one of the major purposes of physical therapy is improvement of muscle flexibility and 
 consequently increase of the range of motion ( rom ) of the affected joint . from the suite of 
 manual skills available , joint range of motion exercises and stretching are more commonly 
 selected . from the available selection of physical agents , thermal therapy and electrical 
 stimulation therapy have been actively used to improve tightened soft tissue , alleviate 
 pain , and reduce the muscle spasms that limit rom . the aims of applying a physical agent and 
 stretching are similar to each other , such as to decrease pain and aid relaxation ; 
 therefore , they are often used in combination with each other in order to be complementary 
 and with the expectation of synergistic effects . currently the main combination is 
 thermotherapy and therapeutic exercises , such as hot packs or therapeutic ultrasound with 
 stretching . thermotherapy modalities , when applied before therapeutic exercises , promote 
 improvement in connective tissue ( such as collagen fiber ) extensibility ; this improvement is 
 thought to enhance the effect of stretching . when the pain is the primary complaint recorded 
 and thus restricted rom is present , transcutaneous electrical nerve stimulation ( tens ) is 
 initially applied to relieve the pain before the application of rom exercises and stretching 
 in the clinical setting . in our previous study1 , we clarified that 
 tens , when applied to the medial hamstrings in young healthy subjects , led to an increased 
 pressure pain threshold ( ppt ) , which resulted in decreased muscle hardness ( mh ) and an 
 increased straight leg raising ( slr ) range of motion(rom ) . this is consistent with the 
 findings of espejo2 , who applied 
 interferential current through bipolar application with a frequency of 4 khz and a frequency 
 modulation amplitude of 100 hz . they reported that their electrical stimulation group and 
 stretching group had greater effects on improvement of knee extension range when compared 
 with their control group . when deep connective tissues , such as the skeletal muscle and the joint capsule , are 
 injured , excitement of the nociceptor is induced , and this leads to hyperactivity in the 
 muscle spindle and lowering of the muscle spindle threshold . this condition of increased 
 muscle tone is called muscular guarding . with regards to circulatory dynamics , persistent 
 muscle contraction induces a state of ischemia in the muscle , which produces algesic 
 substances , such as inflammatory mediators , and induces excitation of the nociceptor3 . kubo4 stated that improvement in the rom as caused by stretching 
 accompanied changes in muscle - tendon unit viscoelasticity and increased pain tolerance at 
 the end of the stretching range . when passively stretching muscles , increased pain would be 
 a factor limiting rom in addition to shortening of the muscle and soft tissue . given the 
 above , we hypothesized that tens could enhance the stretching effect in a synergistic manner 
 by decreasing mh and increasing both muscle flexibility and the pain threshold . the purpose 
 of this study was to clarify the immediate effects of tens in combination with stretching on 
 mh , ppt , and the rom in slr in healthy individuals . fifteen healthy young males ( age , 1826 years ; height , 168.8  4.7 cm ; body weight , 67.8 
  13.7 kg ) volunteered to participate in this study . the inclusion criteria were as 
 follows : 1 ) no history of neuromuscular disease or musculoskeletal injury involving the 
 lower extremities and 2 ) less than 70 degrees rom in slr . all subjects were notified of 
 the procedures and purpose of this study , and informed written consent was obtained from 
 all of the volunteers before initiation of the study . the protocol was approved by the 
 koriyama tohto academy educational foundation ethics committee ( approval number : 
 r-1401 ) . subjects performed two protocols , which were 1 ) stretching alone ( s group ) and 2 ) tens 
 and stretching ( ts group ) . the ts group received a 20-minute tens application and 
 immediately afterwards performed 10 minutes of stretching , while the s group completed 
 only 10 minutes of stretching . both protocols included 10-minute rest , periods with the 
 subjects in the supine position before and after the protocols . static stretching ( i.e. , 
 stretching ) was used as the stretching method in our study . stretching was done at 90% of 
 the maximum slr rom for 30 seconds followed by a 30-second rest a total of 10 times over 
 10 minutes . electrical stimulation was generated via a tens unit ( eu-940 , ito co. , ltd . , the tens parameters were s symmetrical biphasic pulse current waveform , 100 hz 
 frequency , and 200 s pulse duration . the stimulation 
 intensity was set at 80% of the sense of pain threshold and in which a muscular 
 contraction was also produced . tens was delivered through two self - adhesive carbon rubber 
 electrodes ( 5 cm  9 cm ) positioned over the medial hamstrings and attached to the middle 
 portion between the ischial tuberosity and the popliteal region , and they were located 20 
 cm apart . three parameters were used for the experiment : ppt , soft tissue hardness as expressed by 
 mh , and slr rom . the data were collected before tens ( t1 ) , before stretching ( t2 , 
 immediately after the end of tens application for the ts group ) , immediately after 
 stretching ( t3 ) , and 10 minutes after stretching was concluded ( t4 ) . the clinometer was placed on the 
 patellar ligament and adjusted to zero at the starting point of the slr rom , and then the 
 maximum slr rom was measured manually . the knee and hip joints of the contralateral leg 
 were maintained in full extension . for the measurements of mh and ppt , an outcome 
 evaluation system ( oe-220 , ito co. , ltd . , tokyo , japan ) was used5 , and mh and ppt were measured on the medial hamstrings at 
 an angle of 90% of the maximum slr rom . in order to maintain this angle , the distal part 
 of the lower leg was suspended in a sling , and the sling was fixed at the position in 
 which the traction force was applied perpendicular to the lower leg through a pulley 
 ( fig . 1fig . 1.block diagram of the experimental setupin the position of 90% slr rom , the muscle hardness ( mh ) and pressure pain 
 threshold ( ppt ) were measured on the medial hamstrings , and the measurement sensors 
 were attached to the lower leg . ) . the 
 experimental room was controlled at a temperature of 25 c and a humidity of 50% . the two 
 protocols were performed on two separate occasions at least one day apart but no more than 
 seven days apart . block diagram of the experimental setup in the position of 90% slr rom , the muscle hardness ( mh ) and pressure pain 
 threshold ( ppt ) were measured on the medial hamstrings , and the measurement sensors 
 were attached to the lower leg . statistical analysis with a paired t - test was used to compare the 
 differences in values at t1 in the ts group and at t2 in the s group . additionally , a 
 one - way repeated measures analysis of variance ( anova ) was performed to determine the 
 time - dependent changes ( between t1 , t2 , t3 , and t4 ) in each group , and a bonferroni s test 
 was undertaken at the point at which a significant main effect was observed . the 
 difference values were calculated based on t2 in the s group and t1 in the ts group . difference value 1 ( d1 ) indicates the difference between t2 and t3 in the s group or 
 between t1 and t3 in the ts group , while difference value 2 ( d2 ) indicates the difference 
 between t2 and t4 in the s group or between t1 and t4 in the ts group . these differences 
 can be expressed as d1=t3t2 and d2=t4t2 for the s group and as d1=t3t2 and d2=t4t2 for 
 the ts group . following calculation of the difference values , a paired t - test was 
 performed to detect significant differences between the s and ts groups . ibm spss 
 statistics for windows , version 22.0 ( ibm japan , ltd . , tokyo , japan ) , was used , and 
 statistical significance was accepted for values of p < 0.05 in all analyses . the changes in mh , ppt , and slr rom ( mean  sd ) in the s and ts groups are 
 shown in table 1table 1.changes in mh of each group and comparison between groups  meansd ( n ) 
 n=15t1t2t3t4d1d2 s group3.60.53.40.43.50.40.20.20.10.1ts group 3.60.53.10.53.10.53.40.50.50.2**0.30.2**s group : stretching alone ; ts group : tens and stretching . * * d1&d2 : s group vs. ts group ( p<0.01 ) , 2table 2.changes in ppt of each group and comparison between groups  meansd ( n ) 
 n=15t1t2t3t4d1d2 s group15.11.315.61.315.21.30.61.20.11.0 ts group 14.71.921.91.920.62.317.31.15.92.0**2.61.4**s group : stretching alone ; ts group : tens and stretching . * * d1&d2 : s group vs. ts group ( p<0.01 ) , and 3table 3.changes in slr rom of each group and comparison between groups  meansd ( degrees ) 
 n=15t1t2t3t4d1d2 s group65.32.470.73.069.82.85.42.04.52.2ts group 64.81.873.23.276.94.173.10.412.22.8**8.31.9**s group : stretching alone ; ts group : tens and stretching . * there was no significant difference between t2 in the s group and t1 in the 
 ts group . therefore , the starting values were relatively similar between the s group and the 
 ts group for all three parameters . * d1&d2 : s group vs. ts group ( p<0.01 ) s group : stretching alone ; ts group : tens and stretching . * * d1&d2 : s group vs. ts group ( p<0.01 ) s group : stretching alone ; ts group : tens and stretching . * * d1&d2 : s group vs. ts group ( p<0.01 ) in the s group , one - way repeated measures anova revealed significant main effects in mh and 
 slr rom ( mh , f = 11.07 , p < 0.01 ; ppt , f = 2.07 , p = 
 0.146 ; slr , f = 66.50 , p < 0.01 ) . multiple comparison revealed 
 significant differences in t3 compared with t2 in mh ( p < 0.01 ) and in t3 and t4 compared 
 with t2 in slr rom ( p < 0.01 ) . on the other hand , there were significant main effects in 
 mh , ppt , and slr rom in the ts group ( mh , f = 65.54 , p < 0.01 ; ppt , 
 f = 107.56 , p < 0.01 ; slr , f= 162.40 , p < 0.01 ) . multiple comparison showed significant differences in t2 , t3 , and t4 compared with t1 in all 
 parameters ( p < 0.01 except p < 0.05 at t4 in mh ) . with regards to the results of mh , d1 in the s group was 0.2  0.2 n , and d1 in the ts 
 group was 0.5  0.2 n ; thus , there was a significant difference between the groups ( p < 
 0.01 ) . n , and d2 in the ts group was 0.3  0.2 
 n ; thus , there was a significant difference between the groups ( p < 0.01 ) . secondly , the changes in ppt were as follows : d1 in the s group was 0.6  1.2 n , and d1 in 
 the ts group was 5.9  2.0 n ; there was a significant difference between the groups ( p < 
 0.01 ) . d2 in the s group was 0.1  1.0 n , and d2 in the 
 ts group was 2.6 1.4 n ; there was a significant difference between the groups ( p < 
 0.01 ) . in terms of slr rom , d1 in the s group was 5.4  2.0 , and d1 in the s group was 12.2  
 2.8 ; there was a significant difference between the groups ( p < 0.01 ) . similarly , d2 in 
 the s group was 4.5  2.2 , and d2 in the ts group was 8.3  1.9 ; there was a significant 
 difference between the groups ( p < 0.01 ) . we examined the influence of tens on muscle hardness and pressure pain threshold in a 
 previous study1 . as a new initiative , this 
 study aimed to clarify the immediate effects of the combination of tens , which is widely 
 known to have a pain inhibitory effect , and stretching by using the three parameters of 
 muscle hardness , pressure pain threshold , and slr rom . the main findings in this study show 
 that the combination of tens and stretching is more effective than stretching alone , since 
 the results for all three parameters , muscle hardness , pressure pain threshold , and slr rom , 
 in the ts group showed significantly beneficial changes compared with the s group . a large number of studies have reported the pain inhibitory effect of tens6,7,8,9,10,11,12,13,14 . previous studies10 , 15 , 16 have shown that the tens parameter used in this study has 
 a high hypoalgesic effect . as in these studies , the present study also revealed 
 significantly increased values for pressure pain threshold in the ts group , although the s 
 group did not show such a significant change . calculation of the percentage ( the rate of 
 change ) of d1 based on t1 in the ts group ( based on t2 in the s group ) revealed that the ts 
 group had a 41.0  15.9% difference and that the s group had a 3.8  7.9% difference in 
 pressure pain threshold . from the above , the pain inhibitory effect of tens and stretching 
 when used in combination was readily recognized , and the results indicated that stretching 
 might not achieve the same amount of inhibitory effect when performed solely . there are several earlier reports that examined the method and time parameters and 
 conditions of stretching17,18,19,20,21,22,23 . akagi17 reported that 2 minutes of stretching of 
 the gastrocnemius significantly increased rom and decreased muscle - tendon unit stiffness ; 
 however , it did not change the joint torque . nakamura18 assessed passive torque as an index of muscle - tendon unit 
 resistance and myotendinous junction displacement as an index of muscle extensibility using 
 ultrasonography and a dynamometer during 5 minutes of stretching . their results suggested 
 that stretching for more than 2 minutes effectively increased muscle extensibility , which in 
 turn decreased muscle - tendon unit resistance . matsuo19 and umegaki20 
 also reported that 5 minutes of stretching , the protocol for which comprised of 5 repeated 
 1-minute stretching periods with a 30-second rest period between each stretching period , 
 decreased the stiffness of the hamstring muscle - tendon unit . additionally , magnusson21 also reported the effectiveness of 5 
 minutes of stretching in improving the flexibility of the hamstrings . based on the above 
 studies , the time set for stretching in this study ( a total of 5 minutes of stretching over 
 an allocated 10-minute period ) seemed to be adequate to obtain a sufficient effect . with regards to the actual method of stretching for the hamstrings , umegaki24 studied the use of ultrasonic shear wave 
 elastography and reported that adding hip rotation to the stretching position for the 
 hamstrings ( i.e. , internal rotation for the medial hamstrings and external rotation for the 
 lateral hamstrings ) may not have a significant effect on muscle elongation of each 
 hamstring . given the above , we stretched the medial hamstrings at the intermediate position 
 between internal and external rotation in the present study . regarding the application of a combined physical agent and stretching protocol , there are 
 many studies that have reported on the combination of thermal therapy , such as hot packs , 
 and stretching . a meta - analysis completed by nakano25 reviewed 12 randomized clinical trials involving 352 healthy 
 participants and concluded that treatments combining thermal therapy and stretching improved 
 rom when compared with stretching alone . additionally , they stated that this is due to 
 increased collagen extensibility and reduced connective tissue viscosity and viscoelasticity 
 of muscle fiber at higher temperatures as caused by the thermal therapy . as for the combined usage of tens and stretching , general clinical recommendations are as 
 follows : stretching is applied after reducing pain by tens in orthopedic disorders , and 
 stretching is applied during or after electrically stimulating the antagonist of muscles 
 displaying high levels of spasticity in central nervous system disorders26 , 27 . however , the mechanisms of their effects have not been fully 
 clarified . hou et al.6 reported on the combination 
 of physical agents and therapeutic exercises and stated that some combinations were 
 effective for easing cervical myofascial pain and increasing cervical rom . they compared 
 five therapeutic combinations of ischemic compression , tens , stretching , interferential 
 current or the myofascial release technique combined with hot packs and active rom exercise . the results showed that a combination of a hot pack plus active rom exercise coupled with 
 tens or interferential current was more effective for pain relief and that a combination of 
 a hot pack plus active rom exercise coupled with stretching or the myofascial release 
 technique was more effective in increasing cervical rom than other combinations . these 
 results partially support our findings , particularly regarding the results indicating a pain 
 inhibitory effect caused by the combined usage of tens and the therapeutic exercise . however , the precise influence of tens on muscle hardness and rom is not clear . as for the effect of tens on muscle hardness , our previous study15 showed that tens significantly reduced muscle hardness 
 values , although at 10 minutes after application , the reduction level was not as much as 
 that immediately after application . in the present study , we could maintain the tens effect 
 for 10 minutes after application by adding stretching after tens . the results show that the 
 change rate of d1 for muscle hardness was 14.1  4.7% in the ts group and 6.3  5.0% in 
 the s group ; the ts group had a significant decrease in the muscle hardness compared with 
 the s group . regarding the slr rom , the change rates of d1 were 18.7  4.0% and 8.0  3.0% in the ts and 
 s groups ; thus there was almost a 19% increase in slr rom in the ts group . espejo2 reported that interferential current and 
 stretching both had significant effects on improvement of rom in the knee joint compared 
 with the control group , although there was almost no difference between the two 
 interventions . the reason why a significant difference was not seen between the two 
 interventions is likely due to the length of the interferential current application and 
 stretching . in the study , the length of the stretching procedure was 80 seconds , which 
 included 2 sets of 30-seconds of stretching and 10 seconds of rest . the length of 
 electrotherapy also matched the procedure for stretching , i.e. , a total of 80 seconds , which 
 included 2 sets of 30-seconds of stimulation with 10 seconds of rest . in our experiment , we 
 applied tens for 20 minutes ; therefore , this could have led to a significant effect compared 
 with the previous study . furthermore , the combined application in our study was found to be 
 more effective for increasing hamstrings extensibility than a single solely applied 
 protocol . passive rom has been recognized as a less reliable index in assessment of improvement in 
 muscle flexibility and the effects of stretching . this is because it tends to be affected by 
 psychological factors , such as increased tolerance of pain and repeated extension 
 stimulation28 . because of this , several 
 measurement methods , such as measuring passive torque when moving a joint passively and 
 measuring the amount of displacement of the muscle - tendon junction , have been developed 
 recently as new tools for evaluating muscle flexibility and the effects of stretching . however , these methods require expensive devices , such as an isometric dynamometer and 
 ultrasonography ; additionally , passive torque is generally used only as a parameter of 
 limited joint movement . nakamura29 
 reported that significant positive correlations were observed between muscle hardness and 
 passive torque or displacement of the myotendinous junction and suggested that muscle 
 hardness when measured by a muscle hardness meter is both an accurate index and an 
 easy - to - use method . in our study , a muscle hardness meter was used to assess the changes in 
 muscle hardness while maintaining the same slr rom before and after the intervention . since 
 a significant decrease in muscle hardness was shown post intervention , it could be deduced 
 that the intervention significantly increased muscle tissue flexibility . in our previous study1 , an increased 
 pressure pain threshold was observed after applying tens . based on this finding , the 
 mechanism of the decrease in muscle hardness might be explained as follows : firstly , 
 decreased input from the sensory system caused by the pain inhibitory effect of tens led to 
 a decline in -motor neuron activity and thus a decline in muscle spindle sensitivity . therefore , the overactivity of muscle tone was controlled , and this resulted in decreased 
 muscular viscoelasticity and an increase in rom . in addition to the above mechanism , further increased muscle flexibility resulting from 
 stretching could be the reason for the significant increase in the slr rom . increased muscle 
 flexibility caused by stretching has been indicated to be due to an increase in the 
 flexibility and movement of the aponeurosis and the connective tissue , such as the 
 endomysium , perimysium , and epimysium , and not to be due to lengthening of muscle fiber30 , 31 . firstly , we did not investigate how the 
 physical stimulation of tens had influenced the connective tissue , including the perimysium 
 and the epimysium , because of the parameters used in this study . secondly , since the 
 intensity of the electrical stimulation used in this study was set at a level that induced 
 muscle contraction , electrical stimulation might have additionally produced local muscle 
 fatigue . furthermore , this study targeted healthy young males as subjects , and so further clinical 
 study is required to clarify the effects of combined usage of tens and stretching on elderly 
 people and people with pain and muscle tone abnormalities . we investigated the immediate outcomes caused by combined application of tens and 
 stretching on muscle hardness , pressure pain threshold , and slr rom . the findings in this 
 study revealed that combined usage led to a significant decrease in muscle hardness , a 
 significant increase in the pressure pain threshold , and a significant increase in slr rom 
 when compared with stretching alone . therefore , it is suggested that a combination of tens 
 and stretching is an effective method of increasing muscle flexibility .
[ purpose ] this study aimed to clarify the immediate effects of a combined transcutaneous 
 electrical nerve stimulation and stretching protocol . 
 [ subjects ] fifteen healthy young 
 males volunteered to participate in this study . 
 the inclusion criterion was a straight leg 
 raising range of motion of less than 70 degrees . 
 [ methods ] subjects performed two 
 protocols : 1 ) stretching ( s group ) of the medial hamstrings , and 2 ) tanscutaneous 
 electrical nerve stimulation ( 100 hz ) with stretching ( ts group ) . 
 the ts group included a 
 20-minute electrical stimulation period followed by 10 minutes of stretching . 
 the s group 
 performed 10 minutes of stretching . 
 muscle hardness , pressure pain threshold , and straight 
 leg raising range of motion were analyzed to evaluate the effects . 
 the data were collected 
 before transcutaneous electrical nerve stimulation ( t1 ) , before stretching ( t2 ) , 
 immediately after stretching ( t3 ) , and 10 minutes after stretching ( t4 ) . [ results ] 
 combined transcutaneous electrical nerve stimulation and stretching had significantly 
 beneficial effects on muscle hardness , pressure pain threshold , and straight leg raising 
 range of motion at t2 , t3 , and t4 compared with t1 . [ conclusion ] these results support the 
 belief that transcutaneous electrical nerve stimulation combined with stretching is 
 effective in reducing pain and decreasing muscle hardness , thus increasing range of 
 motion .