Whole-body vibration (WBV) has been gaining more prominence in recent years among medical and sports specialists. Considered to be a safe and practical method, training associated with vibration has been shown to be an alternative and efficient exercise modality in treating patients with functional limitations, as well as for high performance training in athletes (Albasini et al., 2010; Ritzmann et al., 2010; Pollock et al., 2010).
Evidence that the vibratory stimulus is capable of increasing neuromuscular activity has been demonstrated by studies involving surface electromyography (Lienhard et al., 2014; Ritzmann et al., 2010; Wirth et al., 2011). Although there is no consensus about the mechanisms by which the vibratory stimulus affects the neuromuscular system, it has been suggested that the cause of the increase in motor unit recruitment is an excitatory response of the muscle spindles, due to the stretch reflex mechanism (Bosco et al., 1999a; Cardinale and Lim 2003; Pollock et al., 2012; Torvinen et al., 2002; Seidel, 1988; Xu et al., 2015).
It has been demonstrated that neurophysiological factors involved in the response to vibratory stimulus have an important contribution of the oscillation frequency at which body structures are exposed (Bazett-Jones et al., 2008; Cardinale and Lim 2003). In a review analyzing the influence of vibration characteristics on neuromuscular performance, Luo et al., (2005) recommended that the most effective frequencies to achieve greater muscle activation are between 30 and 50 Hz.
It is suggested that increased muscle activity during exercises associated with vibration is related to the use of higher frequencies and amplitudes (Hazell et al., 2007; Pollock et al., 2010). Contrary, Cardinale and Lim (2003) reported that the 30 Hz vibration frequency was responsible for providing greater activation of the vastus lateralis (VL) muscle when compared to 40 and 50 Hz.
Despite its wide use in sports training and patient rehabilitation, there is still controversy regarding the factors that initiate neurophysiological responses in skeletal muscle during WBV. In addition, the different types of vibration (synchronous and alternate) and the wide range of frequency and amplitude combinations reported in various studies (Bazett-Jones et al., 2008; Bosco et al., 2009b; Cardinale and Lim, 2003; De Ruiter et al., 2003; Torvinen et al., 2002; Ritzmann et al., 2013), besides the inconsistent results make it difficult to interpret the efficacy of WBV.
Some authors have demonstrated the need for greater caution in carrying out studies involving surface electromyography analysis during vibration (Abercromby et al., 2007; Fratini et al., 2009). Electromyographic signal contamination during the vibratory stimulus occurs due to the presence of motion artifacts caused by the performed oscillation frequency (Lienhard et al., 2014; Sebik et al., 2013; Wirth et al., 2011). Although often neglected in some studies, using additional filters during signal treatment is recommended to remove such vibration-induced artifacts that can lead to misinterpretation of the analyzed data (Lienhard et al., 2015b; Sebik et al., 2013).
Riztmann et al., (2010) conducted an analysis about the factors that may compromise the quality of the sEMG signal. These authors suggested that the contribution of motion artifacts are insignificant and not representative compared to vibration-induced reflex muscle activation, showing that it is possible to use sEMG data collected during vibration without applying additional filters.
Considering the above, this study was designed to analyze the effects of WBV on VL sEMG amplitude in healthy subjects using two different commonly used training frequencies (30 Hz and 50 Hz) as well as to verify the existence of vibration-induced artifacts and the influence of additional filters on the analyzed signal's characteristics. To our knowledge, few studies (Abercromby et al., 2007; Fratini et al., 2009; Lienhard et al., 2015a; Ritzmann et al., 2010; Sebik et al., 2013) have analyzed this issue making it difficult to give an exact quantification of sEMG activity during the WBV, which may compromise the quality and reliability of the analyzed data. Moreover, most of them were restricted to aspects related to the frequency spectrogram, without taking into account the effects on the amplitude of the electromyographic signal and its responses to different frequencies of vibratory stimulus after proper sEMG signal processing.
Therefore, we hypothesized that without adequate removal of vibration-induced artifacts from the electro-myographic signal, VL muscle activity during a vibration exercise protocol would be higher in comparison in the same exercise without vibration, with no difference between vibration frequencies. In contrast, we believe that a possible increase in sEMG amplitude would no longer be detected after appropriate signal treatment and removal of such vibration-induced artifacts.
The study sample consisted of forty physically active women (average age: 22.9 [+ or -] 2.8 years; body mass index--BMI: 23 [+ or -] 2.5 Kg/[m.sup.2]) recruited in a non-probabilistic way from a local university. The inclusion criteria were: healthy female, involved in recreational physical activity at least three times a week without training at a competitive level (Pincivero et al., 2003), aged between 18 and 28 years. Exclusion criteria were inability to understand protocol commands, incorrect execution of assessment procedures or presence of pain, discomfort, vertigo or dizziness during the tests and interventions. Following these criteria, there were no reported exclusions for this study (Figure 1).
This study was approved by the local university Research Ethics Committee (protocol number 752.291) and conforms to ethical aspects based on Resolution 466/12 of the National Health Council and Declaration of Helsinki. All volunteers that participated in the study gave their written informed consent after being explained about the research objectives, risks and benefits. This study is registered at www.clinicaltrials.gov under the number NCT02416362.
Design of the study
This is an observational cross-sectional analytical study with a two-group, repeated measures design. The volunteers were randomly divided, using the method of randomly permuted blocks from the website (www.randomization.com). The participants were allocated into two different groups of 20 individuals each: 30 Hz and 50 Hz group.
All participants underwent a preliminary isometric evaluation of the non-dominant lower limb to normalize the electromyographic signal. Participants were seated in a chair of a computerized...