User:Sunnyred.lin/Neuromuscular response to whole body vibration

Whole body vibration (WBV) uses a strategy that exposes whole body to mechanical vibration while standing on a vibration platform. In the past, vibration has been considered as an occupational hazard in daily life, overdose vibration could cause excessive torque placed on the spine. However, low amplitude and low frequency mechanical stimulation has been suggested as a safety and effective way to exercise musculoskeletal structures [1]. With proper vibration input, nowadays WBV is commonly used in gym and clinical with the purpose of increasing muscular performance, such as flexibility, power and strength [1, 2]. Some clinic even used the vibration to train elders to improve their balance or reduce pain [2]. Although WBV is being broadly available to exercisers and patients, this exercise modality is still largely unknown to the scientific community, and there is still limited comprehensive review available about the control mechanisms to allow further understanding. Up to now, central nervous information processing of vibration is the most common mechanism to explain potential vibration effect. With applying vibration directly on the tendon or whole body, most people have believed that the stimulation will lead to certain neurophysiological responses. The purpose of this article is tried to summarize the related articles through literature review.



Vibration Mode
Vibration is a mechanical ossification, such as periodic alteration of force, acceleration and displacement over time [2]. The oscillating plats currently available for two systems: (a) reciprocating vertical displacements on the left and right side of a fulcrum; (b) the whole platform vibration consistently up and down. Compared with synchronous vibration on both feet, side-alternating vibration would evoke rotational movements along the lower extremity and then result in larger amplitude [4]. According to Abercrombie et al. study, they found both vibration system were able to elicit electromyography (EMG) activation significantly of lower extremity [5]. However, they agreed that the asymmetric and non-vertical vibration direction may cause different impact on muscle stretch, postural change, and tissue vibration [5]. Besides, the intensity of the vibration stimulation was determined by frequency, amplitude, and duration. In Pollock et al. study, they pointed out that motor unit firing (MUs) response is reflexive and phase locked to the vibration cycle. In other words, specific motor unit only be firing by corresponding phase of the vibration cycle. Therefore, the extent of the oscillatory motions would cause different neuromuscular response under certain compression and expansion exposure.



Frequency

 * Low frequency vibration may have greater effect in vibration training. In contract, relatively high frequency might decrease the muscular performance.
 * However, Marin et al. found that surface electromyography activity (sENG) and rating of perceived exertion (RPE) increased with the acceleration from 25 Hz to 45 Hz vibration
 * MU discharge was related to the vibration cycle. In Pollock et al. study, 30 Hz vibration was used to continually discharge Ia afferent.
 * Vibration frequency responsible for timing MU discharge and retain a pulsatile quality.
 * In Pollock et al. study, they found muscle activation tended to increase linearly with frequency between 5-30 Hz . Furthermore, they noticed that low amplitude vibration had essentially similar EMG activity as the lowest frequency.

Amplitude

 * WBL vibration with larger amplitude might active muscle more effectively. Possibly a threshold to elicit motor reaction.
 * The magnitude of the WBV effect was higher on high amplitude mode (3.1 mm) than low mode (1.0 mm) for sEMG activity.
 * In Pollock et al. study, they have compared the effects of high (5.5 mm) and low (2.5 mm) amplitude whole body vibration. The results indicated that muscle increased 5-50% of maximal voluntary contraction with increasing electromyography activity.
 * High frequency had a trend of increasing EMG but not significant

Duration
Since 1990, high frequency and prolong directly vibration exposure have already been proved to have a side effect on physiology and body structure [10]. In Bongiovanni et al. study, they found 1 min sustained superimposed muscle vibration with high frequency (150 Hz) will reduce the EMG activity for maximal voluntary contraction [10]. In addition, motor unit firing rate, especially high-threshold motor unites, decreased after the prolong vibration, which result in muscle fatigue [6, 10]. The reduction of motor output were possibly caused by presynaptic inhabitation in monosynaptic pathways and then reduce the accessibility of α motoneurone pool [10]. Furthermore, prolong vibration primarily affect the ability of generating or maintaining high firing rate in high-threshold motor unit, which units consist of fast twist muscle fibers and supply powerful [11], the reduction of these MUs activity could lead to substantial force loss. Recently study conducted by Marzo et al. in 2011, they compared 30s, 60s, and 90s with fixed frequency and amplitude (30 Hz and 4 mm) whole body vibration. They found 1 minute had the greatest improvement on jump ability and muscle power, but over 1 minute elicits neuromuscular fatigue [12]. Furthermore, 3 and 6 sets of intervention had better performance than 9 sets [12]. As a result, prolonged vibration might decrease the neuromuscular performance of maximal contraction through the inhabitation of motor unit recruitment.

Motor Unit

 * According to Rittweger literature review published in 2010, the evidence showed that acute superimposed vibration seem to elicit a specific warm-up effect [2]. In Rittweger et al. study, they found subjects rapidly reach the comparable degree of exhaustion and muscular fatigue with the vibration intervention [13]. The vibration stimulation appears to alter neuromuscular recruitment which enhances neuromuscular excitability [13]. Although there was no direct interaction between EMG activity and muscle power, the difference in EMG frequency suggest predominately large motor units will be recruited with vibration stimulation [13].

Stretch reflex

 * WBV exercise interacts with spinal reflex loops, which increased excitability input from muscle spindles exposed to vibration [5]. In Rittweger et al. study, increase in the stretch reflex amplitude was noted in vibration group compared with the baseline group [13]. However, the effect of stretch reflex faded away after 10-20s [13]. The stretch effect was further supported by Abercromby et al. study that leg extensor muscle group had greater EMG activity than the flexor muscle group during WBV for squatting exercise. The extensor muscle group is stretched as the upward motion of the vibration platform and enhance muscle activation [5]. According to Rittweger et al. viewpoints, differences in stretch reflex amplitude after vibration is caused by enhanced center motor excitability, especially with respect to the fast twitch fibers and motor units [13].

Tonic vibration reflex (TVR)

 * TVR is also commonly used to explain the neuromuscular response after mechanical vibration. The stimulation of vibration is possible able to twitch muscle length shortly and then elicit a reflex response. In Hagbarth et al. tendon vibration study, the tonic vibration reflex (TVR) could activate muscle spindle through mediation of Ia afferents signals. Muscle fiber could later be activated through large α motoneuron pathway [14].In addition, TVR is thought to be elicited via the spindle loop and then mitigate reflex level [13]. It is possible that presynaptic inhabitation in the group Ia excitatory pathways and then induce neural adaption [10]. However, TVR can only be elicited by passively vibrator with the relaxed segment. As a result, voluntary movement for maintaining posture might dispute the whole body vibration response. Furthermore, H and tendon reflexes are inhibited by locally TVR due to presynaptic inhabitation in predominantly monosynaptic reflex (low-threshold). In contract, all muscles are vibrated stimulatory during WBV. Vibration on agonist muscle may result in reciprocal inhabitation of antagonist. WBV might be able to excite polysynaptic pathways (high-threshold) and then increase muscle activity [6].

Chronic Adaption
For the chronic training effect, Delecluse et al. had compared 12weeks period of WBV (amplitude: 2.5-5 mm, frequency: 35-40 Hz, 3 times/week) and resistance training (RES) on muscle strength. They found that isometric and isokinetic strength was significantly improved after WBV and RES (+9% - 16.6%). However, only WBV group had better jump height (+7.6%) but no different on RES group. The WBV training effect even maintained at least 72 hours after the last training session [15]. It is likely that WBV elicits a biological adaption on neural potentiation. While extensive sensory input was estimated, muscle might be reflexively elicited through positive proprioceptive feedback [15]. According to Komi vivo force study in human skeletal muscle, the stretch reflex contributes significantly to muscle stiffness and force generation in stretch-shortening activity [16]. Therefore, stimulation sensory receptors and the Ia afferent with WBV might thus lead to efficient response of stretch reflex for jumping.

Limitation

 * There is a lack of strictly controlled studies in the vibration training effect.
 * Fatigue affects not only force generation, but also the temporal characteristics of the neuromuscular mechanism.
 * Difference in muscle length of each subject may responsible for different time spend to travel along the reflex arc.
 * Most study was conducted by muscle vibration , but a few study specifically focused on whole body vibration.
 * The acceleration tends to decrease around 0.2-9 g with distance from the platform. Muscle which is closest to the platform had great vibration effect and muscle activity [4]. The precise mechanism of on each body part during whole body vibration is still unclear. Transmissions of vibration through the whole body is changed by different tissue and related position, such as bone, cartilage, synovial fluid, joint kinematics and muscular activity [14]. Therefore, mechanical energy could be stored and returned from the elastic structures during soft tissue vibration [14]. Further, different starting position could also change the effect of whole body vibration. According to Pollock et al. study, leaning forward will reduce the load through the heels, increasing muscle activity and ankle joint stability, and offering further damping on ankle and hip [4].
 * According to Cardinale et al. review, whole body vibration is unlikely to stimulate hypertrophy in young healthy population, but may be sufficient in weak and inactive subjects, such as non-alethic or elders through neural adaptions [15].

Conclusion
The human body is not a rigid body, which consisted of several spring-like soft tissues that store and release mechanical energy. The transmission of vibration could be affected by starting position and the distance from vibration source. In addition, there still remain speculations as to how WBV affects performance of the neuromuscular system. Once the evidence researches support the training effect, vibration could be applied as part of intervention in clinic or gym.