**1. Introduction**

The force exerted by the body on the ground when standing is mirrored by a reaction force. The use of footwear during many activities of daily living not only provides a level of protection but also modifies the pressure distribution characteristics of the feet and, therefore, the forces that act on the foot. Depending on the type of footwear, these forces may be attenuated or "dampened". Additionally, the shape and construction of the sole, insole and heel may all modify the forces and load experienced by different foot regions [1–3].

From a mechanical perspective, balance preservation during upright standing is quite complex as the human body is never in a condition of perfect equilibrium and that balance must be maintained via two points of contact (both feet). The center of all external forces acting on the plantar surface of the foot is known as the center of pressure (COP). While providing a base of support, the feet can independently induce changes in COP trajectory in the coronal (anteroposterior direction) and sagittal (mediolateral direction) planes and, therefore, obfuscate right and left COP to create a condition of asymmetry. While COP displacement in the anteroposterior direction can be understood as bilaterally symmetrical when the trajectory (forward or backward) is concurrently equal between both feet, the level of bilateral symmetry in the mediolateral direction is far more problematic to measure. One possible method for assessing bilateral symmetry in mediolateral COP trajectory is by considering foot structure. Albeit an oversimplification, this paradigm allows us to treat COP displacement as symmetrical when left and right foot mediolateral COP both shift either to the inside or outside of the feet.

The literature is abound with studies investigating balance preservation during quiet standing primarily by analyzing COP-related variables. However, the vast majority is based on using a single force plate and thus measure the exerted force concurrently for both feet [4–10]. Few investigations have addressed the magnitude and distribution of force separately for the right and left foot with the use of more than one force platform. Although Soangra and Lockhart [11] and Brauer et al. [12] investigated the similarities and dissimilarities between right and left foot COP trajectories, no studies have ye<sup>t</sup> addressed COP displacement in regards to the level of symmetry (or asymmetry) between the left and right foot. As COP has been identified as a measure of the neuromuscular response to maintain balance, di fferences in right and left foot COP trajectories can serve as a measure of sensorimotor control and function. In this way, the respective COP trajectories for either foot and the congruence between both points of application signify the ability of the central nervous system to integrate information from the sensory systems and then activate di fferent postural muscles (exerting pressure at a specific foot region) so that upright stance is preserved.

While the foot can be assumed to hold two degrees of freedom relative to the lateral gastrocnemius (plantarflexion/dorsiflexion and inversion/eversion), COP displacement can independently shift not only in the anteroposterior and mediolateral directions but also combinations of the two, such as an anterior or posterior slant to either the medial or lateral side. If analyzed as a temporal series, this bivariate approach for each foot could help identify patterns in left and right foot COP trajectory. Knowledge of COP displacement between both feet across di fferent population cohorts can help identify postural pathologies including foot deformities resulting from improper footwear or decreased neuromuscular control by some deficiency in central nervous system function.

Therefore, the purpose of the study was to define the incidence of right and left foot symmetry or asymmetry via displacement in COP trajectory by identifying what associations exist between left and right foot COP direction in an asymptomatic population. By knowing the value of the symmetry between left and right foot COP displacement, it could be possible to formulate criteria for evaluating postural balance during upright standing. We hypothesized that the temporal and spatial characteristics of right and left foot COP trajectories during upright stance would show little variability in a sample of healthy young adults.

#### **2. Material and Methods**
