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PhD position University of Nice Sophia antipolis France

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  • PhD position University of Nice Sophia antipolis France

    Analysis and processing of human electrical and physiological signals
    during whole body vibration exercise

    PhD position
    University of Nice-Sophia Antipolis
    Laboratory Human Motricity, Education Sport and Health


    Over the last decade, vibration exercise has become one of the most popular alternative exercise modality for recreational subjects, high-skilled athletes, the aged and health compromised individuals. This growing interest for this exercise modality has been driven by companies who started to develop and commercialize vibration platforms. While the use vibration is not new (vibration as a medical technique was mentioned in ancient Greco-Roman sources and vibratory massage was popular among physicians in the 19th century), the notion that vibration can be beneficial is relatively new. Indeed, long-term vibration exposure has traditionally been regarded as only detrimental in occupational physiology, especially for workers who are constantly and continually exposed to vibrations from different types of machinery (Mester et al., 2002). In the neurosciences, by contrast, direct muscle or tendon vibration is a standard tool of investigation of the neuromuscular system (Eklund and Hagbarth, 1966). Nevertheless, only few authors have postulated therapeutic effects by vibratory stimuli in the past in humans. Among them, the first were Sanders (1936) and Whedon et al. (1949), who performed some studies on an oscillating bed, meant to counteract cardiovascular and musculoskeletal de-conditioning. Nazarov and Spivak (1985) were the first to apply vibration as a training modality for athletes. More recently, the NASA (National Aeronautics and Space Administration) has focused on this exercise modality to find an effective way to counteract the loss of bone density occurring during spaceflight missions (Rubin et al., 2001). Therefore, this led to an emerging scientific interest in vibration as an exercise modality, mainly referred as “whole body vibration” (WBV) in the literature.
    Nowadays, although WBV exercise is broadly available to exercisers and patients, it seems that this exercise modality is still largely unknown to the scientific community. Quite confusingly, numerous studies and reviews have been published on the topic (Cardinale and Bosco, 2003; Cardinale and Wakeling, 2005; Luo et al., 2005; Nordlund and Thorstensson, 2007; Marín and Rhea, 2010a; 2010b; Rittweger, 2010). Unfortunately, the diversity of platforms and the heterogeneity of experimental procedures (e.g., protocols and settings, devices, subjects’ pretraining status, gender and inter-individual differences…) used lead to many controversies and unanswered questions, which have resulted in many physical therapists and coaches applying WBV in an empirical manner often based on personal beliefs. The lack of scientific consensus and international recommendations is particularly crucial because platforms are available to everyone (patients, competitive and leisure sports people, sedentary subjects, etc.), and the commercial pressure from manufacturers has reinforced the almost magical feature of this exercise modality. The important question therefore is whether the current interest in whole body vibration as an exercise modality is only due to unduly perceived exertion, or whether it can really constitute a physiological exercise stimulus.
    Vibration is a mechanical oscillation, i.e., a periodic alteration of force, acceleration and displacement over time. Vibration exercise, in a physical sense, is a forced oscillation, where energy is transferred from an actuator to a resonator. The majority of contemporary WBV devices produce periodic sinusoidal oscillations, where energy is transferred from the vibratory device (i.e. the actuator) to the human body, or parts of it (i.e., the resonator) while standing on the platform. When being vibrated on a platform, most people report an unusual perception that is often compared to the ranging and banging of the feet during downhill skiing. This sensation is partly due to a movement illusion and there is also a perception of exertion, when the energy created by the platform is transferred through the human body. Among the different platforms, two different types of energy transfer have to be discerned. One type transfers vibration to both feet synchronously, whilst the second type operates in a side-alternating way, so that the right foot is lowest when the left foot is highest. As the human body is not a rigid body, muscles and tendons act as spring-like elements that store and release mechanical energy. In such a spring-mass system, compression occurs during the vibration upstroke, and expansion during the down stroke. Beside, considering to the human anatomy, vibrations are transmitted from one segment to the next, i.e. from the foot to the calf, from the calf to the thigh etc. The amount of vibration energy transmitted will depend on musculoskeletal stiffness and damping. As a consequence, because each human being has unique musculoskeletal properties, WBV exercise must be individualized.
    The load that WBV imposes to the human body during exercise is characterized by the interaction of four parameters: frequency, amplitude, acceleration, and duration (Mester et al., 2002; Luo et al., 2005). Due to the wide combinations of these parameters it is extremely challenging to establish optimal recommendations for effective WBV exercise. It was initially suggested that WBV sessions incorporating low frequency and low peak-to-peak displacement were effective (Cardinale and Bosco, 2003; Cardinale and Wakeling, 2005). Luo et al. (2005), however, concluded that it was not possible to make recommendations because no study had directly assessed a variety of peak-to-peak displacement settings. Indeed, numerous reviews of the literature, based on both acute and chronic WBV studies, have reported contradictory conclusions regarding the effectiveness of WBV exercise. More recently, in two meta-analyses of the literature, Marín and Rhea (2010a; 2010b) have attempted to propose optimal settings, but these settings differed depending on the outcome (e.g., muscle strength or muscle power). Finally, their analysis was performed on healthy individuals, and cannot be generalized to other populations (e.g., the aged and health compromised individuals).
    In addition to the unsettled question on the effectiveness of WBV exercise, there is currently no clear consensus on the mechanisms by which WBV enhances muscle strength, power, balance, joint range of motion, bone mineral density… The most often cited mechanism proposed with WBV exposure is related to neural potentiation, as it may elicit a “tonic vibration reflex”, classically observed when vibration stimulus is directly applied to a muscle tendon. In order to investigate this hypothesis, some these studies have used surface electromyography (sEMG) to measure the level of the neuromuscular activity during WBV (Abercromby et al., 2007; Cardinale and Lim, 2003; Fratini et al., 2009; Ritzmann et al., 2010). The nature and validity of sEMG recordings during WBV has to be examined carefully before any deductions can be drawn based on the EMG data. Indeed, a potential source of error is the occurrence of motion artefacts caused by the applied vibration. Currently, literature focuses on the motion artefact as it pertains to clinical recording, such as electrocardiograms, electroencephalograms, sEMG, electrical impedance pneumography,.... In electrocardiography, motion artefact voltage amplitude can result in values ten times larger than the measured signal, which can be particularly troublesome either in ambulatory recordings and or during exercise testing. Fortunately, as the typical power density of these types of artefacts is confined at very low frequencies, they can be largely attenuated using a high-pass filter (Clancy et al., 2002) with limited loss of signal content. n classical clinical EMG recordings the frequency content of motion artefact is also considered to be below 10–20 Hz; therefore, a high-pass filter is often applied (e.g. with a cut-off frequency of 20 Hz), resulting in minimal loss of the sEMG signal power while rejecting most of the motion artefact. However, in particular situations such as WBV treatment, the power of motion artefacts is not confined below 10–20 Hz and standard high-pass filters are not suitable for filtering out this artefact. Although many studies have used sEMG during WBV exposure, surprisingly enough, there are only four studies addressing the problem of motion artefacts in sEMG signals analysis and processing (Abercromby et al., 2007; Fratini et al., 2009; Ritzmann et al., 2010). Again, the discrepancies in the results observed and authors’ conclusions reinforce the need of standardized procedures. As a consequence, it is unknown if sEMG could be an appropriate measure to individualize each person’s optimal WBV settings.
    Other authors have observed that metabolic demand is increased during WBV exposure in comparison with sham conditions (Rittweger, 2010). In addition, the modification of WBV settings also influences energy metabolism. However, in terms of energy expenditure these authors reported that energy turnover and cardiorespiratory responses were moderate. One would expect that the vibration-related increase in muscular energy metabolism leads to the generation of additional heat. Indeed, intramuscular temperature increases during whole body vibration exercise have recently been reported (Cochrane et al., 2008). Hence, given that vibration exercise stipulates muscular energy turnover and heat production, there is an obvious need for perfusion to comply with increased demands. During WBV exercise heart rate skin and blood flow can be enhanced (Rittweger, 2010). However, these latter studies have focused on post vibration exercise measurements. Few studies have attempted to assess metabolic responses (e.g., heart rate, muscle oxygenation...) during WBV exercise, and when it was performed, little or no information concerning data analysis and processing related to motion artefact removal was provided. One innovative research would be to study the benefit of modern source separation techniques to achieve this removal.
    Therefore, the purpose of the present project is to highlight the contribution of motion artefact in electrical (i.e., sEMG), and physiological (i.e., heart rate, blood flow, muscle oxygenation) recordings during WBV exercise. As recently, suggested local muscle acceleration will be assessed to quantify muscle displacement during WBV exercise (Fratini et al., 2009). The influence of WBV settings (e.g., various combination of frequency, peak-to-peak displacement of the platform) will be assessed, as well. The outcome of this project relies on the complementary expertise in the respective scientific field of investigation of the two laboratories involved in the project, i.e., investigation of mechanisms and effects during whole body vibration exercise for the LAMHES (Colson et al., 2009; 2010; Petit et al., 2010), and analysis and processing of physiological and biomedical signals for the I3S (Farina et al., 2001; Blain et al., 2009; Meste et al., 2009)

    For application CV and motivation letter
    Dead line August 20

    Contact : Pr Brisswalter , brisswalter@unice.fr
    Pr Colson , colson@unice.fr
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