Conventional biomechanical analyses of human movement have been generally derived from linear mathematics. While these methods can be useful in many situations, they fail to describe the behavior of the human body systems that are predominately nonlinear. For this reason, nonlinear analyses have become more prevalent in recent literature. These analytical techniques are typically investigated using concepts related to variability, stability, complexity, and adaptability. This review aims to investigate the application of nonlinear metrics to assess postural stability. A systematic review was conducted of papers published from 2009 to 2019. Databases searched were PubMed, Google Scholar, Science-Direct and EBSCO. The main inclusion consisted of: Sample entropy, fractal dimension, Lyapunov exponent used as nonlinear measures, and assessment of the variability of the center of pressure during standing using force plate. Following screening, 43 articles out of the initial 1100 were reviewed including 33 articles on sample entropy, 10 articles on fractal dimension, and 4 papers on the Lyapunov exponent. This systematic study shows the reductions in postural regularity related to aging and the disease or injures in the adaptive capabilities of the movement system and how the predictability changes with different task constraints.

Osteoporosis is a prevalent health concern among older adults and is associated with an increased risk of falls that may result in fracture, injury, or even death. Identifying the risk factors for falls and assessing the complexity of postural control within this population is essential for developing effective regimes for fall prevention. The aim of this study was to assess postural control in individuals recovering from osteoporotic vertebral fractures while performing various stability tasks. Seventeen individuals with type II osteoporosis and 17 healthy subjects participated in this study. The study involved maintaining balance while standing barefoot on both feet for 20 s on an Advanced Mechanical Technology Inc. (AMTI) plate, with eyes open, eyes closed, and eyes closed in conjunction with a dual-task. Another three trials lasting 10 s each were undertaken during a single-leg stance under the same conditions. Fall risk was assessed using the Biodex Balance platform. Nonlinear measures were used to assess center of pressure (CoP) dynamics in all trials. Reducing the support area or elimination of the visual control led to increased sample entropy and fractal dimension. Results of the nonlinear measurements indicate that individuals recovering from osteoporotic vertebral fractures are characterized by decreased irregularity, mainly in the medio-lateral direction and reduced complexity.

Introduction: Postural control is a complex system that combines many subcomponents. The central nervous system’s connection with the muscular system allows to execute the voluntary movements and provides appropriate tonus and contractions of postural muscles. The aim of this study was to assess the effect of voluntary muscle contraction on the stability of standing posture. Material and methods: Seventeen young adults (24.11 ± 3.05 years) completed 4 bipedal standing attempts lasting 20 seconds each. Measurements were performed on AMTI plate and involved maintaining balance while standing barefoot on both feet with eyes open and closed. Two additional tests with provoked isometric voluntary muscle contraction were performed under the same visual conditions. Results: For examined healthy subjects, the lack of visual feedback in combination with increased muscle tension caused a significant increase (p < 0.05) in the peak of sway in the anterior-posterior (AP) and medio-lateral (ML) direction. On the other hand, the values of the center of pressure (CoP) path length and its velocity due to the increased muscle contraction did not significantly decrease their values for trials with eyes open and closed. Conclusions: This study indicates that voluntary muscle contraction can influence the standing posture mainly in combination with a lack of visual feedback, causing its deterioration.

Given the high postural control demands of sport Jiu-Jitsu, prolonged training in this discipline may result in sport-specific adaptations, particularly in positions closely related to combat scenarios. This study aimed to evaluate the differences in reaction time and postural control between elite Jiu-Jitsu athletes and untrained individuals, highlighting the potential influence of specialized training on these critical performance attributes. This study was conducted on thirty-one young participants (fifteen Brazilian Jiu-Jitsu athletes and sixteen non-athletes). Reaction time was measured using the Fit Light Trainer system in two positions. Postural control was assessed during 30 s bipedal and single-leg standing, both with and without a dual task involving tracking a randomly moving point on a screen. Results indicated that athletes demonstrated significantly faster reaction times (p = 0.0242) and greater complexity in postural control mechanisms, as evidenced by higher fractal dimension values during single-leg standing with dual tasks in the anterior–posterior direction (p = 0.0011). These findings suggest that Brazilian Jiu-Jitsu athletes possess enhanced neuromuscular and cognitive–motor integration, crucial for managing the complex demands of martial arts. This study highlights the importance of incorporating dual-task scenarios in training to optimize athletic performance and postural control in high-demand sport contexts.

Objectives: This study aimed to explore the impact of irritating sounds on the postural control of healthy adults, considering both linear and nonlinear parameters, subjective assessments, and gender differences. Methods: Thirty-four young participants (17 females, 17 males) completed three 30 s bipedal standing stability tests on a balance platform: one with visual control (EO), another without visual control (EC), and a third without visual control but accompanied by irritating sounds (ECS). Additionally, participants filled out a questionnaire evaluating their sound sensitivity. Linear and nonlinear parameters from each balance test were considered for statistical analysis. Results: The findings reveal significant gender-based variations in sensitivity to sound, with women exhibiting higher sensitivity. No statistically significant differences in postural control were observed between males and females, except for a notable increase in irregularity (SampEn values) in the anterior–posterior direction for females in the ECS trial. Correlation analyses revealed a moderate and statistically significant correlation between SampEn values in the AP direction and SE scores. Conclusions: This study highlights the intricate relationship between sensory stimuli, attention, and the body’s ability to maintain balance. The presence of irritating sounds led to increased irregularity in postural control, particularly in the absence of visual control.

Postural control is a term used to describe how the central nervous system regulates sensory information from other systems to produce adequate motor output to maintain a controlled, upright posture. Emotions (fear, anxiety) and thus personality type can affect the strategy of body control. This study aimed to evaluate the impact of personality on postural control. Thirty-three healthy individuals participated in this study. The big-five model was used to examine personality traits. Each participant performed four different standing tasks (one and both legs standing with eyes open (eo) and closed (ec): 2eo, 2ec, 1eo, 1ec). We showed that the dominant personality traits in the study group were extraversion and agreeableness. There were significantly low negative associations between nonlinear parameters and personality traits. A moderate correlation was noted for the 1eo trial between Openness and the Lyapunov exponent. In conclusion, nonlinear measures provide a possible link between personality and postural control. The relationships detected are weak. It shows that factors such as visual control and the size of the support area rather than personality will play a significant role in describing postural control.