***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Abstract Transcutaneous electrical nerve stimulation ( TENS ) reduces pain through central mechanisms involving spinal cord and brainstem sites . Since TENS acts through central mechanisms , we hypothesized that TENS will reduce chronic bilateral hyperalgesia produced by unilateral inflammation when applied either ipsilateral or contralateral to the site of muscle inflammation . Sprague – Dawley rats were injected with carrageenan in the left gastrocnemius muscle belly . Mechanical withdrawal threshold was tested bilaterally before and 2 weeks after carrageenan injection . After testing withdrawal thresholds at 2 weeks , rats received TENS treatment either ipsilateral or contralateral to the site of inflammation . In each of these groups , rats were r and omized to control ( no TENS ) , low frequency ( 4 Hz ) , or high frequency ( 100 Hz ) TENS treatment . TENS was applied for 20 min at sensory intensity under light halothane anesthesia . Mechanical withdrawal thresholds were re‐assessed after TENS or ‘ no TENS ’ treatment . Unilateral injection of carrageenan to the gastrocnemius muscle significantly reduced the mechanical withdrawal threshold ( mechanical hyperalgesia ) bilaterally 2 weeks later . Either low or high frequency TENS applied to the gastrocnemius muscle ipsilateral to the site of inflammation significantly reversed mechanical hyperalgesia , both ipsilateral and contralateral to the site of inflammation . Low or high frequency TENS applied to the gastrocnemius muscle contralateral to the site of inflammation also significantly reduced mechanical hyperalgesia , both ipsilateral and contralateral to the site of inflammation . Since ipsilateral or contralateral TENS treatments were effective in reducing chronic bilateral hyperalgesia in this animal model , we suggest that TENS act through modulating descending influences from supraspinal sites such as rostral ventromedial medulla ( RVM ) This study aim ed to evaluate the effectiveness of transcutaneous electrical nerve stimulation ( TENS ) for treatment of  postoperative pain in patients who underwent cardiac surgery . In addition , we sought to determine whether TENS would be related to improved pulmonary function and muscle electrical activity in this patient population . Forty-five patients , 32 males and 13 females , aged 41 - 74 years were r and omly allocated to receive TENS ( n=23 ) or sham treatment ( n=22 ) during 4 h on the third postoperative day . A 0 - 10 visual analogic scale was used to assess pain ; lung function was evaluated by spirometry and surface electromyography ( n=10 in each group ) was used to quantify electrically-induced muscle activity ( trapezius and pectoralis major ) . TENS was associated with significant reductions on spontaneous and cough-induced postoperative pain as compared to sham ( P<0.05 ) . There was also improvement in chest wall-pulmonary mechanics after TENS with proportional increases in tidal volume and vital capacity ( P<0.05 ) . In addition , electrical activity of both muscle groups was enhanced after TENS , but not post sham ( P<0.05 ) . TENS is a valuable strategy to alleviate postoperative pain following cardiac surgery with positive effects on pulmonary ventilatory function and electrical activity of thoracic and girdle muscles OBJECTIVE To investigate to what extent a single 60-minute session of transcutaneous electrical nerve stimulation ( TENS ) would modify chronic clinical pain , acute experimental pain , and the flexion reflex evoked in chronic low back pain patients . STUDY DESIGN Thirty young subjects with chronic low back pain were r and omly allocated to two groups , receiving either TENS or placebo stimulation to the lumbosacral region for 60 minutes . The flexion reflex was elicited by an electrical stimulation applied to the subject 's right sole and recorded electromyographically from the biceps femoris and the tibialis anterior muscles . MAIN OUTCOME MEASURES Subjective sensation of low back pain and the electrically induced pain were measured by two separate visual analog scales , termed VAS(LBP ) and VAS(FR ) , respectively . Data obtained before , during , and 60 minutes after TENS and placebo stimulations were analyzed using repeated measures ANOVA . RESULTS The VAS(LBP ) score was significantly reduced to 63.1 % of the prestimulation value after TENS ( p<.001 ) , but the reduction was negligible after placebo stimulation ( to 96.7 % , p = .786 ) . In contrast , no significant change was found in the VASFR score ( p = .666 ) and the flexion reflex area ( p = .062 ) during and after stimulation within each group and between the two groups ( p = .133 for VASFR and p = .215 for flexion reflex area ) . CONCLUSIONS The same TENS protocol had different degrees of antinociceptive influence on chronic and acute pain in chronic low back pain patients UNLABELLED We investigated the effect of transcutaneous electrical nerve stimulation ( TENS ) for  inguinal herniorrhaphy postoperative pain control in a prospect i ve , r and omized , double-blinded , placebo-controlled study . Forty patients undergoing unilateral inguinal herniorrhaphy with an epidural anesthetic technique were r and omly allocated to receive either active TENS or placebo TENS .  Postoperative pain was evaluated using a st and ard 10-point numeric rating scale ( NRS ) . Analgesic requirements were also recorded . TENS ( 100 Hz , strong but comfortable sensory intensity ) was applied for 30 minutes through 4 electrodes placed around the incision twice , 2 and 4 hours after surgery . Pain was assessed before and after each application of TENS and 8 and 24 hours after surgery . In the group treated with active TENS , pain intensity was significantly lower 2 hours ( P = .028 ) , 4 hours ( P = .022 ) , 8 hours ( P = .006 ) , and 24 hours ( P = .001 ) after the surgery when compared with the group that received placebo TENS . Active TENS also decreased analgesic requirements in the postoperative period when compared with placebo TENS ( P = .001 ) . TENS is thus beneficial for postoperative pain relief after inguinal herniorrhaphy ; it has no observable side effects , and the pain-reducing effect continued for at least 24 hours . Consequently , the routine use of TENS after inguinal herniorrhaphy is recommended . PERSPECTIVE This study presents the hypoalgesic effect of high-frequency TENS for postoperative pain after inguinal herniorrhaphy . This may reinforce findings from basic science showing an opioid-like effect provided by TENS , given that high-frequency TENS has been shown to activate delta-opioid receptors Transcutaneous electrical nerve stimulation ( TENS ) is commonly used for pain relief . However , little robust research exists regarding the combination of parameters required to provide effective doses . This study investigated the hypoalgesic effects of different parameter combinations , applied simultaneously at two sites ( segmental and extrasegmental ) , on pressure pain threshold ( PPT ) in pain‐free humans . Two‐hundred and eight volunteers ( median age 22 years , range 20–26 ) were r and omized to eight groups : six active TENS groups , placebo and control . Parameter combinations were such that frequency always differed at each site ( 110Hz or 4Hz ) , but intensity could be either the same or different levels : high ( to tolerance without pain ) or low ( strong but comfortable ) . TENS was administered to the forearm over the radial nerve and the ipsilateral leg below the fibular head for 30min with monitoring for 30 further minutes . PPT measurements were taken bilaterally from the mid‐point of first dorsal interosseous muscle , by an independent blinded rater , at baseline and at six subsequent 10‐min intervals . Log‐transformed data were analysed using repeated‐ measures analysis of covariance ( baseline values and gender as covariates ) . Those groups using high‐intensity stimulation at the segmental stimulation sites showed significantly greater hypoalgesia  than placebo ( p<0.025 in each case ) . The largest hypoalgesic effect was for simultaneous high‐intensity stimulation at segmental and extrasegmental sites , using different frequencies . These results reaffirm that high‐intensity stimulation ( regardless of frequency ) is of fundamental importance in effective dosage BACKGROUND AND PURPOSE It is not fully understood how transcutaneous electrical nerve stimulation ( TENS ) intensity affects mechanical pain threshold . METHOD Sixty-six healthy volunteers ( 13 male , 53 female ; 132 h and s ) without prior experience of TENS  participated in the study , which comprised a r and omized single-blind controlled trial . TENS was administered for 20 minutes through electrodes ( 25 x 25 mm ) placed on the h and s and forearms with a fixed frequency of 100 Hz and pulse duration of 150 micros . TENS intensity was r and omized and allocated in a concealed manner so that one arm received TENS with stimulation intensity set at participants ' subjective sensory threshold and the other received TENS with stimulation intensity continuously adjusted by physiotherapists to a strong but comfortable non-painful stimulation . Observers were blinded to stimulation intensity levels . RESULTS Mechanical pain threshold increased significantly , by a mean total of 0.79 kg/cm2 ( 95 % confidence interval [ 95 % CI ] : 0.54 - 1.04 ) ( p < 0.001 ) on the strong but comfortable non-painful stimulation side . The mean change in mechanical pain threshold on the sensory threshold side was 0.19 kg/cm2 and did not reach statistical significance ( 95 % CI-0.15 to 0.51 ) . The mean stimulation intensity level for sensory threshold was 6.7 mA ( 95 % CI : 5.65 to 7.83 ) which was significantly lower ( p < 0.001 ) than the mean stimulation intensity for the strong stimulation , which was 20.5 mA ( 95 % CI 16.6 to 24.4 ) , respectively . The strong stimulation levels were , on average , 3.05 times higher than sensory threshold , but individual variations were large ( range 1.2 - 6.1 ) . CONCLUSIONS TENS administered at a strong but comfortable non-painful stimulation intensity increases mechanical pain threshold ipsi-laterally in healthy subjects , whereas TENS administered at sensory threshold intensity does not . TENS may be ineffective if electrodes are placed contralaterally or distant to the pain site and if stimulation intensity levels are not titrated to subjective strong levels . Further clinical trials are needed to clarify if these findings may also be generalized to population s of chronic pain sufferers OBJECTIVE To determine the release pattern of serotonin and noradrenaline in the spinal cord in response to transcutaneous electric nerve stimulation ( TENS ) delivered at low or high frequency . DESIGN Prospect i ve r and omized allocation of 3 treatments . SETTING Research laboratory . ANIMALS Male Sprague-Dawley rats ( weight range , 250 - 350 g ) . INTERVENTION Knee joints of rats were inflamed with a mixture of 3 % carrageenan and 3 % kaolin for 24 hours prior to placement of push-pull cannulae into the dorsal horn of the spinal cord . Push-pull sample s were collected in 10-minute intervals before , during , and after treatment with low-frequency TENS ( 4 Hz ) , high-frequency TENS ( 100 Hz ) , or sham TENS . TENS was applied to the inflamed knee joint for 20 minutes at sensory intensity and 100-mus pulse duration . Push-pull sample s were analyzed for serotonin and noradrenaline by high performance liquid chromatography with coulemetric detection . MAIN OUTCOME MEASURES Spinal concentrations of serotonin and noradrenaline . RESULTS Low-frequency TENS significantly increased serotonin concentrations during and immediately after treatment . There was no change in serotonin with high-frequency TENS , nor was there a change in noradrenaline with low- or high-frequency TENS . CONCLUSIONS Low-frequency TENS releases serotonin in the spinal cord to produce antihyperalgesia by activation of serotonin receptors Background Transcutaneous electrical nerve stimulation ( TENS ) is an effective adjunctive therapy for postoperative pain ; however , effects of different frequencies of stimulation have not been systematic ally investigated . Laparoscopic sterilization ( LS ) causes significant pain in the early postoperative period and requires substantial postoperative medication . Therefore , we studied the effects of TENS on postoperative pain after LS through placement of Yoon fallopian rings in a prospect i ve , r and omized , double-blinded , and placebo-controlled study . Methods Sixty-four patients undergoing LS for uterine tube ligation were r and omly allocated to receive either active TENS or placebo TENS .  Postoperative pain was evaluated using a st and ard 11-point numeric rating scale and the McGill Pain Question naire (MPQ)-pain rating index and number of words chosen . Both high frequency ( 100 Hz ) and low frequency ( 4 Hz ) TENS , at strong , but comfortable sensory intensity , were applied for 20 minutes through 4 electrodes placed around the surgical incision immediately after surgery . Pain was assessed before and after application of TENS when patients were at postanesthesia care unit ( PAC 
    Output:
    
Basic science studies using animal models of inflammation show changes in the peripheral nervous system , as well as in the spinal cord and descending inhibitory pathways , in response to TENS .
Translational studies show mechanisms to prevent analgesic tolerance to repeated application of TENS .
This review also highlights data from recent r and omized , placebo-controlled trials and current systematic review s. Clinical trials suggest that adequate dosing , particularly intensity , is critical to obtaining pain relief with TENS .