The effect of whole-body vibration training on electromyographic signals in stroke patients

Objective. Was to verify the influence of vibration training on the electromyographic signal of the rectofemoral (RF) and tibialis anterior (TA) muscles in stroke patients. Method. This is clinical trial, with 43 hemiparetic stroke patients, who were randomized into two groups: control (CG, n=19) and intervention (IG, n=24). The instruments used for evaluation were the Mini Mental State Examination, the Fugl-Meyer Evaluation Scale, and surface electromyography (EMG) in maximum voluntary isometric contraction (MVIC) of the RF and TA muscles bilaterally and simultaneously. Whole-body vibration therapy (WBV) (tri-plane statistical analysis used was the independent t test to compare the baseline characteristics of the CG and IG. The Shapiro – Wilk test was applied to verify the normality of the data and subsequently the Wilcoxon test to compare times and the Mann-Whitney test to compare groups. Results. The results show that EMG RMSn activity did not change intra-group nor inter-group. Conclusion. Vibration training did not influence the EMG signal of the RF and TA muscles in stroke patients. posteriormente la prueba de Wilcoxon para comparar tiempos y la prueba de Mann-Whitney para comparar grupos. Resultados . Los resultados muestran que la actividad EMG RMSn no cambió intragrupo ni intergrupo. Conclusión: El entrenamiento vibratorio no influyó en la señal EMG de los músculos RF y TA en pacientes con ictus.


INTRODUCTION
Whole body vibration (WBV) is a neuromuscular exercise modality that is gaining prominence both in physical training centers as a therapeutic modality for rehabilitation and in research 1 . WBV is an intervention involving a vibrating platform and the mechanical oscillation of force alternating between acceleration and displacement. The most common types of alternation are lateral (oscillatory) and vertical (linear). These stimuli are transmitted to the body and are captured by sensory receptors. The stimuli cause a complex neurophysiological spinal and supraspinatus reaction called tonic vibration reflex. This reflex is responsible for muscle activation and increased functional performance 2,3 .
Studies point to promising results of this therapy for patients affected by stroke. It has been shown to improve motor function, increase blood perfusion 4 , and potentiate muscle activation 5,6 . However, these mechanisms are still not clearly elucidated, and there are strong recommendations for more research to better understand and substantiate the mechanism of biomechanical and physiological changes in the effect of vibration on stroke patients 7 .
According to the World Health Organization, stroke is an important public health problem in Brazil. It is a leading cause of death in the country and has a strong impact on the functional capacity of survivors 8 . Therefore, we propose implementing WBV in the rehabilitation of stroke patients and the use of surface electromyography (EMG) as a method of assessing the effects of WBV on this population 9,10 .
EMG is a technique that allows the recording and analysis of electrical signals produced in muscle cells. It can be used to quantify the relationship between the strength of electromyographic signals, the level of muscle activation, the moment of muscle activation, and the fatigue signal 9 . It was observed an increase in muscle activity of 5%-50% of the

Randomization process
Researcher 1 created a randomization list sequentially numbered by the computer and stored in a sealed envelope.
The researcher subsequently carried out a random draw allocating participant to the CG and the IG ( Figure 1).

Assessment tools
Participants were assessed using the following instruments. Sensorimotor function was evaluated using the EMG was used to assess the muscle electrical activity of the RF and TA muscles in MVIC 9 . EMG showed satisfactory reliability in assessing the electrical activity of the muscles 17 .

Electromyographic evaluation procedures
Electromyographic signals were collected using a four-

Processing of the EMG signal
To analyze the activity of the investigated muscles, the root mean square of the EMG signal amplitude was calculated. All the signals were processed using MATLAB software routines (MathWorks Inc, Natick, MA). The EMG signal length was 5s, but the first and last seconds were disregarded; therefore, a period of 3s was analyzed. This was used to establish the reference value, the maximal root mean square, of each analyzed signal before and after the intervention to achieve reliable measurements. The reference value was divided by the greatest value of all the analyzed signals and then multiplied by 100.

Statistical analysis
Descriptive statistics were used to characterize the sample relative to the clinical and demographic variables. An independent t test was used to compare the baseline characteristics of the GC and the GI. A Shapiro-Wilk test was performed to determine the normality of the data. In addition, a Wilcoxon test was used for within-group comparison, and a Mann-Whitney test was used for between-group comparison. All analyses were executed using the SPSS package (version 20.0). The probability level for statistical significance in all tests was set at p<0.05.

RESULTS
Forty-three individuals completed the entire intervention protocol, with a sample loss of seven participants due to missing the intervention. Table 1 shows the clinical characteristics of the remaining sample. There was no statistical difference regarding age, level of cognition, and motor dysfunction.  Table 2 shows the muscular activity of the RF and TA of the paretic side in a comparison before and after the intervention and between the groups. The EMG activity normalized root mean square (RMSn) did not change after the intervention with the WBV or when compared to the CG.
Statistical analysis confirmed these non-significant observations, with p≥0.05 and a low effect size.

DISCUSSION
The main finding in the present study reveals that WBV training did not influence the electromyographic signals in the RF and TA muscles. This finding confirms that the EMG RMS of the VL and RF also did not change after vibration treatment 25,26 . However, the functional performance of athletes and stroke patients has been shown to improve after vibration treatment 27 . Vibration treatment can be justified by the improvement in neuromuscular efficiency leading to a reduction in EMG activity due to the increase in muscle resistance 26  Another point to be analyzed relates to the frequency used in the intervention protocol, 50Hz, which was not able to modify the electromyographic signal. Some researchers have observed that vibration is able to induce increased EMG activity due to the synchronization of motor units, which may be dependent on the frequency of vibration [39][40][41] . I was used a frequency of 20 Hz-55 Hz, with 55 Hz producing little muscle activation of the lateral gastrocnemius as opposed to the vastus lateralis 37 . It is suggested that the increased frequency of vibration leads to a reduction in EMG activity due to the presynaptic inhibition caused by the constant vibration stimulus. Some studies that highlight the frequency of 30Hz found a greater effect on the electromyographic signal in patients with central injuries and in athletes [42][43][44] .
Researchers report that frequency below 20 Hz can cause a resonance effect on the body, being not indicated in these cases 45,46 .
Contrary to the findings of the present study, others have found an increase in the EMG activity of several lower limb muscles 43,[47][48][49] . This may be the result of the collection performed during training different from that in the present study, where data collection was performed before and after the intervention. The RMS is very susceptible to interference from the external environment and therefore data normalization is necessary 9 .
Regarding the effect of vibration on the CG and the effect of intergroup and intragroup intervention on the paretic and non-paretic lower limb, there was no statistical difference, and the electromyographic values were similar on both sides after the vibration training. In contrast, it was found a significant increase in electromyographic activity in both limbs for the vastus lateralis and gastrocnemius muscles during different exercises in chronic stroke patients during vibration training 43 . Similar answers were also found in a study for the TA muscles and biceps femoris during WBV 47 and in a study after the intervention 49 .
Some limitations should be pointed out, such as the need to change protocols such as frequency variation, use of isotonic exercise, long-term re-evaluation to estimate the best protocol for the rehabilitation of stroke patients, and a follow-up to verify the permanence of the results.

CONCLUSION
It is concluded that vibration training did not influence the EMG signal of the RF and TA muscles in stroke patients.

ACKNOWLEDGMENT
This work was funded by the Coordenação de