Group differences in postural balance skills
The present study indicates that there are significant differences in the COP parameters between the three following samples: athletes (EA), football players (FP), and control sample (CS). The FP group demonstrated significantly higher body variability in the ML direction under visual control conditions than EA and CS. Entropy in this direction was significantly higher in the CS group in comparison to EA and FP, and also in the AE group as compared to FP. However, in the AP direction, the FP showed a higher speed of body sway than persons from the CS group. An analysis of changes in COP without visual inspection showed that the body sway of the FP group was higher than in the EA sample, with regards to the variability of COP. In contrast, entropy was significantly higher in the CS and AE group, as compared to FP. This may indicate a specific adaptation of posture control among FP, developing consistently with the requirements of the sport (movement and other motor activities on the pitch). Our research is consistent with Paillard’s previous findings and also showed a greater dependence of posture control on eyesight in elite track and field athletes and football players than in the control group (CS) .
Consistent with previous research , balance skills are not related to gender. Body posture control is an individual feature depending on body build, age, and level of training. Numerous studies show that balance-oriented exercises can improve adaptive posture control and that, the higher the level of competition, the more stable the posture in soccer [13,17,50,58]. The postural regulation of subjects with a better sports performance level was less disturbed by sensorial manipulation than that of sportsmen with a lower level of performance. There is a close relationship between the level at which the sport is played and the effectiveness of postural regulation . Barone et al. showed that, in soccer, the higher the level of competition, the more stable the posture and the less visual information required for postural maintenance .
COP parameter values testify to global coordination abilities concerning the equilibrium system . Higher COP variability may indicate various regulatory mechanisms manifesting in the ability to maintain balance by triggering random movements. The test results of Yamada et al.  indicated that, in football players, along with the increase in sports advancement, the instability in the AP plane increases. Other studies  indicate that specific training contributes to a decrease in the variability of body excretions. COP parameter values testify to global coordination abilities being related to the equilibrium system . Higher COP variability may indicate various regulatory mechanisms, manifesting in the ability to maintain balance by triggering random movements.
Our findings differ from Biec and Kuczyński , who stated that footballers have better body balance control compared to nonathletes. The difference in our results may mean that the age of the respondents, and thus the level of training, can explain the differences and dependencies in the changes in COP values. Our research is consistent and supports the data presented by Wojciechowska-Maszkowska et al. . The other studies indicate that football players and gymnasts are not very different in terms of static or dynamic control .
Group differences in personality and temperament
Surprisingly, neither personality nor temperament traits differ significantly between the EA, FP, and CS samples in this study, except for agreeableness, as was demonstrated by the results of the one-way ANOVA. A recent study  indicated that academic team sports players scored significantly lower for neuroticism, openness, and agreeableness, and higher for extroversion than students studying for a physical education major. This study seems partially consistent since agreeableness was significantly lower in both EA and FP samples when compared to the CS group (which includes physiotherapy students). The other study  compared personality traits between individual and team sport athletes. Higher agreeableness in team sports players was found when compared to individual sports samples. Similar to the present study, no two groups of athletes were the same for neuroticism, extraversion, conscientiousness, and openness. On the other hand, Talyabee et al.  showed that nonathletes scored lower than an academic sample of athletes for agreeableness, extroversion, and conscientiousness, and higher for neuroticism. Agreeableness seems to be a very valuable trait in team sports. A systematic review  indicates that sport participants with high levels of agreeableness report more favorable relationships with their teammates and coaches. Highly agreeable athletes also demonstrate less team-related conflict and better cooperation and relationships with other team members . The role in the team (i.e., leader, star) may also determine the usefulness of higher or lower agreeableness, as Kim et al. indicated . It should be said that high inconsistency exists between studies, which may be determined by cultural differences, or maybe the specific sport discipline.
Nevertheless, the present study indicates that an essential trait that differs from physical education students from professional athletes is agreeableness. People who score high on agreeableness are perceived as empathetic and altruistic, while a low agreeableness score relates to selfish behavior and a tendency to compete with others rather than cooperate . Because sporting is fundamentally related to competition, a lower level of agreeableness may be a beneficial trait in sports achievement . Thus, lower agreeableness may be related to frequent participation in competitions, a high level of aspiration, and a need for achievement.
However, it is important to note that the above interpretation may be false, since the results of the two-way ANOVA did not confirm the above group differences. Instead, gender differences were found, which may explain the previously mentioned group differences. In the CS women dominated, and scored higher on agreeableness in this study. Furthermore, gender was a significant predictor of agreeableness, explaining approximately 20% of its variance. Moreover, gender differences were found in openness to experience. The higher levels of agreeableness and openness among women, when compared to men, are consistent with previous studies about individual differences in personality [23,24]. On the other hand, the present study did not find significant sex differences for neuroticism, extroversion, and conscientiousness, as well as for all temperamental traits, which is not consistent with previously reported studies [23,24,35]. This result may be caused by the specifics of the participants in the study, not the large sample size.
Finally, the two-way ANOVA also showed that the FP sample reported lower scores in endurance as a temperamental trait than the EA and CS groups. Endurance manifests in the tendency to react adequately in situations that require long-term activity or intense behavior. This result is not consistent with the previous study, which did not show any significant differences between team and individual sports in terms of temperamental traits . Independent of the sports discipline, athletes should be characterized by a generally high level of endurance. However, the differences found in this study were small, so it may be a random effect of a small sample size.
Association of postural balance with traits of personality and temperament
The main goal of the study was to look for relationships between COP indicators and temperament and personality traits. The novelty of this study was the combination of biomechanical and psychological data for understanding postural behavior. Although a series of correlations was found between select COP parameters and traits of personality and temperament, the associations were weak in the present study and may not be considered sufficient, given multiple comparisons. Indeed, a series of regression analyses did not confirm the previous relationships. Surprisingly, among all personality and temperamental traits, only emotional reactivity and openness demonstrated a significant association with the selected COP parameters, but in the ML plane and EO condition exclusively. The predictors of emotional reactivity were high SD and low RA, whereas predictors of openness were high RA and low SD and MV.
The regulatory function of temperament is best manifested in the ER trait: seeking or avoiding stimulation and adjusting the level of stimulation to individual needs. People with low levels of emotional reactivity are resistant to stress, show less neurotic behavior, cope better with stress, have better indices of mental health, and prefer events with a high level of stimulation, when compared to highly ER individuals . Thus, a low level of ER seems to play a crucial role in sports. Indeed, studies indicated that athletes demonstrate a significantly lower level of ER than a nonathlete population . ER was found to be an essential trait for predicting physical activity and high sports achievement [36,37,71,72]. Emotional reactivity facilitates quick reactions to emotional stimuli and heightened emotional sensitivity and intensity . The present study indicates that the pattern of coping with a loss of equilibrium is based on increasing SD and decreasing RA in the ML plane, while standing on the stabilographic platform with eyes open, in highly emotionally reactive individuals.
In contrast, people with higher scores for openness to experience demonstrate the opposite strategy. They cope with the loss of balance by reducing SD and MV in the ML plane, and increasing RA when their eyes are open. Open people actively seek new and varied experiences and are apt to be particularly reactive and thoughtful about the ideas they encounter. The openness trait involves motivation, a need for variety, cognitive sentience, and a need for deep understanding, which may lead to higher academic performance in undergraduates . Research indicates that openness is related to physiological responses to chronic social stress . Herzhoff and Tackett  found three facets of openness in children: intellect, imagination, and sensitivity. Openness is the one personality trait that is the most strongly related to creativity and intelligence. Moreover, the heritability of openness mirrors the heritability of intelligence . Conversely, low openness to experience was found as a preclinical marker of incipient cognitive decline in older adults . Intelligence has a strong genetic basis and manifests at the neuronal level in a shorter time and with a more accurate response for stimulus. Thus, openness and balance share a common neuronal mechanism responsible for adaptation to the environment. This needs to be tested in future research, since COP parameters may explain 35% of openness variance.
It is important to note that the nervous system is much more involved in the control of the position of the body in the ML plane in comparison to the AP plane. Control of the human body posture is characterized by greater stability in the AP plane, because of two-legged support of the body in vertical standing. Furthermore, research showed that conscious control of the body posture is less effective than the automatic [78‒80]. The range (RA) and variability (SD) of body sway in vertical posture are dependent on each other. An increase in the values of both parameters may indicate less stability of the equilibrium system, and a decrease in the value of both parameters may indicate better stability. On the other hand, lower values of mean velocity (MV) indicate a certain calm control . We can speculate that people with heightened openness to experience tended toward a higher risk of losing balance by increasing their range of sway, but concurrently they tried to restore balance control through decreasing SD and MV.
On the other hand, highly emotionally reactive people may react more emotionally and automatically than consciously, by reducing RA, but simultaneously increasing SD. This may be related to the worst cognitive flexibility. Low emotional reactivity was found to be associated with higher-order flexibility and cognitive control , more successful self-control , high energetic arousal and hedonic tone of arousal, and also low tense arousal . Conversely, highly ER people seem to present higher emotional tension and cognitive stiffness.
The present study is not consistent with previous research. Wojciechowska- Maszkowska et al.  found a relationship between body balance and conscientiousness among football players. SD, AP, EC were significantly higher for more conscientious athletes, whereas less conscientious athletes showed higher SD, AP, and EO. Previous research found an interaction between static balance and sensory sensitivity in a sample of football players . Individuals with heightened sensory sensitivity showed lower SD, ML, and EC and higher SD, AP, and EC. There may be an infinite number of variables that mediate the relationship between postural balance and personality or temperament. Future research should focus on further exploration of this association.
Limitations of the study
There are some limitations of the study. First, the sample size was not large because there is a small total number of elite athletes in the country. Further studies should consider a larger sample size, e.g., by including elite athletes from various countries. Secondly, the age of the participants was limited to early adulthood. Further studies should include people with a wide range of ages and experience of sports training. The group of athletes consisted of two selected sports disciplines; thus, the results of this study may not be generalized to other individual and team sports. Further studies may include other sports disciplines. Finally, personality and temperament traits were measured using a self-reported survey. The other experimental methods for assessing behavior concerning personality and temperament traits may be taken into consideration in future research.