the pain threshold ( pt ) is the minimum amount of pain or unpleasant feeling experienced by 
 an individual , and it may be changed by continuous stimulation1,2,3 .
the neurophysiological theory proposes that continuous stimulation 
 will induce a high pt because pain will inhibit pain by inhibiting neurons in the dorsal 
 horn of the spinal cord .
second , the cognitive theory , an adaptive theory , proposes that if 
 a subject often experiences uncomfortable sensations , their ability to perceive pain 
 improves , which leads to a chronically increased pt , and , the hypersensitivity theory 
 suggests that the pt will decrease due to exaggerated reactions in patients with chronic 
 pain6 .
a trigger point is a tender point in the muscle fibers that divides active and potential 
 trigger points .
an active trigger point is a point which moves during the performance of 
 activities of daily living , while a potential trigger point is that one that is not felt 
 until it is stimulated , e.g. by the application of pressure on the skin3 , 7 , 8 .
most people have similar potential trigger points despite having 
 different lifestyles , which may be the main reason for pain1 .
therefore , physiotherapists must consider the potential trigger 
 points when they create exercise plans for healthy people as well as patients .
aerobic exercise increases oxygen consumption and induces pain relief by activating 
 endocannabinoid receptors that can easily pass the blood vessel barrier9 . sparing et
al.10 reported that college students running on a treadmill or pedaling 
 on a stationary bike at 7980% of the maximum heart rate exhibited sedation and analgesia 
 via activation of the endocannabinoid system .
hoffman et al.11 also found that pain perception changed after a minimum of 10  min 
 of aerobic exercise with a 75% maximal oxygen uptake ( vo2max ) .
koltyn et al.12 suggested that an acute bout of exercise 
 induced a lower systolic blood pressure , a higher heart rate , and changed the pain 
 perception .
however , isometric exercise increased blood pressure , but it is not related to 
 pain perception13i.e .
, neurotransmitter 
 and blood pressure levels were changed and then changes in pain perception were induced .
muscle strength exercises have a positive effect on pt ; however , some exercises such as 
 weightlifting can worsen pain14 .
kolton 
 and arbogasr15 reported that the pt 
 increased after 5  min of strengthening exercise with 75% of one - repetition maximum , and 
 returned to normal after 15  min of strengthening exercises .
bartholomew et al.16 reported that pain tolerance increased 
 after 20  min of self - selected exercise , indicating an analgesic response after exercise . 

koltyn et al.17 found that the pt of 
 women increased and the pt of men did not change after isometric strengthening exercise .
as mentioned above , the analgesic effect of aerobic exercise requires a minimum of 10  min ; 
 however , strengthening exercise did not require the same amount of time . furthermore , most 
 studies selected an exercise intensity associated with maximum resistance , and the time for 
 which exercise was performed differed among studies .
hence , a study of the effect of 
 exercise duration using mild intensity of exercise was required .
moreover , in most previous 
 studies , it was unclear whether a potential or active trigger point was used . studies of the 
 potential trigger point are essential , since it could lead to disease progression if it is 
 not treated . therefore , in the present study , we aimed to elucidate the manner in which 
 aerobic and strengthening exercises affect pts over time , and to identify appropriate 
 exercises that clinicians can use for managing pain in patients .
one 
 participant dropped out of the study during the second test for personal reasons .
all the 
 subjects provided their written informed consent to participation in the experiment in 
 accordance with the ethical principles of the declaration of helsinki .
this study also 
 excluded subjects who took any medication for musculoskeletal pain , in order to eliminate 
 interference effects .
the pain pressure thresholds ( ppts ) were measured using an electronic algometer ( jtech 
 medical , usa ) over potential trigger points .
the examiner identified a trigger point by 
 palpating and exerting vertical pressure on the skin .
the examiner increased the pressure at 
 the rate of 1  kg / s until the subject experienced a pain or an unpleasant feeling , and the 
 ppt values were measured 3 times for each muscle1 .
the participants were instructed to verbally express their first 
 perception of pain or an unpleasant feeling , at which point the application of pressure was 
 stopped18 . the ppt has been proven to 
 be a reliable measure ( interclass correlation , 0.780.93)19 .
the subjects in the aerobic group walked on a treadmill for 10 and 40  min at 6.5  km / h .
the 
 subjects in the strength group performed 10 and 40  min of circulate training exercises that 
 included a bench press , lat pulldown , biceps curl , triceps extension , and shoulder press 
 based on the perceived exertion . the subjects in the control group rested in a quiet room 
 without exercising for 10 and 40  min .
the ppts were measured before exercise as well as immediately after 10 and 40  min of 
 exercise using an electronic algometer at the potential trigger points of 5 muscles .
the 
 measurements were recorded at 2-day intervals to minimize fatigue - related errors and the 
 mean values were then calculated .
the 5 muscles examined were the trapezius , supraspinatus , teres major , rhomboid , and 
 levator scapulae .
most individuals experience problems , such as sudden muscle pain , in these 
 muscles due to computer work and sedentary lifestyles , and therefore , these muscles were 
 selected .
friedman s test was used to identify the change in the ppts according to time , and the 
 kruskal - wallis test was used to confirm differences among the 3 groups .
the mann - whitney 
 test was performed using spss 18.0 ( ibm , korea ) .
a significance level of 0.05 was chosen for 
 the friedman s and kruskal - wallis tests and 0.001 for the mann - whitney test .
the participant s mean age was 25.2  years , their mean height was 176  cm , and their mean 
 weight was 75  kg ( table 1table 1 .
general characteristics of the subjects ( n=14 ) ( m(sd))groupscharacteristicsag*sg**cg***height ( cm)1767.7174.53.7176.83.1weight ( kg)75 13.97810.27810.2age ( year)25.2 0.871.205.324.30.5 * ag : aerobic exercise group , * * sg : strengthening exercise group , * * * cg : control 
 group , * * * * m : mean , sd : standard deviation ) . *
ag : aerobic exercise group , * * sg : strengthening exercise group , * * * cg : control 
 group , * * * * m : mean , sd : standard deviation the ppts of all the muscles of all the subjects decreased immediately after 10  min of 
 exercise or rest , although no significant differences in these values were noted .
the ppts 
 of all the muscles decreased immediately after 40  min only in the control group , and 
 significant differences were observed in the right trapezius and left levator scapulae 
 muscles ( table 2table 2 .
the changes of ppts with time and different exercise types ( m(sd))times typesag ( n=5)cg ( n=5)sg ( n=5)r tra baseline28.42.331.63.629.65.42.210  min24.64.225.34.323.81.40.240  min29.65.423.95.123.33.90.21.57.6 * 6.0*l tra baseline28.02.327.93.230.55.11.010  min24.72.926.44.223.95.80.440  min25.32.423.65.224.14.70.52.05.26.5*r lev baseline32.01.129.63.434.84.34.410  min28.52.928.95.427.07.30.640  min26.46.326.04.925.93.80.37.6 * 5.26.0*l lev baseline32.72.234.13.133.14.50.710  min30.92.730.75.726.54.62.440  min29.44.227.84.428.16.10.35.28.4 * 6.0*r tere maj baseline25.25.822.42.324.15.01.010  min21.73.722.06.119.84.90.540  min20.86.421.75.021.36.20.046.4 * 0.73.5l tere maj baseline22.96.223.33.626.57.20.710  min20.85.021.86.219.46.00.240  min20.15.721.95.620.85.70.51.61.26.5*r rom baseline30.16.526.63.827.02.21.210  min26.26.225.56.525.84.20.140  min26.08.224.86.125.86.00.034.81.41.5l rom baseline30.66.027.55.129.54.10.310  min28.06.026.87.326.77.80.340  min25.58.325.85.625.66.10.17.6 * 2.82.0r supra baseline30.25.031.54.333.73.21.710  min29.43.530.46.728.87.30.540  min28.46.129.36.229.08.60.31.20.41.5l supra baseline30.32.531.74.031.51.71.910  min28.65.130.56.227.57.70.840  min29.44.429.85.129.710.90.51.20.72.0 * p<0.05 , * * ag : aerobic exercise group , * * * sg : strengthening exercise group , 
 * * * * cg : control group , * * * * * m : mean , sd : standard deviation .
tra : trapezius , lev : 
 levator scapulae , tere ma : teres major , rom : romboid major , supra : supra spinatus , r : 
 right ,
* p<0.05 , * * ag : aerobic exercise group , * * * sg : strengthening exercise group , 
 * * * * cg : control group , * * * * * m : mean , sd : standard deviation .
tra : trapezius , lev : 
 levator scapulae , tere ma : teres major , rom : romboid major , supra : supra spinatus , r : 
 right ,
l : left although the ppts of both the trapezius and left supraspinatus muscles increased 
 immediately after 40  min of exercise in the aerobic exercise group , the differences were not 
 significant .
furthermore , in the strengthening exercise group , the ppts of the left 
 trapezius , left levator scapulae , left and right teres major , and left and right 
 supraspinatus muscles increased ; however , significant differences were only noted for the 
 left trapezius , left levator scapulae , and left teres major muscle ( table 2 ) .
moreover , no significant differences in the change of 
 the ppts were observed among the groups .
pain perceptions are changed 
 by exercise in patients with chronic low back pain ; however , the effect of anesthesia 
 differs according to the type of exercise20 .
most of the previous studies have examined aerobic exercise , 
 whereas only a few studies have focused on strengthening exercise , therefore , we also 
 focused on strengthening exercise in the present study . in the present study ,
treadmill walking was chosen as the aerobic exercise mode and circuit 
 exercise was chosen as the strengthening exercise mode , because these forms of exercise are 
 frequently chosen as self - exercise . the same testing method was employed for the 2 tests 
 that differed in duration ( 10  min and 40  min ) , and it was repeated at a 2-day interval . 

hoffman et al.21 suggested that ppt 
 reproducibility did not evoke a significant stress - induced analgesic effect , therefore , they 
 did not measure potential trigger points .
however , in the present study , ppt reproducibility 
 at potential trigger points was measured .
droste et al.22 reported that the ppts 
 of 3 groups ( cycle ergometer , naloxone , and placebo groups ) did not change significantly , 
 whereas hoffman et al.11 found that pain 
 ratings were decreased 5  min after 30  min of exercise at 75% vo2max , but had 
 returned to baseline by 30  min after 30  min of exercise in healthy people .
hoffman et al.21 reported that ppts were 
 decreased after 30  min of aerobic exercise , consisting of leg cycling , among patients with 
 chronic low back pain .
however , koltyn et al.12 showed that the ppts of a cycle ergometer group increased after 15 , 
 20 , and 30  min of exercise due to lowered systolic blood pressure and a higher heart rate . 

kosek et al.23 found that the pt of 
 healthy subjects was increased after isometric exercise at a reading of 21% on a peak flow 
 meter ; however , the pt remained unchanged in patients with fibromyalgia .
koltyn and 
 arbogast15 suggested that the pt after 
 45  min of strengthening exercise did not significantly differ between a strengthening 
 exercise group and a control group . in the present study ,
the ppts had decreased immediately after 10  min of exercise or rest 
 in the aerobic , strengthening exercise , and a control groups decreased after 10  min , 
 although the differences between the values were not statistically significant among the 
 groups .
first , the patients pain perceptions were 
 changed and they could recognize the pain due to the application of pressure over the skin 
 by the examiner 
i.e . , the subjects recognized potential pain after the examiner exerted 
 pressure on the skin , and the potential trigger point was changed to an activating trigger 
 point . when these subjects performed exercise , they still experienced the pain as they had 
 already identified the trigger point .
in fact , the subjects may have remembered the trigger 
 point during short duration of the exercise .
second , the measurements were made repeatedly and may have represented the summation of 
 findings at the trigger points .
the subjects repeatedly experienced pain at the trigger 
 points , and may have easily remembered the pain at the time of exercise completion .
this 
 would indicate that pain is a sensory experience , involving a summation of psychological 
 experience and cognitive experience .
the ppts of 3 muscles in the aerobic exercise group and 6 muscles in the strengthening 
 exercise group increased immediately after 40  min of exercise , but none of these values 
 indicated a significant difference , except for the ppts of the left trapezius , left levator 
 scapulae , and left teres major in strengthening exercise group .
therefore , 40  min of 
 exercise was more effective than 10  min of exercise , although the change in the ppts was 
 mild .
although we used mild 
 intensity exercise in the present study , most previous research has used maximum intensity 
 exercise .
maximum intensity exercise may increase the blood supply to the muscle , 
 sufficiently enough to control pain .
however , the intensity of the exercise used in the 
 present study was perhaps in sufficient to ensure adequate blood supply to the muscles for 
 pain control .
the ppts of all of the muscles in the control group decreased immediately after 40  min of 
 rest , but no significant differences were noted .
this finding may be explained the fact that 
 the subjects could easily perceive the trigger points after the examiner exerted pressure at 
 these points .
the subjects remembered the pain that was caused by the pressure applied by 
 the examiner , and were continuously aware of the pain during the rest period .
this may have 
 been one reason why the subjects responded quickly when an examiner measured the pain 
 threshold again .
physiotherapists may need to treat patients who complain of a greater amount of pain after 
 therapy in the clinic , because a potential trigger point may have changed to an active 
 trigger point in these patients .
the results of the present study indicate the potential for 
 change in the condition of the trigger points after therapy .
therapists need to determine 
 whether pain originates from an activated or potential trigger point .
we believe that the measurement of ppts in the sitting position is a limitation of this 
 study , as the examiner needs to apply a greater amount of pressure when the subject is in 
 the sitting position than when the subject is lying on a table .
moreover , the sample size 
 was small , and therefore , the findings are difficult to generalize . in conclusion , ppts decreased immediately after 10  min of exercise or rest , but the ppts of 
 some muscles were increased immediately after 40  min of exercise .
moreover , no significant 
 differences in these values were noted according to the type of exercise .
therefore , both 
 strengthening exercise as well as aerobic exercise of mild intensity were unable to 
 effectively control pain .
hence , we suggest that 40  min is an effective exercise time , but 
 that an increased intensity of exercise is required to control pain . thus , more specific exercises need to be developed in the future , including exercises with 
 varying intensity and duration .
moreover , the transition of a potential trigger point to an 
 activated trigger point should be identified to prevent potential disease
   [ purpose ] we assessed the effects of aerobic exercise and strengthening exercise on pain 
 pressure thresholds ( ppts ) over time . [ subjects and methods ] fifteen healthy participants 
 were recruited and randomly divided into 3 groups : aerobic exercise , strengthening 
 exercise , and control . 
 the subjects in the aerobic group walked on a treadmill for 40  min 
 at 6.5  km / h . the subjects in the strength group performed circuit training that included 
 bench press , lat pull down , biceps curl , triceps extension , and shoulder press based on 
 the perceived exertion for 40  min . the subjects in the control group rested without any 
 exercise in a quiet room for 40  min . 
 the ppts of 5 potential muscle trigger points before 
 exercise , and immediately after 10 and 40  min of exercise or rest were measured using an 
 electronic algometer ( jtech medical , usa ) . 
 the friedman s , kruskal - wallis , and 
 mann - whitney tests were performed using spss 18.0 ( ibm , korea ) . [ results ] the ppts of all 
 subjects decreased after 10  min of exercise , but the difference was not statistically 
 significant . 
 the ppts of the control group decreased after 40  min . 
 furthermore , the ppts 
 of 3 muscles increased after 40  min of aerobic exercise and of 6 muscles after 40  min of 
 strengthening exercise . no significant difference in ppts 
 was noted among the groups . 
 
 [ conclusion ] the results show that 40  min is a more appropriate exercise time , although 
 the efficacy of controlling pain did not differ between strengthening exercise and aerobic 
 exercise . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION