Independent bimanual coordination improves gradually from ages 5 to 13, as assessed by the bimanual circles–lines coupling (BC) task.

Bimanual coordination skills are essential for countless everyday activities such as typing, meal preparation, and driving (Gooijers and Swinnen, 2014). Many of these daily tasks require not symmetrical but simultaneous and asymmetric use of both hands. It is well established that performing asymmetric bimanual movements often results in interference effects, where the motor programs for each hand influence each other (Chan and Chan, 1995). This intermanual interference is considered to reflect interactions between two distinct motor programs or between perceptual goals (Heuer, 1993; Mechsner et al., 2001; Wolpert and Ghahramani, 2000).

A well-known paradigm for quantitatively measuring this interference is the bimanual circles–lines coupling (BC) task (Franz et al., 1991; Franz, 1997; Garbarini et al., 2012, 2013, 2014). In this task, participants are asked to draw circles and vertical lines simultaneously under two experimental conditions. In the unimanual (UM) condition, participants repeatedly draw vertical lines with their preferred hand. In the bimanual (BM) condition, participants simultaneously draw circles with their non-preferred hand while continuing to draw vertical lines with their preferred hand. Typically, the vertical lines drawn in the BM condition are distorted into elliptical shapes compared to those in the UM condition, due to the interference from the circular motor program. The magnitude of this interference is referred to as the bimanual coupling effect (BCE), calculated by subtracting the ovalization index (OI) in the UM condition from that in the BM condition (Garbarini et al., 2012, 2013, 2014; Piedimonte et al., 2014, 2018). A lower BCE indicates a higher ability to independently control each hand during simultaneous movement (Piedimonte et al., 2014).

However, the developmental trajectory of the BCE remains largely unclear. To date, only one study has investigated the age-related changes in BCE using the BC task. Piedimonte et al. (2014) examined BCE across four age groups—children aged 6 and 10 years, young adults aged 20–30 years, and older adults aged 60–80 years—and found that the BCE was highest in the 6- and 10-year-old children and lowest in the young adult group. These findings suggest that the ability to independently control both hands simultaneously improves with age, but the developmental progression during childhood remains to be clarified.

Several neuroscientific studies suggest that BCE relies on the activity of the fronto-parietal network. Garbarini et al. (2014) reported that during the BM condition of the BC task (i.e., one hand drawing lines, the other drawing circles), the supplementary motor area (SMA) and posterior parietal cortex (PPC)—key regions of the fronto-parietal network—showed greater activation compared to symmetrical bimanual movements (e.g., both hands drawing lines). Similarly, Iester et al. (2025) demonstrated that the BM condition elicited stronger activation in the primary sensorimotor cortex (Brodmann areas [BA] 3 and 4), premotor cortex (BA6), superior parietal lobule (BA7), and inferior parietal lobule (BA39, 40) than symmetrical conditions. These authors proposed that the BM condition involves greater engagement of both sensorimotor and associative areas to manage the competition and coordination between two independent motor programs (Iester et al., 2025). In particular, the inferior parietal lobule is thought to play a central role in sensorimotor integration, behavioral monitoring, and intermanual information mediation (Iester et al., 2025).

Furthermore, while the primary sensorimotor cortex matures relatively early, by around age six, the fronto-parietal network—including the prefrontal cortex and parietal associative areas—undergoes gradual maturation during adolescence (Casey et al., 2005). Gogtay et al. (2004) similarly reported a sequential maturation pattern from sensory-motor regions to parietal association cortices and, finally, to the dorsolateral prefrontal cortex from age 4 to 16. These findings suggest that the BCE, reflecting the ability to independently control each hand, is likely to undergo developmental changes during childhood. Nevertheless, its precise developmental course remains unclear.

Additionally, because the BC task assesses the ability to perform independent simultaneous hand movements, it is expected to relate to upper-limb motor abilities, particularly fine motor skills. However, it remains unknown which specific fine motor skills are associated with BCE. A widely used standardized battery to assess children’s coordination abilities is the Movement Assessment Battery for Children–2nd edition (M-ABC2; Henderson et al., 2007). The manual dexterity component of M-ABC2 provides age-adjusted quantitative evaluations of four fine motor skills: preferred-hand skill, non-preferred-hand skill, bimanual skill, and handwriting skill.

Therefore, in the present study, we administered the BC task to children aged 5 to 13 years to investigate the developmental changes in their ability to perform independent simultaneous bimanual movements. We also evaluated their fine motor skills using the M-ABC2 and examined the relationship between BCE and each type of fine motor skill.

Table 1 summarizes the data obtained for each age group. There were no significant differences in sex [χ²(8) = 4.623, p = 0.797, φ = 0.176], and preferred hand [χ²(8) = 9.630, p = 0.292, φ = 0.253] among the age groups.

In all age groups, BM-OI significantly increased compared to UM-OI (5-year-old, z = −2.073, p = 0.038, r = −0.691; 6-year-old, z = −3.459, p = 0.001, r = −0.755; 7-year-old, z = −3.702, p < 0.001, r = −0.849; 8-year-old, z = −4.286, p < 0.001, r = −0.875; 9-year-old, z = −3.549, p < 0.001, r = −0.837; 10-year-old, z = −3.408, p = 0.001, r = −0.880; 11-year-old, z = −3.621, p < 0.001, r = −0.878; 12-year-old, z = −2.903, p = 0.004, r = −0.838; 13-year-old, z = −3.408, p = 0.001, r = −0.880).

Figure 2 shows the results of the age group comparisons for the three variables measured in the BC task (UM-OI, BM-OI, and BCE). The Kruskal–Wallis test revealed a significant difference in UM-OI among the age groups (p < 0.001) (Figure 2A). Multiple comparisons in post-hoc analysis showed that UM-OI significantly decreased in the 10-year-old group compared to the 5-year-old (p = 0.024, r = −0.627) and 6-year-old groups (p = 0.004, r = −0.666). The UM-OI in the 11-year-old group significantly decreased compared to the 5-year-old (p = 0.008, r = −0.576), 6-year-old (p = 0.001, r = −0.645), and 7-year-old groups (p = 0.042, r = −0.536). The UM-OI in the 12-year-old group significantly decreased compared to the 6-year-old group (p = 0.016, r = −0.684). The UM-OI in the 13-year-old group significantly decreased compared to the 5-year-old (p = 0.037, r = −0.59) and 6-year-old groups (p = 0.008, r = −0.532).

The Kruskal–Wallis test also revealed a significant difference in BM-OI among the age groups (p < 0.001) (Figure 2B). Multiple comparisons in post-hoc analysis showed that BM-OI in the 5-year-old group significantly increased compared to the 9-year-old (p = 0.037, r = −0.614), 11-year-old (p = 0.030, r = −0.608), 12-year-old (p = 0.026, r = −0.667), and 13-year-old groups (p = 0.044, r = −0.566).

In contrast, there was no significant difference in BCE among the age groups (p = 0.365) (Figure 2C).

There were significant negative correlations between age and the variables measured in the BC task: UM-OI (rs = −0.510, p < 0.001) (Figure 3A), BM-OI (rs = −0.426, p < 0.001) (Figure 3B), and BCE (rs = −0.186, p = 0.022) (Figure 3C).

Table 2 presents the results of the Pearson partial correlation analysis, controlling for age, between the three variables measured in the BC task (UM-OI, BM-OI, and BCE) and the four types of fine motor skills (preferred hand skill, non-preferred hand skill, bimanual skill, and handwriting skill). BM-OI showed significant negative partial correlations with preferred hand skill (rₚ = −0.262, p = 0.001), non-preferred hand skill (rₚ = −0.286, p < 0.001), bimanual skill (rₚ = −0.369, p < 0.001), and total fine motor skill (rₚ = −0.342, p < 0.001). Additionally, BCE showed significant negative partial correlations with non-preferred hand skill (rₚ = −0.263, p = 0.001), bimanual skill (rₚ = −0.353, p < 0.001) (Figure 4), and total fine motor skill (rₚ = −0.295, p < 0.001).

The present study investigated the developmental trajectory of the bimanual coupling effect (BCE) and its association with fine motor skills in children aged 5 to 13 years. Across all age groups, a significant increase in ovalization index (OI) from the unimanual (UM) to the bimanual (BM) condition was observed, indicating the presence of BCE. Age group comparisons revealed that both UM-OI and BM-OI significantly decreased with increasing age. Furthermore, correlation analyses demonstrated significant negative associations between age and UM-OI, BM-OI, and BCE. These findings suggest that not only unimanual motor performance, as reflected by UM-OI, but also the ability to control both hands independently and simultaneously, as indicated by BM-OI and BCE, improves with age from 5 to 13 years.

In early infancy, children can only perform simple symmetrical bimanual movements. However, by around 2 years of age, more complex asymmetric bimanual actions begin to emerge (Fagard and Jacquet, 1989). Developmental studies focusing on the contrast between symmetrical and asymmetrical bimanual coordination have shown that seven-year-olds perform symmetrical movements more accurately than asymmetrical ones, but this difference diminishes significantly between the ages of 8 and 10 (Fagard and Pezé, 1992; Fagard et al., 2001; Fagard, 1987). Indeed, symmetrical or mirror movements are frequently observed in young children (Utley and Steenbergen, 2006), but they dramatically decrease and stabilize by around 8.5 years, becoming rare by age 9 (Cohen et al., 1967; Lazarus and Todor, 1987). In addition, the speed and accuracy of bimanual coordination tasks requiring each hand to control movement along a different axis, such as drawing along x- and y-coordinates, have been shown to improve significantly between ages 6 and 13 (Marion et al., 2003; Steese-Seda et al., 1995).

Neuroimaging studies have revealed that asymmetric bimanual movements, such as those required in the BM condition of the BC task, activate the fronto-parietal network—including the supplementary motor area, premotor cortex, superior parietal lobule, and inferior parietal lobule—more strongly than do symmetrical bimanual movements (Garbarini et al., 2014; Iester et al., 2025; Sadato et al., 1997; Wenderoth et al., 2004). Moreover, the fronto-parietal network involved in bimanual control continues to mature between ages 6 and 10 and reaches full maturation during adolescence (Casey et al., 2005; Gogtay et al., 2004).

In addition, the BC task requires dual attention control, as it involves simultaneously drawing straight lines with one hand and circles with the other. This necessitates high-level cognitive control centered in the prefrontal cortex, particularly executive functions and working memory. These functions are known to be still immature at approximately 6 years of age and are reported to stabilize around the age of 8 (Brocki and Bohlin, 2004; Frick and Möhring, 2016).

Furthermore, the age-related improvement in BCE observed in this study may be explained by the structural maturation of transcallosal motor fibers (TCMFs) and the accompanying enhancement of interhemispheric inhibition (IHI). Indeed, Kwon et al. (2014) demonstrated that TCMFs originating from the corticospinal tract begin to appear after age 6 and significantly increase in prevalence around age 10, suggesting a marked structural development of interhemispheric motor communication during middle childhood. Thus, the age-related decrease in BCE observed in this study may reflect enhanced capacity to suppress involuntary mirror movements as a result of the maturation of IHI mechanisms.

In line with these findings, the present study provides further evidence that the ability to perform simultaneous and independent bimanual movements improves throughout middle childhood. Notably, although age-related improvements in UM-OI and BM-OI were demonstrated both through age group comparisons and correlation analyses, age-related improvement in BCE was observed only in the correlation analyses. This suggests that the development of the ability to control each hand independently during simultaneous movement progresses gradually rather than abruptly between ages 5 and 13. One possible explanation for the lack of significant group differences in BCE is that changes in BCE across adjacent age groups may be subtle, and the high inter-individual variability combined with limited group sizes may have reduced the statistical power to detect between-group differences. In contrast, correlation analyses treat age as a continuous variable and are more sensitive to detecting linear developmental trends across a wide age range. Therefore, we considered the correlation analysis to be a more appropriate and powerful method for capturing the developmental trajectory of BCE in this study.

Another aim of the present study was to examine which specific fine motor skill(s), among four measured using the M-ABC2, are associated with the ability to perform independent simultaneous bimanual movements, as indicated by BM-OI and BCE. Interestingly, although UM-OI reflects the degree of ovalization during repetitive straight-line drawing with the preferred hand, no significant association was found between UM-OI and preferred hand skill. In contrast, BM-OI was significantly associated with both preferred and non-preferred hand skills, and BCE was significantly associated with non-preferred hand skill. These results suggest that the ability to draw straight lines with the preferred hand while simultaneously performing a conflicting circular movement with the non-preferred hand, as indexed by BM-OI, may reflect a higher level of skill in independently coordinating each hand. That is, greater independence between the hands is likely to be achieved when both the preferred and non-preferred hands are sufficiently proficient. Similarly, the significant association between BCE and non-preferred hand skill implies that greater control over the non-preferred hand may contribute to reducing the interference effect between hands during simultaneous movements.

Most importantly, partial correlation analyses controlling for age revealed that among the four types of fine motor skills assessed, both BM-OI and BCE were significantly associated with bimanual skills. This indicates a meaningful relationship between the ability to independently control both hands and overall bimanual coordination skill. However, this result is not surprising. In this study, the bimanual skills subtest of the M-ABC2 included age-appropriate tasks: the threading beads task for ages 5–6, the threading lace task for ages 7–10, and the triangle with nuts and bolts task for ages 11–13. Each of these tasks requires both the preferred and non-preferred hands to perform different yet complementary movements toward a shared goal. Thus, the finding that better performance in the BC task is associated with higher bimanual skill appears to be highly valid.

Moreover, this result supports the utility of the BC task as a method for evaluating bimanual coordination skills. While M-ABC2 is a well-established assessment tool for fine motor abilities, other standardized tests such as the Bruininks–Oseretsky Test of Motor Proficiency, Second Edition (BOT-2; Bruininks and Bruininks, 2005) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio and Fewell, 2000) are also frequently used. These tests evaluate various fine motor skills—such as cutting along a line with scissors, copying shapes, sorting cards, stacking blocks, or open the lid of a container—and require different sets of materials depending on the age of the child. In contrast, the present findings suggest that the BC task may serve as a practical alternative for assessing bimanual coordination skills across a wide age range, without the need for task variation or extensive material preparation.

This study has several limitations. First, it was not possible to equalize the sample size across age groups. This may have been a primary factor contributing to the lack of statistically significant differences in BCE across age groups. Future studies should aim to balance the sample size across age groups and increase the overall number of participants. Second, in the present study, we did not measure the repetitive circular trajectories performed by the non-preferred hand during the BM condition of the BC task. Simultaneous measurement of both the vertical line trajectories of the preferred hand and the circular trajectories of the non-preferred hand may allow for a more detailed assessment of the BCE. Third, the findings of this study suggest that the BC task has potential as a quantitative measure of bimanual coordination skills. However, to further validate this potential, future research should incorporate additional standardized assessments of children’s fine motor skills beyond the M-ABC2 (Henderson et al., 2007), such as the BOT-2 (Bruininks and Bruininks, 2005) and the PDMS-2 (Folio and Fewell, 2000). Lastly, while this study revealed developmental changes in the ability to perform independent simultaneous bimanual movements during childhood, it did not directly examine the relationship between these changes and the underlying neural mechanisms —such as the maturation of the fronto-parietal network and the increase in IHI—that are assumed to support this ability. Future research addressing this question would contribute to a more comprehensive understanding of the development of upper-limb motor control in childhood.

This study behaviorally demonstrated that the bimanual coupling effect (BCE)—as measured by the BC task, and reflecting the ability to perform independent simultaneous bimanual movements—improves between the ages of 5 and 13. It also revealed a significant relationship between this ability and bimanual coordination skills. Future studies that take into account the limitations of the present research are needed to further investigate the developmental changes in both the ability to move both hands independently and its underlying neural mechanisms, as well as to evaluate the potential of the BC task as a standardized assessment tool for bimanual coordination skills.