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The Effect of Footwear on Free Moments During a Rotational Movement in Country Swing Dance.

Spinning, sliding, and jump-landing activities are commonly performed by dancers, and different dance styles use different specialized shoes. Previously, investigators have quantified the effect of footwear on postural control, pressure distribution, and ground reaction forces in dancers. (1-5)

Understanding the biomechanical interaction between dancers and floors could also provide insight for performance training in dance. (6) One area that has not been studied is how footwear may affect the twisting loads applied to the lower extremities during rotational movements.

Previous efforts have focused on understanding how dancers produce angular impulse around the vertical axis during rotational movements such as fouette, pirouette, and pique turns. (6-9) Dancers interact with the ground to initiate and stop angular impulse through modulating ground reaction forces and free moments in the transverse plane. (6,10) As Holden and Cavanagh (11) explained, the free moment is a force couple about the vertical axis that passes through the center of pressure, representing a frictional moment between the individual and ground. From a bottom-up inverse dynamics perspective, free moments can be transferred proximally and result in loads on segments and joints such as the tibia and knee, (8) and rotational loads can strain ligaments around the knee joint. (12) Although the free moment has been studied as a factor associated with dance performance, (6) more research is warranted.

Country swing dance is a form of country-western dance, and the largest swing dance event ever was attended by 1,184 dancers in 2015. (13) There is a lack of kinematic or kinetic research in country swing dance and the typical shoe wear utilized. One unique characteristic of country swing dance is that dancers typically dance in "cowboy" or "cowgirl" boots, which create a unique shoe-floor interaction. Typical boots include leather-bottom or rubber-bottom varieties. Beginners may choose to practice swing dance with athletic shoes or barefoot. Previous studies have shown that various shoe-floor interactions are likely to demonstrate different coefficients of friction (14-17) and result in different lower extremity kinematics, kinetics, and performance during dynamic movements. (14,15,18) Quantifying how footwear with different coefficients of friction may affect free moments during rotational movements will provide evidence for future longitudinal studies to quantify how footwear selection may affect injury rates and performance in dance. As a step in that direction, the purpose of the current study was to quantify the coefficients of friction of different footwear and examine the effect of different footwear on peak and average free moments during a rotational movement in country swing dance. It was hypothesized that the footwear with the lowest coefficient of friction would result in the lowest peak and average free moments during the rotational movement.

Material and Methods

Participants

Seven male (age: 21.3 [+ or -] 1.0 years; height: 1.80 [+ or -] 0.03 m; mass: 74.5 [+ or -] 8.1 kg; dance experience: 2.8 [+ or -] 3.0 years; preferred shoe size: 10.1 [+ or -] 0.4) and 8 female (age: 20.0 [+ or -] 1.6 years; height: 1.66 [+ or -] 0.03 m; mass: 63.7 [+ or -] 5.9 kg; dance experience: 5.8 [+ or -] 5.0 years; preferred shoe size: 8.1 [+ or -] 0.5) country swing dancers participated in the current study. An individual was excluded from this study if he or she had not regularly participated in country swing dance for more than 3 months in his or her life, could not complete the rotational movement, could not fit the shoes provided for the study, had a history of major lower extremity injuries that required surgical treatment, had known osteoporosis or bone metabolism disorders, possessed any other conditions that would prevent him or her from participating in maximal effort activities, or was pregnant. The current study was approved by the University of Wyoming Institutional Review Board, and participants signed consent forms prior to participation.

Data Collection

Free Moments

The shoes (Fig. 1) included one pair each of male and female rubber-bottom boots (Justin Boots, Fort Worth, Texas), one pair each of male and female leather-bottom boots (Justin Boots, Fort Worth, Texas), and one pair each of male and female running shoes with rubber bottoms (Brooks Sports, Bothell, Washington). The size of male footwear was men's 10 (US size), and the size of female footwear was women's 8 (US size). Participants were allowed to wear multiple socks to maintain their preferred shoe tightness during data collection. Two force plates (FP4060-05-PT, Bertec Corp, Columbus, Ohio) were set up parallel to each other to collect three dimensional ground reaction forces, moments, and center of pressure data at a sample frequency of 1,000 Hz using Digital Acquire 4.12 software (Bertec Corp, Columbus, Ohio).

For the rotational movement (Fig. 2), participants started at rest with both feet pointing forward. They took one step onto the first force plate using the left foot and then took another step onto the second force plate with the right foot. Subsequently, participants pivoted approximately 360[degrees] on the right forefoot before stepping off of the second force plate and onto a central tape line. A metronome was set at 75 beats per minute, and participants made each foot contact with a force plate or the ground with a beat of the metronome. Participants had unlimited practice trials to ensure familiarity with the task. Three official trials were performed for each footwear condition: rubber-bottom boots, leather-bottom boots, running shoes, and barefoot. Any trial that did not meet the description of the rotational movement was discarded and repeated. The testing order of different footwear was randomized. Participants had a minimum of 30 seconds rest between each trial and were allowed to have longer rest as needed to avoid fatigue.

Coefficients of Static Friction

Prior to the beginning and after completion of data collection with all participants, coefficients of static friction were measured between the shoes and surface of the force plate using the maximum achievable incline method. (16) Only the shoes of the right side were tested because participants spun on their right forefoot during the rotational movement. During the measurement, a shoe was placed on a force plate and an investigator slowly lifted one end of the force plate until the shoe started to slide (Fig. 3). A weight was placed on the top of boots to ensure that only the forefoot of the bottom contacted the force plate since the materials of the heel were different from the sole. The angle between the surface of the force plate and the ground when the shoe started to slide was measured by another investigator using a goniometer. The coefficient of friction was calculated by taking the tangent of the angle. (16) Five trials were performed for each shoe.

Data Reduction

Ground reaction forces, moments, and center of pressure data (Figs. 2 and 4) were exported from the software and subsequently filtered using a fourth-order, zero-phase-shift Butterworth filter at a low-pass cut-off frequency of10 Hz. (9) The free moment (Figs. 2 and 4) was calculated using the following equation: Free moment = Mz + COPx * Fy - COPy * Fx where:

Mz = ground reaction moment around the z axis

COPx = center of pressure position along the x axis

COPy = center of pressure position along the y axis

Fx = ground reaction force along the x axis

Fy = ground reaction force along the y axis

The free moment was expressed as the frictional moment the ground applied to the dancer. For the left leg, the peak and average positive free moments (Fig. 2) that acted to generate the rotational movement were extracted from the first force plate. For the right leg, the peak and average negative free moments (Fig. 2) that acted to resist the rotational movement were extracted from the second force plate. Calculations were performed using subroutines developed in MATLAB[c] software version 2013a (MathWorks[R], Natick, Massachusetts).

Statistical Analysis

The statistical effect size for the comparison between the rubber-bottom boots and other footwear conditions was expected to be larger than 1 based on a pilot study. Assuming the effect size for a pairwise comparison was no less than 1, a sample size of 8 was needed for a type 1 error no greater than 0.05 and a power no less than 0.8. As such, the number of 15 participants was sufficient to meet the expected statistical power.

Data of three official trials for each footwear condition were averaged for analysis. Free moments were compared using repeated-measures analyses of variance (ANOVA), with the footwear condition as a within-participant factor. When the sphericity assumption in repeated-measures ANOVAs was violated, the Greenhouse-Geisser correction was performed. If an ANOVA showed a significant main effect, paired t-tests were performed between each pair of two footwear conditions. The Benjamini-Hochberg procedure was applied to all paired t-tests to control the study-wide false discovery rate to be 0.05. (19) Statistical analyses were performed using the IBM SPSS Statistics software version 22 (IBM, Armonk, New York).

Results

As shown in Figure 5, coefficients of static friction were greater for the rubber-bottom boots and running shoes than the leather-bottom boots for both males and females during both pre-test and post-test. Coefficients of static friction tended to increase from pre-test to post-test for male and female rubber-bottom boots, female leather-bottom boots, and male running shoes.

ANOVAs showed significant main effects (p [less than or equal to] 0.003) for peak free moments and average free moments for both left and right legs. Hence, paired t-tests were performed between each pair of conditions for all four dependent variables, for a total of 24 paired t-tests. After adjustment for the overall type 1 error rate, the largest p value for a significant paired t-test was 0.029.

For the left leg, paired t-tests demonstrated that both peak and average positive free moments were greater for the running shoe and rubber-bottom boot conditions than the barefoot and leather-bottom boot conditions (Fig. 6). For the right leg, paired t-tests demonstrated that both peak and average negative free moments were greatest for the running shoe condition, second greatest for the barefoot and rubber-bottom boot conditions, and least for the leather-bottom boot condition (Fig. 7).
Figure 6 Mean and standard deviations of peak and average positive free
moments for the left leg during the rotational movement for different
footwear conditions. The magnitude for Group A was significantly
greater than group B.

          Peak Free Moment (N.m)  Average Free Moment (N.m)

Barefoot  B
Running   A
Rubber    A
Leather   B

Note: Table made from bar graph.


Discussion

The purpose of the current study was to quantify the coefficients of friction of different footwear and examine the effect of different footwear on the peak and average free moments during a rotational movement in country swing dance. The findings support the authors' hypothesis since the leather-bottom boots, which demonstrated the lowest coefficient of friction, also resulted in the lowest peak and average free moments among the four footwear conditions.
Figure 7 Mean and standard deviations of peak and average negative free
moments for the right leg during the rotational movement for different
footwear conditions. The magnitude was the greatest for Group A, the
second greatest for Group B, and the least for Group C.

          Peak Free Moment (N.m)   Average Free Moment (N.m)

Barefoot  B
Running   A
Rubber    B
Leather   C

Note: Table made from bar graph.


Previous investigations have quantified different properties of footwear on dance biomechanics with a focus on landing forces, postural control, and foot pressure distribution. (1-5) These findings provide insight into shoe selection as a strategy to decrease loads imposed on dancers. In the current study, the free moments that acted to generate the rotational movement for the left leg and resist the rotational movement for the right leg were quantified. The peak free moments were generally below 20 Nm; the findings indicate that the leather-bottom boots may decrease twisting loads resulting from free moments.

Coefficients of static friction have been commonly used to characterize smoothness between two objects. A previous study has shown that a higher coefficient of friction between the shoe and floor results in lower extremity biomechanics associated with increased loads to the anterior cruciate ligament (ACL) during dynamic cutting tasks. (14) In this study, the coefficients of friction were measured by quantifying the horizontal pulling force and normal force when a shoe is moving on an even floor. The coefficient of friction between a disposable shoe cover and a laminate floor surface was 0.38, and the coefficient of friction between an athletic shoe and a rubber mat was 0.87. Morio et al. (17) utilized similar methods to quantify the coefficients of friction for 12 combinations of shoe-floor interfaces comprised of six different walking shoes on a bare force plate (steel) or a wooden floor. The coefficients of friction ranged from 0.69 to 1.46. Decreasing the coefficient of friction between the shoe and floor has been proposed to limit rotational loads applied at the knee. (20)

In the current study, the coefficient of static friction was measured by quantifying the maximum achievable angle, (16) and the coefficients of friction for the running shoes were within the range of data reported in previous studies. (14,17) The changes in peak and average free moments are generally consistent with the findings of coefficients of static friction, as the leather-bottom boot had the lowest coefficient of friction. It should be noted, however, that the coefficients of static friction were tested during linear motion and the free moments were measured during angular motion. While the rubber-bottom boots and running shoes had similar coefficients of static friction, the running shoes may be more resistant to angular motion. Another study has shown that footwear tread groove parameters, including groove orientation, width, and depth, could affect coefficients of friction. (21) Thus, the tread groove of the running shoes might have a greater rotational coefficient of friction, resulting in greater free moments. Different from quantifying free moments, which requires force plates and sophisticated calculations, measuring coefficients of static friction may provide a low-cost and convenient method for characterizing the shoe-floor property for shoe selection. Furthermore, even though the differences among shoes were consistent between pre-test and post-test, small increases in coefficients of friction were observed for several shoes, which were likely worn out during the data collection.

The findings may also provide implications for swing dance performance. Zaferiou et al. (6) quantified how angular impulse about the vertical axis was generated during single and double pique turns. Both free moments and ground reaction forces that acted away from the center of mass contributed to the generation of angular impulse. Meanwhile, as the demand of angular impulse increased from single to double turns, the increased moment was primarily due to the changes of direction and magnitudes of the ground reaction forces for both pushing and turning legs. (6) In the current study, the magnitudes of peak free moments were similar to those reported by Zaferiou et al. (6) The positive free moments applied to the left foot acted together with ground reaction forces to generate the angular impulse, and the negative free moments applied to the right foot primarily acted to decrease the angular impulse. The decreased performance demand of generating angular momentum from free moments for the leather-bottom boots may help transitions between the rotational movement and subsequent movements. On the other hand, sufficient friction and free moments are needed for timely initiation and cessation of movements, especially during activities that involve sudden acceleration and deceleration. Coefficients of friction and free moments that are too low may cause a slippery shoe-floor interaction, resulting in increased risk of fall (22) and decreased quality of performance. (15)

Pedroza et al. (15) quantified the time participants needed to complete an agility task under different coefficient of friction conditions. The various shoe-floor interactions were created by using different combinations of linoleum tile, vinyl flooring, wax, and soles. It was found that a coefficient of friction less than 0.5 was associated with increased time to complete the task, highlighting the importance of adequate coefficient of friction for performance. As running and sidestepping are commonly performed in dance, sufficient coefficients of friction and free moments are needed to prevent falls and maintain dance performance. The most effective coefficients of friction and resistance to rotational movements are likely dependent on specific dance movements, dance expertise, and the goal of training and performance.

The current study had several limitations. First, only one rotational movement during country swing dance was studied. Further studies may include a variety of dance movements. Second, linear coefficients of friction were measured between shoes and force plates but not between shoes and actual dance floors. The magnitudes of coefficients of friction may be different for dance flooring. In addition, quantifying angular coefficients of friction may be more relevant for assessing free moments during rotational movements. Third, only footwear was manipulated in the current study. Future studies may investigate the effect of both footwear and platform surfaces on free moments. Fourth, shoes were likely worn out during the data collection, which may affect free moments during dynamic tasks. Future studies are needed to quantify how different levels of wear may affect free moments and dance performance. Fifth, dancers with different body mass, height, and dance experience were included. Although a within-participant design was utilized, individual differences may affect the influence of footwear on dance performance. Last, the current study quantified the changes in free moments, which is only one mechanism that may load the lower extremities. Future longitudinal studies are warranted to investigate the potential cause-effect relationships among footwear, injury rates, and dance performance.

Conclusion

Leather-bottom boots had the lowest coefficient of static friction and resulted in the lowest peak and average free moments during a rotational movement in country swing dance compared with rubber-bottom boots, running shoes, and barefoot. The leather-bottom boots may decrease twisting loads that result from free moments. Coefficients of friction and free moments may be considered for future longitudinal studies to investigate the cause-effect relationships among footwear, injury potential, and dance performance.

References

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Hunter D. Perala, BS, Margaret A. Wilson, PhD, and Boyi Dai, PhD

Hunter D. Perala, BS, and Boyi Dai, PhD, Division of Kinesiology and Health, University of Wyoming, Laramie, Wyoming, USA. Margaret A. Wilson, PhD, Department of Theatre & Dance, University of Wyoming, Laramie, Wyoming, USA.

Correspondence: Boyi Dai, PhD, Division of Kinesiology and Health, University of Wyoming, Laramie, Wyoming 82071, USA; bdai@uwyo.edu.

Caption: Figure 1 Female rubber-bottom boots (top left), male rubber-bottom boots (bottom left), female leather-bottom boots (top middle), male leather-bottom boots (bottom middle), female running shoes (top right), male running shoe (bottom right). Pictures were taken after the completion of data collection of all participants.

Caption: Figure 2 The rotational movement (top) and corresponding vertical forces (bottom-left) and free moments (bottom-right) from the left-foot (force plate 1) contact to the right-foot (force plate 2) takeoff.

Caption: Figure 3 Measuring coefficients of static friction for a rubber-bottom boot (left), a leather-bottom boot (middle), and a running shoe (right) by quantifying the maximum achievable angle ([??]).

Caption: Figure 4 Positive directions of ground reaction forces, moments, center of pressure, and free moment. FM: free moment; Mz: ground reaction moment around the z axis; COPx and COPy: center of pressure position along the x and y axes; Fx, Fy, and Fz: ground reaction force along the x, y, and z axes.

Caption: Figure 5 Mean and standard deviations of coefficients of static friction of different shoes during pre-test and post-test.

doi.org/10.12678/1089-313X.22.2.84

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Author:Perala, Hunter D.; Wilson, Margaret A.; Dai, Boyi
Publication:Journal of Dance Medicine & Science
Article Type:Report
Date:Apr 1, 2018
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