Open access peer-reviewed chapter
Abstract
Motor imagery allows patients with difficulty in voluntary movements to mentally practice a target motor task. Numerous neurophysiological studies have investigated the mechanisms underlying the benefits of motor imagery, but many aspects remain unclear. Since both central and spinal neural function need to be leveraged to improve various motor functions, we have investigated motor imagery and spinal neural functions. Our previous research demonstrated a facilitation effect of motor imagery on spinal neural function and an immediate effect on muscle strength. Specifically, a mild imagined muscle contraction strength may be sufficient to enhance the excitability of spinal motor neurons. In addition, kinesthetic imagery or combined action observation and motor imagery may substantially enhance the excitability of spinal motor neurons. Also, keeping a position of the upper or lower extremities close to the desired movements leads to greater enhancement of the excitability of spinal motor neurons during motor imagery.
Keywords
- motor imagery
- action observation
- sensory modality
- F-waves
- spinal neural function
- muscle strength
1. Introduction
Motor imagery (MI) is a cognitive process that is realized when an individual imagines that he/she is creating specific motor actions without actually performing the action or contracting muscles [1, 2, 3]. MI does not require special equipment and it can be easily performed at any time and location. Therefore, it can be used in a variety of settings, for example, sports medicine or post-operative and post-stroke rehabilitation. A systematic review and meta-analysis of randomized control trials conducted by Zhao et al. [4] revealed that combined MI training and physical therapy is more beneficial for improving the lower limb motor function, gait function (i.e., gait speed, stride length, and cadence), and activities of daily living (ADL) compared to physical therapy alone in post-stroke patients with lower limb motor dysfunction. Similarly, a systematic review and meta-analysis conducted by Li et al. [5] revealed that MI training is effective for improving range of motion, muscle strength, and independence in ADL and to reduce pain in post-operative patients who underwent total knee arthroplasty. In addition, research has demonstrated the effectiveness of MI both on muscle strength for young and old healthy adults [6] and upper limb motor function in post-stroke patients [7]. The number of systematic reviews and meta-analyses on MI training has grown in the past years. While the clinical use of MI is expected to increase in the future, the evidence of its effects and knowledge about the underlying neurophysiological mechanisms are not yet sufficient. In this chapter, we provide an overview of our previous studies and we discuss MI training in evidence-based physical therapy, with a special focus on benefits related to spinal neural function.
2. Central and spinal neural excitability during MI
Various neurophysiological studies using positron emission tomography, functional magnetic resonance imaging, and near-infrared spectroscopy have revealed mechanisms underlying the effects of MI on the central nervous system. Specifically, research has shown that MI-induced activity in the primary motor cortex (M1), supplementary motor area (SMA), premotor area (PM), prefrontal cortex (PFC), parietal cortex, and subcortical areas such as the basal ganglia and cerebellum [8]. The SMA, PM, basal ganglia, and cerebellum, are key structures in motor control, particularly the planning and preparation stages [9, 10]. Furthermore, these areas are activated during both MI and motor execution (ME). Although numerous previous studies have provided evidence that MI and ME share many common neural networks [8, 11], the activity of M1 during MI is controversial. Some studies found a significant increase in the activity of M1 during MI [12, 13, 14, 15, 16] whereas others did not [10, 17, 18, 19]. On the other hand, by evaluating the corticospinal excitability using transcranial magnetic stimulation (TMS) with high temporal resolution, a significantly increased amplitude of motor evoked potentials (MEPs) was observed during MI compared with the rest condition, suggesting elevated corticospinal excitability [20, 21, 22, 23, 24]. These results indicated that MI can increase corticospinal excitability. However, corticospinal excitability is lower during MI compared to ME [22]. Similarly, the observed M1 activity has been reported to be lower during MI compared to ME (i.e., less than a half) [13]. Although many motor-related brain areas are activated during MI, the role of M1 in MI is still unclear.
In addition, research suggests that it is important to assess spinal neural function during MI. Lower motor neurons of the spinal cord, namely α motor neurons, directly command muscle contraction. Descending pathways comprising upper motor neurons in the motor cortex or brainstem centers such as the reticular formation modulate the spatial and temporal patterns of activation of α motor neurons. Therefore, α motor neurons are the final common pathway for integrating various excitatory and inhibitory commands from upper motor neurons and for transmitting the information to the skeletal muscles [25]. To effectively improve motor function, it is necessary to facilitate both the central nervous system and spinal neural function. Indeed, a significant reduction of corticospinal excitability including spinal motor neurons was observed in post-stroke patients [26, 27, 28]. Also, the function of the corticospinal pathway is associated with the acquisition of dexterous motor skills [29, 30]. Therefore, assessing the excitability of spinal motor neurons during MI could help toward a better understanding of the neurophysiological aspects underlying the beneficial effects of MI.
3. Assessing the spinal neural function
The F-wave, H-reflex, and T-reflex are typically used to assess the spinal neural function. In our laboratory, we mainly used the F-wave to assess the excitability of spinal motor neurons during MI under various conditions. The F-wave is defined as compound action potential resulting from the re-excitation (“backfiring”) of spinal anterior horn cells by an antidromic impulse following distal electrical stimulation of α motor neurons [31, 32, 33]. When electrical stimulation is applied to α motor neurons, compound muscle action potentials orthodromically propagate through α motor neurons and can be recorded from the corresponding muscles (M-wave). In addition to α motor neurons, the H-reflex and T-reflex involve the Ia fiber or muscle spindle in their reflex pathway, but the reflex pathway involved in the F-wave generation consists only of α motor neurons. Therefore, the F-wave can be suitable to assess the spinal motor neurons excitability. The main indexes used to estimate the excitability of spinal motor neurons are
4. Excitability of spinal motor neurons during MI
4.1 Background
The observed effects of MI on the excitability of spinal motor neurons are inconsistent across studies. For example, research has shown that the excitability of spinal motor neurons was significantly decreased after 3 hours intentional relaxation and that it recovered quickly after ME. In addition, when sustained intentional relaxation and MI of thumb abduction were performed simultaneously, the excitability of spinal motor neurons was maintained at pre-relaxation levels [35]. Also, excitability increased during MI of isometric ankle plantar flexion movement [36, 37]. These results indicate that MI may enhance the excitability of spinal motor neurons. However, studies have shown that MI of thumb abduction [21, 38, 39] or palmar flexion [20, 40] did not alter the excitability of spinal motor neurons. Another study on speed skaters performing MI in a competition showed varying responses in participants as both increased and decreased excitability was observed in different participants compared to that at rest [41].
In the following section, we will introduce previous research from our laboratory regarding the excitability of spinal motor neurons during MI under various conditions. The protocols were approved by the Research Ethics Committee at Kansai University of Health Sciences and conducted in accordance with the Declaration of Helsinki. In addition, written informed consent was obtained from all subjects prior to participation.
4.2 Influence of sensory inputs on the excitability of spinal motor neurons during MI
The influence of sensory inputs on the excitability of spinal motor neurons during MI was investigated in a sample of 11 healthy volunteers (8 males, 3 females; mean age, 34.0 years) [42]. Subjects were in a supine position on a bed and were asked to fix their eyes on the display of the pinch meter (Digital Indicator F340A; Unipulse Corp., Tokyo Japan). The F-wave was recorded using a Viking Quest electromyography machine ver. 9.0 (Natus Medical Inc., Pleasanton, CA, USA). A pair of silver EEG cup electrodes (10 mm diameter; Natus Medical Inc., Pleasanton, CA, USA) were placed over the thenar muscles and the base of the first dorsal metacarpal bone. The skin was cleaned with an abrasive gel (Neprep® Skin Prep Gel; Weaver and Company, Inc., Aurora, CO, USA). The F-wave was evoked from the left thenar muscles by delivering supramaximal electrical stimulation to the left median nerve at the wrist. Supramaximal stimulus intensity was set at a level 20% higher than the maximal stimulus intensity that could elicit the largest M-wave amplitude.
The baseline excitability of spinal motor neurons was determined by recording the F-wave during 1 minute of relaxation. Then, the holding trial was performed. Specifically, the F-wave was recorded while the participant was holding the pinch meter sensor between the left thumb and index finger. Following the holding trial, participants were instructed to press the pinch meter sensor between the left thumb and index finger with maximal effort for 5 s. Then, for the motor task, subjects were instructed to exert pinch force at their target pinch force [i.e., the pinch force at 50% maximal voluntary contraction (MVC)] for 10 s with visual feedback. In addition, subjects learned isometric left thenar muscle contraction at 50%MVC by performing the motor task for 1 minute. In the MI with sensor trial, subjects performed MI of the isometric left thenar muscle contraction while holding the pinch meter sensor for 1 minute, whereas in the MI without sensor trial, subjects performed the MI without holding the sensor for 1 minute. The F-wave was recorded during MI in both conditions.
Thirty F-waves were recorded in each trial. For the F-wave analysis, the amplifier gain for the M-wave was set at 5 mV per division, 200 μV per division for the F-wave, and a sweep of 5 ms per division. The bandwidth filter ranged from 20 to 3 Hz. The minimum peak-to-peak amplitude of the F-wave was 20 μV [43]. Persistence and F/M amplitude ratio were estimated from the recordings. In addition, we calculated the relative data obtained by dividing persistence and the F/M amplitude ratio during two MI trials with those obtained at rest.
With regard to the MI with sensor condition, the values of persistence during the holding and MI trials were significantly higher than the values obtained at the baseline. The F/M amplitude ratio during the MI trial was significantly higher than that obtained at the baseline. Regarding the MI without sensor condition, the values of persistence observed during the holding and MI trials were significantly higher than that obtained at the baseline. The F/M amplitude ratio during the holding and MI trials was slightly higher than that obtained at the baseline, but the observed differences were not significant. The relative data for persistence and F/M amplitude ratio during MI with the sensor condition were higher than those obtained during MI without the sensor condition.
Overall, the observed changes of F-wave during MI suggest that MI could increase the excitability of spinal motor neurons. In addition, somatosensory inputs, including tactile and proprioceptive, can help increase the effect of MI on the excitability of spinal motor neurons. However, substantial inter-individual variability in excitability was observed and future studies are needed to further elucidate these mechanisms.
4.3 Influence of imagined muscle contraction strength on the excitability of spinal motor neurons during MI
Our first study using MI of isometric thenar muscle activity at 50%MVC showed that MI can enhance the excitability of spinal motor neurons, as measured by the F-wave. In ME, the excitability increased linearly with the strength of muscle contraction [44]. To test the hypothesis that MI and ME share common neural networks and, thus, that the excitability of spinal motor neurons should increase with the imagined muscle contraction strength, we performed an experiment of MI of isometric thenar muscle activity at varying %MVC level (i.e., 10, 30, 50, and 70%MVC) [45].
Ten healthy volunteers (5 males, 5 females; mean age, 28.7 years) participated in the study. Subjects were in a supine position on a bed and were asked to fix their eyes on the display of the pinch meter (Digital Indicator F340A; Unipulse Corp., Tokyo Japan) throughout the experiment. The recording condition for the F-wave was the same as in [42]. The F-wave recordings were conducted under three trials termed rest, MI, and post. In the rest trial, subjects kept relaxation for 1 minute. Before the MI trial, subjects learned the isometric left thenar muscle contraction while exerting the pinch force at 10%MVC for 1 minute. In the MI trial, subjects were instructed to imagine the isometric left thenar muscle contraction at 10%MVC for 1 minute. After the MI trial, subjects kept relaxed without performing MI for 1 minute again. The above process was defined as the 10% MI condition. This process was also performed under three other conditions (i.e., the 30, 50, and 70% MI conditions, respectively). The four MI conditions were randomized and performed on different days.
Persistence F/M amplitude ratios under the four MI conditions were significantly higher than those at the rest trial. Both persistence and F/M amplitude ratio in the post-trial under the four MI conditions were similar to those observed at rest.
Also, Figure 1 shows the relative values of persistence and the F/M amplitude ratio measured in the four MI conditions. Results showed that the relative data for persistence and the F/M amplitude ratio were similar among the four MI conditions.
Figure 1.
Relative data for persistence and F/M amplitude ratio under the four MI conditions. There were no differences in relative data for persistence and F/M amplitude ratio among the four MI conditions. (adapted from reference [
Overall, these results suggested that MI of isometric thenar muscle contraction under various imagined strengths could enhance the excitability of spinal motor neurons and that the imagined strength did not seem to influence the observed effect. Thus, it may be sufficient to perform MI at mild (e.g., 10%MVC) imagined muscle contraction strength to enhance the spinal motor neuron excitability. However, similar to our previous research [42], there was a lot of variability in the excitability of spinal motor neurons during MI and further research is needed.
4.4 Influence of sensory modality on the excitability of spinal motor neurons during MI
Our previous research [42, 45] revealed that MI of isometric thumb opposition movement enhanced the excitability of spinal motor neurons. However, substantial inter-individual variability was observed. It is hypothesized that this variability could be, at least in part, related to different sensory modalities experienced by the tested subjects.
Sensory experience during MI can be classified into two modalities, specifically kinesthetic and visual. Kinesthetic imagery (KI) is defined as a subject imagining muscle contraction corresponding to actual movements. On the other hand, visual imagery (VI) is defined as a subject imagining movements performed by himself/herself (i.e., internal VI) or performed by another person (i.e., external VI). Previous research has shown that KI can activate motor-related brain regions to a larger extent than VI [46]. In addition, a significantly higher MEPs amplitude (i.e., higher corticospinal excitability) was observed during KI compared to VI [22, 23]. Therefore, we hypothesized that the sensory modality experienced during MI may affect the excitability of spinal motor neurons.
We administered a survey to 85 subjects to investigate the sensory modalities used while performing MI [47]. Results showed that three types of sensory modality were used during MI: kinesthetic, tactile/pressure, and visual, used either alone or combined. In Ref. [48], we investigated the possible influence of the sensory modality of MI on the excitability of spinal motor neurons by using kinesthetic and somatosensory perception.
Fourteen healthy volunteers (10 males, 4 females; mean age, 23.4 years) participated in the study. Subjects were in a supine position on a bed and were asked to fix their eyes on the display of the pinch meter (Digital Indicator F340A; Unipulse Corp., Tokyo Japan) during the experiment. The recording settings for the F-wave were the same as in the above described study and are reported in [42, 45]. The baseline of the excitability was determined in a rest trial by recording the F-wave during relaxation for 1 minute. Next, subjects were instructed to exert isometric left thenar muscle contraction at 50%MVC for 1 minute with visual feedback. Subjects were trained with two sensory modalities: kinesthetic (i.e., thenar muscle contraction while exerting the pinch force at 50%MVC) and somatosensory (i.e., tactile and pressure perception of thumb finger pulp while pressing the pinch meter sensor). Following training, subjects performed a randomized sequence of KI, somatosensory imagery (SI), and combined kinesthetic and somatosensory imagery (SKI), which consisted of imagining kinesthetic and somatosensory perception simultaneously. The F-waves were recorded for 1 minute in each trial. Following the MI trials, subjects were asked how vivid could they imagine the three imagery tasks on a five-point Likert scale ranging from 1 (very hard to image vividly) to 5 (very easy to image vividly).
Persistence and the F/M amplitude ratio were analyzed, as in the above-mentioned studies [42, 45]. Results are summarized in Figure 2. Persistence during KI and SI was significantly higher compared to the baseline. The F/M amplitude ratio during KI was significantly higher than that at rest. Both persistence and the F/M amplitude ratio during SKI were higher than that at rest, but the observed differences were not significant. The index of the imagery vividness, as measured using a five-point Likert scale, was significantly lower in the SKI trial than in the KI and SI trials.
Figure 2.
Persistence and F/M amplitude ratio during SI, KI, and SKI trials. Persistence was significantly increased during SI and KI compared to rest (**
Overall, the results of this study indicated that kinesthetic perception may be a more effective sensory modality to enhance the excitability of spinal motor neurons during MI compared to the other sensory modalities investigated. The excitability of spinal motor neurons during SKI was similar to the baseline. This may be, at least in part, related to the relatively low imagery vividness score in the SKI trials, possibly suggesting that subjects could not imagine kinesthetic and somatosensory perceptions simultaneously.
4.5 Influence of combined action observation and MI on the excitability of spinal motor neurons
Previous research [42, 45] has shown inter-individual differences in the spinal motor neurons excitability during MI, which may be influenced by sensory modality during MI or by imagery vividness [48].
Recently, action observation (AO) has received much attention in the fields of sports and neurorehabilitation. AO shares neural substrates with MI and ME, including the PM, parietal cortex, and somatosensory cortices [49]. Numerous TMS studies showed a significant increase in corticospinal excitability during AO [50, 51, 52, 53]. In addition, greater enhancement of brain excitability [54] and corticospinal excitability [55, 56, 57] were observed during combined AO and MI. AO has also been reported to enable more vivid MI [58]. The study outlined below aimed to investigate whether combined AO and MI can enhance the excitability of spinal motor neurons to a larger extent than AO alone [59].
Thirty-one healthy volunteers (23 males and 8 females; mean age, 23.5 years) participated in this study. Subjects were sitting comfortably in a chair with their left forearm resting on the armrest during the experiment. The left forearm was fully supinated, and the hand was kept open and relaxed with the palm facing upward. Prior to F-wave recording, cyclic left thumb opposition movements at 1 Hz were video-recorded using an iPad (Apple, Inc., Cupertino, CA, USA) for 1 minute. Then, F-wave recordings were performed for three trials, specifically: rest, AO, and combined AO and MI (AO + MI). In the rest trial, the F-wave was recorded during relaxation for 1 minute to determine the baseline. In the AO trial, subjects viewed the video of their own cyclic left thumb opposition movements. To ensure subjects recognized the video as if it was their thumb moving to realize the AO modality, the size and position of the hand displayed on the video were adjusted to the size and position of the left hand of the tested subject. For the AO + MI trial, subjects were asked to imagine the left thenar muscle activity while viewing left thumb opposition movements displayed on the iPad for 1 minute. To counterbalance the order effect, 15 subjects performed the tasks in the following order: rest, AO, and AO + MI, whereas the others performed the task in the following order: rest, AO + MI, and AO.
Results are summarized in Figure 3. Persistence during AO and during AO + MI was significantly higher than that at rest. The F/M amplitude ratio during AO + MI was significantly higher than that at rest. Overall, these results indicated that AO + MI may facilitate the spinal motor neurons excitability.
Figure 3.
Persistence and F/M amplitude ratio during AO and AO + MI trials. Persistence was significantly increased during AO and AO + MI trials compared to rest (**
5. Immediate effect of MI on muscle strength
In addition to the study of the spinal motor neurons excitability during MI under various imagery conditions outlined in the above sections, we performed a study on the effects of MI on muscle strength [60].
The first study on the effect of MI on muscle strength by Yue & Cole [61] demonstrated a 22% increase in muscle strength following MI training of little finger abduction at MVC for 4 weeks. Similarly, Sidaway et al. [62] demonstrated a 17% increase in muscle strength of ankle dorsiflexion following MI training of ankle dorsiflexion at MVC for 4 weeks. In addition, MI training for 1 week showed a 10% increase in muscle strength of ankle plantar flexion [63]. Overall, these findings suggest that MI training can increase muscle strength in the upper and lower limbs.
It is well known that physical performance improves immediately after MI, and muscle strength may be involved in this process. However, there were no studies demonstrating the immediate effect of MI on muscle strength. In addition, while previous studies [61, 62, 63, 64] adopted MI at MVC for MI training, no study investigated the effect of MI at submaximal muscle contraction strength on muscle strength. The study summarized in the following aims to investigate the immediate effect of MI at 50% MVC on muscle strength.
Thirty-six healthy volunteers (20 males, 16 females; mean age, 21.8 years) with no history of orthopedic and neurological disease participated in this study. Subjects were randomly divided into two groups using stratification (i.e., 10 males and 8 females were assigned to each group): MI group (mean age, 22.1 years; mean body weight, 56.7 kg); and control group (mean age, 21.6 years, mean body weight, 59.2 kg). There was no statistical difference in age and body weight between the two groups. The dominant leg of all subjects was the right one. The maximal ankle plantar torque was recorded using a Biodex System 3 dynamometer. Subjects were seated in the adjustable chair of the dynamometer. The right ankle joint was kept in neutral position, and the axis of the dynamometer was aligned with the rotation axis of ankle dorsiflexion/plantar flexion. After warm-up, subjects performed 5 s isometric ankle plantar flexion movement with maximal effort for three times as a pre-intervention trial. Next, all subjects were trained on the isometric right ankle plantar flexion movement at 50%MVC using visual feedback for 1 minute. After training, subjects in the MI group performed MI of isometric right ankle plantar flexion at 50%MVC for 1 minute, whereas subjects in the control group kept relaxed for 1 minute without performing MI. In the post-intervention trial, subjects performed 5 s isometric ankle plantar flexion movement with maximal effort for three times.
Maximal ankle plantar flexion torque (Nm) was determined as the average value of peak ankle plantar flexion torque in the three repeated flexion movements. The maximal ankle plantar flexion torque was normalized to body weight and expressed as Nm/kg. Relative indexes were computed by dividing the normalized maximal ankle plantar flexion torque measured in the post-intervention trial to that obtained in the pre-intervention trial.
In the MI group, the normalized maximal ankle plantar flexion torque at the post-intervention trial was significantly higher than that measured in the pre-intervention trial. In the control group, no significant difference in normalized maximal ankle plantar flexion torque between the pre- and post-intervention trials was observed. The relative index for normalized maximal ankle plantar flexion torque computed in the MI group was significantly higher than that computed in the control group (Figure 4).
Figure 4.
Relative data for normalized maximal ankle plantar flexion torque during MI and control groups. Relative data for normalized maximal ankle plantar flexion torque in the MI group was significantly higher than that in the control group (*
These results indicated that MI of isometric ankle plantar flexion at 50%MVC could increase maximal ankle plantar torque immediately after MI. However, this study did not investigate different imagined muscle contraction strengths, and the effects of MI on muscle strength as a function of the imagined muscle contraction strength need to be investigated in more detail.
6. Application of MI in clinical settings
Our previous findings [42, 45, 48] indicated that MI enhances the excitability of spinal motor neurons and that a mild (e.g., 10%MVC) imagined muscle contraction strength may be sufficient for MI training. However, research also suggested that sensory modality and vividness of MI may influence the excitability of spinal motor neurons during MI.
Different techniques can be introduced to make MI more effective. The first method to enhance the effect of MI is providing sensory inputs, including tactile and proprioceptive perception while performing MI. As discussed above, the excitability of spinal motor neurons during MI while holding the pinch meter sensor was significantly higher than that observed during MI while not holding the sensor [42]. Similarly, Mizuguchi et al. [65] demonstrated that the corticospinal excitability during MI of squeezing a ball while touching the ball was higher than that observed during MI while not touching the ball. Noticeably, no increase in corticospinal excitability was observed while touching the ball with no MI. Accordingly, it is important to perform MI training with the position of the upper or lower extremities close to the actual movements. Secondly, combined AO and MI can be effective. Although, the excitability of spinal motor neurons was increased during AO alone, combined AO and MI increased the excitability to a larger extent than AO alone [59]. These methods may activate motor-related neural function more effectively due to synergistic effects of “MI and sensory inputs” or “MI and AO”, and may allow individuals to imagine the movements vividly.
In addition, duration of MI should be considered for MI training. We investigated time-dependent change in the excitability of spinal motor neurons during MI for 5 minutes using the F-wave, and we observed that the excitability of spinal motor neurons at 1-minute and 3-minutes after MI initiation was significantly higher than the baseline, returning to the baseline level after 5 minutes. In addition, subjects could not perform MI vividly for 5 minutes after MI initiation [66]. Mental fatigue caused by repetitive MI of the hand grip movement decreased the corticospinal excitability [67]. These findings indicate that the benefit of MI on the excitability of spinal motor neurons is not necessarily a function of the length of MI. In general, the optimal duration or number of sessions for MI training has not been determined yet, and a detailed investigation is required. In clinical settings, condition of MI training, including sensory inputs, intensity, sensory modalities, the use of AO, and duration, should be set according to the purpose of rehabilitation and individual MI ability.
Finally, the maximal isometric ankle plantar flexion torque was significantly increased immediately after MI at 50%MVC for 1 minute, suggesting that MI at 50%MVC may be beneficial in rehabilitation to improve muscle strength. When utilizing MI in rehabilitation, it is necessary to learn the desired movements in advance, but this may be difficult for specific patients, for example, patients with heart failure who are not able to perform exercise at high intensity. Therefore, MI at 50%MVC may be able to increase the excitability of spinal motor neurons and muscle strength with relatively less physical load than MI at MVC.
7. Conclusion
In this chapter, we presented the influence of MI under various conditions on the excitability of spinal motor neurons and the immediate effect of MI on muscle strength. The F-wave data reported here may provide evidence for elucidating the neurophysiological mechanisms underlying the observed MI effects. Further research is needed to investigate optimal settings for MI in physical therapy and support future, effective application of this technique in clinical settings.
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