pain is one of the principal motives to consult a health care professional . among the analgesic modalities used by rehabilitation professionals , transcutaneous electrical nerve stimulation ( tens ) is commonly used , as it is noninvasive , safe , and affordable ; it has no undesirable side effects and can be easily administrated in clinical settings as well as by the patients themselves . during or following tens application , categorized as either conventional tens or acupuncture - like tens ( al - tens ) , parameters for conventional tens include high - frequency ( 50100  hz ) , low - intensity ( nonpainful paraesthesias ) stimulations , and a small pulse width ( 50200  s ) . on the other hand , al - tens parameters include low - frequency ( 210  hz ) , high - intensity ( above pain threshold ) stimulations , and a longer pulse width ( 100400  s ) . conventional tens analgesia is explained by the gate control theory [ 3 , 4 ] , whereas the al - tens pain modulation mechanism is explained by descending endogenous opiate system . the later pain modulation mechanism modulates nociceptive transmission by inhibiting ascending nociceptive pathways in the dorsal horn of the spine at multiple segmental levels as a result of descending efferents [ 68 ] . although other pain control mechanisms might be involved in al - tens , such as gaba and serotonin ( demonstrated in animal models ) , muscle contraction , the main analgesic effect arises from this classic opioid - dependent descending inhibitory pathway . although the pain modulation mechanisms involved in al - tens are well known , there is limited evidence in the literature regarding the duration of analgesia induced by al - tens . the current data is arising from clinical studies [ 8 , 1113 ] , studies involving experimental pain stimuli in healthy subjects [ 1418 ] , and animal studies [ 1921 ] . the results range from 30 minutes to  at least an hour  postapplication and it is generally accepted that al - tens analgesia is of longer duration than the analgesia induced using the conventional tens . however , the results regarding the duration of al - tens pain modulation in the above studies have to be interpreted with caution since the duration of analgesia was not the main outcome tested in any of these studies . a better understanding of al - tens analgesia , specifically duration , would be beneficial since the duration of the analgesia induced by al - tens can influence the therapeutic options chosen by the rehabilitation professionals ( i.e. , al - tens versus conventional tens ) . with this in mind , the main objective of this study was to describe the duration of analgesia and magnitude of analgesia induced by the application of al - tens on healthy subjects with an experimentally induced heat pain stimulus . the secondary objective was to verify if the duration or magnitude of analgesia induced by the al - tens was influenced by the duration of the application of the al - tens ( 15 and 30 minutes ) . a repeated - measures , intrasubject randomized experimental design was used , where each participant was his / her own control . participants were randomly assigned to either begin with the 15 ( t15 ) or 30 ( t30 ) minutes application of al - tens . this counterbalancing of participants minimized a potential sequence effect . since this was an intrasubject design , concealment of allocation was not possible  all subjects knew that they would receive the 15 or 30 minutes treatment during the second session . the study was conducted in accordance with the helsinki declaration , and after approval from the ethics review board of the centre de recherche clinique tienne lebel ( chus ) ; data was collected from 22 healthy volunteers , 8 men and 14 women . inclusion criteria were healthy and pain - free volunteers aged 18 years or above , being able to understand and follow instructions . volunteers were excluded if they were presenting any painful pathology , loss of sensibility to heat , and pain on treatment areas or had any contraindication to al - tens ( i.e. , wearing a pacemaker or cardioverter defibrillator , wearing a metal implant under the treatment area by the al - tens , suffering of epilepsy , and presenting a haemorrhagic risk , a malignancy , or damaged skin under the treatment area ) . all participants were advised to avoid any kind of analgesic medication for a period of 12 hours before the experimentation and avoid caffeinated beverages 4 - 5 hours before experimentation since caffeine can block the analgesic effect of tens [ 23 , 24 ] . all participants were recruited on a voluntary basis and were nave to al - tens . all subjects signed an informed consent form and received a free parking voucher for each visit . the study took place at the school of rehabilitation of the universit de sherbrooke between january 2011 and november 2011 . all five students were trained by the principal investigator ( ytl ) in order to standardize the data collection process . all participants received the experimental procedures in the same sequential order ; familiarization , determination of test - stimulus intensity , test stimulus , al - tens application , and assessment of al - tens analgesia . all subjects received al - tens application in two separate sessions ( on different days ) of either 15 or 30 minutes duration by random assignment . we arbitrarily chose 15 and 30 minutes treatment times since they are the typical treatment durations . both experiments were separated by at least 6 days to limit any possible carryover effect from session 1 to session 2 .   a sustained ( tonic ) experimental heat pain test was chosen since it is safe , easily reproducible and may be more comparable to clinical pain than to brief ( phasic ) pain tests . the noxious stimulus was induced by a 9  cm thermode ( tsa ii , neurosensory analyzer , medoc instruments , north carolina , usa ) . they were then familiarized with the experimental ( thermal ) pain ( test stimulus ) induced by the thermode on the palm of the dominant hand . subjects were advised that the thermode temperature would gradually increase to a maximum of 51c and reassured that it could not induce any burn . this procedure was repeated twice and the subjects had to verbally report when they began to feel pain ( pain threshold ) and when the pain was intolerable ( pain tolerance ) . to determine the test - stimulus intensity ( temperature of heat pain ) , the thermode was applied on the volar aspect of the dominant forearm . pain perception was assessed with a computerized visual analogue scale ( covas ) graduated from 0 ( no pain ) to 100 ( maximum tolerable pain ) linked to the thermode . this instrument has a good validity and reliability and was previously used in other experimental pain studies . the subjects had to move the cursor when they reported initial pain sensation ( covas score : 1/100 which corresponds to the pain threshold ) . as the temperature of the thermode increased , subjects moved the cursor towards the right . when the maximum temperature they could tolerate was reached , the cursor needed to be at the extreme right ( covas score : 100/100 which corresponds to the pain tolerance ) . the thermode was placed on another area of the forearm to avoid overstimulation and possible local sensitization . subjects were given at least two trials of test stimulus to practice heat pain ratings with the covas and to ensure consistent responses between trials . then , the temperature rated as 50/100 on the covas was used for the next part of the experimentation , the test stimulus ( one - minute constant stimulation ) . for the test stimulus , the thermode was applied to the volar aspect of the dominant forearm one minute at the temperature corresponding to 50/100 pain perception on covas . subjects were told that they had to move the cursor left or right as decreased function of the pain perceived . in this part of the test , the temperature always increased to reach the temperature corresponding to 50/100 pain perception and then remained constant for one minute . all subjects were blinded to the temperature used during the test stimulus and to the study 's hypothesis . the pain perception within this period had to be between 40/100 and 60/100 on the covas . below or above these values , the temperature of the thermode was adjusted 0.5c to fit in this reasonable interval . when the temperature of the thermode produced a pain perception of approximately 50/100 , al - tens was then applied . the al - tens ( eclipse , empi ) was applied on the posterolateral aspect of the nondominant shoulder of all patients with the following parameters : 2  hz frequency and 180  msec impulse duration ( 1 channel , 2  2 inches nonadhesive , reusable , black carbon rubber electrodes ) . this specific location was chosen for two reasons : ( 1 ) for standardization across all participants and most importantly ( 2 ) for demonstrating that al - tens induces a diffuse analgesic effect rather than a gate - control ( segmental ) analgesic effect . the pain intensity and unpleasant perception were recorded at 2 minutes intervals during the al - tens application by a numerical verbal scale ranging from 0 ( no pain ) to 100 ( maximum tolerable pain ) . since al - tens application needs to be painful in order to induce analgesia , the intensity of al - tens was increased until the subjects reported a perceived pain of at least 30/100 with the verbal numeric pain scale since it is shown that this type of descending analgesia can only be activated by a noxious stimulus .   to measure the duration of al - tens analgesia , series of one minute test stimulus were performed with the contact thermode one minute after the al - tens application and repeated at five minutes intervals until the mean perceived pain intensity was equal or above the perceived heat pain during the first test stimulus ( before al - tens application ) . when this occurred , we considered that there was no more analgesia induced by al - tens and the experiment was terminated . the thermode was placed on another area of the dominant forearm between each repetition to avoid overstimulation . to measure the magnitude of al - tens analgesia ( the amount of pain modulation produced by al - tens ) , we calculated the difference between the mean pain intensity score of the heat pain stimulus before and after the al - tens . we considered that analgesia was no longer present when individual mean pain ratings of the post - al - tens test stimulus were above minus 0/100 ( or a positive score ) . descriptive statistics are presented as median and interquartile range ( iqr ) [ 25th and 75th percentile ] in the text and as median and standard error in the figures . in order to measure the duration of analgesia induced by the al - tens , a kaplan - meier survival analysis was used , with median duration as well as 95% confidence intervals . the mcnemar test ( non parametric ) was used to determine if the duration of application of the al - tens ( 15 or 30 minutes ) influenced the duration and/or magnitude of analgesia induced by the al - tens . during the 15 minutes application of al - tens , the mean pain intensity was 34.12  17.7/100 and the mean pain unpleasantness 58.12  20.9/100 . for the 30 minutes application of al - tens , the mean pain intensity was 31.95  16.0/100 and the mean pain unpleasantness 58.4623.6/100 . the median duration of the analgesia induced by al - tens was 10 minutes following both applications of either 15 or 30 minutes of al - tens ( t15 = 10,00 [ 1,6818,31 ] ; t30 = 10,00 [ 0,0023,58 ] ) . the analgesia induced by al - tens ranged from 060 minutes for the 15 minutes application and from 080 minutes for the 30 minutes application . figure 1 illustrates the number of subjects that still had analgesia ( analgesia versus no analgesia criteria ) through time for both treatment times . the survival analysis performed showed no significant difference in the duration of analgesia between the 15 minutes and the 30 minutes application of al - tens as the two curves intersect throughout the graph ( p = 0.991 ) . only half of the participants still had heat - pain analgesia induced by the al - tens at 15 minutes postapplication following both t15 and t30 groups ( figure 1 ) . finally , we observed that al - tens application did not induce analgesia in all participants . there were 3 participants in each group , the same subjects in the t15 and the t30 groups , who did not have a pain reduction during the first test stimulus after al - tens application . the magnitude of al - tens analgesia was calculated for each 1-minute test stimulus following the al - tens application for the first 15 minutes post - tens application since more than 50% of the subjects did not show analgesia after this point . we observed a statistically significant reduction in pain ratings after the application of al - tens ( p = 0.016 and p = 0.03 for the t15 and t30 groups , resp . ) . the percentage of reduction in pain intensity for the t15 group ranged between 26% and 36% ( figure 2 ) . similarly , the percentage of reduction in pain for the t30 group during the same period ranged between 20% and 31% . no participant had a complete analgesia ( 100% in reduction pain ratings ) with al - tens , as presented in figure 2 . we observed that either 15 or 30 minutes application of al - tens induced comparable magnitudes of analgesia at all points in time ( all p values > 0.05 ) ( see figure 2 ) . this study used a standardized experimental pain procedure and an intrasubject design to specifically determine the duration of the analgesia induced by al - tens . according to our findings , the median duration of al - tens analgesia on experimental heat pain was 10 minutes . although the maximum duration was 80 minutes for the t30 and 60 minutes for the t15 , the median durations for the two treatments time were comparable . some experimental studies point out that at higher intensities al - tens produces inhibition of central nociceptive transmission for at least 1 hour [ 8 , 13 , 16 , 19 , 20 ] . the duration of analgesia we observed was much shorter , considering that it lasted less than 15 minutes for 50% of the sample as shown by the survival analysis . one previous study   explored the duration of noxious conditioning pain modulation ( which relies on the same pain modulation mechanism ) on healthy volunteers where the noxious conditioning pain was the cold pressor test ( cpt ) . in this study , the mean duration of cpm analgesia was 35 minutes following the 7c cpt and 10 minutes following the 12c cpt . these results regarding the duration of analgesia are within the same range as we observed and further suggests that descending inhibition may represent an all - or - none phenomenon . this is not surprising given that comparable methodology and population were used to assess the duration and magnitude of the cpm . as part of the second objective , we wanted to determine if the duration of application of the al - tens ( 15 versus 30 minutes ) influenced the duration of analgesia . our survival analysis showed no significant difference between the analgesia duration following the 15 or 30 minutes application of al - tens . no previous study tried to determine if a longer application time of al - tens produced longer lasting analgesia . since the duration of al - tens application did not seem to influence the analgesia duration , we can suppose that 15 minutes of al - tens application would be enough to produce analgesia which would allow the clinician ( i.e. , physical therapist ) to maximize the efficiency of his intervention . as we observed ( see figure 1 ) , it seems that the application of al - tens for 30 minutes tends to produce longer analgesia duration for some patients . along with the duration , we wanted to determine if the magnitude of analgesia induced by the al - tens was influenced by the duration of application of the al - tens ( 15 and 30 minutes ) . as expected with al - tens application , we observed a reduction in the perceived pain intensity with the application of either 15 or 30 minutes of al - tens . however , there was no significant difference in the percentage of reduction of pain intensity between the 15 and 30 minutes application and this at each observation point after al - tens application . furthermore , we found a large variation in the magnitude of reduction in pain perception between subjects . for example , in the t15 group the reduction between subjects varied between 67% and 10% at the 10 minutes observation . however , the large variability we observed in the magnitude of analgesia might explain the fact that the results were comparable at all points in time . this is the first study to explore the analgesia duration of al - tens with standardized protocol of experimental pain , which assure that all participants received comparable noxious stimuli during al - tens . furthermore , we used an intrasubject design , which compares the results of the same participants following a 15 and a 30 minutes application of al - tens , where each participant was his own control . the main limitation of this study is linked to the fact that it was done on healthy subjects and used an experimental pain model that does not mimic clinical pain ; it is wise not to extrapolate the present findings to clinical use and therefore the external validity is limited . another limitation of this study is that most of the 22 participants were young ( mean age 25.41  9.33 years old ) ascribable to a possible voluntary response bias . the application of al - tens on the shoulder above the pain threshold reduced perceived pain intensity of experimental heat pain at the dominant forearm in healthy participants for approximately 10 minutes . the analgesia induced by al - tens ranged from 060 minutes for the 15 minutes application and from 080 minutes for the 30 minutes application . furthermore , using an intrasubject design , we observed that a longer application of al - tens ( 30 minutes versus 15 minutes ) induced neither longer duration of analgesia nor greater magnitude ( greater analgesic effect ) . this experimental study is the first step leading to the investigation of al - tens analgesia duration on a population suffering from clinical pain . further studies could also explore precisely the impact of al - tens intensity on analgesia . this could help narrow down the best way to use al - tens in physical therapy .
background . 
 acupuncture - like tens ( al - tens ) is a treatment modality that can be used to temporarily reduce pain . 
 however , there is no clear data in the literature regarding the specific duration of analgesia induced by al - tens 
 . objectives . to describe and quantify the duration and magnitude of al - tens analgesia on experimental heat pain in healthy subjects and verify if the duration or magnitude of analgesia induced by the al - tens was influenced by the duration of the application of the al - tens ( 15 versus 30 minutes ) . methods . 
 a repeated - measures , intrasubject randomized experimental design was used , where each participant was his / her own control . 22 healthy volunteers underwent heat pain stimulations with a contact thermode before ( pretest ) and after ( posttest ) al - tens application ( 15 and 30 minutes ) . 
 outcome measures included subjective pain during al - tens , duration , and magnitude of al - tens - induced analgesia . results . 
 survival analysis showed that the median duration of al - tens analgesia was 10 minutes following the application of either 15 or 30 minutes of al - tens . 
 the magnitude of analgesia following either application was comparable at all points in time ( p values > 0.05 ) and ranged between 20% and 36% pain reduction . 
 conclusion . 
 only half of the participants still had heat - pain analgesia induced by the al - tens at 15 minutes postapplication .