Translating the Base of Support a Mechanism for Balance Maintenance during Rotations in Dance

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Abstract
Few studies have investigated how balance is maintained while the body rotates in an upright posture. Under the basic definition of balance, the body's center of mass (CoM) lies along a vertical line over the base of support (BoS); therefore, balance can be regained by moving the position of the CoM or the BoS. In a ballet pirouette, extreme movement of the BoS is aesthetically unacceptable; however, subtler BoS translations may be a viable strategy for balance maintenance. The results of this study suggest that translation of the BoS is not only utilized but correlated with a larger number of revolutions, n, in a pirouette. Although increasing n is not the primary goal of the pirouette, scientific understanding of how dancers maintain balance while performing large n turns can help dance educators teach safer, more effective pirouettes of any n. To investigate the relationship between the number of revolutions completed in a pirouette and 1. physical properties of the dancers, 2. how the pirouettes were initiated, 3. maximum deviations from equilibrium, and 4. movement of the BoS, this study determined whether n was correlated with several relevant biomechanical parameters. Motion analysis of 11 skilled female dancers performing standard ballet pirouettes determined CoM and BoS kinematics. Six variables were computed from these data: initial topple angle from vertical (4.5° ± 2.0°), initial margin of stability (-4.4 ± 2.8 cm), initial spin rate (2.19 ± 0.37 rev/s), maximum topple angle (9.8° ± 6.8°), normalized BoS distance traveled per revolution (7.3% ± 3.3%), and dancer effective pendulum length (1.172 ± 0.023 m). A significant positive correlation was found between n and normalized BoS distance traveled per revolution (r = .873, p < 0.001). No significant correlations were found between n and any of the other five variables. When the foot rotates against the ground, as in a pirouette, friction is kinetic (lower coefficient of friction than no relative motion between surfaces), and body manipulations for balance maintenance lead to greater likelihood of BoS translation. These results suggest that dancers should be encouraged to make adjustments that could lead to subtle BoS translations during rotations. Given this new information, more research is needed to determine the best practices for teaching balance maintenance during rotations in dance.

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