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Paper text:
in daily life , humans are constantly faced with new postural control tasks to which they 
 have to adapt and adjust . to detect motion and adjust voluntary and reflexive muscle 
 responses
, the balance system uses sensory information from visual , vestibular and 
 somatosensory receptors1 . nevertheless , 
 decline in the function of proprioceptive sensation
proprioception in the leg and trunk muscles plays an important role in maintaining postural 
 stability2 .
past studies have reported 
 that proprioception and vibration sensation in the lower limbs decrease during normal 
 aging3 , 4 , and reported postural instability has been observed in elderly 
 people5 .
also , both muscle strength and 
 lower postural stability decline with aging6 , 7 .
therefore , a vibratory stimulus that 
 matches the response frequency of the receptors present in skeletal muscle may influence 
 body sway .
the representative receptor and response frequencies are 30  hz in the meissner 
 corpuscle , 60  hz in the muscle spindle , and 240  hz in the corpuscle of vater - pacini .
the acquisition of a motor skill is generally a gradual process requiring many repetitions 
 over a period of time8 .
in addition , 
 several reports have shown that body movements induced during repeated exposure to postural 
 disturbances gradually decrease9 . 

furthermore , it is probable that sensory deficits result from either location or physical 
 performance reduction , and might be a causative factors of postural instability .
past cross - sectional and longitudinal studies have reported that the course of decline for 
 measures of muscle strength , gait and balance accelerates with advancing age10 , 11 .
for instance , there are reports that grip strength may begin to 
 decline from the age of 3012 .
likewise , 
 measures of balance function , gait and motor function have been reported to deteriorate 
 after the age of 4011 .
therefore , it is 
 important to measure muscle strength , balance , gait speed and response to proprioceptive 
 input during middle age for a comprehensive assessment of age - related changes in physical 
 performance , health and balance ability .
the aim of this study was to investigate whether a vibratory stimulation applied to the 
 proprioceptive inputs of the lumbar multifidus and the gastrocnemius muscles affects 
 postural stability and physical performance of middle - aged subjects .
the subjects were 25 ( 13 males , 12 females ) middle - aged people ( age = 46.0  3.0 ) .
inclusion criteria were : middle - aged 
 at the time of the examination in 2013 , living at their home in aichi or gifu , and had not 
 participating in another study .
we measured subjects height ( to the nearest 0.1  cm ) and 
 weight ( to the nearest 0.1  kg ) additional inclusion criteria were : absence of low back pain , 
 ability to perform the task , and absence of neuromuscular or orthopedic diseases , artificial 
 pacemakers , digestive diseases , or symptoms of dehydration .
the center of pressure ( cop ) was recorded using a wii balance 
 board ( nintendo co. , ltd . ,
recently , the wii balance board has been much used 
 in the field of medical research , and it has been reported that results from the wii balance 
 board correlate closely with those of commercially available force plates13,14,15,16 . 
 by fixing two of the vibrators from the vibration device on the participants lumbar 
 multifidus and gastrocnemius muscles ,
the subjects stood barefoot on the wii balance board with their feet together 
 and their eyes closed .
they were instructed to remain still and relax in a standing posture 
 with their arms hanging loosely at their sides .
the amplitude of the vibration was 1.6  mm , 
 and the frequencies were 30 , 60 and 240  hz .
each subject s cop was measured under six 
 conditions , the two different muscles and the three different frequencies of vibratory 
 stimulation : ( 1 ) 30  hz on the lumbar multifidus ; ( 2 ) 30  hz on the gastrocnemius muscles ; ( 3 ) 
 60  hz on the lumbar multifidus ; ( 4 ) 60  hz on the gastrocnemius muscles ; ( 5 ) 240  hz on the 
 lumbar multifidus ; and ( 6 ) 240  hz on the gastrocnemius muscles .
the measurement time was 30 
 seconds , which was divided into two intervals of 15 seconds .
grip strength of the subjects 
 dominant hands was measured in kilograms using a digital handheld dynamometer ( mcz-5041 , 
 macros ltd , tokyo , japan ) .
gait function was assessed by gait time tests conducted on a 
 defined path subjects usual gait speed was measured in seconds with a stopwatch . 

participants were asked to walk on the flat , straight surface at their usual gait speed .
two 
 markers were used to indicate the start and end of the path , and a 5  m and over approach was 
 allowed before reaching the start marker so that participants could attain their usual gait 
 pace before reaching the timed path .
they were instructed to continue their gait past the 
 end of the path for a further 5  m and over to ensure that the gait pace was consistent 
 throughout the task .
informed consent was obtained from all the subjects prior to their inclusion in the study , 
 and the ethics committee of the national center for geriatrics and gerontology at the nagoya 
 institute of technology approved the study protocol .
pearson correlation coefficients were calculated to determine the relationships between 
 gait speed , grip strength and postural sway during vibratory stimulations of 30 , 60 , or 
 240  hz applied to the lumbar multifidus or gastrocnemius muscles .
we defined the change in 
 anteroposterior diplacement of cop as : y = y(during )  y(pre ) . where y is the displacement of 
 the y - coordinate of the cop recorded by the wii balance board , and y(pre ) and y(during ) are 
 the mean values of the time series data of y for the first 15 sec and the last 15 sec , 
 respectively .
these calculations were using performed a program we wrote using matlab ( the 
 mathworks , inc . ) .
multiple linear regression analysis was used to examine whether postural sway during 
 vibratory stimulations was associated with physical performance .
postural sway during each vibratory stimulation was also 
 investigated using the stepwise method of multiple linear regression analysis .
all analyses were performed using ibm spss statistics software ( version 20 ; spss inc . , 

the characteristics of the subjects are shown in table 
 1table 1.physical characteristics of the study participantsvariablesparticipants ( n = 25)age ( years)46.0  3.0height ( cm)165.6  8.3weight ( kg)61.1  11.2grip strength ( kg)33.1  7.6usual gait speed ( m / s)1.4  0.2center of pressure : gmv 30  hz , during - pre ( cm)0.23  0.30center of pressure : lmv 30  hz , during - pre ( cm)0.08  0.43center of pressure : gmv 60  hz , during - pre ( cm)0.44  0.50center of pressure : lmv 60  hz , during - pre ( cm)0.79  0.58center of pressure : gmv 240  hz , during - pre ( cm)0.29  0.53center of pressure : lmv 240  hz , during - pre ( cm)0.23  0.51values are meansd .
center of pressure displacement ( averages  deviation ) of the participants 
 during vibration of the gastrocnemius muscles , lumbar multifidus .
positive values 
 signify anterior body sway , negative values indicate posterior body sway .. during upright stance with vibratory stimulations , participants showed 
 increased body sway in the anteroposterior direction ( table 1 ) .
subjects showed a negative correlation between cop displacement of the 
 30  hz gastrocnemius muscles vibration ( gmv ) and their gait speed ( r = 0.50 , p < 0.05 ) 
 ( table 2table 2.correlation coefficients of center of pressure sway changes induced by gmv ( 30 , 
 60 , 240  hz ) , and lmv ( 30 , 60 , 240  hz ) and grip strength and gait speedvariablesgrip strengthusual gait speedcenter of pressure : gmv 30  hz , during - pre0.200.50*center of pressure : lmv 30  hz , during - pre0.340.18center of pressure : gmv 60  hz , during - pre0.010.02center of pressure : lmv 60  hz , during - pre0.060.39center of pressure : gmv 240  hz , during - pre0.010.12center of pressure : lmv 240  hz , during - pre0.040.01the asterisk denotes a significant differences , * p < 0.05 .
the negative 
 correlation indicates that subjects gait speed increased with increasing posterior 
 shift of center of pressure sway . ) , and the regression coefficient was significant ( r 25% , = 0.50 , 
 p < 0.01 ) .
gmv , gastrocnemius muscles vibration ; lmv , lumbar multifidus 
 vibration . center of pressure displacement ( averages  deviation ) of the participants 
 during vibration of the gastrocnemius muscles , lumbar multifidus .
the negative 
 correlation indicates that subjects gait speed increased with increasing posterior 
 shift of center of pressure sway .
there was no significant correlation between body sway or grip strength at this level of 
 proprioceptive stimulation ( gmv of 30  hz ) , even though there was a significant correlation 
 between postural sway and gait speed .
a 30  hz proprioceptive stimulation elicited as a change in the average displacement of cop 
 away from the center of support , in the posterior direction ( negative value ) .
the faster the 
 subjects habitual gait , the larger the change in cop displacement of 30  hz gmv became . in 
 the stepwise multiple regression analysis ,
only the change in cop displacement of 30  hz gmv 
 was identified as a significant independent variable related to gait speed .
previous studies have reported that vibration perception and proprioception decline with 
 increasing age3 , 4 .
sensory information from receptors in the muscles and joints 
 provide feedback regarding joint position sense , movement and touch .
sensory information 
 from the lower extremity is arguably the most important contributor to standing balance , 
 because the proprioceptive threshold is lower than the threshold for the perception of the 
 speed of center of foot pressure sway17 . 

functional decline in the somatosensory system occurs with aging and these changes are 
 associated with postural instability . during gait ,
joint and muscle mechanoreceptors provide 
 information to help adjustment with each step and achieve the ideal foot placement .
previous 
 studies have reported that elderly people have decreased responses to vibration compared 
 with younger people5 , 18 .
a previous study demonstrated that measures of balance and gait 
 function deteriorate from the ages of 40 and 60 , respectively11 .
kilner et al . reported the presence of coherent oscillatory activity in the human hand and 
 forearm muscles , and showed that the 1530  hz component has task - dependent modulation19 .
other studies of vibration frequencies 
 that are usually applied for exercise purposes have reported effects on the vastus 
 lateralis , and showed that emg amplitudes are larger at stimulation of 30  hz20 .
additionally , the frequency for eliciting 
 appearances and disappearances of postural effects has been reported to be in the range of 
 2530  hz for the leg and trunk muscles21 .
the cutaneous and subcutaneous mechanoreceptors that innervate glabrous or hairless skin 
 are the rapidly adapting meissner corpuscles . it has been suggested that the stimulation of the gastrocnemius muscles influences gait 
 speed and body sway . because tactile information from the sole affects postural control22 , the number of meissner corpuscles has 
 been shown to reduce with age23 , with a 
 corresponding decrease in gastrocnemius muscle vibration perception and gait speed .
daly et al.24 reported the importance of 
 promoting exercise interventions to improve muscle strength from the age of 50  years and 
 onward , with the need to develop more targeted programs designed to enhance both balance and 
 gait speed in the elderly .
gait and balance exercise might be a key factor in postural 
 stability , as previously shown in several studies , and such exercise could be important for 
 preventing falls10 , 11 .
han et al.25 
 reported that vibration may have a positive effect on the maintenance of static balance by 
 activating proprioceptors .
the results of the present study suggest that 30  hz gmv is an 
 important proprioceptive stimulus and that exercise with 30  hz vibratory stimulation , 
 particularly for middle - aged people , would help to improve their gait speed and balance 
 function by stimulating the meissner corpuscles of the gastrocnemius muscle .
taken together , 
 these data support the possibility that reduced gait function in middle - age results from a 
 reduction in the number of meissner s corpuscles in the gastrocnemius muscle .
thus , a 
 decrease in cop sway induced by 30  hz gmv may indicate a reduction in gait speed due to 
 increasing postural instability .
this suggests that variables derived from cop sway measured 
 during 30  hz gmv can be used as an index for predictive factors of gait function in middle 
 age .
a limitation of this study was that only healthy middle - age people were surveyed . 

additional study is needed to determine whether the age difference in the age - related 
 postural control decline of proprioceptive sensitivity is caused by declines in physical 
 performance or other characteristics .
[ purpose ] the purpose of this study was to examine the relationship between physical 
 function and postural sway during local vibratory stimulation of middle - aged subjects in 
 an upright position . 
 [ subjects ] the subjects were 25 healthy community - dwelling 
 middle - aged people . [ methods ] we measured postural sway using a wii board while vibratory 
 stimulations of 30 , 60 , or 240  hz were applied to the subjects lumbar multifidus or 
 gastrocnemius muscles . 
 physical function was evaluated by 5-m usual gait speed and grip 
 strength . 
 [ results ] gait speed was strongly correlated to the anteroposterior body sway in 
 the upright position during 30  hz gastrocnemius muscles vibration ( gmv ) . 
 [ conclusion ] 
 postural sway during 30  hz gmv was strongly associated with gait speed and showed a 
 posterior displacement . 
 these findings show that the lower leg s response to balance 
 control under 30  hz proprioceptive stimulation might be a good indicator of declining gait 
 function .