Conflicting Mechanical Objectives during Turns: During turns, each leg plays a distinct role in generating the 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 are appeased.
To the right, you can click through videos and images to walk-through a turn's conflicting mechanical goals and an exemplar dancer's strategy to successfully accomplish these goals. (Graphics were generated using custom MATLAB software to display multiple reaction force vectors- dark green to light green- with each corresponding video frame.)
Model System: Turns performed by dancers provided a unique opportunity to understand how skilled individuals successfully satisfy competing mechanical objectives. I have determined how dancers satisfy the momentum and balance requirements of fundamental dance turns (Pique and Pirouette turns) with inherent variations of rotation and translation. Each served as a well-practiced turn that can be performed with increased rotational requirements (e.g. single, double, triple). Additionally, both turns imposed a challenge during the turn phase: the dancer must spin with single-limb support on an extended leg (extended knee and plantarflexed ankle), while supported by a very small base. The pique turn requires lateral body translation and is initiated using sequential foot contacts, whereas, the pirouette requires minimal body displacement and is initiated using double-limb support. The differences between turns affords a systematic investigation of varied mechanical goals. By using a within-subject experimental design, I was able to identify an individual’s control preferences across tasks, which can be used in the design of personalized feedback to improve performance.
Overview of Results: As rotational requirements increase in both turns, dancers generated more angular impulse (integral of moment applied over time) while maintaining linear impulse (integral of reaction force over time) generated during the turn initiation phases. Despite changes in reaction forces to increase angular impulse generation, each leg was coordinated such that the total linear impulse generated maintained alignment with the desired center of mass trajectory. Subject-specific strategies were used to increase angular impulse, including regulation of reaction force magnitude, direction, position vector length, and/or duration of moment applied.
Subject-specific strategies identified encourage the use of customized feedback to an individual performer, given the performer's preferred solution spaces, rather than a "one-size-fits-all" approach to training interventions.
Zaferiou, A.M., Flashner, H., Wilcox, R.R., McNitt-Gray, J.L. (in press). Lower Extremity Control during Turns Initiated With and Without Hip External Rotation, Journal of Biomechanics. doi: 10.1016/j.jbiomech.2016.12.017
Zaferiou, A.M., Wilcox, R.R., McNitt-Gray, J.L. (2016). Modification of Impulse Generation during Piqué Turns with Increased Rotational Demands, Human Movement Science 47: 220-230. doi:10.1016/j.humov.2016.03.012
Zaferiou, A.M., Wilcox, R.R., McNitt-Gray, J.L. (2016). Modification of Impulse Generation during Pirouette Turns with Increased Rotational Demands, Journal of Applied Biomechanics 32(5):425-32. doi:10.1123/jab.2015-0314
Zaferiou, A.M. and McNitt-Gray, J.L. (2016). Whole-Body Balance Regulation during the Turn Phase of Piqué and Pirouette Turns of Varied Rotational Demands, Medical Problems of Performing Artists 1. 31(2): 96-103. doi:10.21091/mppa.2016.2017
TURNING WHILE WALKING
During turning-while-walking, the trajectory of the body's center of mass is controlled relative to the base of support via ground reaction forces generated. These forces must be coordinated during foot contacts such that the linear and angular momentum requirements of the task are satisfied while maintaining balance. Achieving these mechanical objectives during turning-while-walking can be challenging to older adult fallers, as indicated by the high incidence of falls and serious injuries. As turn speed increases, the mechanical demands required to change both linear and angular momentum increases, which increases task difficulty, particularly for older adult fallers.
As a Research Scientist at the VA of Greater Los Angeles during my studies at USC, I determined the role of each leg during turning-while-walking tasks performed by older-adult fallers. I also performed a model validation study assessing joint degree of freedom requirements and body segment parameter selection.