in korea , close to 86.8% of the elderly population ( aged 65 or older ) has chronic 
 degenerative disease1 . among these 
 degenerative diseases , arthritis has the highest prevalence rate . osteoarthritis mainly 
 occurs in the hip joints and knee joints , which are weight - bearing joints , and most elderly 
 persons aged 65  years or older have this disease1 , 
 2 . osteoarthritis induces the growth of 
 osteophytes , leading not only to pain but also to the loss of movement and often to a state 
 of lethargy , in which the individual avoids walking1 . this lethargy hinders the individual from performing functional 
 activities , often resulting in muscle weakening and atrophy , including decrease in muscle 
 force and motor unit activation of the muscles around the joint affected by 
 osteoarthritis3 . muscle weakening and 
 atrophy further contribute to reduced walking and functional abilities required for daily 
 living3 . flexion - relaxation phenomenon ( frp ) refers to the appearance of muscular - electrical silence 
 on the surface of the erector spine during complete trunk flexion4 , which could be affected by several factors , including 
 lumbopelvic postures5 , repetition of 
 tasks6 , and muscle fatigue7 . chronic osteoarthritis , therefore , often leads to stooped postures 
 as a result of the tension experienced when assuming postures in which the erector spine is 
 elongated3 , 4 . the erector spinae muscle is a posture - maintaining muscle that is 
 used in all daily living activities in which trunk forward - tilting and straightening motions 
 are performed . normal frp of the erector spinae is an important precondition for smoothly 
 performing the functional activities of daily living and is an important measurement 
 variable in the evaluation of treatment effects at clinics8 . there are currently insufficient studies examining the effects of osteoarthritis on the frp 
 of the erector spinae and on how the frp of the erector spinae observed in young adults is 
 different from that of the elderly in whom osteoarthritis has occurred . therefore , the 
 present study compared the frps of the erector spinae occurring during ( 1 ) trunk flexion , 
 ( 2 ) complete trunk flexion , and ( 3 ) trunk extension motions in both healthy , young females 
 and elderly females with chronic knee osteoarthritis . in addition , data regarding whether 
 the frp of the erector spinae can be used as a method of evaluating the degree of pain in 
 chronic osteoarthritis in the elderly will be presented . if the erector spinae 
 flexion - relaxation rates can be used as a pain scale , it may assist healthcare professionals 
 to improve elderly patients quality of life . this study was a randomized controlled trial , which included healthy adult females 
 attending a university and elderly females aged at least 65  years diagnosed with chronic 
 osteoarthritis in the leg , among the elderly community residing in a metropolitan area of 
 chungcheongnam - do in south korea . the selection criteria for elderly subjects were as 
 follows : those who had no orthopedic or neurologic disease affecting the leg other than 
 chronic osteoarthritis , had not undergone surgical treatment of the leg within the last 6 
 months , had not experienced any fracture in the leg in the last 6 months , had corrected 
 visual acuity not lower than 0.8 and had no blurred vision , could understand experimenters 
 instructions and cooperate with the experimenters , and had understood the purpose of the 
 present study and voluntarily agreed to participate . the study was approved by the 
 institutional review board of hanyang university , and written informed consent was obtained 
 from all patients prior to the experiments . the general characteristics of the subjects that 
 participated in the present study are shown in table 
 1table 1.general characteristics of study subjects ( n=30)elderly females with chronicosteoarthritis 
 ( n=17)healthy adult females ( n=13)age ( years ) , m  sd 75.8  12.0 * 21.1  0.8height ( cm ) , m  sd153.5  6.3159.6  3.7weight(kg ) , m  sd56.5  8.752.9  5.4*p<0.05 . in the present study , surface electromyography ( emg ) ( trigno wireless system , delsys inc . , 
 boston , ma , usa ) was used to measure the muscle activity of the healthy adult females and 
 elderly females with chronic knee osteoarthritis while they were performing trunk flexion , 
 complete trunk flexion , and trunk extension . the emg signals measured by the trigno sensor 
 and wirelessly transmitted to the trigno base station were analyzed using emgworks 4.0 
 ( delsys inc . , boston , ma , usa ) software . all measured muscle activity values were calculated 
 as percentages of maximal or submaximal voluntary isometric contraction values . the emg 
 signal - sampling rate was 1,024  hz , and the signals were band - pass filtered within a range of 
 10500  hz . metronomes were used to enable the experimental subjects of the present study to perform 
 trunk flexion , complete trunk flexion , and trunk extension for 5  s each over a total of 15 
 s. the beat was set to 60 bpm , and seconds were counted so that the subjects could hear and 
 perform each motion for 5 s. before attaching the emg electrodes , sweat and foreign matter in the experimental muscle 
 regions were removed in order to minimize skin resistance . the attachment points for 
 measuring the thoracic erector spinae were 5  cm to the left and right of the spinous process 
 of the t9 , while the attachment points for measuring the lumbar erector spinae were 2  cm to 
 the left and right of the spinous process of the l2 . the manual muscle force postures for 
 measuring of the maximal voluntary isometric contraction ( mvic ) in healthy adults and the 
 submaximal voluntary isometric contraction in elderly persons with chronic knee 
 osteoarthritis were determined following the method of osullivan et al6 . the values of each subject were measured 3 times for 5  s , 
 in succession , over a total of 15 s. measurements during the first and last seconds of each 
 measurement period were excluded , and the maximum values during the middle 3  s were used for 
 the quantification of the root mean square ( rms ) . to minimize muscle fatigue , 1 the subjects were randomly measured in the 
 order of voluntary participation . during the measurement of the flexion - relaxation rates , three motions were performed : ( a ) trunk flexion , ( b ) complete trunk flexion , and ( c ) trunk 
 extension . posture 1 was a standing posture , posture 2 was a trunk - bending motion , and 
 posture 3 was a maximum trunk - bending motion . in the complete trunk flexion section , the 
 subjects were instructed to bend the trunk maximally within the range of bending with no 
 pain before measurement . preliminary instruction was provided to the subjects by the 
 experimenter , who performed the motions in front of the subjects and helped the subjects 
 practice the motions at least one time . the flexion - relaxation rates were determined using both the values obtained by dividing the 
 data for trunk extension by that of complete trunk flexion ( c / b ) and by dividing the data 
 for trunk flexion by that of complete trunk flexion ( a / b ) . the mean values of the three 
 trunk flexion extension motions were used to determine the flexion - relaxation rates . the means and standard deviations of 
 the general characteristics of the study subjects ( age , gender , height , and weight ) were 
 calculated using descriptive statistics . in the present study , the independent variable was 
 the presence or absence of chronic knee osteoarthritis , and the dependent variable was the 
 frp of the erector spinae . independent t - tests were used to examine the erector spine 
 flexion - relaxation rates according to whether the presence or absence of chronic knee 
 osteoarthritis . to test the statistical significance , was set to 0.05 , and the collected 
 data were analyzed using pasw ver . the effect of chronic knee osteoarthritis on the frp of the erector spinae was first 
 examined . the values obtained by dividing the data for trunk extension by that of complete 
 trunk flexion were not different for the left and right lumbar erector spinae or right 
 thoracic erector spinae ; however , they were significantly different for the left thoracic 
 erector spinae ( p<0.05 ) ( table 2table 2.flexion-relaxation phenomenon ( frp ) values obtained by dividing the data for 
 trunk extension by those for complete trunk flexion from healthy adults and elderly 
 individuals ( units : % ) musclesmean  sdelderly females withchronic osteoarthritishealthy adultfemalesleft thoracic erector spine1.67  0.61 * 1.22  0.45right thoracic erector spine1.87  0.822.31  1.35left lumbar erector spine2.35  1.312.08  1.36right lumbar erector spine2.46  1.031.85  1.81*p<0.05 ) . the values obtained by dividing the data for trunk flexion by that of 
 complete trunk flexion , were not different for the left and right lumbar erector spinae and 
 right thoracic erector spinae , but were significantly different for the left thoracic 
 erector spinae ( p<0.05 ) ( table 3table 3.flexion-relaxation phenomenon ( frp ) values obtained by dividing the data for 
 trunk flexion by those for complete trunk flexion from healthy adults and elderly 
 individuals ( units : % ) musclesmean  sdelderly females with chronic osteoarthritishealthy adult femalesleft thoracic erector spine1.47  0.55 * 1.12  0.31right thoracic erector spine1.41  0.491.99  1.43left lumbar erector spine2.00  1.082.58  2.75right lumbar erector spine1.76  0.901.62  1.31*p<0.05 ) . in the present study , the frp of the erector spinae in elderly females with chronic knee 
 osteoarthritis and healthy young females were measured to determine whether the erector 
 spinae flexion - relaxation rates could act as a useful pain evaluation tool for patients with 
 knee osteoarthritis . although no significant differences were evident in the 
 flexion - relaxation rates of the left and right thoracic erector spinae or right lumbar 
 erector spinae , significant differences were shown in the flexion - relaxation rates of the 
 left thoracic erector spinae . degenerative arthritis develops in most people aged 65 or older , with a higher prevalence 
 in females than in males . lower - limb osteoarthritis is 
 associated with foot postures and affects the dynamic alignment of the lower limbs9 . reilly et al.9 reported that patients with knee joint osteoarthritis showed changes 
 in the ankle joint postures when compared with healthy individuals , and according to the 
 study of hu et al.10 , foot posture and 
 stance widths affected the load - sharing mechanism of the lumbar muscle tissues . therefore , 
 knee osteoarthritis was believed to affect the frp of the lumbar erector spinae . the frp can act as a quantitative measure for the evaluation of changes in the 
 neuromuscular system functions11 . varying 
 flexion - relaxation rates among patients are a useful clinical tool that can help in 
 diagnosing chronic lower back pain in patients3 . this argument is supported by strong evidence that those subjects 
 with low back pain do not exhibit the frp during complete trunk flexion12 . watson et al.13 demonstrated the validity of measuring the frp of the lumbar 
 vertebrae to diagnose lower back issues and reported high specificity and sensitivity . in 
 addition , the flexion - relaxation rates are a valuable tool for distinguishing between 
 patients with cervical pain and those with no cervical pain , and to indicate changes in the 
 neuromuscular control in patients with cervical pain . the flexion - relaxation rates of the 
 cervical spine were reported to be significantly lower in patients with cervical pain14 . shin et 
 al.15 advised that the knee angles and 
 individuals flexibility levels affected the frp of the lumbar muscles . they reported that 
 in the case of subjects with high flexibility , knee angles affected the flexion - relaxation 
 rates when the trunk angle became 90 , and in the case of those with moderate flexibility , 
 the foregoing reactions appeared when trunk angles were 7090. they also reported that in 
 the case of those with poor flexibility , knee angles did not affect the flexion - relaxation 
 rates . according to the study by hashemirad et al.16 , muscle activity pattern of the erector spinae varied with 
 flexibility during trunk flexion / extension . the flexion - relaxation reactions are affected by 
 several factors , including trunk loads , lumbosacral postures , trunk angular speed , number of 
 task repetitions , and muscle fatigue6,7,8 , 17 . although previous studies were actively 
 conducted on the effects of loads , angular speed , muscle fatigue , and postures on the 
 flexion - relaxation rates , studies on the effects of changed lumbosacral postures of patients 
 with chronic knee osteoarthritis on the frp of the erector spinae have been insufficient . therefore , the present study intended to examine the differences in the frp of the erector 
 spinae between elderly persons with degenerative knee osteoarthritis and healthy adults . 
 although not clinically significance , the results showed significant differences only in the 
 left thoracic erector spinae , but not in the left and right lumbar erector spinae or right 
 thoracic erector spinae . first , the study sample was small and was limited to females 
 with chronic knee osteoarthritis second , 
 although the same voice signals were given using a metronome during trunk flexion and 
 extension , trunk speeds were different among individual subjects . third , the levels of knee 
 pain were different among individual patients with chronic knee osteoarthritis , and the 
 effects of medication and previous treatment could not be eliminated . to further determine 
 the frp of the erector spinae in elderly females with chronic knee osteoarthritis , further 
 studies that account for the limitations of the present study may be required .
[ purpose ] this study investigated the flexion - relaxation phenomenon of the erector spinae 
 in elderly women with chronic knee osteoarthritis and determined whether the 
 flexion - relaxation phenomenon can be used as a pain evaluation tool in such cases . 
 [ subjects and methods ] seventeen elderly females with chronic knee osteoarthritis and 13 
 healthy young females voluntarily participated in this study . 
 they performed three 
 postural positions in 15  s : trunk flexion , complete trunk flexion , and trunk extension , 
 each for 5 s. while these positions were held , muscle activation of the thoracic and 
 lumbar erector spinae were measured using surface electromyography . 
 the flexion - relaxation 
 rate was determined by dividing the values for trunk extension by those of complete trunk 
 flexion and by dividing the values for trunk flexion by those of complete trunk flexion . 
 [ results ] according to our results , the flexion - relaxation phenomenon was different 
 between healthy young and elderly females with chronic knee osteoarthritis . 
 specifically , 
 there was a difference in the left thoracic erector spinae muscle , but not in the left and 
 right lumbar erector spinae or right thoracic spinae muscle . 
 [ conclusion ] our study 
 demonstrated that the erector spinae muscle flexion - relaxation phenomenon can be used as a 
 pain evaluation tool in elderly females with chronic knee osteoarthritis .