Introduction
Bimanual coordination in older adults is crucial for participating in daily activities and live independently [1]. Elderly people may experience difficulties in performing activities that require significant bimanual coordination [2]. According to previous studies, the supplementary motor area (SMA) plays an important role in planning and executing sequential and continuous bimanual motor tasks. It also has a crucial role in motor planning prior to movement initiation [3]. The parietal cortex also plays a significant role in bimanual coordination, particularly in complex tasks [4]. Given the changes that occur in old age, providing safe and low-risk solutions to cope with these changes is highly important to reduce the treatment costs and time for elderly people. It has been reported that transcranial alternating current stimulation (tACS), as a non-invasive brain stimulation, facilitates changes in neural activity and promotes neuroplasticity in healthy people [5]. Considering the decline in motor function among the elderly in fine motor tasks and their difficulties performing these tasks in their lives, which are important indicators of functional independence [6], an effective stimulation method to improve bimanual motor skills can be beneficial for the rehabilitation of older adults. Therefore, this study aims to investigate the effects of tACS applied over the right posterior parietal cortex (P4) and SMA on bimanual coordination in elderly women.

Methods
In this quasi-experimental study, 31 healthy elderly women (Mean age 65.26±4.01 years) without a history of neurological or psychological disorders participated. They had normal or corrected-to-normal vision, with no previous experience of tACS. They completed the Montreal Cognitive Assessment Test (MoCA) [7] and the Edinburgh Handedness Inventory (EHI) [8]. The Purdue Pegboard Test (PPT) was used to assess bimanual coordination ability. This task required the participants to use both hands while assembling one pin, one collar, and two washers. They were given a total of 5 minutes to complete the test, and the total score was determined by the number of correctly assembled units at the end of the trial. In this study, a 2-channel brain electrical stimulator (NeuroStim2, Medina Teb, Iran) was used for sham and online brain stimulations. The right parietal cortex (P4) and the SMA (3.0 cm anterior to the central sagittal midline) were identified according to the 10–20 international standard system [9]. The participants received intervention under three conditions: tACS over SMA, tACS over P4, and sham. Each condition was performed in one session with at least a one-week interval. The acquisition phase included four trials of the PPT which lasted 20 min, with a 2-minute interval. During this training, participants received either 20-Hz beta-band tACS or sham stimulation. In the online stimulation protocol, a low intensity of 1 mA at 20 Hz (peak-to-peak intensity) was used. The used waveform was sinusoidal, and each trial lasted for 5 minutes. Throughout the stimulation, each participant underwent four trials, with a 2-minute interval between each trial. For the sham stimulation protocol, the current was gradually increased over a 30-second period at a frequency of 20 Hz, maintained at 1 mA for 10 seconds, and gradually decreased for 30 seconds at 20 Hz. The statistical analyses were conducted in SPSS software, version 26. The normality assumption was assessed using the Shapiro-Wilk test. The sphericity assumption was examined using Mauchly’s test. A repeated measures ANOVA was conducted to determine the difference in the scores of bimanual coordination, followed by a Post hoc test using the Bonferroni test. The significance level was set at 0.05.

Results
Figure 1 shows participants’ bimanual coordination scores in trials 1-4 under three different conditions. According to the results of the Shapiro-Wilk test, the distribution of data was normal (P>0.05), Mauchly’s Test confirmed the sphericity assumption for the main effect of condition (X2(2)=2.878, P=0.237), but not for the main effect of trial (X2(5)=22.558, P<0.001) or the interaction effect of condition and trial (X2(20)=54.197, P<0.001); Therefore, the Greenhouse–Geisser correction was used for the main effect of trial and the interaction effect.



According to the results of ANOVA, the main effect of condition (F(2, 52)=1.867, P=0.165, ŋ2p=0.067) and the interaction effect of condition and trial were not significant (F(3.278, 85.24)=1.855, P=0.138, ŋ2p=0.067), but the main effect of trial was significant (F(1.868,48.562)=42.781, P<0.001, ŋ2p=0.622). Therefore, there was no significant difference in bimanual coordination among different conditions. Based on the results of pairwise comparison using Bonferroni post hoc test for the effect of trial (Table 1), bimanual coordination significantly increased trial-to-trial.




Conclusion
The findings of the present study indicated that active stimulation of the P4 and SMA using β-tACS (20 Hz) did not cause a significant difference in bimanual coordination compared to the sham stimulation. However, the results of pairwise comparisons using Bonferroni post hoc test showed that bimanual coordination significantly improved during the online stimulation. This suggests that although the brain stimulation itself may not have had a significant effect, the training accompanied with it lead to improvements in bimanual coordination. These findings are consistent with previous studies conducted by Berger et al. [10] and Choe et al. [11], but inconsistent with the results of Pogosian et al. [12] and Miyaguchi et al. [13]. A review and meta-analysis study by Hu et al. [14] reported that factors such as stimulation mode, location, and frequency can affect the outcome of tACS regarding motor performance. They also found that online tACS had a significant positive effect on motor performance in healthy individuals compared to sham stimulation. Previous research has shown that tACS can modulate neural activity not only after stimulation but also during stimulation, specifically under the electrode [15]. Furthermore, studies have demonstrated that β-tACS over the motor cortex during task performance can also help stabilize practice related performance improvement. It is worth to note that age may be a factor contributing to the discrepancies between the present study and previous studies. Older adults tend to have decreased dexterity and neural plasticity, which might have eliminated the effects of brain stimulation in this study. Additionally, it is important to consider the duration of brain stimulation. The after-effects of tACS are usually observed over a duration longer than 10 minutes. Given that the present study used only 5 minutes of tACS, it is possible that the after-effects were not fully produced. However, the potential repeated effects of the 5-minute tACS should not be neglected, as suggested by Miyaguchi et al. who used a 1-minute tACS and the PPT on young people [13]. Therefore, further research is needed to fully understand the relationship between tACS-induced changes in neural networks and changes in motor performance. The non-significance effect of tACS on bimanual coordination can be due to the complexity of the bimanual coordination task and the unclear mechanisms underlying bimanual coordination [10]. Some studies have highlighted the importance of beta-band activity in interhemispheric coordination of movements [16], while others did not find any effect of tACS on brain oscillations in the beta frequency [10]. Furthermore, although both the amplitude of motor evoked potentials and the amplitude of beta oscillations in the motor cortex are related to cortical excitability, they do not seem to be strongly related to each other [17]. On the other hand, various studies have shown the importance of the dominant hemisphere in bimanual coordination [18]. However, it is still speculative whether this factor can explain the physiological side effects of tACS without changes in bimanual coordination performance.
In general, according to the findings of the present study, tACS over SMA and P4 had no significant effect on older women’ bimanual coordination. Based on the improvement of bimanual coordination in elderly women after using the Purdue pegboard task, it seems that practice can be safely used to improve bimanual coordination in elderly people. These practices are cost effective and accessible, making them suitable for use at home, rehabilitation centers, and other care centers for the elderly. By improving the quality of life and maintaining the independence of the elderly, these practices can also help reduce the additional costs associated with elderly care.

Ethical Considerations

Compliance with ethical guidelines
The study was approved by the Ethics Committee of Ferdowsi University of Mashhad (code: IR.UM.REC.1400.177). Prior to the study, the participants were given the necessary explanations about the study objectives and procedure and signed a consent form. Their personal information was kept confidential, and they were free to leave the study at any time.

Funding
This study was extracted from the doctoral dissertation of Asiyeh Dehghani, registered by the Faculty of Sports Sciences, Ferdowsi University of Mashhad. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Authors contributions
Conceptualization, design, data collection, data analysis, and writing: Asiyeh Dehghani; Review & editing, methodology, and supervision: All authors. 

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgments
The authors would like to thank all participants for their cooperation in this study.