###### Abstract  
Examining the dynamic interplay of muscle contributions to postural stability enhances our understanding of the neuromuscular mechanisms underlying balance control. This study examined the similarity in shape (using cross-correlation analysis) between seven individual lower limb electromyographic (EMG) signals and center-of-pressure (COP) displacements (i.e., EMG-COP correlation) in 20 young adults (25.2 \(\pm\) 4.0 years) performing bipedal balance tasks on both stable and multi-axially unstable surfaces, testing the effects of four factors--leg dominance, surface stability, sway direction, and foot position--on individual EMG-COP correlations. The results revealed significant effects of leg dominance (\(p\) = 0.004), surface stability (\(p\)\(\leq\) 0.001), and sway direction (\(p\)\(\leq\) 0.001) on specific muscles. Notably, balancing on the non-dominant leg resulted in a stronger correlation between tibialis anterior activity and postural sway compared to the dominant leg. On a stable surface, postural sway showed stronger correlations with the rectus femoris, semitendinosus, biceps femoris, gastrocnemius medialis, and soleus muscles than on an unstable surface. Additionally, anteroposterior postural sway exhibited a greater correlation with semitendinosus and tibialis anterior activity compared to mediolateral sway. These findings underscore the importance of specific muscles in maintaining bipedal balance, with implications for improving balance performance across various populations. p 1 2024 18 January 2025 1  
ground reaction forces and moment-of-force outcomes [4]. To address this gap, direct analyses of movements and muscle activations offer a deeper understanding of postural control mechanisms beyond COP-based measures [8]. Surface electromyography (EMG), which records electrical muscle activity during tasks, provides a more accurate assessment of muscle contributions to postural control [9; 10]. By capturing subtle muscle activity during quiet stance and analyzing its temporal relationship with COP displacements through cross-correlation analysis, the EMG-COP coordination approach offers unique insights into muscle activation patterns associated with postural adjustments [11; 12; 13; 14; 15]. This method highlights the complex temporal and coordination dynamics of muscular contributions to stability, advancing an understanding of the neuromuscular control mechanisms underpinning postural stability during balance tasks [16; 17]. In this regard, the current study aimed to enhance our understanding of neuromuscular control during bipedal stance--a fundamental part of daily activity--focusing on testing the effects of leg dominance, surface stability, sway direction, and foot position through detailed examination of individual EMG-COP correlations. First, bipedal motor tasks require the coordinated engagement of both hemispheres to maintain an upright posture through synchronized limb movements [18]. Leg dominance influences the symmetry of neuromuscular control essential for postural stability, seen as different movement control strategies of movement components [19; 20], and serves as one of the risk factors for sports-related lower limb injuries [21; 22; 23; 24]. Analyzing the symmetrical contributions of leg muscles during bipedal tasks is therefore crucial for understanding neuromuscular control. Second, unstable surfaces like foam pads and wobble boards challenge postural stability by reducing sensory input and impairing corrective ankle torque [25; 26; 27; 12]. The role of muscle activity during anticipatory postural control is critical, as lower limb and trunk adjustments counteract disturbances [11]. Understanding surface stability's effects can offer valuable insights into neuromuscular control, with implications for clinical balance training. Third, upright postural stability varies between movement planes (sagittal and frontal), with specific lower limb muscles playing distinct roles along these axes [28; 29; 30].