transcutaneous electrical nerve stimulation ( tens ) is commonly used in the clinical 
 management of chronic pain , such as for patients with low back pain and knee 
 osteoarthritis1,2,3 . typically for chronic pain 
 management , tens
is applied to the peripheral site of the lesion , with the stimulation 
 administered for at least 15  min4,5,6 . 

these general stimulation parameters produce a diffuse inhibition of pain , likely to be 
 mediated by descending pain control pathways7 , 8 . as well , a dose - response relationship of 
 tens on pain control has been reported , with stronger intensities of stimulation having a 
 greater effect on pain relief9 .
these 
 reported effects of tens on pain control could be of clinical benefit in the management of 
 post - operative pain .
yet , tens is seldom used after surgery as the required stimulation 
 period is typically too long and the placement of electrodes on the surgical site difficult 
 in the early period of post - operative rehabilitation .
therefore , the purpose of our study 
 was to investigate the effects of short - duration , high - intensity and low - intensity tens on 
 pain thresholds in healthy adults , where pain thresholds were evaluated on the side 
 contralateral to the tens application .
twenty - five healthy adults ( 16 males and 9 females ; mean age , 24.5  4.8  years ) were 
 recruited for the study .
participants were screened for relevant contraindications , injury 
 or nerve damage to the upper and lower limbs , chronic illness , pregnancy , cardiac pacemaker , 
 sensitivity to the tens electrodes , current use of pain medication , skin conditions or 
 impairments in skin sensation in the region of electrode placement .
the methods and 
 procedures for this study were approved by the institutional ethics committee of kochi 
 university .
the study was conducted in compliance with the declaration of helsinki , and all 
 participants provided written informed consent .
tens was administered using a nihon kohden unit ( nihon kohden corporation , japan ) , using 
 the following parameters of stimulation : 10  hz frequency , 500 s pulse width , 30  s 
 stimulation duration .
this stimulation was delivered at two intensity levels , a low 
 intensity ( sensory ) level and a high intensity ( motor ) level . for the motor - level
, the 
 intensity of the stimulation was set at 1.5 times the motor threshold of the muscle , while 
 for the sensory - level , the intensity was set at the sensory threshold of the common peroneal 
 nerve .
the two intensity levels of stimulation were administered on separate days , with 
 participants randomly selecting the order of stimulation by drawing random numbers .
the tens stimulation was applied to the peroneal nerve of the dominant leg , through two 
 circular , self - adhesive electrodes ( 2.5  cm diameter ; vitrode d , nihon kohden , japan ) , 
 positioned on the head of the fibula and over the popliteal fossa .
the effects of tens 
 stimulation on pain thresholds was measured using a pressure pain threshold ( ppt ) technique 
 previously described in the literature10 , 11 . to determine the ppt ,
pressure was 
 applied perpendicularly to the skin , at a rate of 50 kpa / s , using a flat , circular , 
 1  cm probe tip .
a 100-point visual analog scale ( vas ) was used to quantify the ppt , with 
 anchors at  0 ,  no pain , and  100 , 
worst pain. as the pressure was applied , 
 participants were asked to rate the associated pain on the vas .
the vas was considered to be 
 an appropriate measure of the ppt based on findings by kemp et al . who reported that even 
 pressure stimulation below the pain threshold was consistently perceived and rated 
 significantly higher than  0 ,  no pain , on the vas12 .
ulrika et al . reported a vas score between 30 and 40 to be 
 representative of the ppt in healthy adults13 .
based on these previous studies , the vas score of 30 was set as a 
 reference point , a priori , in our study , and participants were asked to push a button when 
 they perceived the applied pressure to exceed this pain threshold .
ppts were assessed at two 
 different sites on the non - dominant limbs : 1 ) the belly of the deltoid muscle of the upper 
 limb , at a location 5  cm distal to the acromion , along the midline of the muscle , and 2 ) the 
 tibialis anterior ( ta ) muscle of the lower limb , at a location 5  cm distal and 3  cm lateral 
 to the tibial tuberosity , along the midline of the tibia .
these two ppt sites provided 
 information on the segmental ( i.e. , ppt of the ta ) and non - segmental , or diffuse , ( i.e. , ppt 
 of the deltoid muscle ) effects of tens .
ppt was recorded before , during , and 30  min after 
 the application of tens ( i.e. , before - tens , during - tens , and after - tens , respectively ) . 

three ppt measurements , taken 30  s apart , were recorded before and after tens , with the 
 average of the three ppt values used for analysis ; one ppt measurement was made during the 
 30  s application of tens ( fig .
three ppt values were obtained , at 30  s intervals , before- and 
 after - tens application , with the average ppt of the three measures used in the 
 analysis .
a single ppt measurement was obtained during the 30  s duration of tens 
 application . ) .
the ppt values were highly consistent , with calculated intra - class correlations of 
 0.94 , for the deltoid muscle site , and 0.82 , for the ta .
three ppt values were obtained , at 30  s intervals , before- and 
 after - tens application , with the average ppt of the three measures used in the 
 analysis .
a single ppt measurement was obtained during the 30  s duration of tens 
 application .
all participants completed the study protocol and all data points were included in the 
 analysis .
the percent change in ppt ( % ppt ) was 
 calculated ( i.e. , ( during - tens / before - tens )  100 ) to compare the effects of the 
 sensory - level and motor - level tens .
all statistical analyses were performed using spss 21.0 
 software . the level of significance was set at p < 0.05 for all tests .
ppt measures are reported in table 1table 1.pressure pain threshold ( ppt ) of each time period of tens application ( unit : 
 kpa)motor - levelsensory - level(a ) ta muscle sitebefore248.8  21.6256.5  22.1during329.4  30.2 * 272.0  18.6after260.3  22.1244.5  20.7(b ) deltoid muscle sitebefore161.4  21.7174.2  19.9during217.4  24.2 * 200.6  23.9*after161.4  18.4171.6  21.2mean  se ; * , significant difference between during- and before - tens ; , 
 significant difference between during- and after - tens ( p<0.05 ) .
there were no differences in baseline ppt measures ( i.e. , before - tens ) for 
 the sensory - level and motor - level tens conditions , both at the deltoid and the ta muscle 
 sites .
motor - level tens produced significant increases in the ppt at both the deltoid and ta 
 muscle sites ( i.e. , segmental and diffuse effect ) , whereas sensory - level tens increased the 
 ppt only at the deltoid muscle site ( i.e. , diffuse effect only ) .
the ppt at both muscle 
 sites decreased immediately after stimulation for both levels of tens application .
mean  se ; * , significant difference between during- and before - tens ; , 
 significant difference between during- and after - tens ( p<0.05 ) percent changes in the ppt ( % ppt ) during tens application are reported in table 2table 2.percentage of change in ppt during tens ( unit : % ) motor - levelsensory - levelta135.8  8.0 * 112.0  5.9deltoid147.2  10.0 * 116.3  4.0meanse ; * , p<0.05 .
there were significant differences in % ppt for both the motor- and 
 sensory - level tens at both muscle sites , deltoid ( motor level : 147.2%  10.0% , sensory 
 level : 116.3%  4.0% ) and ta ( motor - level : 135.8%  8.0% , sensory - level : 112.0%  5.9% ) .
in this study , we investigated the immediate effect of two intensity levels of tens 
 application , motor - level and sensory - level , on the modulation of the ppt of the 
 contralateral lower limb ( i.e. , segmental effect ) and upper limb ( i.e. , diffuse effect ) .
the 
 most important outcome of this study was a finding of the segmental and diffuse effects of 
 motor - level tens , increasing the ppt at both deltoid and ta muscle sites .
motor - level tens 
 also produced a larger increase in the ppt ( i.e. , higher % ppt ) , compared to the 
 sensory - level tens intensity .
these results are in agreement with findings from claydon et 
 al.14 who reported high - intensity tens 
 stimulation to be of fundamental importance to effective dosage , regardless of the frequency 
 of stimulation . to the best of our knowledge ,
no other studies have shown the immediate 
 effects of high - intensity , motor - level , tens application on the ppt at contralateral sites . 

positive findings of the diffuse and segmental effects of motor - level tens on contralateral 
 ppts provides evidence of the feasibility of including tens as one component of an effective 
 pain management strategy in the early post - operative phase of rehabilitation , and laying a 
 foundation for a novel approach to pain management in physical therapy . as motor - level tens produced an increase in the ppt at both segmental and diffuse 
 contralateral sites , it is reasonable to suppose that motor - level tens activated a systemic 
 pain modulating response , rather than only a local response .
animal studies have 
 demonstrated a positive correlation between nociception ( i.e. , application of painful 
 stimuli ) and activation of diffuse noxious inhibitory controls ( dnic)15 . in humans , pud et al.16 reported both a local ( i.e. , segmental ) and diffuse effect of 
 endogenous analgesia . for their noxious stimuli ,
pud et al . asked subjects to immerse 
 themselves into noxious cold water ( 1  0.5  c ) for 30 s. in our study , we demonstrated that 
 a 30  s of motor - level tens was an effective stimulus to trigger a diffuse , pain analgesic , 
 effect .
the intensity of the motor - level tens stimulus was a  very strong but subjectively 
 still  non - painful stimulation ( i.e. , no participants withdrew from the painful stimulus ) , 
 as used in previous studies17 .
we 
 hypothesize that the intensity of the motor - level tens was sufficient to activate a 
 dnic - like mechanism which contributed to the generalized increase in ppts .
exercise - induced hypoalgesia ( eih ) , using a variety of exercise modalities , has been 
 reported to be effective in humans .
although many studies have used high - intensity exercise 
 ( e.g. , aerobic exercise or exhaustive isometric exercise ) to produce hypoalgesia18,19,20 , positive analgesic effects of low 
 intensity exercise ( i.e. , hand grip or raising leg straight ) have also been reported21,22,23 . in the present study ,
although it was not a voluntary contraction , this stimulus may 
 have produced an eih effect .
placebo analgesia is activated by expectation and distraction of 
 attention . in our study , it is possible that the tens stimulation distracted attention away 
 from the pressure point .
however , analysis of this specific mechanism lies outside the scope 
 of this study .
reported that , during voluntary contraction of the quadriceps at an intensity of 20% to 
 25% of maximal voluntary contraction , ppts increased in both the contralateral quadriceps 
 ( i.e. , segmental ) and infraspinatus ( i.e. , diffuse ) muscles , compared to baseline27 . in our study , we demonstrated that these 
 segmental and diffuse analgesic effects can be achieved even with a low intensity muscle 
 contraction .
the baseline ppt 
 indicated a higher sensitivity of the deltoid to pressure , compared to the ta muscle .
this 
 result was consistent with previous studies28 , 29 , with the higher sensitivity of the 
 deltoid to effects of tens contributing to the significance of ppt measures at this site 
 only .
the limitations of our study must be considered in the interpretation of the outcomes . 

foremost , participants in our study were healthy adults and , therefore , it is unclear 
 whether similar results would be obtained in patients .
as well , we did not measure ppts on the ipsilateral side 
 of stimulation and , therefore , comparisons to measured contralateral effects are not 
 possible .
finally , the effects for only two levels of stimulation intensity were evaluated 
 and , therefore , the  most effective intensity to achieve an analgesic effect can not be 
 determined . in conclusion ,
our study provided evidence of the beneficial effects of motor - level tens to 
 produce an immediate and large increase in ppt , compared to sensory - level tens .
while 
 additional research is required , outcomes of this study provide a foundation for a novel 
 approach for post - operative pain management in physical therapy .
 [ purpose ] this study aimed to investigate the differential effects of high - intensity and 
 low - intensity transcutaneous electrical nerve stimulation on the contralateral side on the 
 pain threshold in healthy subjects . 
 [ subjects and methods ] twenty - five healthy adults , 
 volunteers received two intensity levels ( motor - level , 1.5 times the muscle motor 
 threshold ; sensory - level , sensory threshold of the common peroneal nerve ) , for 30  s on 
 separate days . pressure pain threshold was recorded on the contralateral tibialis anterior 
 and deltoid muscle before , during , and after stimulation . 
 [ results ] motor - level 
 stimulation significantly increased the pressure pain threshold at both muscle sites , 
 while effects of sensory - level stimulation on pressure pain thresholds were significant 
 only at the deltoid site . the percent change in pressure pain thresholds at both sites was 
 significantly higher during motor - level stimulation . [ conclusion ] motor - level stimulation , 
 applied unilaterally to one leg , produced immediate contralateral diffuse and segmental 
 analgesic effects . 
 this may be of therapeutic benefit in patients for whom transcutaneous 
 electrical nerve stimulation can not be directly used at the painful site . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION