repetitive lifting and unloading as well as moving with excessive weights have been known 
 to be major causes of various musculoskeletal disorders and low back pain1 . such carrying or actions may increase the 
 load on the spine while decreasing the pressure that protects the spine , thereby increasing 
 the risk of low back pain and injury to the spine2 .
therefore , to ensure workers safety and prevent injury , cumulative 
 fatigue induced in repetitive work environment needs to be in studies using various 
 interventions for recovery from fatigue . in this study ,
muscle fatigue , tone of the erector spinae , and blood levels of ck ( creatine 
 kinase ) and ldh ( lactate dehydrogenase ) , two blood serum enzymes , were measured before and 
 after the intervention and their results were compared to analyze the cumulative 
 fatigue .
it has been widely used to 
 improve reduced muscle function , muscle strength , and joint range of motion , as well as the 
 muscle activities of neurological patients . however , if long - term tens is applied to the 
 human body , it can cause muscle fatigue and the accumulation of waste matter , and serious 
 muscle damage .
it has also been reported that tens does not have a significant effect on 
 fatigue after high - intensity exercise3,4,5 . 

therefore , although therapies using tens may have a variety of advantages , more study is 
 required to conclusively determine the effect tens has on muscles with cumulative 
 fatigue .
microcurrent stimulation plays a role in restoring cells or facilitating healing by 
 increasing the creation of adenosine triphosphate ( atp ) and proteins , and curing wounds 
 through the supply of electric energy at the cell level , which restores the potential of the 
 cell membrane to normal potential6 . in 
 recent years
, a number of studies of microcurrent stimulation have been performed for in a 
 wide range of conditions such as fracture and fatigue recovery , and various diseases 
 involving cancers and diabetic nerve damage7,8,9,10 . in particular ,
microcurrent stimulation 
 is known to be effective for cumulative fatigue recovery as it reduces ck in the blood of 
 patients with delayed onset muscle soreness11 . however , a study by cheng et al . reported that microcurrent 
 stimulation had no positive effect on the recovery of an arm with delayed onset muscle 
 soreness12 . in this study , microcurrent stimulation and tens
were administered to subjects with 
 cumulative fatigue to analyze their effects on fatigue recovery .
thirty - two male persons in their 20s participated in this study and they were randomly 
 divided into three groups : an mc group of 12 persons who underwent microcurrent stimulation , 
 a tens group of 10 persons who underwent transcutaneous electrical nerve stimulation , and a 
 con group of 10 persons who only rested .
the subjects had no musculoskeletal disorders or 
 related disease histories and were restricted to right - hand dominant males .
prior to 
 participation in this study , subjects were given a detailed description of the experimental 
 and safety procedures , and each subject signed an informed - consent form .
the following tables show the 
 general characteristics of the subjects and the homogeneity test results for the two groups 
 ( tables 1table 1.general characteristics of the subjectsmc ( n=12)tens ( n=10)con ( n=10)age ( yr)21.81.822.41.521.51.8height ( cm)173.64.9176.25.9174.85.3weight ( kg)70.19.869.73.165.46.4meansd , mc : microcurrent group , tens : tens group , con : control group   and 2table 2.pre-intervention values of the three groupsmc ( n=12)tens ( n=10)con ( n=10)rmf ( hz)72.610.6073.34.6073.34.7lmf ( hz)74.37.6071.26.1070.55.7rmt ( kg / mm)12.20.9012.40.8012.31.4lmt ( kg / mm)12.51.8014.16.0012.21.2ck ( u / l)119.017.4126.561.1115.332.8ldh ( u / l)417.140.6421.241.2429.353.7there were no significant differences : anova , p>0.05 , mc : microcurrent group , 
 tens : tens group , con : control group , rmf : right median frequency , lmf : left median 
 frequency , rmt : right myotonometry , lmt : left myotonometry , ck : creatine kinase 
 lactate , ldh : lactate dehydrogenase ) . meansd , mc : microcurrent group , tens : tens group , con : control group   there were no significant differences : anova , p>0.05 , mc : microcurrent group , 
 tens : tens group , con : control group , rmf : right median frequency , lmf : left median 
 frequency , rmt : right myotonometry , lmt : left myotonometry , ck : creatine kinase 
 lactate , ldh : lactate dehydrogenase before the test , subjects wore comfortable dresses and were informed about the correct 
 motions . to maintain the same work condition , a 75-cm - high small table was prepared and a 
 30-cm - wide , 30-cm - deep , and 25-cm - high box was filled with a 10-kg dumbbell13 .
as soon as subjects had completed 
 repeated lifting and lowering of the box 100 times during 15 minutes with a symmetrical 
 posture in the sagittal plane14 , the 
 pre - test measurements were conducted . immediately after the pre - test , the tens group was 
 administered transcutaneous electrical nerve stimulation ( es-420 , ito , japan ) after 
 connecting two rectangular radiofrequency catheters ( 5090 mm ) .
the longissimus and 
 iliocostalis were treated for 20 minutes ( monophasic pulsed direct current of 80  hz with 
 phase duration of 10  s , pulse width of 300  s and on / off time 10  s/50  s ) .
the mc group 
 received microcurrent from a microcurrent stimulator ( es-420 , ito , japan ) at 100 ma and 0.3 
 hz on the same spot for 20 minutes .
the con group took a rest for 20 minutes while lying 
 prone on the floor . immediately after the intervention , emg , muscle tone , ck , and ldh were 
 measured .
the temperature and humidity in the laboratory were maintained at 23  c and 
 60% .
muscle activating signal was checked by using surface emg machine ( mp100 , biopac , usa ) .
the 
 sampling rate of the signal was 100  hz and bandpass filtering was performed between 
 20500  hz . the digitized signal was processed with fft ( fast fourier transformation ) using 
 acqknowlege 3.91 software on a personal computer to obtain the median frequency ( mf ) . 

the closest two of the recorded values were averaged and used as the mvc value of 
 the subjects .
after the mvc measurement , 60% mvc was calculated for the quantification of 
 the collected measurement values from the experiments so that measurements before and after 
 the treatment were performed with the above method .
the electrode was attached to the right 
 and left erector spinae of the right- and left - side back and the longissimus and the 
 iliocostalis around the hip . to measure the emg of these muscles ,
a pair of surface 
 electrodes was attached horizontally in the right and left directions to the muscle belly 
 5  cm away from the neurocentral joints of thoracic vertebrae t10 and lumbar vertebrae 
 l214 . in order to measure muscle tone of the erector spinae , a myotonometer ( missoula , mt , 
 neurogenic technologies inc . ,
while subjects rested on a bed , the longissimus 
 and the iliocostalis at the waist were measured .
the measurement probe consists of inner and 
 outer cylinders . according to the resistance of the tissue , the distance between two 
 cylinders changes and the resistance of the tissue
the force 
 applied to the cylinder was divided into eight stages ( 0.25 , 0.50 , 0.75 , 1.00 , 1.25 , 1.50 , 
 1.75 , 2.00  kg ) so that measurement at each position of the level of displacement could be 
 made .
force - displacement curves were created using computational software and the areas 
 under the curve ( auc ) were calculated from the obtained data .
the area under the curve ( auc ) 
 is indicating of level of muscle tone15,16,17 .
spss / win 18.0 was used for all statistical processing . means and standard deviations of all 
 measurements were calculated and analysis of variance ( anova ) was conducted to test the 
 homogeneity of the experimental groups . in order to analyze the effect of the treatment on 
 muscle fatigue , muscle tone , and ck and ldh serum levels ,
the paired t - test was used to 
 examine within group pre- and post - test differences , while analysis of covariance ( ancova ) 
 was used to compare the mc and tens groups with the con group .
microcurrent stimulation , tens , and rests were given to subjects who had fatigues and then 
 emg , muscle tones , ck and ldh were measured and analyzed . the results are shown in tables 3table 3.the effects of intervention on the muscle fatigue indexgrouppre - interventionpost - interventionrmf * ( hz)con70.710.667.47.9mc 72.610.663.88.5tens73.34.7065.93.8lmf * ( hz)con70.811.463.79.7mc 75.47.659.110.4tens70.55.768.55.3*paired t - test ( p<0.05 ) , anova , mc group  con group ( p<0.05 ) , mc : 
 microcurrent group , con : control group , tens : tens group , 4table 4.the effects of intervention on the myotonometry indexgrouppre - interventionpost - interventionrmt * ( kg / mm)con12.31.416.51.8mc 12.20.918.81.2tens12.40.816.21.3lmt * ( kg / mm)con12.21.215.22.4mc 12.51.819.31.3tens14.16.016.71.1*paired t - test ( p<0.05 ) , anova , mc group  con group ( p<0.05 ) , mc : 
 microcurrent group , con : control group , ts : tens group group , tens : tens group , and 5table 5.the effects of intervention on fatigue markers in the bloodgrouppre - interventionpost - interventionck * ( u / l)con119.040.691.636.4mc126.561.185.729.8tens139.342.095.521.8ldh * ( u / l)con421.241.2374.741.1mc417.140.6355.031.1tens433.355.8395.044.6*paired t - test ( p<0.05 ) , mc : microcurrent group , con : control group , tens : tens 
 group . *
paired t - test ( p<0.05 ) , anova , mc group  con group ( p<0.05 ) , mc : 
 microcurrent group , con : control group , tens : tens group * paired t - test ( p<0.05 ) , anova , mc group  con group ( p<0.05 ) , mc : 
 microcurrent group , con : control group , ts : tens group group , tens : tens group * paired t - test ( p<0.05 ) , mc : microcurrent group , con : control group , tens : tens 
 group all the three groups showed significant changes in muscle fatigue , muscle tone , and blood 
 levels of ck and ldh , between before and after the intervention ( p<0.05 ) .
the comparison 
 result of the mc and con groups found a significant difference in muscle fatigue and muscle 
 tone in the right and left erector spinae ( p<0.05 ) .
however , no significant difference 
 was found in the blood levels of ck and ldh .
in addition , no significant difference was 
 found between the tens and con groups .
a study by dolan et al . reported that 100 times of lifting and lowering a 10-kg weight for 
 12.1 minutes resulted in increase of muscle fatigue14 .
furthermore , lee et al . revealed that after repetitive work of 
 more than six times per minute with a sub - maximal weight of 15% , it took more than five 
 minutes for the workers backs to recover from cumulative fatigue to the initial condition . 

when cumulative fatigue occurs , loads on the vertebrae continue to increase during the 
 repetitive work whereas pressure that protects the vertebrae starts to reduce thereby 
 increasing the risk of low back pain2 .
clarkson et al . reported that when subjects aged between 18 and 40  years performed repeated 
 of lifting and lowering , 50 times , with maximum eccentric contraction , muscle damage to the 
 biceps brachii was induced along with increases in both ck and ldh level in the blood18 .
the accumulation of markers of fatigue in 
 the blood and muscle fatigue due to repetitive lifting and lowering work is known as one of 
 the main reasons for injury during work19 .
an increase in muscle tone indicates an abnormal state in the musculoskeletal system and it 
 is used as an indicator of muscle fatigue20 .
reported that with additional lifting and lowering of 
 heavy goods , muscle tone of workers increased further , whereas workers without such 
 repetitive work showed relatively less muscle tone21 .
in addition , muscle tone was shown to correlate with emg frequency 
 and a decrease in muscle fatigue increases muscle tone as well22 . in this study , cumulative muscle fatigue and muscle tone 
 showed significant changes after the intervention .
in particular , subjects administered 
 microcurrent stimulation showed significant changes in muscle fatigue and muscle tone .
weber et al . administered microcurrent stimulation to subjects with delayed onset muscle 
 soreness and reported no significant difference in muscle fatigue
reported that patients with plantar fasciitis who wore shoes that produced microcurrent 
 stimulation for six weeks showed a significant reduction in , muscle fatigue in the tibialis 
 anterior as assessed by the median frequency6 .
their result is similar to that of our present study in which 
 muscle fatigue in the right and left erector spinae was reduced significantly .
microcurrent 
 stimulation can induce cell responses in damaged tissues through bioelectricity thereby 
 assisting with the healing of tissues .
it can also facilitate endogenous bioelectric current 
 which would reduce resistance in the damaged portion , and promote conduction of bioelectric 
 current assisting with the recovery of homeostasis of the human body .
that is , microcurrent 
 enhances electrical and chemical processes , which assist the healing process , returning 
 damaged muscle tissues to a normal state23 .
reduction of the median frequency value indicates a reduction in 
 muscle fatigue22 , 24 , and muscle tone and microcurrent stimulation is more effective for 
 the recovery of muscle fatigue and muscle tone than only taking a rest .
the microcurrent 
 therapy increased collagen formation speed in the recovery of damaged cells more than in the 
 tens therapy , which is why the microcurrent stimulation had a more positive effect on muscle 
 fatigue and muscle tone .
no significant differencs were found after the intervention between the tens and con 
 groups .
this result is in agreement with park s study in which an administration of 100 pps 
 to finger muscles for 30 minutes had no effect on muscle fatigue and muscle strength25 .
although appropriate stimulation through 
 tens can help to prevent fatigue accumulation by increasing the local blood flow , the 
 temporary increase in local circulation was not sufficient enough to have a significant 
 effect on muscle fatigue recovery .
a study by lim reported that after heavy exercise , ck and ldh decreased slightly or 
 increased for up to 30 minutes , together with aggravation of the pain in patients with 
 delayed onset muscle soreness26 , 27 .
furthermore , a study that can 
 provide a generalized conclusion through a large number of subjects and various measurement 
 tools shall be conducted in the future .
 [ purpose ] the aim of this study was to determine the effect of low - frequency electrical 
 stimulation on fatigue recovery of the erector spinae with cumulative fatigue induced by 
 repeated lifting and lowering work . [ subjects ] thirty - two healthy men volunteered to 
 participate in this study and they were randomly divided into three groups : a mc group of 
 12 persons who underwent microcurrent , a tens group of 10 persons who underwent 
 transcutaneous electrical nerve stimulation , and a control group of 10 persons who only 
 rested . [ methods ] cumulative fatigue was induced and then , emg , muscle tone , ck and ldh 
 serum levels of the erector spinae were measured . 
 each group then underwent the assigned 
 intervention and was re - measured . to analyze the differences in fatigue between before and 
 after the intervention 
 , the paired t - test was conducted , while groups were compared using 
 analysis of covariance with a control group . 
 [ results ] the mc groups showed a significant 
 reduction in muscle fatigue and decreased muscle tone when compared to the control group . 
 
 however , no significant differences were found between the tens and control groups . 
 
 [ conclusion ] these results suggest that microcurrent stimulation was effective for 
 recovery from cumulative muscle fatigue while tens had no effect . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION