip joint moments of the gesture leg hip showed minor changes, while the support leg moments decreased. The demand level was not a limiting factor for the novice dancers' use of the pelvis and leg vertical angle.
3. During the execution of the grand rond de jambe en l'air, more burdens were placed on the support leg. The support leg hip abductor showed the largest peak moment among the muscle groups. The hip abductors were identified as important muscles in performing grand rond de jambe en l'air.
Relevance of the Study
Net joint moments provide dance scientists insight into the role of the muscles at the joints and the muscular effort or "cost" for each individual joint in the execution of complex movements such as grand rond de jambe en l'air. Net joint moment patterns also provide information regarding the dominant muscle groups at different time points throughout the motion cycle. This information is used to understand the relative contributions of the muscles in the movement; identifying critical muscles from net joint moment data enables dance teachers to better prepare their dancers for the demands of technique.
Information generated from this study benefits teachers of dance and dancers in general. Understanding the contribution of pelvic motion in creating a specific range of motion for the gesture leg is vital. While exaggerated movement of the pelvis is not desired, allowing the pelvis to move helps the dancer achieve the desired range of motion. Kinematic analyses have demonstrated that using the pelvis to facilitate positioning of the gesture leg is a potential distinction between the skilled and novice dancers.3,4 However, this study revealed that the focus on pelvic motion should not be limited to the gesture leg. While the support leg demand did not increase with increased vertical demand, the demand on the support leg abductors was noteworthy.
The message to include emphasis on the support leg in teaching grand rond de jambe en l'air (and other movement requiring full range of motion at the hip) is clear. Addressing the support leg to facilitate range of motion of the gesture leg provides two advantages for the dancer. First the abductor muscles (tensor fascia lata, gluteus medius, minimus, assisted by the anterior superior fibers of the gluteus maximus) serve to orient the pelvis over the support leg. This orientation of the pelvis facilitates the second advantage, balancing movement of the pelvis in relation to the gesture leg. In other words, focusing on the support leg would limit excess or undesired movement of the pelvis. Therefore the emphasis on teaching movements that require large ROM and balance should be on both the support and the gesture leg. While their actions are different, their contribution to the desired movement is the summation of their individual roles. Achieving an aesthetic ideal in dance requires synthesis of whole body.
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(10.) Kwon YH: The effects of body segment parameter estimation on the experimental simulation of a complex airborne movement. Doctoral dissertation. Pennsylvania State University, University Park, PA, 1993.
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Young-Hoo Kwon, PhD., and Joong-Hyun Ryu, M.S., are in the Biomechanics Laboratory, Texas Woman's University, Denton, Texas. Margaret Wilson, Ph.D., is in the Department of Theatre and Dance, University of Wyoming, Laramie, Wyoming.
Correspondence: Young-Hoo Kwon, Ph.D., Biomechanics Laboratory, Texas Woman's University, P.O. Box 425647, Denton, Texas 76204-5647; ykwon@ twu.edu.
This work was presented at the 16th Annual Meeting of the International Association for Dance Medicine and Science held in West Palm Beach, Florida, USA, in October 2006, and published as an extended abstract in the Annual Meeting Proceedings.
Table 1 Participant Characteristics Mass Height Age Career (kg) (cm) (yrs) (yrs) Skilled Mean 54.7 168.3 26.9 10.2 (N = 8) (SD) (3.5) (2.7) (7.7) (5.9) Novice Mean 59.2 166.5 24.0 4.2 (N = 6) (SD) (8.7) (3.5) (2.6) (2.6) Table 2 Peak Gesture Leg Hip Joint Moments (in Nm/kg) Skilled (N = 8) Dominant Peak Muscle ([section]) 90[degrees] 105[degrees] Flexor GF Mean 0.37 0.36 (SD) (0.03) (0.03) Extensor GE Mean 0.32 0.31 (SD) (0.05) (0.05) Abductor GB Mean 0.37 0.38 (SD) (0.03) (0.04) Hor. Abductor GHB1 Mean 0.13 0.14 (SD) (0.03) (0.03) GHB2 Mean 0.13 0.14 (SD) (0.04) (0.04) Novice (N = 6) Dominant Peak Muscle ([section]) 90[degrees] 105[degrees] Flexor GF Mean 0.41 0.40 (SD) (0.09) (0.07) Extensor GE Mean 0.33 0.35 (SD) (0.11) (0.12) Abductor GB Mean 0.40 0.45 ([dagger]) (SD) (0.07) (0.10) Hor. Abductor GHB1 Mean 0.14 0.15 (SD) (0.03) (0.03) GHB2 Mean 0.14 0.18 ([dagger]) ([double dagger]) (SD) (0.03) (0.03) ([section]) See Figure 2 for the location of the peaks; ([dagger]) Significantly different from the skilled group (p < .05); ([double dagger]) Significantly different from the 90[degrees] condition (p < .05) Table 3 Peak Support Leg Hip Joint Moments (in Nm/kg) Stilled (N = 8) Dominant Peak Muscle ([section]) 90[degrees] 105[degrees] Extensor SE Mean 0.42 0.33 ([double dagger]) (SD) (0.09) (0.08) Abductor SB Mean 1.11 0.97 ([double dagger]) (SD) (0.19) (0.17) Lat. SL Mean 0.41 0.40 Rotator (SD) (0.12) (0.08) Novice (N = 6) Dominant Peak Muscle ([section]) 90[degrees] 105[degrees] Extensor SE Mean 0.41 0.29 ([double dagger]) (SD) (0.22) (0.22) Abductor SB Mean 1.04 0.83 ([double dagger]) (SD) (0.27) (0.22) Lat. SL Mean 0.48 0.46 Rotator (SD) (0.16) (0.13) ([section]) See Figure 3 for the location of the peaks; ([double dagger]) Significantly different from the 90[degrees] condition (p < .05)
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|Title Annotation:||Original Article|
|Author:||Kwon, Young-Hoo; Wilson, Margaret; Ryu, Joong-Hyun|
|Publication:||Journal of Dance Medicine & Science|
|Date:||Jul 1, 2007|
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