Specificity of foot configuration during bipedal stance in ballet dancers

Highlights

• Benefits from ballet training were tested for stances with different foot positions.

• Dancers had benefits for a familiar foot position, regardless the stance difficulty.

• Specificity regarded both the stability and the structure of the postural control.

Abstract

Background and Aim

Learning highly specialized upright postures may be of benefit for more common as well as for novel stances. In this study, we asked whether this generalization occurs with foot configurations previously trained or depends on a generic increase in balance difficulty. We also explored the possibility that the benefit may concern not only the level of postural performance but also the structural organization of the upright standing.

Methods

Ten elite professional ballet dancers were compared to ten untrained subjects, measuring the motion of the center of pressure (COP) across a set of five stances with different foot configurations. The balance stability was measured computing the area, the sway path, and the root mean square of the COP motion, whereas the structure of the postural control was assessed by compute approximate entropy, fractal dimension and the mean power frequency. The foot position included common and challenging stances, with the level of difficulty changed across the configurations. Among these conditions, only one foot configuration was familiar to the dancers.

Results

Statistically significant differences between the two groups, for all the parameters, were observed only for the stance with the foot position familiar to the dancers. Stability and structural parameters exhibited comparable differences.

Conclusions

We concluded that the benefit from classical ballet is limited to a specific foot configuration, regardless of the level of stance difficulty or the component of postural control.

Introduction

Most of the everyday actions, such as walking or reaching, require an accurate spatio-temporal control of the standing posture. Generally, the nervous system elaborates the postural commands based on the planning of upcoming movements [1], [2]. Thus, the formation of the postural orders to modulate the upright stance is functionally linked to the specificity and complexity of the action execution. This association is important for those sports or artistic activities, such as gymnasts or ballet, where the control of body orientation and equilibrium are critical for the performance optimization. A debated topic is whether the specific postural training, required for specific movements, can be of benefit for the standing control during common stances and/or in novel challenging postures.

Several authors who addressed this question, found that subjects practicing activities with a specific demand of postural ability exhibited improvements in stance stability mainly in contexts for which their practice was related to [3], [4], [5], [6]. For example, ballet dancers perform better balance control than untrained subjects only for specific foot configurations associated with the ballet training, such as point and demi-pointe [3].

However, in these studies two important elements were overlooked. First, the most challenging postures were also the most specific, whereby it was problematic to distinguish the effects of the task specificity from a generic influence of the postural difficulty. Second, the standing performance was evaluated with respect to the level of stability, neglecting a possible generalization of the strategic components of the postural control, i.e. the ways in which the upright balance is maintained.

To deal with the first point, we compared ballet dancers and untrained subjects using a set of five foot configurations with increasing level of postural instability. The two most difficult postures included a familiar stance for the dancers but not for the no-dancers, and an unfamiliar stance for both the groups. The hypothesis that the benefit from a previous training is limited to specific postures would be validated if the differences between the two groups will occur only for the foot configuration familiar to the dancers. Conversely, if only the dancers will show a performance improvement parallel to the increase of postural challenge, it will mean that the level of standing difficulty contributes to the postural ability generalization, regardless of the task specificity.

To further explore the possibility of generalization of postural skills learned by ballet dancers, the upright performance was evaluated not only measuring the stance stability, but also using parameters regarding the dynamic structure of the motion of the center of pressure (COP). Thus, we provided data on the frequency spectrum, the temporal, and the spatial dynamic of the sway oscillations, making clearer and more complete the information on the postural benefit associated with the ballet.