*2.1. Subject*

An age-homogeneous sample of 102 university students (54 females and 48 males) was recruited. Mean age was 21.08 ± 1.08 years, height 172.89 ± 9.56 cm and body mass 68.09 ± 13.12 kg. All individuals provided their written informed consent to participate in the study and ethical approval was obtained. All procedures were performed at a biomechanics research laboratory at the Opole University of Technology in Opole, Poland. The participants signed informed consent and were informed of the protocol and procedures for the experiment prior to the exercise. The study was approved by the Human Ethics Committee.

#### *2.2. Ground Reaction Force (GRF) Measurement Procedure*

The study protocol involved measuring ground reaction force during two 45-s trials of static standing. Ground reaction force (GRF) data were synchronously collected on two 600 × 400 mm piezoelectric force platforms (Kistler Type 9286B; Kistler Instruments AG, Winterthur, Switzerland) placed under each foot. Four tri-axial force sensors located in the corners of each platform quantified the ground reaction force signals at a sampling frequency of 50 Hz (measurement range was from 10 kN to 20 kN). The force platforms were calibrated before use and integrated with a base transceiver station (BTS) Smart optoelectronic system (BTS Bioengineering, USA) to register the force–time characteristics. During signal acquisition, the participant was asked to assume a relaxed upright posture (minimizing head and trunk movements) with the upper extremities resting freely against the trunk and to fixate on a point placed at eye level 3 m from the subject. Only running shoes were allowed, and the participant stood with their feet completely parallel (no ankle rotation with a 30 cm stance width).

#### *2.3. COP Measurement*

GRF signals were recorded 10 s after trial commencement for 45 s. The two trials were executed one after the other with no change in foot position. The Bioware software automatically calculated *x*and *y*-axis COP location (in mm) separately for right and left foot using the equations:

$$\mathbf{Ax} = (\mathbf{Fx}^\* \mathbf{a} \mathbf{z} 0 - \mathbf{M} \mathbf{y}) \text{/Fz} = -\mathbf{M} \mathbf{y}' / \mathbf{Fz} \tag{1}$$

where:

> Ax path of the lateral direction of the force component


$$\mathbf{A}\mathbf{y} = (\mathbf{F}\mathbf{y}^\star \mathbf{a}\mathbf{z}0 + \mathbf{M}\mathbf{x})\mathbf{/}\mathbf{F}\mathbf{z} = \mathbf{M}\mathbf{x}'\mathbf{/}\mathbf{F}\mathbf{z},\tag{2}$$

where:

> Ay way of action of force component back and forth


In order to aid the quantification of COP trajectory as a function of time, the initial COP location was shifted to the mean COP location (calculated for each trial).

A mathematical coordinate system was used to present the test results. The *x*- and *y*-axes show the direction of action of ground reaction forces caused by foot pressure on the ground. This is called phase plane (ground). The *x*-axis represents the action of these forces in the left and right directions. The *y*-axis represents the action of these forces in the forward and reverse directions. In the context of these axes, anterior foot pressure means that the subject transfers weight to the toes and hind toes. There are people who transfer weight to the foot in opposite directions: outside, to the left foot to the left and to the right foot to the right; or inwards, to the left foot to the right and to the right foot to the left. Hence the term "mid-lateral".

The COP data (2250 measures per trial) were then plotted as a statokinesigram for COP spatial trajectory in the mediolateral and anteroposterior directions as well as a stabilogram showing the temporal domain of COP in both directions. An example of the COP trajectory as a stabilogram and statokinesigram is illustrated in Figures 1 and 2, respectively.

**Figure 1.** Exemplary stabilogram of center of pressure (COP) time series data in the mediolateral (Ax) and anteroposterior (Ay) directions.

**Figure 2.** Exemplary statokinesigram of COP spatial data in the mediolateral (Ax) and anteroposterior (Ay) direction.

The selection of a right COP shift trajectory of the right and left foot may illustrate the motor abilities of the subject. The human may determine and choose a right COP displacement trajectory for the right or left foot. Namely, the human may decide on part of a foot which will put pressure on the surface.

#### *2.4. Statistical Analysis*

Basic descriptive statistics were calculated (means ± standard deviations). The Shapiro–Wilk test was used to determine if the data set was well-modeled by a normal distribution. Differences between the obtained values were assessed using Student's *t*-test. The level of symmetry or asymmetry in COP displacement trajectory between the right and left foot in both the mediolateral and anteroposterior directions was assessed with Pearson's correlation coefficients. Correlations between right and left foot COP trajectory were independently calculated in the mediolateral and anteroposterior directions for each participant. Additionally, the correlation coefficients of COP trajectory between the mediolateral and anteroposterior directions were also calculated independently for the right and left foot. The level of significance was set at α = 0.05. All data processing was performed with the Statistica 10.0 software package.
