1.    Introduction
Balance and postural control are among the most fundamental neuromuscular functions in sports, enabling athletes to manage body positioning during jumping and landing, and helping to prevent injuries (1). Maintaining balance during landing is particularly crucial in sports such as volleyball, where repeated jumping and landing are essential components of the game (2). Epidemiological studies have shown that a high percentage of volleyball-related injuries occur during landings following blocks or attacks (3). For instance, several recent investigations have reported that landings after blocking and spiking account for more than half of volleyball injuries (4, 5). In NCAA Division I women’s volleyball between 2014 and 2019, lower limb injuries were also highly prevalent (3), with most being non-contact and occurring during the landing phase (6). During landing, large ground reaction forces (GRFs) are transmitted through the body, which, if not properly controlled, can result in acute injuries (7). Thus, how an athlete lands and maintains balance post-jump is a critical factor in injury prevention.
The ability to quickly regain postural stability following perturbation is a key element of success among elite athletes. Professional volleyball players typically demonstrate rapid postural responses, effectively adjusting their body position to prevent falls or injury (8). In contrast, less experienced players tend to exhibit poorer postural stability and may struggle with balance control during landing (9). Studies across various sports have shown that novices exhibit greater sway amplitudes than experts. For example, in rifle shooting, novices show larger and more variable postural sway than elite marksmen (10). Similarly, elite volleyball players have been found to exhibit smaller CoP excursions in standing tasks compared to non-athletes, reflecting better postural control (11). This is likely due to sport-specific training enhancing neuromuscular mechanisms and reducing unnecessary body movements. One of the key indicators of postural stability during landing is the analysis of foot center of pressure (CoP) trajectories (12). The CoP represents the point of application of the resultant ground reaction force and offers valuable insight into balance strategies when measured via force plates. Abnormal CoP displacement during landing may overload lower limb structures and increase injury risk (13). Therefore, analyzing CoP dynamics in volleyball can reveal control differences between expert and novice players, helping to identify areas of instability in less experienced athletes. The purpose of this study was to compare CoP movement patterns during defensive landing between professional and novice volleyball players. Specifically, this study analyzed CoP sway range, RMS displacement, and velocity in both anterior–posterior (AP) and medial–lateral (ML) directions. Based on our hypothesis, novice players were expected to exhibit greater instability across these parameters, reflecting weaker balance control and potentially higher injury risk. 
2.    Methods
This cross-sectional descriptive study was conducted in a biomechanics laboratory. Twenty male volleyball players participated, including 10 professionals from national leagues and 10 novices with at least two years of playing experience. All participants were injury-free and not under medication prior to testing. Ethical approval was obtained, and informed consent was signed. A Vicon T20 motion capture system (100 Hz) and two synchronized Kistler force plates (1000 Hz) recorded kinematic and kinetic data. Reflective markers were placed according to the Plug-in Gait model. Five randomized blocking tasks were performed: static jump, step-side block to the right and left, and long cross-step block to the right and left. Players landed bilaterally, ensuring each foot contacted a separate force plate. Each task was repeated six times with 2-minute rest intervals. Signals were low-pass filtered (4th-order Butterworth, 20 Hz) and normalized to body weight. Three center of pressure (CoP) parameters were analyzed during the main landing phase: (1) displacement range (AP and ML), (2) root mean square (RMS), and (3) mean velocity. Data normality was tested with the Shapiro–Wilk test. A three-way ANOVA (group × task × direction) was used to assess differences, with significance set at p < 0.05 using SPSS v21.
3.    Results
Postural Sway at Landing
Between-group comparisons revealed significant differences in postural sway in the mediolateral (ML) direction during the static block jump (p = 0.026, F = 5.90) and the short-step jump from the left (p = 0.005, F = 10.35). However, within-group analysis indicated that sway amplitude was not significantly influenced by jump type. Interestingly, although professional players showed greater sway overall, the difference between groups was not statistically significant. In contrast, a significant main effect of direction was observed (η² = 0.69, p < 0.001, F = 40.28), with larger sway in the ML direction compared to the anteroposterior (AP) direction. A significant interaction between direction and jump type was also found (η² = 0.39, p < 0.001, F = 11.76), indicating that sway in all jumps was greater in the ML direction except during the static jump, where AP sway was higher.
RMS of CoP at Landing
RMS values were significantly higher in novice players for most jump types, except for the right-step jump and the short and long jumps from the left in the ML direction. RMS was significantly affected by jump type (η² = 0.78, p < 0.001, F = 13.01) and the interaction between jump type and group (η² = 0.63, p = 0.003, F = 6.50). RMS was also greater in the ML than AP direction (η² = 0.91, p < 0.001, F = 192.28), with a significant interaction between direction and jump type (η² = 0.51, p = 0.022, F = 3.97) (Table 1).

CoP Velocity at Landing
No significant between-group differences were found for CoP velocity. However, significant main effects were observed for jump type (η² = 0.63, p = 0.003, F = 6.35) and direction (η² = 0.35, p = 0.006, F = 9.68). Velocity was generally greater in the ML direction and lower during static jumps compared to other tasks. A significant jump type × direction interaction (η² = 0.47, p = 0.040, F = 3.30) indicated reversed trends during the left-side jump (AP > ML), while no directional difference was seen in the right-side jump.
4.    Conclusion
This study aimed to evaluate and compare the center of pressure (CoP) movement patterns during landing after block jumps between professional and novice volleyball players. The findings revealed that although novice players exhibited significantly lower postural sway in certain tasks, they showed greater RMS values compared to their professional counterparts. Interestingly, CoP sway was greater in professionals, particularly during middle and left-side block jumps. According to Bernstein’s degrees of freedom theory, novices tend to "freeze" joint motion to simplify motor control, whereas experienced athletes utilize greater freedom to execute more adaptive and refined postural adjustments (15,16). Previous studies, such as Caballero et al. (2020), have similarly shown that skilled athletes exhibit broader joint movements, which may explain their increased postural sway (17). Carpenter et al. (2010) further argued that such sway could enhance sensory integration under dynamic conditions, serving as an adaptive strategy (18). Therefore, increased sway among professional players—especially given their higher jump heights—reflects not instability, but a more advanced dynamic postural control strategy (19).
Significant between-group differences in RMS were observed across most tasks, with novices showing higher variability. While variability is often needed for adaptability, excessive or insufficient variability can hinder performance. As per Lipsitz and Stergiou, an optimal "U-shaped" level of variability supports environmental adaptability, while deviations from this norm may signal reduced stability (20–22). Interestingly, despite higher sway, professionals exhibited lower variability, implying better neuromuscular coordination. Moreover, the direction of movement significantly influenced CoP behavior: sway was generally greater in the mediolateral (ML) direction, except during the middle jump, where anteroposterior (AP) sway dominated—likely due to forward momentum. These findings align with Wikstrom et al. (2008), who found that lateral and oblique jumps increase ML instability. Professional volleyball players demonstrated greater CoP sway but lower variability during post-block landings, suggesting refined, stable postural strategies. Additionally, movement direction was found to alter postural control patterns, underscoring the importance of training multidirectional landing skills. Future programs should prioritize enhancing dynamic balance through targeted neuromuscular strategies to reduce injury risk, especially in novice athletes. 

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
Funding
This research did not receive any grants from funding agencies in the public, commercial, or non-profit sectors.
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declared no conflict of interest.