Somatosensory &M otor Research

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Somatosens M o t Res, 2015; 32(1): 1 -7

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© 2015 Inform a UK Ltd. DOI: 10.3109/08990220.2014.937413

ORIG INAL ARTICLE

EMG activity of finger flexor muscles and grip force following low-dose transcutaneous electrical nerve stimulation in healthy adult subjects Michal Kafri, Nir Zaltsberg, & Ruth Dickstein Department o f Physical Therapy, Faculty o f Social Welfare and Flealth Sciences, University o f Haifa, Haifa, Israel

A b s tra c t

K e yw o rd s

Somatosensory stimulation modulates cortical and corticospinal excitability and consequently affects motor output. Therefore, low-amplitude transcutaneous electrical nerve stimulation (TENS) has the potential to elicit favorable motor responses. The purpose of the tw o presented pilot studies was to shed light on TENS parameters that are relevant for the enhancement o f tw o desirable motor outcomes, namely, electromyographic (EMG) activity and contraction strength o f the finger flexors and wrist muscles. In 5 and 10 healthy young adults (in Study I and Study II, respectively) TENS was delivered to the volar aspect o f the forearm. We manipulated TENS frequency (150 Hz vs. 5 Hz), length o f application (10, 20, and 60 min), and side of application (unilateral, right forearm vs. bilateral forearms). EMG amplitude and grip force were measured before (Pre), immediately after (Post), and following 15 min of no stimulation (Study I only). The results indicated that low-frequency bursts o f TENS applied to the skin overlying the finger flexor muscles enhance the EMG activity o f the finger flexors and grip force. The increase in EMG activity of the flexor muscles was observed after 20 min o f stimulation, while grip force was increased only after 1 h. The effects of uni- and bilateral TENS were comparable. These observations allude to a modulatory effect o f TENS on the tested motor responses; however, unequivocal conclusions o f the findings are hampered by individual differences that affect motor outcomes, such as in level o f attention.

EMG, muscle force, motor function, transcutaneous electrical nerve stimulation

In tro d u c tio n

The importance of the somatosensory system to motor function is well recognized (Miles 2005; Floel and Cohen 2006; Dobkin 2009; for recent reviews, see Bergquist et al. 2011; Pleger and Villringer 2013). In physical rehabilitation, frequent attempts are made to utilize somatosensory stimuli to facilitate or enhance motor output (Floel et al. 2004; Hummel and Cohen 2005; Floel and Cohen 2006). In fact, several treatment approaches applied by physical therapists include specific procedures of touch and proprioceptive stimulation in order to guide and shape favorable motor responses (Knott and Voss 1968). Yet, despite demonstrations of the enhance­ ment of motor functions by somatosensory stimuli (Floel et al. 2004; Wu et al. 2006; Conforto et al. 2008, 2010; Golaszewski et al. 2010, 2012), procedures applying somato­ sensory interventions for enhancing favorable motor out­ puts in physical rehabilitation are not well established (Golaszewski et al. 2012). Obviously, conclusions are ham­ pered by alterable intervention methods, by variability of response between and within individuals, and by the lack of sensitive measurement tools to quantify both the Correspondence: M. Kafri, Department of Physical Therapy, Faculty of Social Welfare and Health Sciences, University of Haifa, Mount Carmel, Haifa 3498838, Israel. Fax: +972 4 8288140. E-mail: kafri.michal@ gmail.com

H is t o r y

Received 7 April 2014 Revised 8 June 2014 Accepted 16 June 2014 Published online 25 July 2014

somatosensory stimuli (such as touch, pressure, or stretch that are applied during therapeutic interventions) and motor outcomes. Transcutaneous electrical nerve stimulation (TENS) is a type of electrical somatosensory stimulation that is frequently used in physical rehabilitation/therapy for pain relief (Belanger 2002; Kara et al. 2010). TENS elicits discharges in type la and type II fibers (Akyuz et al. 1995; Hiraoka 2002) and has the potential to affect CNS (central nervous system) excitability and subsequently influence motor output. The amplitude, frequency, form, pulse width, site, and duration of TENS application can be modified, enabling the administra­ tion of various combinations of stimulus parameters. However, the optimal combination of TENS parameters for enhancing motor output are not yet resolved. The goal of the current pilot work was to test the effects of a combination of stimulation parameters that could potentially enhance the motor performance of unilateral manual grip (i.e., electromyographic (EMG) activity and grip force). We were specifically interested in three parameters: TENS frequency, TENS application duration, and body side of TENS application. These parameters were chosen because of their presumed effect on unilateral motor responses. Briefly, regarding frequency, though the effects of both high- (30 Hz and above) and low-frequency bursts (3-5 Hz) have been reported (Pitcher el al. 2003; Tinazzi et al. 2005;

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Dickstein and Kafri 2008), no explicit recommendation about the enhancement of a specific motor output can be made. Likewise, varying durations of 15 min (Akyuz et al. 1995) to 2h of stimulation (Conforto et al. 2008; Koesler et al. 2008; Klaiput and Kitisomprayoonkul 2009) have been applied in different populations and settings with no unequivocal recommendation. Regarding side, the bilateral nature of the sensory and motor cortical network of each hand justifies the trial of bilateral stimulation as a means to enhance cortical response on either side (Korvenoja et al. 1995; Zhu et al. 2007; Conforto et al. 2008; Borchers et al. 2011). We conducted pilot studies in which the effects of lowdose (sensory threshold) TENS on the enhancement of EMG activity and grip force in healthy adults were tested. In these studies, either TENS frequency (high, low) and/or application side (uni- vs. bilateral) and/or treatment duration were manipulated. We consider these studies as pilot studies because of their preliminary and exploratory nature. In one already published study, we investigated the effects of uni- and bilateral low-dose high-frequency (100 Hz) TENS on finger flexor muscle activation and grip force. The results indicated that both unilateral and bilateral TENS were associated with an increase in the underlying muscle activity and grip force, with gains maintained for 15 min post application (Dickstein and Kafri 2008). In view of inconsistent reports regarding positive responses to low-frequency burst trains stimulation reported by others (e.g., Ridding et al. 2001; Celnik et al. 2007; Klaiput and Kitisomprayoonkul 2009), we performed two additional studies, which are presented below. In the first (Study I), we compared the effects of 10- and 20-min applications of low-frequency TENS to the effect of high-frequency TENS during uni- and bilateral stimulation. In the second study (Study II), only low-frequency stimulation was bilaterally applied; however, based on prior reports (Ridding and Uy 2003; Beekhuizen and Field-Fote 2008; Conforto et al. 2008; Klaiput and Kitisomprayoonkul 2009), length of application was extended to a period of 1 h. Both study protocols, described in detail below, were approved by the local IRB (institutional review board), with each participant signing an informed consent form clearly delineating the rights of subjects. Study I

Methods Participants were five male university students (mean age: 30 years, range: 27-33 years), who were healthy, right-hand dominant by self-declaration. The TENS stimulation protocol This included six stimulation conditions: unilateral and bilateral low-frequency TENS, unilateral and bilateral highfrequency TENS, and unilateral and bilateral control stimulation. These stimulation paradigms were applied on six different randomly ordered days within a 14-day period. Randomization of conditions was performed by generating random numbers in Excel (Microsoft Office 2007) and then

Somatosens Mot Res, 2015; 32(1): 1-7

Figure 1. Placement of the electrodes during the transcutaneous electrical nerve stimulation (TENS) application.

sorting the conditions according to the random number column. The study protocol was similar to that applied in our first study (Dickstein and Kafri 2008), yet with the addition of low-frequency stimulation. In all sessions, subjects were seated in front of a table with their forearms placed on its surface. In the “ unilateral” stimulation condition, TENS was applied unilaterally to the skin overlying the volar aspect (over the forearm flexors) (Tinazzi et al. 2005) of the right forearm (see Figure 1), while in the “ bilateral” condition, stimulation was applied to the same site bilaterally. During the stimulation sessions, TENS (Elpha 3000 II unit, Danmeter, Odense, Denmark, biphasic waves), either high (pulse width 200 ps, frequency of 150 Hz) or low (five bursts of 200 ps) frequency, was delivered for two successive periods of 10 min (separated by a “ mid-test” ; see below) via self-adhesive 25 cm2 electrodes. Intensity was adjusted to the sensory level of each participant, described as a slight tingling sensation (stimulation amplitude was between 5 and 6 mA). During the control stimulation, subjects were told that they were receiving a subliminal dose of TENS while stimulation intensity was set to 0 mA. Testing EMG activity and maximal grip force were obtained from the right upper extremity. In each session, prior to TENS application, EMG electrodes (Delsys Myomonitor, Delsys, Boston, MA, USA) were adhered to the skin overlying the right-sided flexor muscles and extensors of the wrist and lingers (location determined by Basmajian and Blumenstein 1980). A JAMAR dynamometer (Sammons Preston, Bolingbrook, IL, USA) was placed in the palm of the right hand between the fingers and the thumb. Following a familiarization trial, four tests—the “ pre­ test” , “ mid-test” , “ post-testl ” , and “ post-test2” —were performed at fixed time points: the pre-test was conducted before the delivery of TENS stimulation; the mid-test was conducted after 10min of TENS stimulation; the post-testl following 20 min of stimulation; and the post-test2 following a non-stimulation rest period of 15 min that started immedi­ ately after post-testl. During this rest period, subjects were asked to remain seated and to watch a nature video film that was displayed on a computer screen located in front of the subjects.

EMG and grip force following sensory TENS

DOI: 10.3109/08990220.2014.937413

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Table I. Mean root mean square (RMS) values (SD) of EMG amplitude (mV) for each of the study conditions. Control Bilateral Pre Mid Post 1 Post 2

0.07855 0.07547 0.07537 0.08644

(0.05) (0.05) (0.04) (0.03)

High frequency Unilateral

0.05255 0.05968 0.06443 0.05927

(0.04) (0.04) (0.05) (0.04)

Bilateral 0.07037 0.08276 0.08412 0.07809

(0.04) (0.04) (0.05) (0.04)

The testing paradigm in each of the four tests was composed of a 1-s baseline, during which no voluntary contractions were made by the subject, followed by four 1-s-long repetitions of maximal voluntary grip contractions separated by a cessation of 4 s. Performance rate was controlled by audio signals (beeps) delivered through the EMG unit. During the beep period, subjects were encouraged by the examiner to exert maximal force on the dynamometer handle. The raw EMG data (bandwidth 0-1000 Hz) were sampled at 1024 Hz and stored for later analysis. Peak grip force of the four contractions was recorded as well. EMG data management Following preliminary observations, the raw EMG data were exported into a MATLAB program. Using MATLAB, noisy measurements (i.e., SD higher than 1.5 mV) were discarded and a notch filter of 50 Hz was applied to eliminate electricity-net interference. Subsequent to data observation, they were root mean squared using a window of five data points with an overlap of two points. Afterward, a period of 250 ms of EMG activity (651-900 ms from the initiation of each contraction) was delimited for further analysis. The selection of the designated time point and duration was based on observations confirming that for all subjects, the highest EMG amplitude was recorded during that period. Inspection of the baseline data was also performed via the MATLAB program, with removal of noisy information due to suspected movement artifacts. A baseline average was then calculated. This latter value was subtracted from each data point recorded during each of the four repetitions of maximal voluntary grip contractions. Subsequently, the mean value of the 250 data points of interest from each repetition (i.e., altogether 1000 data points) was separately averaged for each one of the four tests (i.e., pre-test, mid-test, post-testl, and post-test2). In order to obtain the change in EMG amplitude, the values of the pre-test were then subtracted from those calculated for the mid-test, post-testl, and posttest2. These final values were used for further statistical analysis. Statistical analysis Descriptive and Wilcoxon signed rank tests were applied to test the effects of stimulation on the amount of mean EMG activity during the designated 250-ms period, as well as on maximal grip force. The analysis was separately performed for the effects at the mid-test, post-testl, and post-test2 time points.

Low frequency

Unilateral 0.07226 0.06719 0.08875 0.07475

(0.04) (0.03) (0.08) (0.04)

Bilateral 0.07302 0.08825 0.07472 0.09913

(0.02) (0.03) (0.03) (0.04)

Unilateral 0.06667 0.07534 0.07465 0.07346

(0.027) (0.031) (0.03) (0.02)

R e s u lts

Effects o f low- and high-frequency stimulation on EMG activity o f the finger flexors Unilateral low-frequency burst stimulation caused an increase in EMG amplitude at post-testl, which was signifi­ cant ( z = —2.02, p —0.04). Bilateral low-frequency stimula­ tion similarly caused an increase in EMG amplitude, which was significant only at post-test2 ( z = —2.02, p = 0.04). On the other hand, the effect of either uni- or bilateral high-frequency stimulation was not significant for any measurements. The effect of uni- or bilateral control TENS on the EMG amplitude of finger flexors was also not significant. Mean group data are presented in Table I. Effects o f uni- vs. bilateral stimulation on EMG activity o f the finger flexors The comparison between the effects of unilateral vs. bilateral stimulation yielded non-significant differences for measure­ ments made at all four time points. The effect o f all stimulation modes on maximal grip force This was not significant at any time point during or after stimulation. S t u d y II R a t io n a l e

The results of Study I pointed to low-frequency bursts enhancing EMG amplitude of the finger flexors, with the effect being statistically significant. In view of the fact that in a previous study we noted EMG enhancement by high-frequency stimulation (Dickstein and Kafri 2008), we were interested in substantiating the positive effect of lowfrequency TENS stimulation in a successive experiment. Thus, in Study II, the number of subjects was increased and the number of intervention variables was reduced. In addition, in an attempt to optimize the effect of low-frequency burst stimulation, we applied the stimulation for I h; this time period is relatively long and better compatible with the stimulation periods reported by others (Ridding and Uy 2003). M e th o d s

Ten healthy university students (3 men, 7 women, right handed, 24-34 years old) with no history of neurological or muscular diseases participated in the study.

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Data collection and analysis These were similar to the method applied in Study I except for the following. Stimulation (1) Only low-frequency 5-Hz bursts were applied with each train composed of stimulations with a pulse width of 200 ps; (2) only bilateral stimulation was applied; (3) total stimulation time lasted 1 h; and (4) while applying the stimuli, patients were allowed to read, listen to music, or watch a movie. They were asked to maintain a sitting position and were not allowed to walk, eat, or drink. During this period, the stimulation intensity was adjusted to maintain a slight tingling sensation. Testing Only pre- and post-test measurements were made. As in Study I, the pre-test was conducted before the delivery of TENS stimulation and the post-test was conducted immediately at termination of the 1-h stimulation period. EMG data management This was performed in two ways. The first completely mimicked the protocol applied for Study I. In the second, the delimited period of the EMG data window that was used for analysis was not fixed; rather, for each of the four voluntary contractions, the maximum average of 250 con­ secutive points was calculated (as opposed to 650-900 points in the first method). Thus, the window chosen for each subject always included the maximal amount of EMG activity regardless of the position of that window within the I s of contraction. Statistical analysis Descriptive and Wilcoxon signed rank tests were used for analysis. The analysis made for the first and for the alternative way of EMG data management yielded the same results; therefore, the mean EMG amplitudes at pre- and post-test that were calculated over the 250-ms period as defined in Study I, are reported. R e s u lts

Mean EMG amplitude at pre- and post-test were 0.78 (0.03) and 0.88 (0.04) mV, respectively. Although group mean values pointed to activity enhancement after stimulation, the statistical analysis indicated that the difference between these values at pre- and post-stimulation was not significant; still noteworthy, however, EMG activity at post-stimulation measurement became enhanced in 7 out of the 10 subjects. As for grip force, the individual results indicated enhance­ ment of force at post-test in 6 out of 10 subjects without force reduction in any of the subjects. Mean group values were 34.2 (8.6) and 35.7 (8.1) kg at pre- and post-stimulation, respect­ ively. These differences were significant (p < 0.03). D is c u s s io n

The results of the two presented studies showed enhancing effects of either uni- or bilateral low-frequency bursts of

Somatosens Mot Res, 2015; 32(1): 1-7

TENS applied to the skin overlying the finger flexor muscles on EMG activity of the finger flexors and wrist muscles (Study I) and on grip force (Study II). The increase in EMG activity of the flexor muscles was observed after 20 min of stimulation, while grip force was increased only after 1 h of stimulation. The control non-stimulation condition had no effect on the EMG flexor muscles’ activity or grip force. These observations point to a modulatory effect of TENS on the tested motor responses. The effects of uni- and bilateral TENS were comparable, while EMG activity of the wrist and finger extensor muscles was not affected by the stimulation. Abundant studies have focused on the effect of TENS on motor functions in patients, especially in individuals post-stroke. The focus of our studies, however, was on the effects of low-amplitude (sensory threshold) stimulation on healthy subjects because gaining insight into the responses of healthy individuals is fundamental for the appraisal of patients’ responses to similar interventions. We discuss below the TENS parameters which were manipulated in our studies (frequency, duration, and sided­ ness), as well as the implications for motor outcome variables associated with such stimulation. TENS fre q u e n c y

Our findings in Study I indicating that the effect of highfrequency TENS on EMG output was not significant, do not support those of our prior work in which such an effect was recorded and maintained for 15 min post-stimulation (Dickstein and Kafri 2008). Yet, it should be noted that, although not significant, there was an increase in mean EMG amplitudes after the high-frequency stimulation and at the 15-min follow-up. Therefore, we conjecture that the current insignificance of the findings for high-frequency stimulation could result from low power emanating from a low effect combined with a small sample size. TENS frequency can modulate cortical and peripheral sensory and motor excitability, with increased excitability associated with an increase in motor output and decreased excitability associated with reduced motor output. The inconsistent direction of the modulatory effect of either high- or low-frequency TENS that we observed is supported by reports in the literature, as demonstrated in the following examples. Inhibitory effects of high-frequency stimulation were reported by Tinazzi et al. (2005), who showed that in healthy subjects, 30 min of 2-s 150-Hz trains TENS caused transient inhibitory effects in the stimulated flexor carpi radialis muscle (though facilitatory effects on the antagonist, the extensor carpi radialis muscle). Mima et al. (2004) applied 30 min of high-frequency (90 Hz) TENS at below motor threshold to the thenar eminence and reported on a transient reduction in motor evoked potentials (MEPs) and increased sensory thresholds. This suggested to the authors that short-term highfrequency TENS might have an inhibitory effect on both the sensory and motor systems. High-frequency TENS that was applied for 60 min to the anterior thigh area overlying the rectus femoris muscle similarly caused a decrease in the MEP of the rectus femoris muscle, which lasted more than 30 min

EMG and grip force following sensory TENS

DOI: 10.3109/08990220.2014.937413

following the stimulation (Leonard et al. 2013). Furthermore, high-frequency TENS to the forearm muscle was shown to decrease the excitability of a limited area of motor cortex, but a wider area of primary somatosensory cortex. The conclusion drawn was that high-frequency TENS to the forearm muscle modulates excitability of the primary som­ atosensory cortex and the motor cortex in a different manner (Murakami et al. 2010). Several other studies showed a facilitatory effect of somatosensory stimulation on cortical excitability: in an fMRI (functional magnetic resonance imaging) study in which mesh glove (whole hand) afferent stimulation at 50 Hz and sub-threshold intensity was applied for 30 min, an increase of movement-related BOLD (blood-oxygen-level dependent) responses was seen within the primary motor and primary somatosensory areas of both hemispheres. Two hours post-stimulation, this modulatory effect diminished to baseline level except within the contra­ lateral primary motor region. It was concluded that the localization of the BOLD response within the sensorimotor cortex reflects an increase in neuronal activity (Golaszewski et al. 2004). Complementary to these findings, other findings show that low-frequency (3 Hz) stimulation of the median nerve at below motor threshold affects SII, with an increase of activity during active attendance to peripheral stimuli (Backes et al. 2000). A different testing paradigm is the application of peripheral stimulation together with central stimulation (trans-cranial magnetic stimulation, TMS). The outcome is assessed by the EMG response to TMS at rest, as an indicator of corticospinal excitability. A common denominator of findings of this group of studies was an increase in MEP after high-frequency TENS, indicating increased cortical excitability. For example, Pitcher et al. (2003) noted facilitation of corticospinal excitability induced by 30-Hz TENS paired with TMS and depression following lowfrequency TENS (3 Hz) paired with TMS. In another study, various schedules of somatosensory electrical stimulation at several sites of the upper extremity paired with TMS were examined. Significant facilitation of MEPs was generally observed; yet, in some instances, MEPs were not enlarged and occasionally were significantly depressed (Charlton et al. 2003). T E N S d u r a t io n

Stimulation time of either peripheral or paired associative stimulation (peripheral stimulation in combination with TMS) varies in different studies, with the most common treatment periods being 15 min (Akyuz et al. 1995), 30 min (Goulet et al. 1997; Ridding and Uy 2003; Golaszewski et al. 2004; Tinazzi et al. 2005; Ridding and Flavel 2006), 6 0 min (Pyndt and Ridding 2004; Leonard et al. 2013), and 2h (Koesler et al. 2008). The rationale for the selection of a particular duration was usually not reported; yet, it seems to be established that adaptation of the sensorimotor cortex to repetitive stimulation is a “ time-consuming” process requiring rela­ tively long stimulation time (Manto et al. 2006; Bliss and Cooke 2011). In our own studies, a stimulation period of 1 h of low-frequency bursts demonstrated no advantage in

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enhancing EMG activity in the finger flexors over the effect of shorter (10 and 20 min) stimulation periods. Yet, a significant increase in grip force was observed only after 1 h of TENS stimulation. S id e d n e s s

In almost all reports, stimulation was one-sided and motor response was recorded on the same side. This includes response modulation to simultaneous stimulation of two points on the same hand (sometimes connoted as paired stimulations (e.g., Ridding and Uy 2003)), as well as studies in which peripheral and central (TMS) stimulation were applied in a predefined time window (with the later paradigm also called paired associative stimulation (Ilic et al. 2011; Krivanekova et al. 2011)). Nevertheless, current evidence of bilateral cortical representation of each hand (Korvenoja et al. 1995; Zhu et al. 2007) justifies bilateral stimulation as a means to enhance cortical response on either side. In addition, the interaction between bilateral sensorimotor cortices during bi-manual coordinated movements (Shibuya and Ohki 2004; Sutherland and Tang 2006) also points to the advantage of bilateral stimulation for facilitating either unilateral response. Even so, we were not successful in demonstrating an advantage of bilateral over unilateral TENS application in the facilitation of unilateral EMG response or grip force. M o t o r o u tc o m e v a ria b le s

The outcome variables used in our study—EMG activity and grip force—were measured during a natural voluntary move­ ment. In contrast, several other studies that evaluated the motor response to somatosensory stimulation used methods in which the response was evoked with the subject being passive (e.g., response to TMS) (Pitcher et al. 2003; Manto et al. 2006; Golaszewski et al. 2012). The underlying assumption in either case is that the motor response is affected by modulation of cortical or peripheral excitability induced by the somatosensory stimulation. However, measurement of the motor outcome of a natural movement is more ecologically valid and is therefore more relevant for further clinical studies. Especially pertinent is the finding of an increase in grip force after 1 h of TENS, which provides direct evidence for improvement in motor behavior. A complementary recent finding indicated that—with the use of peripheral electrical stimulation—voluntary drive was required to induce change in corticomotor excitability in healthy participants (Taylor et al. 2012). Another factor related to the ecological validity of the results is the level of attention of the subject. This factor is hard to control and might explain the variability in our own as well as in other studies. Thus, important knowledge regarding possible enhancing or depressing interferences remains unknown. The difficulty in separating the effects of attention, like spatial attention, from changes in motor performance has been acknowledged by several investigators (Eimer and Forster 2003; Rosenkranz and Rothwell 2004; Stefan et al. 2004). Apparently, experimental paradigms that mimic natural life situations, with subjects’ attention directed towards issuing maximal voluntary response (e.g., Taylor et al. 2012), are more ecologically valid than studies in which only automatic motor responses are monitored.

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Based on our three carefully applied methods, we suggest that the modulatory effect of low-amplitude somatosensory stimulation on a specific motor output in healthy subjects in real-life situations is variable and not amenable to generalization across individuals. Noteworthy, according to King and Sluka (2001), even for pain relief, either low- or high-frequency TENS is equally successful in reducing secondary mechanical hyperalgesia. In the same way, opti­ mal parameters of somatosensory stimuli for ensuring a desirable motor outcome seem to be at least partially dependent on personal attributes. The idea of the existence of individualized unique inhibition-facilitation profiles (Xiaoming et al. 2014) is supportive of that suggestion. Thus, in view of the complexity of the environmental and individual factors that determine motor-functional output, we wonder if the application of prescribed protocols of TENS is advantageous for the optimal enhancement of motor output. Acknowledging the selection of optimal stimulation param­ eters in the neurorehabilitation of patients with hand motor deficits after brain damage, more than one combination of stimulation parameters might be possible (Bergquist et al. 2011; Golaszewski et al. 2012). A major limitation of our studies which stems from their pilot nature is the small number of participants, limiting the statistical analysis to non-parametric procedures and prevent­ ing analyses such us multivariate analysis. However, the findings of statistically significant differences between preand post-TENS, demonstrate that the study was powered to detect changes for both EMG amplitude and grip force. Further studies with larger sample size would more compre­ hensively assess the effect of TENS on motor outcomes and explore interaction between different TENS variables. Another limitation of the study is that Study II had no control condition. However, the design of Study II was based on the findings of Study I which demonstrated no significant effect for the control condition. Conclusions

TENS can modulate motor responses; however, when applied in the clinic, response characteristics should probably be individually monitored in order to optimize stimulation parameters and favorable outcomes. D eclaration o f interest

The authors report no conflicts of interest. References Akyuz G, Guven Z, Ozaras N, Kayhan O. 1995. The effect of conventional transcutaneous electrical nerve stimulation on somato­ sensory evoked potentials. Electromyogr Clin Neurophysiol 35: 371-376. Backes WH, Mess WH, van Kranen-Mastenbroek V, Reulen JPH. 2000. Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention. Clin Neurophysiol 111:1738-1744. Basmajian J, Blumenstein R. 1980. Electrode placement in EMG biofeedback. Baltimore: Williams & Wilkins. Beekhuizen KS, Field-Fote EC. 2008. Sensory stimulation augments the effects of massed practice training in persons with tetraplegia. Arch Phys Med Rehabil 89:602-608.

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DOI: 10.3109/08990220.2014.937413

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EMG activity of finger flexor muscles and grip force following low-dose transcutaneous electrical nerve stimulation in healthy adult subjects.

Somatosensory stimulation modulates cortical and corticospinal excitability and consequently affects motor output. Therefore, low-amplitude transcutan...
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