Whole-body vibration (WBV) training has become a popular exercise method for athletes and patients over the last decade. It is used as a training form in prophylaxis and medical rehabilitation, as well as in standard strength training (Nordlund and Thorstensson, 2007; Rittweger, 2010). WBV exercises are performed while standing on a motor driven oscillating platform device. Hereby, a distinction is drawn between side alternating vibration platform devices, which reciprocate vertical displacements on the left and right side of a fulcrum, and synchronous vibration platform devices, where the whole plate oscillates uniformly up and down (Cardinale and Wakeling, 2005; Rauch et al., 2010; Rittweger, 2010). The mechanical vibration stimulus applied to the muscles and tendons during WBV exercise is characterized by a cyclic transition between eccentric and concentric muscle contractions and leads to a neuromuscular response (Rittweger, 2010). Thus, WBV exercise acutely enhances the pattern of the neuromuscular system due to the physiological mechanism called "tonic vibration reflex" (TVR), where muscle spindle reflexes facilitate the activation of Iamotoneurons, leading to muscle contractions (Hagbarth and Eklund, 1966; Lance et al., 1973). Monosynaptic and polysynaptic pathways have been shown to mediate TVR (Desmedt and Godeaux, 1980; Luo et al., 2005).
Previous studies have shown that WBV has an effect on muscle strength parameters (Delecluse et al., 2003; Issurin et al., 1994; Luo et al., 2005; Schlumberger et al., 2001; Torvinen et al., 2002), performance (Lamont et al., 2009; Wyon et al., 2010) and postural control, especially in older adults (Bruyere et al., 2005; GomezCabello et al., 2013; Kawanabe et al., 2007; Runge et al., 2000). Most of these studies focused on leg muscles and only few studies were found that investigated the effects of WBV exposure on trunk and neck muscles. According to this, it has been reported that WBV training reduces back pain by relaxing the back muscles (Iwamoto et al., 2005; Rittweger et al., 2002). Furthermore, Osawa and Oguma (2013) showed a significant increase in the muscle strength of the back muscles after 13 weeks of WBV training with a synchronous vibrating platform compared to conventional strength training (+51.5 [+ or -] 34.1%, p
During WBV exposure, the magnitude of EMG levels of a specific muscle depends on the vibration load determined by the biomechanical parameters (vibration frequency, peak-to-peak displacement, and joint angle), the exercise parameters (side alternating or synchronous vibration platform devices, acute vs. chronic effects, exercise position) (Abercromby et al., 2007a; 2007b; Krol et al., 2011; Perchthaler et al., 2013; Petit et al., 2010; Roelants et al., 2006), and the anatomical localization of the muscle (Hug, 2011). There are no studies investigating the influence of the combination of these parameters to achieve the highest neuromuscular activity of the trunk and neck muscles. In particular, the acute effect of the variation of peak-to-peak displacement on the neuromuscular response of these muscles has not been investigated. Furthermore, vibrations could be potentially harmful to the soft tissue organs within the head (Rittweger, 2010), and therefore the selection of the biomechanical parameters should be well-considered. In this context, Abercromby et al. (2007b) quantified head accelerations by performing slow dynamic squats with knee angles 10-35[degrees] during WBV and comparing them to ISO 2631-1:1997 standards for potentially harmful vibration exposure. They suggest that potentially harmful vibration transmission to the head is minimized when using a side alternating vibration platform rather than a synchronous device and by squatting with a knee angle of 26-30[degrees]. In addition, Caryn et al. (2014) investigated how changes in WBV frequency and knee angle affect acceleration transmission to the head on a synchronous vibration device. These authors reported that frequencies below 30 Hz combined with knee angles less than 40[degrees] should be avoided to reduce the risk of injury to structures of the head during vibration exercise. Although both studies recommend a vibration frequency of 30 Hz, there are differences in the peak-to-peak displacement of 4 mm (Abercromby, 2007b) and 1-2 mm (Caryn, 2014), respectively. The main contrast between the studies by Abercromby (2007b) and Caryn (2014), however, is a significant discrepancy relating to the recommendations for knee angles (26[degrees]-30[degrees] vs. > 40[degrees]) to avoid harmful head accelerations. In respect to the recommended parameters of these two studies, knee angles of 30[degrees] and 45[degrees] at a frequency of 30 Hz were assumed as biomechanical parameters in the present study. In addition, low and high peak-to-peak displacements (2.6 and 7.8 mm) were included.
The aim of this study was to examine the combination of peak-to-peak displacement and knee angle, which achieves the highest level of activity of the trunk and neck muscles during WBV. We hypothesized that the biomechanical parameters affect the level of neuromuscular activity in different dimensions. We first examined to what extent these muscles were active during WBV. Next, we observed the differences between muscle activity with (WBV) and without (CON) a vibration stimulus of 30 Hz frequency. Finally, we investigated how the level of activation was affected by the biomechanical parameters. These steps were important to provide effective and safe recommendations for WBV training protocols for trunk and neck muscles.
To test the hypothesis, measurements were done to analyze the neuromuscular activity of the trunk and neck muscles during WBV exposure. Trunk muscles analyzed in previous studies addressing acute (Wirth et al., 2011) and chronic (Osawa and Oguma, 2013) effects of WBV were selected for the present trial. In addition, one neck muscle was also included, as data about EMG analyzes of the neck muscles during WBV is lacking in current literature. Therefore, surface EMG was used to record the signals from the upper and lower rectus abdominis muscles, upper and lower erector spinae muscles and from the splenius muscle in different conditions of WBV and without vibration stimulus. The neuromuscular activation levels of the trunk and neck muscles were the dependent variables. The independent variables were the vibration condition (vibration stimulus with a vibration frequency of 30 Hz and no vibration), two peak-to-peak displacements (2.6 and 7.8 mm) and knee angles at 30[degrees] and 45[degrees]. To normalize EMG, muscle activation was recorded during isolated and isometric maximal voluntary contractions (MVC). EMG treatment procedure and data analysis were performed according to Merletti (1999).
Twenty-eight healthy and physically active men (n = 14) and women (n = 14) (age 23 [+ or -] 3 years, height 1.73 [+ or -] 0.17 m, weight 65.5 [+ or -] 19.5 kg, BMI 21.8 [+ or -] 4.2 kg x [m.sup.-2]) volunteered to participate in the present study. Exclusion criteria were fresh fractures, all types of diseases related to gallstones and kidney stones, and acute back pain. No one had to be excluded on the basis of these criteria. All participants gave written informed consent to participate in the experiment. The study was approved by the Human Ethics Committee of the university according to the Declaration of Helsinki.
Surface EMG (Noraxon Telemyo 2400T V2, Scottsdale, AZ) was recorded from the upper (UES) and lower (LES) part of the erector spinae muscle, upper (URA) and lower (LRA) part of the rectus abdominis muscle, and from the splenius muscle (SPL) of a randomized side of the body. Skin was prepared by removing the hair and cleaning the muscle area with fine sand paper and alcohol. Bipolar surface electrodes (Ag/AgCl, 3M Health Care, St. Paul, MN) were applied over the muscle belly using SENIAM recommendations for UES and LES (Hermens et al., 1999), whereby these were not available, e.g. for URA, LRA SPL, referring to literature data (Lehman and McGill, 2001; Konrad, 2005; Wirth et al., 2011). The electrodes were placed with an interelectrode distance of 20 mm on the UES (2 fingers' width lateral from the L1 spinous process), on LES (2-3 cm from the midline at the level of L5, placed on and aligned with a line from the caudal tip of the posterior spina iliaca superior to the interspace between L1 and L2), on URA (2-3 cm lateral from the midline on the second segment of the muscle), on LRA (2-3 cm lateral from the midline, on the fourth segment of the muscles or 2 cm inferior to the umbilicus if the fourth segment could not be palpated), and on SPL (2-3 cm from the midline at the level of C4). A ground electrode was placed over the dorsal process of the seventh cervical vertebra. The preamplified EMG signals were amplified (x1000), bandpass-filtered at 10-500 Hz [+ or -] 2% cut-off (Butterworth/Bessels), and sampled at 1500 Hz. MyoResearch XP software (Noraxon, Scottsdale, AZ) was used to collect and store the data for subsequent analysis. EMG cables were properly fixed on the skin with tape to prevent the cables from swinging and to avoid-movement artifacts.
After attaching the electrodes, MVC was performed to measure the maximum possible EMG level of the muscle. These MVC were performed on special training devices (DAVID 110, 130, and 140, David Health Solutions Ltd., Helsinki), which allow valid and reliable isolated...