Control and dynamics of turning tasks with different rotation and translation requirements

Turning is fundamental to locomotion, yet, not fully understood from a control and dynamics perspective. Without advances in our current understanding, clinical populations may continue to struggle to change direction while walking and bipedal robots will fail to execute turns as performed by humans. This work focused on identifying the mechanical strategies dancers used to satisfy multiple mechanical objectives during fundamental dance turns. The overarching goal of this research was to understand how dancers effectively control their bodies to interact with their environment during turns requiring varied amounts of rotation and translation. With this knowledge, training procedures for dancers can be improved through more specific cuing from dance teachers or through development of interactive technology to provide feedback. While there may be fundamental differences in the capabilities of clinical populations compared to skilled ballet and contemporary dancers, by studying solutions experts use under progressively more challenging conditions, insight is gained regarding control priorities that can be mapped to help other populations. ❧ During turns, each leg plays a distinct role in generating the reaction forces required to control both linear and angular momentum of the body. The reaction force regulation over time, generates linear and angular impulse needed for the desired movement. However, angular impulse about the body's center of mass involves reaction force components that are not aligned with the desired center of mass trajectory, which imposes conflicts in control priorities. Through coordination of each leg’s reaction force generation, the aforementioned conflicts can be appeased. ❧ Turns performed by dancers provided a unique opportunity to understand how skilled individuals successfully satisfy multiple mechanical objectives. Specifically, “piqué” and “pirouette” turns with inherent variations of rotation and translation were studied to determine how dancers satisfy linear and angular momentum requirements, while maintaining balance. Each turn serves as a well-practiced task that can be performed with increased rotational requirements (e.g. single, double). Additionally, both turns impose a specific postural challenge during the turn phase: the dancer must spin while supported by a single leg (extended knee and plantarflexed ankle) and a very small base (the forefoot). The differences in how pirouette and piqué turns are terminated also provided a systematic comparison between balance maintenance during a translating turn (piqué) and a “turn-and-stop” displacement turn (pirouette). By using a within-subject experimental design, control preferences of an individual across tasks were identified, which may be used in the design of personalized feedback to improve performance. ❧ In this series of studies, skilled dancers performed single and double piqué and pirouette turns while each leg was supported by a forceplate. During each turn, reaction forces were measured using dual forceplates (1200 Hz) while three dimensional kinematics were simultaneously captured (100 Hz). At the whole-body level, linear and angular impulse generated by the push and turn legs during turn initiation and contributing factors to impulse generation (resultant horizontal reaction force, sinθ, position vector magnitude) were quantified and compared across the group and within a dancer (α = .05). During the turn phase, balance regulation was compared between piqué and pirouette turns and between single and double turns across the group and within a dancer. At the segment-level, the contributions at the ankle, knee, and hip during impulse generation were compared between pirouette turns initiated with and without hip external rotation. ❧ Modification of Impulse Generation during Piqué Turns with Increased Rotational Demands: The purpose of this study was to determine how skilled dancers (n=10) used the push and turn leg to regulate angular and linear impulse generated during the initiation phase of piqué turns performed with increased rotational demands. During the initiation of a piqué turn, a dancer interacts with the environment to generate lateral and angular impulse in order to satisfy the lateral and rotational momentum requirements. The piqué turn is initiated with single leg support from the “push leg” and the turn phase occurs with single leg support from the “turn leg”. The turn initiation phase ends as the push leg departs the ground, shortly after the turn leg makes initial contact with the ground. ❧ Results indicate that in order to increase the amount of body rotation from the performance of a single piqué turn to a double piqué turn, dancers tended to increase the net angular impulse generated and decrease the mean lateral impulse generated. The push leg contributed more angular and lateral impulse compared to the turn leg in both single and double turns. However, all dancers studied increased angular impulse generated by the turn leg, despite its contact with an extended knee, plantarflexed ankle, and contact only with the forefoot during a short duration (~0.2 s). As number of turns performed increased, dancers increased the push leg’s mean anterior impulse generation and the turn leg’s mean posterior impulse generation, consistent with mechanisms to increase angular impulse. The push leg tended to increase the mean moment applied about the center of mass via redirecting the reaction force, whereas the turn leg used combinations of modulation of the reaction force (magnitude and direction) and increased position vector length (either via decreased center of mass velocity towards the turn leg, and/or by contacting the ground earlier with the turn leg as the center of mass was approaching the turn leg’s ground contact). Control strategies used by dancers during progressively more challenging turning tasks tended to be subject-specific. By coordinating the generation of reaction forces between legs, the net horizontal impulse in the anterior/posterior direction remained minimal, despite impulse regulation used to achieve increased rotational demands. ❧ Modification of Impulse Generation during Pirouette Turns with Increased Rotational Demands: The purpose of this study was to determine how skilled dancers (n=11) used the push and turn leg to regulate angular and linear impulse generated during the initiation phase of pirouette turns performed with increased rotational demands. During the initiation of a pirouette turn, a dancer generates linear and angular impulse in order to satisfy the linear center of mass displacement and angular momentum requirements. The pirouette turn is initiated with double leg support from both the push leg and turn leg. The turn initiation phase ended as the push leg departed from the ground. ❧ Results indicate that as rotational demands increased from performance of a single pirouette turn to a double pirouette turn, dancers increased the mean net angular impulse generated and maintained minimal linear impulse generated towards the turn leg’s base of support. The contribution of each leg to net angular impulse in both single and double pirouettes was linked with stance configuration strategies. While the center of mass horizontal position resides somewhere within the base of support initially, stylistic differences between schools of ballet provided context for some range across subjects regarding the initial center of mass positioning closer to the turn leg or to the push leg. Dancers who initiated the turn with a larger position vector between the center of mass and the turn leg’s base of support generated more mean angular impulse with the turn leg during both single and double pirouettes. Conversely, dancers who initiated the turn with the center of mass closer to the turn leg, generated more angular impulse with the push leg in both single and double turns. However, all dancers studied generated more linear impulse in the direction of the turn leg’s base of support with the push leg than they did with the turn leg. Overall, dancers increased the horizontal reaction force magnitude at one or both legs in order to increase the moment applied about the center of mass. The push leg also tended to increase the mean moment applied about the center of mass via redirecting the reaction force. As was the case during piqué turn initiation, by coordinating the generation of reaction forces between legs, changes in the mean net horizontal impulse perpendicular to the desired center of mass trajectory remained minimal, despite impulse regulation at each leg used to achieve increased rotational demands. ❧ Whole-Body Balance Regulation during the Turn Phase of Piqué and Pirouette Turns: The purpose of this study was to compare balance regulation strategies used by skilled dancers (n=10) during the turn phase between piqué and pirouette turns and between single and double turns. After the initiation of both piqué and pirouette turns, the turn phase required whole body rotation while maintaining balance in a challenging kinematic context. However, differences in how the pirouette and piqué turn is terminated provided a systematic comparison between balance maintenance during a translating turn (piqué) and a “turn-and-stop” or minimal displacement turn (pirouette). As rotational demands increase, it was expected that turns with multiple rotations impose a need to position the CM over the base of support for longer periods of time. Maintaining the CM position over the base of support for a longer period was expected to require more corrective actions that involve regulation of the reaction forces relative to the center of mass. ❧ Comparisons between the piqué and pirouette, for both single and double turns, revealed that the center of mass during the pirouette was more vertically aligned than was the center of mass during the piqué turn. Throughout the turn phase of a pirouette, the dancers’ center of mass was aligned within 15° from vertical (the mean alignment for both single and double pirouettes was less than 5° for all subjects, n=10). As rotational demand increased in both turns, the ground reaction forces were regulated relative to the CM. By controlling the braking force and moment applied about the CM the potential for the CM to overshoot the horizontal positon of the base of support in the primary direction of travel was limited. ❧ Lower Extremity Control during Turns Initiated with and without Hip External Rotation: The purpose of this study was to compare impulse generation strategies used by skilled dancers (n=5) at each leg and the associated muscle recruitment strategies during the turn initiation phase of pirouettes performed with and without hip external rotation. Classical ballet turns are typically initiated and performed with the hips externally rotated, whereas, more modern choreography requires dancers to also be proficient in performing tasks with neutral hip alignment. This provided an opportunity to investigate how dancers satisfy the same mechanical objectives at the total body level when generating ground reaction forces using different leg kinematics. In addition to comparing impulse generation strategies, activation of superficial hip muscles were monitored using surface electromyography (1200 Hz) and joint kinetics during turn initiation were determined for both legs using measured ground reaction forces, 3D segment kinematics, and body segment parameters. ❧ Results indicate that differences in impulse generation between turn conditions were consistent with initial stance configuration. Despite differences in ground reaction force orientations between turn conditions, on average, at least 90% of the ground reaction force was aligned with the respective leg plane for both turn conditions. In addition, a majority of the net joint moment at the ankle, knee, and hip acted about an axis perpendicular to the leg plane. However, differences in tibia segment alignment relative to the leg plane affected the distribution of the knee net joint moment when represented with respect to the tibia versus the thigh reference system. During both turn types, most participants used primarily extensor moments at the ankle and knee, flexor and abductor moment at the push leg’s hip, and extensor and abductor moments at the turn leg’s hip. While muscle recruitment patterns were subject-specific, they were consistent with the knee and hip joint moments across turn conditions. ❧ The technique-related findings of this series of studies indicated that dancers coordinated both legs to regulate the reaction forces during the initiation and turn phases of piqué and pirouette turns. Dancers used subject-specific mechanisms to increase angular impulse generated during turn initiation. However, at the segment-level, during the initiation of pirouettes with varied initial hip kinematics, dancers tended to align their lower extremities with each reaction force in a way that simplified control at the hip across tasks. These whole-body and segment-level results can be used to inform the design of customized feedback technologies that aim to facilitate skill acquisition and improve dance performance.