***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: This study aim ed to assess the influence of different whole body vibration ( WBV ) determinants on the electromyographic ( EMG ) activity during WBV in order to identify those training conditions that cause highest neuromuscular responses and therefore provide optimal training conditions . In a r and omized cross-over study , the EMG activity of six leg muscles was analyzed in 18 subjects with respect to the following determinants : ( 1 ) vibration type ( side-alternating vibration ( SV ) vs. synchronous vibration ( SyV ) , ( 2 ) frequency ( 5–10–15–20–25–30 Hz ) , ( 3 ) knee flexion angle ( 10 ° –30 ° –60 ° ) , ( 4 ) stance condition ( forefoot vs. normal stance ) and ( 5 ) load variation ( no extra load vs. additional load equal to one-third of the body weight ) . The results are : ( 1 ) neuromuscular activity during SV was enhanced compared to SyV ( P < 0.05 ) ; ( 2 ) a progressive increase in frequency caused a progressive increase in EMG activity ( P < 0.05 ) ; ( 3 ) the EMG activity was highest for the knee extensors when the knee joint was 60 ° flexed ( P < 0.05 ) ; ( 4 ) for the plantar flexors in the forefoot stance condition ( P < 0.05 ) ; and ( 5 ) additional load caused an increase in neuromuscular activation ( P < 0.05 ) . In conclusion , large variations of the EMG activation could be observed across conditions . However , with an appropriate adjustment of specific WBV determinants , high EMG activations and therefore high activation intensities could be achieved in the selected muscles . The combination of high vibration frequencies with additional load on an SV platform led to highest EMG activities . Regarding the body position , a knee flexion of 60 ° and forefoot stance appear to be beneficial for the knee extensors and the plantar flexors , respectively BACKGROUND Low back disorders and their prevention is of great importance for companies and their employees . Whole-body vibration is thought to be a risk factor for low back disorders , but the neuromuscular , biomechanical , and /or physiological mechanisms responsible for this increased risk are unclear . The purpose of this study was to measure the acute effect of seated whole-body vibration on the postural control of the trunk during unstable seated balance . METHODS Twenty-one healthy subjects ( age : 23 years ( SD 4 years ) ) were tested on a wobble chair design ed to measure trunk postural control . Measurements of kinematic variance and non-linear stability control were based on seat angle before and after 30 min of seated whole-body vibration ( b and width=2 - 20 Hz , root-mean-squared amplitude=1.15m/s(2 ) ) . FINDINGS All measures of kinematic variance of unstable seated balance increased ( P<0.05 ) after vibration including : ellipse area ( 35.5 % ) , root-mean-squared radial lean angle ( 17.9 % ) , and path length ( 12.2 % ) . Measures of non-linear stability control also increased ( P<0.05 ) including Lyapunov exponent ( 8.78 % ) , stability diffusion analysis ( 1.95 % ) , and Hurst rescaled range analysis ( 5.2 % ) . INTERPRETATION Whole-body vibration impaired postural control of the trunk as evidence d by the increase in kinematic variance and non-linear stability control measures during unstable sitting . These findings imply an impairment in spinal stability and a mechanism by which vibration may increase low back injury risk . Future work should investigate the effects of whole-body vibration on the anatomical and neuromuscular components that contribute to spinal stability The purpose of this study was to investigate the consequences of a  high-speed boat transit  on physical performance .  Twenty-four Royal Marines were r and omly assigned to a  control ( CON ) or transit ( TRAN ) group . The CON group sat onshore for 3 h whilst the TRAN group completed a 3-h transit in open-boats running side-by-side , at 40 knots in moderate-to-rough seas , with boat deck and seat-pan acceleration recorded . Performance tests ( exhaustive shuttle-run , h and grip , vertical-jump , push-up ) were completed pre- and immediately post-transit/sit , with peak heart rate ( HRpeak ) and rating of perceived exertion ( RPE ) recorded . Serial blood sample s ( pre , 24 , 36 , 48 , 72 h ) were analyzed for creatine kinase ( CK ) activity . The transit was typified by frequent high shock impacts , but moderate mean heart rates ( < 45 % HRpeak ) . The TRAN group post-transit run distance ( −219 m , P < 0.01 ) and vertical-jump height ( 5 % , P < 0.05 ) were reduced , the CON group showed no change . The TRAN group post-transit test RPE increased ( P < 0.05 ) , however , HRpeak was similar for each group ( 98 % ) . Post-transit CK activity increased in the TRAN group up to 72 h ( P < 0.01 ) and also , but less markedly , in the CON group ( 24 and 48 h , P < 0.05 ) . Post-transit run and jump performances were reduced despite mean transit heart rates indicating low energy expenditure . The greater TRAN CK activity suggests muscle damage may have been a contributory factor . These findings have operational implication s for Special Forces/naval/police/rescue services carrying out dem and ing , high-risk physical tasks during and immediately after high-speed boat transits Underst and ing of the resonance behaviour of the human body is important in the identification of vibration frequencies and body postures associated with back problems . In this study , experimental modal analysis was applied to whole-body vibration . Eight subjects were exposed to vertical r and om vibration while adopting three different postures on a rigid seat without a backrest .  Motions of the spine , pelvis and viscera in the mid-sagittal plane were derived from skin-mounted accelerometers ; head responses were measured using a bite-bar . Eight modes of vibration response were extracted below 10 Hz . A principal resonance of the human body at about 5 Hz consisted of an entire body mode , in which the skeleton moved vertically due to axial and shear deformations of buttocks tissue , in phase with a vertical visceral mode , and a bending mode of the upper thoracic and cervical spine . A bending mode of the lumbar and lower thoracic spine was found with a pitching mode of the head in the next higher mode located close to the principal mode . The second principal resonance at about 8 Hz corresponded to pitching modes of the pelvis and a second visceral mode . When subjects changed posture from erect to slouched , the natural frequency of the entire body mode decreased , result ing in a decrease in the principal resonance frequency . Shear deformation of buttocks tissue increased in the entire body mode due to the same change of posture . The complex body motions suggest that any forces causing injury from whole-body vibration will not be well-predicted by biodynamic models incapable of representing the appropriate body motions and the effects of body posture . It seems likely that the greatest risk of back problems will arise from the bending deformations of the spine STUDY DESIGN A cross-sectional study in patients with recurrent/chronic low back trouble and healthy control subjects . OBJECTIVE To evaluate the effect of paraspinal muscle fatigue on the ability to sense a change in lumbar position . SUMMARY OF BACKGROUND DATA Protection against spinal injury requires proper anticipation of events , appropriate sensation of body position , and reasonable muscular responses . Lumbar fatigue is known to delay lumbar muscle responses to sudden loads . It is not known whether the delay is because of failure in the sensation of position , output of the response , or both . METHODS Altogether , 106 subjects ( 57 patients with low back trouble [ 27 men and 30 women ] and 49 healthy control subjects [ 28 men and 21 women ] ) participated in the study . Their ability to sense a change in lumbar position while seated on a special trunk rotation unit was assessed . A motor rotated the seat with an angular velocity of 1 degree per second . The task in the test involved reacting to the perception of lumbar movement ( rotation ) by releasing a button with a finger movement . The test was performed twice , before and immediately after a fatiguing procedure . During the endurance task , the participants performed upper trunk repetitive extensions against a resistance , with a movement amplitude adjusted between 25 degrees flexion and 5 degrees extension , until exhaustion . RESULTS Patients with chronic low back trouble had significantly poorer ability than control subjects on the average to sense a change in lumbar position ( P = 0.007 ) , which was noticed before and after the fatiguing procedure . Lumbar fatigue induced significant impairment in the sensation of position change ( P < 0.000001 ) . CONCLUSIONS Lumbar fatigue impairs the ability to sense a change in lumbar position . This feature was found in patients and control subjects , but patients with low back trouble had poorer ability to sense a change in lumbar position than control subjects even when they were not fatigued . There seems to be a period after a fatiguing task during which the available information on lumbar position and its changes is inaccurate Owing to the recent interest in torso stabilization exercises together with many questions regarding the duration of prolonged isometric holds in occupational setting s , the authors attempted to assess the level of back muscle oxygenation during prolonged isometric contractions . Specifically , this study recorded relative oxygen saturation of haemoglobin/myoglobin using Near Infrared Spectroscopy ( NIRS ) in the L3 erector mass during prolonged isometric contractions at intensities from 2 to 30 % of maximum voluntary contraction ( MVC ) . It was hypothesized that available oxygen to these muscles is severely compromised even at moderate levels of activation observed in occupational work . Eight volunteers without a history of lower back pain or injury participated in this study . The exercise task involved isometric contraction of the lower erector spinae at five different levels of each subject 's maximal voluntary contraction : 2 , 5 , 10 , 20 and 30 % MVC , presented in r and om order . Subjects were placed in a sitting position , with a curved plastic plate secured horizontally to the pelvis to minimize movement at the hip joint . During extensor exertions , they were restrained with a harness that was attached at chest level to a load cell . Each isometric contraction was performed for 30 s followed by 1 min of rest . All levels of contraction demonstrated reduction in oxygen . Given the concern for motion artefact on the NIRS signal , sham trials were conducted where the subjects went through the procedure of attaching the pulling cable but no active pull was performed . These trials showed no change in the NIRS signal . At this time NIRS appears to be the only non-invasive instrumentation available to indicate total available muscle oxygen during low level , prolonged work . Although the specific tissue volume sample d by NIRS can not be positively identified , it appears that tissue oxygenation in the lumbar extensor musculature is reduced as a function of contraction intensity , even at levels as low as 2 % of MVC . These data have implication s for prolonged work where postures requiring isometric contractions are sometimes held for hours , and where musculoskeletal illness has been linked to prolonged contraction levels above 2 % MVC — these data suggest a possible biological pathway Exposure to whole-body vibration is implicated as one of the occupational risk factors for lower back disorders ; however , its influence on the lumbar muscle physiology is still poorly understood . The objective of this study was to investigate the effects of backrest support and h and grip contractions on lumbar muscle oxygenation and blood volume responses during seated whole-body vibration using continuous dual-wave near-infrared spectroscopy . Thirteen healthy men were exposed to frequencies of 3 , 4.5 and 6 Hz on a vibration simulator , in r and omized order on separate days . Each day the duration of the protocol was 30 min . During the fifth minute of vibration ' with ' and ' without ' backrest support , participants performed maximal rhythmic h and grip contractions for 1 min . In general , erector spinae oxygenation and blood volume showed a trend to decrease with vibration exposure compared to the control condition . However , these responses were not influenced by the change in vibration frequency ( P > 0.05 ) . Sitting without backrest result ed in a greater decrease in oxygenation ( by 27 % , P = 0.02 ) and blood volume ( by 11 % , P = 0.05 ) than with backrest , implying a deficiency in oxygen supply owing to the sitting posture . Compared to the vibration-only condition , h and grip work decreased both oxygenation ( by 22 % , P = 0.003 ) and blood volume responses ( by 13 % , P = 0.04 ) , suggesting that postural load due to prolonged sitting combined with physical activity during vibration might further burden paraspinal muscles . The influence of adipose tissue thickness of the lumbar muscle on optically derived oxygenation and blood volume changes was inconclusive An experiment is described in which two independent groups of eight subjects each performed a combined continuous and discrete tracking task during exposure to vertical whole-body vibration .  Both groups received sinusoidal and r and om vibration at preferred third-octave centre frequencies of 0.5 - 10 Hz . One group performed the task with the display collimated by a convex lens . Without the collimation , performance was disrupted by both types of vibration at all vibration frequencies ; collimation removed the disruption at frequencies above 1.6 Hz . There were differences in the effects of r and om and sinusoidal vibration at 2.0 and 2.5 Hz , suggesting that compensatory eye movements were assisting performance during exposure to the predictable sinusoidal motion . The results show that continuous control performance was disrupted by visual interference at frequencies above 1.6 Hz ; closed-loop system transfer functions showed that visual interference increased the phase lags which impaired control performance . Possible mechan 
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Discussion Given the inconsistency in the reported responses , the precise changes to human function remain unknown .
However , there is sufficient evidence to warrant the design of studies which investigate occupationally relevant physical performance changes following WBV