Journal of Musculoskeletal J Musculoskelet Neuronal Interact 2021; 21(4):533-541 and Neuronal Interactions Original Article Effects of different vibration frequencies on spinal cord reflex circuits and thermoalgesic perception Hatice Kumru1,2,3, Sergiu Albu1,2,3, Semra Oguz1,4, Narda Murillo1,2,3, Giuseppe Lucente2,3,5, Josep Valls-Sole6 1 Fundación Institut Guttmann, Institut Universitari de Neurorehabilitació adscrit a la UAB, Barcelona, Spain; 2 Univ Autonoma de Barcelona, Bellaterra (Cerdanyola del Vallès), Spain; 3 Fundació Institut d’Investigació en Ciències de la Salut Germans Trias i Pujol, Badalona, Spain; 4 Department of Physiotherapy and Rehabilitation, Faculty of Health Sciences, Marmara University, Istanbul, Turkey; 5 Neuromuscular and Neuropediatric group, Neuroscience department, Hospital Universitario Germans Trias I Pujol, Badalona, Spain; 6 IDIBAPS (Institut d’Investigació Biomèdica August Pi i Sunyer), Barcelona, Spain Abstract Objectives: We studied the effect of different vibration frequencies on spinal cord excitability and heat pain perception. We hypothesized that the effects of vibration on spinal cord reflexes, and, also those on heat pain perception, depend on vibration frequency. Methods: In 9 healthy subjects, we applied vibration over the tibialis anterior muscle at three different frequencies (50, 150, or 250 Hz) on spinal cord reflex excitably, tested with the H reflex and the T wave in the soleus muscle, as well as on sensory and pain perception, tested by measuring warm perception (WT) and heat pain perception thresholds, (HPT) in sites rostral and caudal to vibration. Exams were carried out before, during, and after vibration. Results: The amplitude of the H reflex and T wave significantly decreased during vibration in comparison to baseline. Low frequencies (50 and 150Hz) induced greater reflex suppression than high frequency (250Hz). No significant changes were observed on WT and HPT. Conclusions: The effects of vibratory stimulation can be summarized as frequency-related Published under Creative Common License CC BY-NC-SA 4.0 (Attribution-Non Commercial-ShareAlike) suppression of the spinal cord excitability without an effect on warm and heat pain perception. The present results may help to design vibration-related interventions intended to diminish spinal cord reflex excitability in spastic patients. Keywords: Heat Pain Perception, Lower Limbs, Spinal Cord Reflex Circuits, Vibration Frequencies, Warm Perception Introduction motor activity and facilitate motor learning in patients with spinal cord injury (SCI), stroke, multiple sclerosis and Focal vibratory stimulation, which consists in applying movement disorders (for details see review by Murillo et portable vibrators over the tendon or muscle belly, has al.)3. In rehabilitation, vibratory stimuli effectively reduce been employed in healthy individuals to enhance training spasticity and spinal reflex excitability on soleus muscle in related muscle strength and neuromuscular adaptation1. spinal cord injury subjects4,5. This was the case using vibration In the clinical context, vibratory stimulation is used to at 50 Hz over quadriceps muscle4, or 30 Hz vibration over prevent muscle weakness due to hypoactivity or long-term the tibia6, in healthy volunteers and SCI subjects. Achilles or immobilization2, as well as to reduce spasticity, promote tibialis anterior tendon vibration at 40 and 80 Hz but not 20 or 120 Hz suppresses late spasm-like activity in antagonist muscles in SCI subjects5. A decrease of the H reflex size in The authors have no conflict of interest. healthy subjects has been reported with vibration over the Achilles tendon at 50 Hz frequency7, over the tríceps surea Corresponding author: Hatice Kumru, MD, PhD, Hospital de Neurorehabilitació Institut Guttmann, Camí Can Ruti s/n. Barcelona, 08916 muscles at 50 and 100 Hz8, and over the Achilles tendon Barcelona, Spain or the gastrocnemius muscle at 70 Hz9. In spastic upper E-mail: hkumru@guttmann.com limbs of hemiplegic patients, the flexed elbow position was Edited by: G. Lyritis reduced with vibration at 100 Hz over the triceps brachii in Accepted 23 August 2021 association with physiotherapy10. Improvement of spasticity 533 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception was reported also in post stroke patients with vibration (age range: 20-35 years) and 4 females (age range: 20-49 at 300 Hz applied over triceps brachii and extensor carpi years) were enrolled after signing a written informed consent radialis longus and brevis muscles during voluntary isometric for participation in this research study. contraction11. Vibration has also been reported to alleviate Inclusion criteria were: male or female between 18 and 75 pain, both, chronic musculoskeletal pain and central pain11-13. years old without any neurological disorder and uncontrolled Using vibration at 300 Hz over triceps brachii and extensor disorder which could limit the experiment and had written carpi radialis longus and brevis muscles during voluntary informed consent. Exclusion criteria were: any implanted isometric contraction in post-stroke patients , the authors metallic or electrical devices, or pregnancy. reported an improvement in pain11. In fibromialgia , vibration at 100 Hz reduced pain significantly13. Neurophysiological evaluation Despite the frequent use of muscle vibration in motor We used routine electrodiagnostic equipment and training and rehabilitation, little is known about the accessories (Synergy, Oxford Instruments; Surrey, England). physiological mechanisms underlying clinical effects. Current Ag-AgCl surface recording electrodes were attached to the evidence indicates that oscillatory mechanical stimulation skin overlying the right soleus muscle on a belly-tendon induced by vibration activates muscle afferents and cutaneous arrangement. The amplifier’s gain was predetermined at 1 mechanoreceptors, which in turn modulate spinal cord mV/division and the band-pass filters were 3 Hz to 10 kHz. circuits and areas of sensorimotor integration in the central nervous system. Effects of vibration on spinal cord reflex Stimulation electrodes (0.8 cm diameter and 2.0 cm fixed excitability have been reported, specifically inhibition of the interelectrode distance) were fixed at the site over the tibialis soleus H reflex. This may be due to an increased presynaptic posterior nerve in the popliteal fossa where a stable H wave inhibition, the refractory state of Ia afferent fibers, neuronal could be obtained with repeated single stimuli given with an transmitter depletion at Ia terminals, postsynaptic reciprocal interstimulus interval of 10 s. Stimulus duration was set at and non-reciprocal Ia inhibition, and effects from cutaneous 1 ms. During the preliminary procedure, once the position and other afferent receptors6-8,14. In addition, vibration- of the recording and stimulation electrodes was set, we induced changes have also been reported at corticospinal obtained the supramaximal soleus M wave. level and cortico-cortical inhibitory circuits14-16. The effects Spinal cord reflex excitability changes were assessed by of vibration on pain may also be related to the effects on measuring the following responses: spinal and supraspinal sensory circuits, as inputs carried by 1. H reflex of maximal amplitude (Hmax). This was obtained sensory Aβ fibers may trigger local segmental inhibition17,18 after applying single stimuli of gradually increasing intensity and descending pain inhibition13. in steps of 0.6 mA from the one eliciting the smallest H wave For instance, low-frequency vibratory stimulation (50- to the one eliciting maximal H reflex. 100 Hz) may induce reduction of muscle tone during the 2. H10%. We adjusted stimulus intensity to obtain an stimulation and, therefore, has a predominant anti-spastic H wave with a peak-to-peak amplitude of about 10% of effect4,6,10,19. In contrast, stimulation at 200-300 Hz may the baseline Mmax response (before vibration). The same reduce sarcopenia and increase muscle strength and motor intensity was used to elicit H10% in the examinations performance after repeated applications20,21. during and after vibration. At each time of evaluation, we We studied the physiological effects of different measured the amplitude of the averaged H10% wave out of frequencies of vibration applied over the tibialis anterior 5 consecutive single stimuli separated by 10 s. (TA) muscle on the Soleus H reflex and T wave, as surrogate 3. T wave amplitude (Twave). The T wave was recorded measures of spinal cord reflex excitability, and on warm and with the same setup while tapping with a sweep triggering heat pain perception thresholds in dermatomes rostral and hammer (Kawe reflex hammer Trömner, Germany) over caudal to the level of vibration application, as measures of the Achilles tendon applied by the same researcher (HK) sensory integration systems. Focal vibration to the TA has trying to use similar strength and precision during the whole been used before to examine the after-hyperpolarization experiment. We measured the amplitude of the averaged time course of motoneuron firing22 and to investigate the response of five recordings obtained before, during and after inhibitory effects over cutaneous muscle reflexes5. Here, we vibration. hypothesize that the effects of vibration over TA muscle on spinal cord reflexes, and, possibly, also those on heat pain Quantitative thermal testing perception, depend on vibration frequency. A computer-controlled thermofoil heating system (Pain and Sensory Evaluation system, Pathway System, Medoc Ltd, Methods Ramat Yishai, Israel) was employed. We used the method of limits to assess warm and heat pain perception thresholds The study protocol was approved by the Research Ethics (WT; HPT). The thermode was applied over two skin areas: Committee of the Institute Guttmann and was carried out in at a middle site of the anterior thigh, corresponding to the accordance with the Declaration of Helsinki. dermatome L3, cranial to the site of vibratory stimulation, Nine healthy subjects (mean age: 29.0±5.7 years; 5 males and the dorsum of the foot, corresponding to the dermatome http://www.ismni.org 534 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception L5, caudal to the site of vibratory stimulation. The two sites below) and, therefore, we normalized inter-individual’s data were examined in pseudo-random alternating order in the by expressing Hmax, H10% and Twave as percentages of the three separate sessions but always in the same order in the individual’s Mmax amplitude for each examination time. examinations before, during and after vibration. From each No data reduction was necessary for WT and HPT, which time of evaluation, we calculated the WT and HPT values after values were entered in the statistical analysis once grouped averaging the results of 4 individual stimuli. per examination time and session. In all variables, we obtained data in the examination times before, during and Study design and Vibratory stimulation protocol after vibration. The Shapiro-Wilk’s test was used to examine normality The study included 3 sessions. In each one of them, we of the data. A two-way repeated measures ANOVA was used a different vibratory stimulation frequency (either 50 used to analyse the effects of factors ‘vibration frequency’ Hz, 150 Hz or 250 Hz), applied over the belly of the right (50, 150, 250 Hz) and “examination time” (before, during tibialis anterior (TA) muscle, because vibration over muscle and after stimulation) on Mmax, Hmax, H10%, Twave, WT belly may provide more effective muscle tone reduction and HPT (the Mmax amplitude was included in the analyses as compared to the application over tendon9. According to in absolute values). A one-way repeated measures ANOVA literature and our experience4-8,11-13, we suggested that the was used to study separately the simple main effects of each lowest frequencies (50 Hz150Hz) could reduce spinal cord vibration frequency on the same parameters. The Mauchly excitability more than higher frequencies (250 Hz) and pain test was used to evaluate whether the sphericity assumption perception could be reduced in higher than lower frequencies. was violated, in which case the Greenhouse-Geisser And we decided one low frequency (50 Hz), one intermediate correction was applied to correct the degrees of freedom of frequency (150 Hz) and high frequency (250 Hz). The order the F-distribution. Partial η2 was calculated to measure the of sessions was randomized. The sessions were conducted in effect size. We considered effect sizes of 0.01 small, 0.09 different days, separated by at least 1 week. medium, and 0.25 large. A Bonferroni correction was used Subjects were sitting barefoot, reclined on a comfortable chair with supported feet stands and exposed knee and for multiple comparisons. calf stimulation and recording sites. Vibration was applied A two-tailed test with an alpha level of 0.05 was used for over the belly of the right tibialis anterior (TA) muscle by a all analyses. stimulator Vibra 3.0 (AD Swiss MedTech SA), attached with All statistical analyses were performed with a commercial ribbons over the belly of the right TA muscle. The device was software package (IBM SPSS, version 13.0, SPSS Inc., set to deliver mechano-sound square wave vibrations at an Chicago, IL, USA). intensity of >350 mbar. At each session, we first obtained the Hmax, M response, Results H10% and Twave, and determined WT and HPT (examination time ‘before’). We then started vibratory stimulation All subjects tolerated well the tests, without any dropout at the frequency selected for that session. The same nor incompletion of the study. For the evaluation during neurophysiological and psychophysical tests were carried out vibration, the vibratory stimuli were applied for as long as the during vibration (examination time ‘during’), starting 5 minutes time the test needed to be completed, roughly 30 minutes. after initiation of stimulation when spinal cord reflexes are reduced in healthy and spinal cord injury subjects, as shown Modulation of spinal cord reflex excitability by Murillo et al.4. Then, neurophysiological and psychophysical Raw data on H10%, Hmax, Hmax/Mmax and Twave testing was repeated immediately after finalizing the are given in Table 1 while the same data represented as stimulation (examination time ‘after’). The total duration percentages of Mmax are shown in Figure 1. The results of each study session was around 2 hours and included: of statistical comparison are given in Tables 2A (two-way preparation of the equipment (EMG, vibratory machine and ANOVA) and 2B (one-way ANOVA). Figure 2 shows the thermal stimulation) and the subject (application of recording EMG recording of H10% before, during and post vibration at electrodes for spinal reflexes), vibratory stimulation, and frequency of 50 Hz, 150 Hz and 250 Hz. determination of the H reflex-related parameters and thermal The two way repeated measures ANOVA revealed a perception thresholds before, during and after vibration (20- statistically significant effect of time of evaluation and time x 25 min for each evaluation). frequency interaction on H10%, Hmax and Twave (p<0.05). Simple main effects analyses showed that H10%, Hmax/ Data reduction and statistical analyses Mmax ratio and Twave significantly decreased as compared We measured peak-to-peak amplitude in the Mmax, Hmax, to baseline during stimulation at 50 and 150 Hz (p<0.05) H10% and Twave and calculated their mean and standard and recovered when vibration was stopped (Figure 1, A, deviation (SD) per each individual and examination time. We C, D). Focal muscle vibration at 250 Hz did not induce any expected no effects of vibration on the Mmax response (even significant changes in spinal cord excitability measures at if we ran statistical analyses on Mmax amplitude, as described any time of evaluation. http://www.ismni.org 535 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception Figure 1. A) H10%,; B) Mmax; C) Hmax; D) Twave. H10%, Hmax and T wave are represented as percentages of Mmax. Table 1. Raw data of H10%, Hmax, Mmax, Hmax/Mmax ratio and T wave. Before During After H10% 1680.0 (311.3) 2.2 (2.2) 372.7 (122.9) Hmax 6318.4 (1332.1) 2243.2 (814.0) 6581.1 (1298.4) 50 Hz Mmax 17424.1 (3764.8) 17431 (3764.8) 17594.9 (3180.0) Hmax/Mmax 0.4 (0.1) 0.2 (0.1) 0.4 (0.1) T wave 2973.6 (592.9) 762.4 (264.5) 2721.9 (643.6) H10% 1616.6 (279.7) 825.9 (496.4) 1734.4 (489.2) Hmax 6436.3 (993.3) 3598.0 (1048.0) 6947.1 (989.7) 100 Hz Mmax 13233.2 (1626.5) 13646.3 (1690.7) 14515.2 (1560.6) Hmax/Mmax 0.5 (0.1) 0.3 (0.1) 0.5 (0.1) T wave 2712.9 (652.1) 1673.9 (547.1) 2563.5 (403.8) H10% 1272.9 (148.6) 1566.2 (899.1) 1833.1 (869.1) Hmax 8643.8 (1404.0) 6603.5 (1314.1) 8160.0 (1128.6) 150 Hz Mmax 18474.8 (2198.9) 19295.4 (2317.9) 17486.8 (1344.5) Hmax/Mmax 0.5 (0.1) 0.4 (0.1) 0.5 (0.1) T wave 2765.8 (479.9) 2189.3 (473.1) 2848.3 (505.7) The figures are the mean and one standard deviation (within parenthesis), given in μV for H10%, Hmax, Mmax and T wave, and as a ratio for Hmax/Mmax, for the three evaluation times and the three frequencies of vibratory stimuli. http://www.ismni.org 536 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception Table 2A. Two-way repeated measures ANOVA on the effects of different frequencies of vibration on spinal cord reflexes. Two-way RM ANOVA Variable Source Df/Error F p ƞp2 Time 1.07/25.60 0.23 0.66 0.01 Mmax Time x Frequency 2.13/25.60 0.98 0.40 0.08 Time 2/48 48.82 0.001 0.67 Hmax* Time x Frequency 4/48 2.87 0.033 0.19 Time 2/48 41.07 0.001 0.63 H10%* Time x Frequency 2/48 8.66 0.001 0.42 Time 1.47/35.30 31.82 0.001 1.0 Twave* Time x Frequency 2.94/35.30 3.94 0.016 0.25 * Statistical analysis was done with normalized data by expressing Hmax, H10% and Twave as percentages of the individual’s Mmax amplitude. Table 2B. One way repeated measures ANOVA on the effects of different frequencies of vibration on spinal cord reflexes. PostHoc (P value) Frequency Variable Df/Error F p ƞp2 Before-during Before-after Mmax 1.2/9.8 0.02 0.98 0.01 - - Hmax* 2/16 28.13 0.001 0.78 0.001 1.0 50 Hz H10%* 2/16 43.13 0.001 0.84 0.001 0.04 Twave* 2/16 23.57 0.001 0.75 0.001 0.39 Mmax 1.1/8.7 3.62 0.09 0.31 - - Hmax* 2/16 26.36 0.001 0.77 0.001 0.99 150 Hz H10%* 2/16 12.86 0.001 0.62 0.006 1.0 Twave* 2/16 6.46 0.009 0.45 0.047 1.0 Mmax 1.0/8.2 0.67 0.44 0.08 - - Hmax* 2/16 4.22 0.03 0.35 0.087 1.0 250 Hz H10%* 2.16 1.42 0.27 0.15 - - Twave* 2/16 4.38 0.03 0.36 0.11 1.0 * Statistical analysis was done with normalized data by expressing Hmax, H10% and Twave as percentages of the individual’s Mmax amplitude. Quantitative thermal testing frequency: i.e., low frequencies (50 and 150 Hz) cause greater suppression of H reflex and T wave than high Data of the two-way repeated measures ANOVA are frequencies (250 Hz). Furthermore, we have shown no effect summarized in Table 2C. Muscle vibration did not cause of vibration at any of the frequencies tested on warm and significant changes in WT at any stimulation site (dermatome L3, above the knee and dermatome L5, dorsum of foot), any heat pain perception thresholds, examined above and below evaluation time or any stimulation frequency (p>0.05 for all the dermatome where vibratory stimulation was applied. comparisons) (Figure 3 A, B). There was a significant effect of evaluation time on HPT, Modulation of spinal cord reflex circuits when tested at the dermatome L5, dorsum of the foot We found a significant effect of 50 Hz and 150 Hz on (p=0.001). Further simple main effects analyses revealed an the amplitude of H 10%, Hmax, and Twave and 150 Hz on increase in HPT following stimulation at 50 Hz compared to the Hmax and the amplitude of Twave, while the highest baseline (F[2,16]=4.73; p=0.024) but the post hoc analysis frequency vibration (250 Hz) did not induce changes on the did not reach significance (Figure 3D). spinal cord reflex circuits. Importantly, changes in H reflex and T wave were observed Discussion during the application of vibration, returning to baseline values after the procedure ended. Our results are in line Our results show that changes in spinal cord excitability with previous reports in healthy individuals23 and in patients with focal vibration depend on the vibratory stimulation with motor impairment due to spinal cord injury4. One of the http://www.ismni.org 537 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception Figure 2. EMG recording of H10% before, during and post vibration at frequency of 50Hz, 150Hz and 250Hz. Figure 3. A) Warm perception threshold -L3 B) Warm perception threshold-L5 C) Heat pain perception threshold -L3 D) Heat pain perception threshold -L5. L3: the dermatome L3 (at a middle site of the anterior thigh), cranial to the site of vibratory stimulation. L5: the dermatome L5 (at the dorsum of the foot), caudal to the site of vibratory stimulation. http://www.ismni.org 538 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception Table 2C. Two-way repeated measures ANOVA on the effects of different frequencies of vibration on QST measures. Two-way RM ANOVA Variable Source Df/Error F p ƞp2 Time 1.30/31.29 0.82 0.40 0.03 WT-L3 Time x Frequency 2.61/31.29 0.55 0.63 0.04 Time 1.51/36.16 3.16 0.07 0.12 HPT-L3 Time x Frequency 3.01/36.16 0.97 0.42 0.08 Time 1.55/37.22 2.03 0.16 0.08 WT-L5 Time x Frequency 3.10/37.22 1.45 0.25 0.11 Time 2/48 9.71 0.001 0.29 HPT-L5 Time x Frequency 4/48 0.88 0.48 0.07 novelties of our study is the examination of the effects of more modulatory effects of vibration at 250 Hz in our study. than one frequency. Using vibration at 50 Hz over quadriceps Furthermore, the lack of significant modulation of M wave at muscle, spasticity and spinal cord excitability reduced in the any of the explored frequencies suggests the absence of any ipsilateral knee and H reflex and T wave on soleus muscle in effect of local vibration on distal axonal excitability4,27. spinal cord injury patients4. Achilles or tibial anterior tendon In our study, the effects seen with 5 minutes vibration at vibration at 40 and 80 Hz but not 20 or 120 Hz suppresses 50 and 150 Hz were not maintained after the intervention. late spasm-like activity in antagonist muscles3. Vibration at On the contrary, Miyara et al28 reported effects on F-wave 100 Hz applied to the triceps brachii of a spastic upper limb parameters and spasticity that remained even 20 minutes in association with physiotherapy reduced the spasticity of after 5 minutes of whole body vibration at 30 Hz in stroke. the flexor agonist, biceps brachii in hemiplegic patients10. These differences may be explained by the different Improvement in spasticity was reported also with vibration at stimulation characteristics, study subjects and outcome 300 Hz in post stroke patients11. measures between the two studies. We believe that the It is known that vibratory stimulation at frequencies effects of focal vibration on spinal cord reflexes is limited to between 80-120 Hz produce an involuntary contraction the time that vibration is being applied. in the targeted muscle and a relaxation of the antagonist muscle23. However, the effects of different frequencies on the Modulation of thermoalgesic perception by focal muscle H reflex or T wave have not been examined before. vibration The H reflex and T wave are reliable indirect measures of Our results show only a small effect of focal 50 Hz vibration spinal cord reflex circuits excitability. The H reflex amplitude on heat pain threshold, with no effect on warm perception depends not only on alpha motoneuron excitability but also threshold. The effect, which was not robust enough to on the degree of synchronization of the Ia afferent input sustain significance in the post-hoc analysis, was found and the level of presynaptic inhibition24,25. The Achilles T in the dermatomes below the site of stimulus application. reflex has the same afferent and efferent pathways as the Previous studies investigating the effects of focal vibration H-reflex, except for the site where the Ia fibers are activated: on thermoalgesic perception29-31 have reported different the H-reflex stimulates directly the Ia fibers, bypassing findings. Using a hand grip vibrating device applied over the muscle spindle organs, while the T-reflex activates the the finger of healthy individuals, Hirosawa et al.30 reported muscle spindle organs26. The inhibition induced in spinal cord increased threshold for warm perception, but only a small reflex excitability, specifically in the soleus H and reflexes, effect on perception of cold, with vibration frequencies of may be due, among other causes, to: (1) increased discharge 125 and 250 Hz. On the contrary, Burnstrom et al.29 used threshold by group Ia afferents, (2) enhanced presynaptic frequencies of 31.5 and 12 Hz to find a significant effect of inhibition at Ia terminals, (3) transmitter depletion at Ia vibration on cold perception and no effects on perception of synapses, and/or (4) postsynaptic changes leading to a warm sensation. The contradictory results were interpreted decrease in motoneuron excitability6-8,24. by the authors as due to different vibration frequencies and Vibration at 250 Hz was not able to induce changes in magnitude of the vibrators. However, both studies measured spinal cord excitability. It is possible that the lack of inhibitory only perception of warm and cold sensation, but not of heat effect of high frequency vibration may be due to the fact pain. Also, they measured the effect in the area where that the discharges of Ia afferents become disharmonious vibration was applied, exploring mostly in such a way, the with frequencies of vibration above 150 Hz. Therefore, local effects of vibration. In the study by Yarniskty et al.31, the Ia volley might be unable to activate the intraspinal subjects reported reduced pain intensity to suprathreshold inhibitory circuits, which might explain the minimal or absent heat when vibration was given to the same dermatome on http://www.ismni.org 539 H. Kumru et al.: Effect of vibratory frequency on spinal cord excitability and pain perception the contralateral side or to the dermatome adjacent to that Conclusion receiving heat stimulation, but had no effects on perception of temperature sensation two or more dermatomes above or The effects of vibratory stimulation can be summarized as below the vibration site. A relevant methodological difference frequency-related suppression of the spinal cord excitability between our study and that of Yarniskty et al.31 is in the way without effect on heat pain perception. Low frequencies (50 to assess pain. While Yarnitsky et al.31 used a visual analogue and 150 Hz) determine greater suppression of spinal cord scale, we utilized a quantitative measure of pain threshold. excitability measured by H reflex and T wave but not high Other methodological differences related to duration of frequencies (250 Hz) without an effect on warm perception stimulation12 and frequency of vibration11 could also explain and heat pain perception, above and below the dermatome the different results. where vibratory stimulation was applied. The data presented On the other side, using high-frequency TENS induced here may help designing vibration-related interventions a significant attenuation of both the acute pain and the intended to decrease spinal cord reflex excitability in patients laser evoked potentials induced by stimuli applied on the with spasticity. In accordance with the results of our study, same dermatome32. However, the stimulation technique this would be better accomplished with low frequency and methodology used in Vassal et al.’s study32 is different vibration (50-150 Hz) than with high frequency vibration from ours, which makes difficult to compare the results (250 Hz). between studies. In spite of methodological differences, our results go in References the same line as the cited previous studies to indicate that 1. Alghadir AH, Anwer S, Zafar H, Iqbal ZA. Effect of there can be a small effect of vibration on perception of localised vibration on muscle strength in healthy thermoalgesic stimuli. 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