Limited function of the paretic upper extremity (UE) is one of the most disabling consequences of stroke [1]. Approximately 55% of stroke survivors had no UE movement following initial rehabilitation [2] which made them more of a burden to their relatives and society as a whole. After stroke, functional recovery is associated with interhemispheric communication. Studies using transcranial magnetic stimulation (TMS) reported that the nonlesioned hemisphere had more corticospinal excitability than the lesioned hemisphere [3-5]. The suppression of the recovery in the lesioned hemisphere leads to persistent functional deficits [3,4,6-9]. Therefore, rebalancing the corticospinal excitability of both hemispheres may support restoration of the function.

The existing evidence supports that the application of low-frequency repetitive TMS (LF-rTMS) could reduce the corticospinal excitability (CE) of the non-lesioned hemisphere (e.g., see [8-13]). TMS is a noninvasive technique that modulates excitability of the cerebral cortex [3,6]. Several studies confirmed that reduction of CE by LF-rTMS leads to the improvement of the paretic hand function [8-14]. Among those studies, only a few study added motor training after a single session of LF-rTMS [11,13,14]. Takeuchi et al. [11] analyzed the combined effects of LF-rTMS with 15-minutes pinch training in chronic stroke. They reported improvement in acceleration of pinching and pinch force in the rTMS group as compared to the pinch training alone group. Vongvaivanichakul et al. [13] examined the effect of LF-rTMS on outcomes following reach-to-grasp (RTG) training in chronic stroke. They reported improvement in movement time on the Wolf Motor Function Test and the time of RTG actions in the experimental group, as compared to the control group. Moreover, the results of Takeuchi’s study [11] showed that the improvement of paretic hand maintained for one week. These findings were possible due to the adjunctive treatment protocol [11]. Of particular interest, there was a review paper that reported the use of non-invasive brain stimulation to augment motor training-induced plasticity [15]. Therefore, an adjunctive protocol is needed to promote the recovery of motor performance. However, the adjunctive protocol used in both studies included a simple pinch task, not a real-world activity. Therefore, it is necessary to identify a type of motor training that involves a daily activity, thereby inducing neural plasticity and long-term behavioral changes.

The Accelerated Skill Acquisition Program (ASAP) and constraint-induced movement therapy (CIMT) are types of task-specific training [16-18]. The ASAP is an innovative, yet fully-defined, evidence-based and theoretically defensible therapy program. It is a hybrid combination of CIMT [19] and skill-based/impairment-mitigating motor learning training with embedded motivational enhancements [16,20,21]. The ASAP provides restoration of the paretic limb by employing tasks that involve real training situations, thereby reducing motor impairments and compensatory strategies [16,21]. CIMT focuses on restraining the movement of the non-paretic hand to prevent adaptive behavior, thus overcoming a phenomenon of ‘learned non-use’ [16-18]. Inhibition of the non-paretic hand is related to the mechanism by which LF-rTMS reduces hyperexcitability of the non-lesioned hemisphere. The aforementioned studies [11,13] reported the combined effects of LF-rTMS and motor training in chronic stroke, its effectiveness with subacute stroke (1–6 months post onset) is however still unclear. We hypothesized that if the adjunctive intervention is assigned to early-onset individuals and the motor training is a task-specific one, the improvement would last more than a week. We based the hypothesis on the principle of experience-induced plasticity, i.e., time matters and task specificity [22]. We recently reported part of the results on the immediate effect of LF-rTMS to enhance task-specific training [14]. Therefore, the objectives of this study were two-fold: (1) to examine the long-term effects of a single session of LF-rTMS combined with task-specific training using the ASAP principles on the paretic hand performance in individuals with subacute stroke, and (2) to determine the relationship between changes in corticospinal excitability of the non-lesioned hemisphere and the improvement of paretic hand function.

We screened 35 individuals that had previously experienced a stroke. Sixteen of these individuals met the inclusion criteria, and randomly grouped into two. No participants reported any adverse side effects, and all participants completed the study. The lesion locations in the 16 participants are shown in Table 1. All of the participants had ischemic stroke, except one participant in the experimental group (participant #006). Additionally, there were 4 right hemispheric stroke in the experimental (real TMS) groups and 5 right hemispheric stroke in the control (sham TMS) group. However, the one nonmatched pair of lesion side between the two groups did not impact their average paretic hand impairment (Fugl-Meyer score) as shown in Table 2. There were no differences in age, level of upper extremity impairment, time since stroke, or cognitive function between the experimental and the control groups (Table 2).

This study examined the effects of a single session of LF-rTMS combined with task-specific training using the principles of ASAP and CIMT on paretic hand performance following subacute stroke. There was a reduction in MEP amplitude of the non-lesioned hemisphere by the LF-rTMS. This study extended the findings of the studies of Takeuchi [11] and Vongvaivanichakul et al. [13] by evaluating the corticospinal excitability of the nonlesioned hemisphere after task-specific training following subacute stroke. The results of this study showed that the reduction in MEP amplitude induced by the real LFrTMS was maintained immediately after task-specific training. Importantly, the improvement in paretic hand performance after task-specific training was enhanced by LF-rTMS and the effects of conjunctive intervention on the hand function persisted for at least 2 weeks.

The possible mechanisms underlying neural plasticity and improvement of the hand performance by the combination of LF-rTMS and task-specific training are explained in this section. One potential mechanism underlying this effect is that the downregulation of excitability of the non-lesioned hemisphere following LF-rTMS enhances the restoration of balanced interhemispheric inhibition (IHI) [6,8,10,26]. There is evidence that interhemispheric interactions are modulated by the kinematics of the movement [27]. It is possible that downregulation of the non-lesioned M1 decreased the IHI to the ED representational area, thereby improving the hand functions. LF-rTMS leads to the release of the gammabutyric acid (GABAA), which is related to long-term depression (LTD). This process results in the modulation of corticospinal excitability within the human cortex via an inhibitory circuit [20,28]. Similarly, motor training can promote neural plasticity [29-31]. A recent meta-analysis brought about the positive impact of motor training on upper extremity functions. In the same analogy with TMS, task specific training could induce the modulation of NMDA activation receptor and GABAergic inhibition. Applying LF-rTMS before task-specific training might be the prime functional networks for the training [23,32-34]. Therefore, one possible mechanism is that their combination might maximize the effect of each other [15]. Additionally, preventing movement of the non-paretic hand during task-specific training may have contributed to decrease excitability of the non-lesioned hemisphere. This have may enhanced the effect of LF-rTMS with the similar mechanism. These findings are consistent with the results of previous studies [16-18].

The difference in the average MEP amplitude of the non-lesioned hemisphere was insignificant 1 week and 2 weeks after training between the two groups. The MEP amplitude in the experimental group remained decreased after 1 week and 2 weeks of training, whereas, there was no change in the mean MEP amplitude in the control group. These findings are different from the results of the study by Takeuchi et al. [11], who reported that the percentage change in average MEP amplitude of the non-lesioned hemisphere ultimately returned to the baseline level. The simple pinch task used in their study may not have been sufficient to exert a long-term effect.

In addition to corticospinal excitability, hand performance was another variable to reflect the behavioral outcomes of post-stroke rehabilitation. We examined the combined effects of LF-rTMS and task-specific training on the function of the paretic hand using the WMFT. Task-specific training was conducted after a single session of LF-rTMS in both groups. The results showed that compared to the control group, the experimental group showed a greater decrease in TMT of the paretic hand relative to baseline immediately after receiving the real LF-rTMS. The TMT of the experimental group was further decreased after training. Thus, these findings indicate that the benefit of task-specific training was augmented by LF-rTMS. Our protocol was similar to that used in the studies by Takeuchi et al. [11] and Vongvaivanichakul et al. [13]. However, we further extended the protocol of motor training employed by these two studies to more daily functional tasks, i.e., reaching and grasping. This study demonstrated the beneficial combined effect of LFrTMS and task-specific training resulting in the sustained decrease in TMT for at least 2 weeks.

This study’s results are different from those of the study by Takeuchi et al. [11] because their improvements in motor performance lasted for 1 week while ours were longer. Thus, it is essential to combine task-specific training with LF-rTMS. In 2013, Higgins et al. [35] applied 1,200 pulses of LF-rTMS and 90 minutes of task-specific training based on daily activities, 2 sessions/week for 4 weeks. Their results revealed insignificant improvements in WMFT time or grip strength between or within the two groups after the intervention or at the 4-week follow-up visit. The lack of significant improvement may have been due to the small sample size used in that study (n=9). Subsequently, in 2014, Rose et al. [26] applied LF-rTMS combined with functional task practice to chronic stroke patients. All participants received 16 sessions of either active LF-rTMS or sham LF-rTMS followed by 1-hour functional task training of the paretic hand. The results showed insignificant changes in WMFT time or grip force either from baseline to after the intervention or from after the intervention to the 1-month follow-up visit between the two groups. These results are consistent with the study by Higgins et al. [35], although Rose et al. [26] used a larger sample size. Thus, these unexpected findings were possibly due to timing from stroke onset. Both studies [26,35] recruited participants with chronic stroke. Perhaps the combined effects of TMS and training were not sufficiently robust to induce an improvement in paretic hand function following chronic stroke. In contrast to those studies, this study investigated participants with subacute stroke. Accordingly, our findings suggested that the combined effects of LF-rTMS and task-specific training should be assigned to early-onset stroke patients, supporting the principle of experience-induced plasticity, i.e., ‘the earlier the better’ [22]. Although Kakuda et al. [27] reported improvement of UE function in chronic stroke after the combination of rTMS and occupational therapy, the efficacy of the intervention should be confirmed with the control group included. Therefore, this study is the only controlled study that demonstrated the persisted effects of LF-rTMS to augment task-specific training-induced plasticity following subacute stroke.

Regarding hand performance, the movement time on the WMFT included six items. The results of this study showed an insignificant difference in TMT between or within the two groups except for the can lifting item of the experimental group, whose results showed a significant difference in the MT of the paretic hand immediately, 1 and 2 weeks after task-specific training as compared to the baseline. On the contrary, the control group insignificant differences in the MT. Therefore, these findings are possibly due to the similarity of the can lifting task to the task-specific training. The postulation is supported by the analysis of each participant such that most of the participants selected grasping a glass without a handle as their training tasks. Another possible explanation for our result is the target of TMS stimulation, i.e., the wrist extensor muscle, is the essential mediator of lifting a can, so this stimulation may resolve the impairment in the performance of the can lifting task.

Regarding grip strength, there were insignificant differences between the two groups at each testing time. However, there was a significant improvement from immediately after LF-rTMS to 2 weeks after training in the real rTMS group but not in the sham TMS group. These findings indicate that the application of LF-rTMS alone or immediate effects of LF-rTMS combined with task-specific training was insufficient to improve the grip strength of the paretic hand. In the long run, the improvement of the paretic hand in the real rTMS group may have been induced by the combined effects of LF-rTMS, task-specific training, and daily activities. Participants were motivated to build up the confidence and self-management which might have encouraged them to use their hand in their daily activities [21]. The enhanced effect through the mesocortical pathway motivated them to continuously use the affected hand thereby improved grip strength at the retention test of week 2 after the training. During the follow-up period, both groups performed daily activities and received physical therapy. However, the effects of daily activities and physical therapy alone were insufficient to change the grip strength as shown in the sham group.

In conclusion, this study established that the effects of a single session of LF-rTMS on the non-lesioned hemisphere combined with task-specific training led to the paretic hand improvement following subacute stroke. This improvement may be as a result of maximizing each individual effects thereby induced the long-term effects persisted for at least 2 weeks. This study suggests that combining the protocol is a useful intervention for individuals with subacute stroke who has mild to moderate impairment.