motor imagery ( mi ) is a process by which working memory is produced without actual motion . the activation of the primary sensory motor cortex , supplementary motor area , and premotor 
 area during mi have been demonstrated1 , 2 . in our previous study , the excitability of 
 spinal neural function during mi at 50% maximum voluntary contractions ( mvc ) was 
 significantly higher than that during rest3 . regarding effect of mi on movement , page et al.4 reported that upper - limb function was improved using a 
 combination of physical therapy and mi . in addition , dickstein et al.5 reported that gait was improved using mi in post - stroke 
 hemiparesis . however , it is unclear whether mi affects the accurate control of muscle force . in our 
 previous study , the excitability of spinal neural function using f - waves during mi and the 
 effect of mi on the accuracy of the movement was examined . an f - wave is a compound action 
 potential obtained as a result of re - excitation of an antidromic impulse following distal 
 electrical stimulation of motor nerve fibers of the spinal anterior horn cells . the 
 correction time , ( total time of 50%mvc  5% ) was defined as the index of accurate control of 
 muscle force . we studied the accurate control of muscle force by including the pinch force 
 rising phase in the pinch force graph . in our previous study , persistence during mi was 
 significantly greater than that at rest . the correction time is not shown to be 
 significantly different before and after mi6 . accordingly , mi increased the excitability of the spinal neural 
 function ; however , it did not affect the accurate control of muscle force . with regard to 
 the cause , the first hypothesis that we did not fully understand at which point a subject 
 would subjectively determine they had reached 50%mvc . therefore , we studied accurate control 
 of muscle force , including the pinch force rising phase . we examined including aim at 50%mvc 
 and out of aim at 50%mvc by subjects . the subject of difficult to imagine the 50%mvc , those 
 subjects decrease the correction time . otherwise , the correction time is decreased by taking 
 additional time for 50%mvc , but the subject is aim at 50%mvc by mi . accordingly , it was the 
 limit for awareness of improving the ability of actual motion in case of including the phase 
 pinch force rising in the pinch force graph . the second hypothesis , the subjects did not 
 learn 50%mvc for 10s , the mi under 50%mvc of the subjects was different from that under an 
 authentic 50%mvc . we should have examined within the subject aimed at 50%mvc . in this study , 
 after the point at which subjects subjectively determined they had reached 50%mvc , we 
 studied the excitability of spinal neural function using f - waves recorded during mi as well 
 as the effect of mi on the accuracy of the movement by new subjects . thirty healthy subjects ( males , 15 ; females , 15 ; mean age , 20.3  1.0  years ) participated 
 in this study after providing informed consent . this study was approved by the research 
 ethics committee at kansai university of health sciences ( approval number : 15 - 04 ) . the study process was as follows . in a resting condition , a holding sensor condition , and a 
 mi condition f - waves during isometric contraction of the opponens pollicis muscle were 
 recorded . the pinch force without visual feedback before and after mi was measured . in the 
 resting condition , f - waves from the left thenar muscles during relaxation were recorded . in 
 the holding sensor condition , the f - wave was recorded while the subject held the pinch 
 pressure sensor [ digital indicator f340a ( unipulse medical inc . ) ] between the thumb and 
 index finger . in the mi conditions , subjects were first required to learn 50%mvc during 
 isometric contraction of the opponens pollicis muscle while holding pinch pressure sensor . the magnitude of mvc was numerically recorded on the pinch pressure sensor display after 
 reaching 50%mvc , and subjects were instructed to keep the 50%mvc value on the pinch pressure 
 sensor display for 10 s. subsequently , we measured the pinch force without using visual 
 feedback for 10  s with the subjects subjectively determining when they had reached 50%%mvc 
 ( task 1 ) . subjects were then asked to imagine the contraction while holding the pinch 
 pressure sensor between their thumb and index finger ( mi while holding sensor ) . 
 additionally , f - waves were recorded during mi . ultimately , we measured the pinch force 
 without using visual feedback for 10  s with the subject subjectively determining when they 
 had reached 50%mvc ( task 2 ) . on a different day , while not using mi ( without - mi condition 
 under the holding sensor ) in similar process , f - waves were recorded ( fig . 1fig . 1.the experimental process . in the resting condition and holding sensor condition , we 
 recorded f - waves . next , the subjects learned 50% mvc during isometric contraction of 
 the opponents pollicis muscle . subsequently , we measured pinch force without using 
 visual feedback before and after motor imagery . additionally , f - waves were recorded 
 during the motor imagery . ) . the experimental process . in the resting condition and holding sensor condition , we 
 recorded f - waves . next , the subjects learned 50% mvc during isometric contraction of 
 the opponents pollicis muscle . subsequently , we measured pinch force without using 
 visual feedback before and after motor imagery . the subjects were comfortably 
 placed in the supine position with both shoulder joints externally rotated and both forearms 
 in an eversion position . a viking quest electromyography machine ( natus medical inc . ) the skin was prepared with abrasive gel to keep the impedance below 
 5 k. f - waves of the left thenar muscles were recorded using a pair of disks attached with 
 collodion to the skin over the belly of the thumb , and the bones of the metacarpophalangeal 
 joint of the thumb . the stimulating electrodes comprised a cathode placed over the left 
 median nerve 3  cm proximal to the palmar crease of the wrist joint and an anode placed 2  cm 
 more proximally . the maximal stimulus was determined by delivering 0.2-ms square - wave pulses 
 of increasing intensity to elicit the largest compound muscle action potentials . supramaximal shocks ( adjusted up to the value of 20% higher than the maximal stimulus ) were 
 delivered at 0.5  hz for acquisition of f - waves . the bandwidth filter ranged from 2 to 3 khz . 
 by stimulating the left median nerve at the wrist , f - waves were 
 analyzed with respect to persistence , the f / m amplitude ratio , and latency using 30 stimuli 
 ( fig . 2fig . f - waves were analyzed with respect to persistence , f / m 
 amplitude ratio , and latency . ) . persistence was defined as the number of measurable f - wave responses divided by 30 
 supramaximal stimuli . f / m amplitude ratio was defined as the mean amplitude of all responses 
 divided by the amplitude of the m - wave . latency was defined as the mean latency from the 
 time of stimulation to the onset of a measurable f - wave . f - waves were analyzed with respect to persistence , f / m 
 amplitude ratio , and latency . subjects touched the copper board with their right index 
 finger when 50%mvc was reached . the point where subjects subjectively determined where they 
 had reached 50%mvc by a spike on the screen was determined by authors . an error range of 
 plus or minus 5% for the pinch force was adopted . the correction time ( total time of 50%mvc 
  5% ) , was defined as the index of accurate control of muscle force . additionally , the 
 coefficient of variation , which was extent of pinch force transition , was defined as the 
 index of accurate control of muscle force . therefore , exerted apposite and sustained force 
 of pinch action by the correction time ( the total time of 50%mvc  5% ) , and coefficient of 
 variation within after the point at which subjects subjectively determined they had reached 
 50%mvc , was examined . the correction time and coefficient of variation from task 1 and 2 the pinch force using electromyogram recording software vitalrecorder2 ( kissei 
 comtec ) was calculated in addition to the correction time using a versatile biological 
 analysis system bimutas - video ( kissei comtec ) ( fig . we studied that exerted apposite and 
 sustained force of pinch action by the correction time ( the total time of the pinch 
 force of 50%mvc  5% ) and coefficient of variation . ) . we studied that exerted apposite and 
 sustained force of pinch action by the correction time ( the total time of the pinch 
 force of 50%mvc  5% ) and coefficient of variation . the data were 
 not recognized as showing normal distribution thus the friedman test was used to compare 
 results between resting and other conditions . thereafter , the scheffe test was used to 
 compare the results across all three conditions . wilcoxon signed - rank test was used to 
 compare correction time and coefficient of variation between tasks 1 and 2 . with regard to changes in the f - wave , persistence significantly increased in the holding 
 sensor and mi conditions compared to the resting condition , and persistence was 
 significantly higher in the mi condition than in the holding sensor condition ( * p<0.05 , 
 * * p<0.01 , respectively ; table 1table 1.changes in the f - wave during mvc50% ( mi group)restholding sensormipersistence ( % ) 63.4  22.778.1  17.290.5  9.6f / m amplitude ratio ( % ) 1.2  0.61.8  0.82.2  1.5latency ( ms)23.5  1.723.4  1.723.2  2.0mean  sd . * significant ( p<0.05 ) , * * significant ( p<0.01 ) vs. at rest . the f / m amplitude ratio was significantly higher in the holding sensor and 
 mi conditions than in the resting condition . no significant differences were found in the 
 f / m amplitude ratio , and between the holding sensor and mi conditions ( * p<0.05 , 
 * * p<0.01 , respectively ; table 1 ) . there were 
 no significant differences in latency among all three conditions ( table 1 ) . persistence was significantly higher in the holding sensor and without - mi 
 conditions than in the resting condition . no significant differences in the persistence was 
 observed between the holding sensor and without - mi conditions ( * p<0.05 , * * p<0.01 , 
 respectively ; table 2table 2.changes in the f - wave during mvc50% ( without mi group)restholding sensorwithout mipersistence ( % ) 47.4  19.172.1  16.567.5  17.6f / m amplitude ratio ( % ) 1.2  0.51.9  1.11.8  1.1latency ( ms)23.9  1.723.5  1.723.5  1.8mean  sd . the f / m amplitude ratio was significantly higher in the holding sensor and 
 without - mi conditions than in the resting condition . no significant differences in the f / m 
 amplitude ratio were observed between the holding sensor and without - mi conditions 
 ( * p<0.05 , * * p<0.01 ; table 2 ) . there were 
 no significant differences in latency among all three conditions ( table 2 ) . significant ( p<0.05 ) , * * significant ( p<0.01 ) vs. at rest . no significant differences were observed in the 
 correction time between tasks 1 and 2 ( table 
 3table 3.changes in the accurate control of muscle force ( mi group)task 1task 2correction time ( s)1.2  1.50.7  1.6coefficient of variation0.1  0.10.1  0.1 ) . no significant differences were observed in the coefficient of variation 
 between tasks 1 and 2 ( table 3 ) . no significant differences were observed in the correction 
 time between tasks 1 and 2 ( table 4table 4.changes in the accurate control of muscle force ( without mi group)task 1task 2correction time ( s)1.2  1.70.7  1.2coefficient of variation0.1  0.10.1  0.1 ) . no significant differences were observed in the coefficient of variation 
 between tasks 1 and 2 ( table 
 4 ) . an f - wave is a compound action potential obtained as a result of re - excitation ( i.e. , 
  backfiring  ) of an antidromic impulse following distal electrical stimulation of motor 
 nerve fibers at the anterior horn . the f / m amplitude ratio reflects the number of backfiring anterior horn cells 
 and the excitability of individual anterior horn cells . therefore , persistence and the f / m 
 amplitude ratio are considered as indices of the excitability of spinal neural function . the 
 persistence under the mi condition was higher than in the resting and holding sensor 
 conditions . we attributed this to the influence of the descending pathways that supply the 
 thenar muscle . in detail , excitatory inputs travels to anterior horn cells through the 
 corticospinal pathway and reticulospinal tract and from the upper motor neurons . suzuki et 
 al.3 reported that the excitability of 
 spinal neural function in the mi condition was influenced by the descending pathways from 
 the cerebral cortex . stephan , luft , and lotze1 , 2 , 7 
 demonstrated the activation of the cerebral cortex , the primary motor area , the primary 
 somatosensory area , the supplementary motor area , premotor area , cerebellum , and basal 
 ganglia during mi . the supplementary motor area , premotor area , cerebellum , and basal 
 ganglia all have roles in planning and preparing movement and have connections with the 
 primary motor area . the bulbar reticular formation , red nucleus , cerebellum , and caudate 
 nucleus have connections to anterior horn cells . the bulbar reticular formation has 
 connections with the primary motor area , the supplementary motor area , premotor area , 
 cerebellum and the red nucleus has connections to the cerebellum . therefore , activation of 
 the cerebral cortex during mi condition presumably increased the excitability of spinal 
 neural function via the corticospinal pathway and extrapyramidal tract . in addition , subjects performed mi while holding the pinch pressure sensor . therefore , the 
 influence of tactile and proprioceptive inputs should be considered . mizuguchi et al.8 , 9 
 reported that the responsiveness of afferent pathways to the primary somatosensory area 
 during mi , while utilizing an object , was modulated by a combination of tactile and 
 proprioceptive inputs while touching the object . tactile and proprioceptive inputs from the 
 periphery are integrated after they are hierarchically processed and then projected to the 
 primary motor area . proprioceptive inputs project to the cerebellar nucleus via the 
 spinocerebellar pathway and to the primary motor area via the red nucleus and thalamic 
 nucleus . besides , suzuki et al.3 reported 
 that the excitability of spinal neural function at mi under the  with sensor  condition was 
 higher than that of the spinal neural function at mi under the  without sensor  condition . 
 it is considered that tactile and proprioceptive inputs , while holding the pinch pressure 
 sensor , may increase the excitability of spinal neural function as part of the synergistic 
 effect . in our previous study , we studied the effect of mi on the accuracy of the movement after 
 learning pinch action for 10s . the correction time , which was the total time of the pinch 
 force in suitable range , was defined as the index of accurate control of muscle force . 
 regarding the effect of mi on the actual movement by pinch action , blefari et al.10 adopted an error range of  5% as the 
 index of accurate control of muscle force . however , the current study and blefari s study were similar 
 in terms of adopting pinch action . on the basis of the above , we adopted an error range of  
 5% by pinch force as the index of accurate control of muscle force . as a result , we studied the accurate control of muscle force by including the phase 
 pinch force rising in the pinch force graph . accordingly , we examined including aim at 
 50%mvc and out of aim at 50%mvc by subjects . the subject of difficult to imagine the 50%mvc , 
 those subjects decrease the correction time . otherwise , the correction time is decreased by 
 taking long time for 50%mvc , but the subject is aim at 50%mvc by mi . the quickly aimed at 
 50%mvc was defined as the index of accurate control of muscle force . however , our purpose is 
 to exert for the pinch force of desired value irrespective of speed . accordingly , it was the 
 limit for awareness of improving the ability of actual motion in case of including the phase 
 pinch force rising in the pinch force graph . the second hypothesis , the subjects did nt 
 learn 50%mvc for 10s , the mi under 50%mvc of the subjects was different from an authentic 
 50%mvc . in this study , 
 we examined the accurate control of muscle force for within the subject aimed at 50%mvc . no 
 significant differences in the correction time were observed between tasks 1 and 2 in the 
 imagery group and the control group , respectively . this was suggested that mi did nt improve 
 the ability of actual motion after learning for 10s . mulder et al.11 reported that mi improved the ability of actual motion only in 
 people with learning that corresponded to the mi task . accordingly , the subjects might not 
 learn 50%mvc for 10s , the mi under 50%mvc of the subjects collided with an authentic 50%mvc . consequently , mi might improve the ability of actual motion on the individual differences . however , the pinch force in task 2 approximated a more authentic 50%mvc than in the nine 
 subjects in task 1 . therefore , we considered the influence of mi improved the ability of 
 actual motion in these subjects . no significant differences were observed in the coefficient 
 of variation between tasks 1 and 2 in the imagery group or the control group , respectively . we posited that mi did not influence the ability to make an adjustment of pinch force before 
 and after the mi . therefore , we concluded that mi improved the pinch force value but not the 
 ability to make an adjustment of pinch force . it is necessary to use mi correctly when the 
 aim is for subjects to improve their ability of actual motion . in detail , it is necessary 
 for subjects to learn complete to the corresponding motor task . mi increases the excitability of spinal neural function suggesting it also affects the 
 accuracy of the movement . it is necessary to do correctly of mi when the subjects aimed at improving the 
 ability of actual motion . in particular , it is necessary to ensure that subjects learn the 
 corresponding motor task and adequate learning time be provided afterward .
[ purpose ] this study aimed to examine the effect of motor imagery on the accuracy of 
 motion and the excitability of spinal neural function . [ subjects and methods ] thirty 
 healthy volunteers ( males , 15 ; females , 15 ; mean age , 20.3  1.0  years ) were recruited . 
 f - waves was recorded at rest , while holding a sensor , and while using motor imagery . 
 next , 
 subjects learned 50% maximum voluntary contraction . 
 the pinch force was measured without 
 visual feedback before and after motor imagery . 
 f - waves were analyzed with respect to 
 persistence and the f / m amplitude ratio . 
 correction time and coefficient of variation were 
 calculated from the pinch force . 
 [ results ] persistence and f / m amplitude ratio ware 
 significantly higher in the holding sensor and motor imagery conditions than in the 
 resting condition . 
 in addition , persistence under motor imagery was significantly higher 
 than that in the holding sensor condition . 
 no significant differences were observed in 
 relative values of correction time and coefficient of variation between the two pinch 
 action conditions . 
 the pinch force in task 2 approximated a more authentic 50%mvc than 
 that in task 1 . 
 [ conclusion ] motor imagery increases the excitability of spinal neural 
 function , suggesting that it also affects accurate control of muscle force .