Balance is a core component of early motor skill development in children and plays a crucial role in maintaining gait stability, improving motor coordination, and preventing injuries (1, 2). As the neuromuscular system matures, children's balance control abilities continue to develop and are optimized. Especially in the preschool stage, the rate of development of balance control in children is significant (3, 4), yet dynamic balance control in walking exhibits great variability (5).

Among the functional tasks designed to assess dynamic balance, narrow beam walking increases the difficulty of walking and places higher demands on body stability control. Not only is this task applicable to assessing balance function (6, 7), but it can also be used to evaluate the effectiveness of balance training (8). Beam walking has been applied to screen motor skill development and early identification of disorders in children (9). Studies have shown that in children with vestibular dysfunction, motor coordination disorders, and intellectual disability, beam walking can be employed as a potential early warning tool to help clinicians detect abnormal tendencies in motor control before symptoms become apparent, thus enabling early intervention and treatment (10). Age 4 is considered to be a critical period for the transition from the initial establishment of gait stability to fine regulation (3). At this age, children's neuromuscular system is still developing, and they may exhibit great gait variability and regulation variations when faced with challenging tasks such as beam walking (11). Reducing walking speed, decreasing step frequency, and increasing lower limb joint range of motion can maintain balance. These compensatory mechanisms often lead to increased energy expenditure and changes in joint loading (12).

Relative to spatiotemporal parameters (e.g., step length and step frequency), center of mass trajectories, and limb kinematics and dynamics (13, 14), the distribution of mechanical loading at the hip, knee, and ankle joints during gait control in preschoolers has not been fully studied. Previous studies have shown that there are significant differences in gait asymmetry among children with different weights (15). Additionally, gait symmetry significantly affects joint loading in the frontal plane (16), potentially leading to asymmetric loading between the dominant and non-dominant legs (17). However, there is currently a lack of quantitative evidence to confirm whether such asymmetry affects joint loading in functional tasks requiring greater frontal stability.

Therefore, this study aimed to investigate the dynamic changes at the hip, knee, and ankle joint reaction forces in the frontal plane during balance beam walking (BBW) and overground walking (OGW) in 4-year-old children, and to reveal the loading regulation strategies of lower limb joints in complex functional balance tasks. We hypothesized that lower limb joint loading adjustments and balance control strategies during BBW in 4-year-old children may exhibit asymmetrical patterns.

Schematic diagrams of the medial and lateral reaction forces at the hip, knee, and ankle joints are shown in Figure 2A₁–A₃. Under the BBW condition, the non-dominant hip joint (Figure 2B₁) exhibited greater medial reaction forces from loading response to mid-stance at 4.82–28.72% (p = 0.001), from initial swing to mid-swing at 68.99–74.11% (p = 0.031), and during the terminal swing phase at 98.91–100.00% (p = 0.039), whereas the dominant hip joint (Figure 2B₂) showed no significant differences. Compared with the dominant knee, the non-dominant knee joint (Figure 2C₁) exhibited lower lateral reaction forces during the transition phase from initial swing to mid-swing at 70.34–81.81% of the gait cycle (p = 0.018). Under the OGW condition, the non-dominant knee joint exhibited a negative reaction force at 70.34% of the gait cycle, indicating that the reaction force acted on the lateral side. However, under the BBW condition, the knee reaction force exhibited a slower transition from the medial side to the lateral side at 79% of the gait cycle. Under both conditions, no significant differences in reaction forces were observed at the dominant knee joint (Figure 2C₂) or ankle joint (Figure 2D₁,D₂).

By comparing the joint reaction forces in the frontal plane under BBW and OGW conditions, we found that 4-year-old children exhibited a unilateral loading shift strategy during dynamic balance control tasks. Children enhanced control of lateral body stability by increasing reaction forces in the mediolateral direction of the non-dominant lower limb (especially the hip and knee joints), thereby maintaining balance. Such adjustments were mainly reflected in the regulation of unilateral lower limb loading rather than symmetric regulation of bilateral loading, indicating a compensatory gait pattern characterized by unilateral regulation and contralateral coordination.

BBW is a functional task that requires highly precise control and regulation of balance and is commonly used to assess motor coordination and stability (28, 29). Compared to normal walking, unstable gait tends to cause the trunk to sway laterally (30), which is negatively correlated with age (31). This instability is particularly pronounced in tasks requiring balance adjustments (32). Previous studies have shown that gait stability can be maintained across different movement patterns through the coordinated action of the hip, knee, and ankle joints along with their muscle synergies (33).

As the primary connection between the trunk and lower limbs, the hip joint serves to generate and transmit force (34). When walking conditions change, gait patterns consequently alter (35, 36). Previous studies have shown that a wider step helps improve lateral stability (37), while a narrower step width increases hip joint reaction forces (38). In the current study, the non-dominant hip joint exhibited greater reaction forces at the early and late swing phases under the BBW condition, which is consistent with the findings of previous studies (38, 39). The reaction force of the non-dominant knee was negative under OGW, indicating that the knee joint was subjected to a force in the medial direction, whereas the reaction force of the left knee gradually transitioned from the lateral to the medial side under BBW, exhibiting a slower transition pattern. Previous studies have found that changes in gait patterns lead to shifts in joint loading patterns (40–43), such as a slower transition from the stance to the swing phase and a significant increase of double support time, thereby reducing joint pressure and avoiding rapid loading impacts to prevent joint injuries (44, 45). The knee joint is primarily responsible for absorbing impact and transmitting ground reaction forces, playing a critical role in overall balance adjustment and fine-tuning of gait during dynamic balance tasks (1, 46). Compared to the hip and ankle joints, the knee joint has a smaller range of motion in the frontal plane and therefore has limited direct control over balance. Its main function is to assist in maintaining postural control (47). Gait adjustment can reduce knee varus moment and medial reaction force, but with little effect on the overall reaction force (48, 49).

The reaction forces at the dominant hip and knee joint did not vary significantly under OGW and BBW conditions. Typically, healthy preschoolers exhibit symmetrical gait patterns when walking normally. However, changes in walking conditions lead to increased unilateral limb loading, resulting in significant gait asymmetry (50, 51). Subsequently, this asymmetry affects the symmetry of the hip joint reaction forces (52). Changes in gait patterns may lead to variability in hip joint loading peaks and the occurrence (38, 39, 53), thereby forming continuous, unstructured adaptive patterns rather than abrupt changes (52, 54). This decentralized loading regulation strategy may reflect that preschool children have not yet established mature motor control patterns in functional tasks of higher difficulty, as evidenced by the dominant hip joint assuming a higher functional role in synergistic regulation. Studies have shown that the dominant leg exhibits more pronounced regulatory functions, while the non-dominant leg contributes more to support assistance, resulting in a unilateral regulation and contralateral assistance gait pattern (55).

Joint loading is influenced by joint kinematics, muscle activation, and neuromuscular control. The greater the joint loading, the higher the gait variability (56, 57). Variations in joint loading appear to be induced by gait variability, but from a control theory perspective, this variability actually reflects the presence of motor redundancy (58). When attempting to perform a motor task, children regulate through various muscle combinations and movement patterns to achieve the same motor goal (59). By increasing antagonist muscle activity, dynamic knee joint stiffness is improved, thereby overcoming knee joint instability (60). Typically, ankle control strategies serve as the primary mechanism for adults to counteract environmental disturbances (61, 62). However, preschoolers exhibit weaker ankle dorsiflexion and inversion-eversion regulatory capabilities, along with differences in muscle activation patterns (63). When faced with unstable conditions, children's ankle joints cannot perform fine-tuning adjustments as effectively as adults (64). Moderate motor variability is not a sign of control failure but rather reflects children's continuous adjustments and optimization of movement patterns in response to environmental conditions. However, long-term irregular distribution of loading on the lower limbs may cause excessive pressure on joints, impairing their ability to absorb impact and ultimately resulting in joint injury, such as osteoarthritis and fractures (65, 66).

The core challenge of beam walking lies in maintaining lateral balance on a narrow support surface (28). Lateral stability is crucial for dynamic balance regulation in gait (67), and the coordination of the hip, knee, and ankle joints and associated muscles significantly influences the adjustment of the center of gravity in the frontal plane (68). The primary mechanism for adults to counteract interference conditions is ankle control strategies, whereas this study found that 4-year-old children seem to prefer hip control strategies for beam walking. This has important implications for understanding the overall balance control mechanisms and injury prevention in preschoolers.

Assessing functional gait tasks through joint reaction forces enables a more comprehensive understanding of preschool children's balance control strategies from a biomechanical perspective. As a functional training tool, beam walking places emphasis on continuity and prevention of excessive lateral sway. This facilitates targeted rehabilitation training for preschool children with impaired gait function. Cognition and attention may influence balance performance (69), which was not considered in the present study, representing a limitation that warrants attention. Cognitive processes related to balance in children vary across tasks, influenced by perceptual characteristics and task specificity. When children focus on task completion, these processes alter gait patterns, thereby increasing gait variability. Children with superior balance abilities tend to perform tasks more efficiently (69–71). However, this relationship requires further confirmation in preschool children. Integrating electroencephalography or eye-tracking technology could analyze the neural mechanisms linking attentional allocation with gait variability in depth (69, 72). This may provide a more comprehensive neurocentral perspective for investigating functional gait task control strategies in preschool children.

Beam walking in 4-year-old children significantly increased the demand for loading regulation in the non-dominant lower limb joints, and the motor control strategies demonstrated pronounced asymmetry. In functional tasks, preschoolers require greater joint loading regulation capabilities to maintain body stability. Children's neuromuscular control system is still developing at this age, but they have already acquired a certain degree of adaptability.