lumbar instability is a common symptom and also one of the causes of low back pain . the definition of lumbar segmental 
 instability is decreased stiffness of the spinal motion segments2 . previous studies have indicated that lumbar instability is 
 associated with pain possibly3 ; and it may 
 be due to a defect in spinal segment movement control , leading to compression of the neural 
 structure4 , 5 . trunk muscular strength may protect the spine during daily activities6,7,8 . weakness of the trunk muscles and poor muscular endurance are also 
 characteristics of lbp9 . the severity of 
 trunk muscular weakness or fatigability may be examined using isokinetic or isometric 
 tests10,11,12 . numerous studies have 
 investigated the difference in trunk muscular strength and endurance between chronic lbp 
 patients and healthy subjects13 , 14 . some reported subjects with lbp have 
 significantly less muscular strength and endurance than healthy subjects13 , 14 . this may be explained by lbp clients being unwilling to make maximal 
 effort during the muscular strength test due to pain or fear of injuries15 , 16 . previous studies have investigated the trunk muscular activation and muscle fatigue of lbp 
 patients using electromyography ( emg ) and compared the results with those of healthy adults . the most frequent site showing pathological changes is the 4th and 5th lumbar vertebrae , 
 where the multifidus muscles are located17 . the median frequency of emg has been used as a fatigue index to 
 compare the back muscular fatigability of the left and right sides between 40% maximal 
 voluntary contraction ( mvc ) and 80% mvc , and the median frequency imbalance was 
 significantly larger in lbp than in healthy subjects16 . regarding exercise training , numerous exercise types have been developed as exercise 
 therapy for clients with lbp , such as hip joint exercises , trunk muscular strengthening 
 exercises , and aquatic exercise18 . it aims to train injured 
 and healthy supporting tissue , facilitate tissue repair and prevent structural weakness from 
 excessive loading19 . exercise training 
 for lbp is not only concerned with positional correctness , but also deep muscle and 
 neuromuscular control training . previous studies have demonstrated that exercise on an 
 unstable surface , such as exercises on a swiss ball and sling exercise training ( s.e.t ) , 
 increases muscular activation levels20,21,22,23 . the activation levels of the internal and 
 external oblique muscles increased during bridge exercise with vibration training on an 
 unstable surface24 . exercise on an 
 unstable surface challenges the motor control system , the increasing the speed and intensity 
 of lumber stabilizers , contraction and the activation ratio as well as improving muscle 
 activation synergy19 , 25 , 26 . postnatal women with 
 pelvic pain reported significantly reduced pain , improved physical function and better 
 quality of life after 20 weeks of s.e.t training27 . baseball players with lbp participated in 12 weeks of exercise 
 training , using ultrasound biofeedback training or s.e.t training . the ultrasound 
 biofeedback training comprised stretching and general trunk muscle strengthening under 
 ultrasound monitoring . the ultrasound biofeedback training helped subjects practice and 
 realize how the deep muscles work . the results showed s.e.t training could improve pain , 
 physical function and lumbar instability more significantly than ultrasound biofeedback 
 training28 . in general , subjects who 
 suffer from lbp show improved trunk proprioception , physical function and trunk stability 
 after six weeks of s.e.t29 . to the best of our knowledge , most studies have measured the effects of interventions for 
 lbp using functional fitness or a subjective questionnaire . few studies have measured the 
 effects of exercise intervention on muscular strength and endurance . in addition , due to the 
 possible influence of psychological factors , using the slope of the median frequency may be 
 a relatively objective way of measuring muscular endurance compared to using maintenance 
 time only . thus , the purpose of this study was to examine the effects of 6 weeks s.e.t for 
 clients with lbp on pain , disability , muscular strength and muscular endurance . this study recruited chronic lbp patients . they were randomly divided into a control group 
 and a training group . the pain area was located between the lower ribs and gluteus folds , 
 and the pain duration had persisted for at least three months . the investigators explained the 
 purposes and procedures of the assessment and obtained basic personal information and 
 informed consent before conducting the experiment . the participants provided their written 
 consent to participation and the study was approved by the kaohsiung medical university 
 ethics committees . participants subjective parameters ( pain intensity and disability levels ) , and objective 
 parameters ( muscular strength and endurance ) participants in the training 
 group performed stabilization exercise using a s.e.t system for six weeks . low scores 
 represented low pain intensity , and high scores represented high pain intensity . the chinese version of the oswestry disability scale for lbp was used to measure the 
 disability level30 . each item is scored from 0 to 5 , with higher the 
 scores indicating greater disability . the maximum score for this questionnaire is 50 points , 
 and the raw scores are converted to percentages . according to fairbank31 , 020% represents mild disability , 2140% represents 
 moderate disability , 4160% represents severe disability , 6180% means most physical 
 functions are impaired by back pain , and it is classified as disabled , and 81100% means the 
 subject is bed - bound . the muscular strength assessment was measured by a custom - designed muscular strength 
 dynamometer . subjects were instructed to carry out the trunk extension and rotation to the 
 left and right in a sitting position . for each movement they aimed to exert maximal 
 voluntary isometric contraction for 5 seconds in three trials ( fig . ( a ) back extension , ( b ) rotation to left and right ) . during the trunk flexion and extension test , the torque sensor was placed on the 
 left of the body with its height parallel to the iliac crest . muscular strength measurements . ( a ) back extension , ( b ) rotation to left and right for the trunk extension test , a fixed bar was set on the spine at the height of the scapula 
 to fix the trunk , and an investigator applied a strength with force along the femoral bone . the torque sensor was placed on the head to measure trunk rotation the fixed bar set at the 
 height of the sternal notch . subjects rested for ten minutes after the maximal muscular strength test , and then carried 
 out the endurance test at about 52% mvic test32 in one trial . subjects performed trunk extension and sustained the 
 contraction until they could not maintain the standard testing posture , their hand touched 
 the bed , or the posture was sustained for 240 seconds . the investigators recorded the time 
 the subject sustained the posture , and the change in the median frequency slope was used as 
 an index of fatigue . participants in the training group attended an exercise course of three sessions a week 
 with each session comprising 30 minutes of training and 10 minutes of warm up . in the training program , participants had to hold the end position for 810 seconds in 
 1215 repetitions of each exercise . the exercise intensity was adjusted by the distance from 
 the point of sling suspension and the distance between the slings and the bed . the 
 repetitions and intensity of exercise investigators considered the 
 fitness levels differed between the participants , thus , the exercise prescriptions were 
 individually decided with different exercise maintenance times and repetitions in the 
 initial training session . investigators instructed participants to perform a pelvic backward 
 tilt and keep their back straight during exercise to prevent injury through excessive 
 loading of the lumbar spine during exercise . exercise ( 1 ) : participants were instructed to flex the elbow to 90 degrees and kneel on the 
 bed under the sling point . exercise ( 2 ) : participants stood on the floor and under the sling point . exercise ( 3 ) : the straps were 30 centimeters apart from the 
 bed . exercise ( 4 ) : 
 participants lay supine on the bed with the ankles under the sling point . exercise ( 5 ) : participants lay prone on the bed and the 
 ankles were under the sling point . straps were set at the height of the participants upper 
 arm to make the body parallel to the bed during exercise . exercise ( 6 ) : participants lay prone on the bed and the 
 ankles were under the sling point . straps were set as high as the participants upper arm to 
 make the body parallel to the bed during the exercise , and the participants supported their 
 body with their forearms and ankles combined with knee flexion . 2.exercise training programs 
 1 ) subjects knelt with their arms in the slings and gradually extended their arms 
 while keeping their arms straight.2 ) subjects stood with their hands in the slings and gradually extended their arms 
 while keeping their backs straight.3 ) subjects lay on the bed with their ankles in the slings and executed a bridging 
 exercise.4 ) subjects lay on the bed with their ankles in the slings , and executed a bridge . then , they gradually flexed their knees to increase the instability.5 ) subjects lay prone on the bed with their ankles in the slings . then , they executed 
 the plank exercise.6 ) subjects lay prone on the bed with their ankles in the slings . they executed the 
 plank exercise , then gradually flexed their knees to increase the instability . 
 . exercise training programs 
 1 ) subjects knelt with their arms in the slings and gradually extended their arms 
 while keeping their arms straight.2 ) subjects stood with their hands in the slings and gradually extended their arms 
 while keeping their backs straight.3 ) subjects lay on the bed with their ankles in the slings and executed a bridging 
 exercise.4 ) subjects lay on the bed with their ankles in the slings , and executed a bridge . then , they gradually flexed their knees to increase the instability.5 ) subjects lay prone on the bed with their ankles in the slings . then , they executed 
 the plank exercise.6 ) subjects lay prone on the bed with their ankles in the slings . they executed the 
 plank exercise , then gradually flexed their knees to increase the instability . subjects knelt with their arms in the slings and gradually extended their arms 
 while keeping their arms straight . subjects stood with their hands in the slings and gradually extended their arms 
 while keeping their backs straight . subjects lay on the bed with their ankles in the slings and executed a bridging 
 exercise . subjects lay on the bed with their ankles in the slings , and executed a bridge . they executed the 
 plank exercise , then gradually flexed their knees to increase the instability . investigators calculated the mean of maximal torque of 10 consecutive data minus the mean 
 baseline torque for 10 consecutive data to calibrate the error . the 10 consecutive data of 
 maximal torque were chosen as the 5 data before the maximal value and the 5 data after 
 maximal value . the value divided by the lever arm was used to obtain the force value . the trunk muscular endurance was measured during extension , and the effects of s.e.t on 
 trunk muscular endurance . the sampling rate of the emg system was 1,500  hz . fast fourier transform ( 512 point , window 
 processing ) were used to convert the time - domain signal to the frequency domain and the 
 median frequency was calculated13 . the 
 calculated median frequency values were fitted using a first - order curve approximation to 
 calculate the slope of the median frequency . a positive slope indicates neuromuscular 
 fatigue did not build up ; a negative slope indicates neuromuscular fatigue is present and a 
 more negative slope indicates a higher level of neuromuscular fatigue16 . the median frequency slope calculation was performed 
 using matlab ( version r2007b mathworks inc . wilcoxon s rank test was used to compare the differences in parameters between pre - and 
 post - intervention . the 
 statistical analyses were performed using jmp statistical software ( version 9.0.0 sas 
 institute inc . ) . this study recruited 16 lbp subjects , who were randomly assigned to the training group and 
 the control group . nine were in the training group and the remaining seven were in the 
 control group . however , two subjects in the training group dropped out of the study due to 
 their work pattern changing , which made them unable to participate in the training sessions . in addition , two subjects in the control group dropped out of the study due to personal 
 reasons . thus , the total number of subjects was 12 , with seven in the training group and the 
 remaining five in the control group . the characteristics of the subjects are listed in table 1table 1.characteristics of the subjectscontrol group(n=5)training group(n=7)age ( yrs)27.6 ( 6.7)27.6 ( 5.6)height ( m)1.65 ( 0.07)1.60 ( 0.04)body weight ( kg)57.6 ( 12.2)57.0 ( 9.9). this study used a visual analog scale to measure the pain intensity . higher scores on this 
 scale indicated higher pain intensity . the pain intensity of the control group and training 
 group before training was 3.75 ( 0.87 ) and 4.29 ( 1.44 ) , respectively , with no significant 
 difference between the groups . in the control group , the pain intensity had reduced after six weeks , but it did not 
 improve significantly ( table 
 2table 2.values of the subjective parameters of each group at pre - and 
 post - trainingcontrol group ( n=4)training group ( n=7)pre trainingpost trainingpre trainingpost trainingpain intensity3.75 ( 0.87)2.38 ( 1.03)4.29 ( 1.44)1.33 ( 1.17)*oswestry ( % ) 19.00 ( 16.20)18.50 ( 11.82)14.29 ( 3.90)8.86 ( 6.41)*the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant difference ( p<0.05 ) . ) . in the training group , after six weeks of training , pain intensity was 
 reduced to 1.33 ( 1.17 ) cm , and there was a significant improvement in pain intensity 
 between pre- and post - training ( p<0.05 ) ( table 
 2 ) . the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant difference ( p<0.05 ) . the training group showed a slightly higher disability level than the control group at the 
 baseline assessment , but there was no statistically significant difference between the 
 groups ( table 2 ) . after six weeks , there was no significant improvement in the oswestry disability index for 
 the control group ( table 2 ) . the disability 
 level in the training group before training was 14.29 ( 3.90 ) percent , and after training , 
 the disability level was significantly lower at 8.86 ( 6.41 ) percent ( p<0.05 ) ( table 2 ) . a comparison of baseline measurements between the groups showed the muscular strengths in 
 extension of the control and training groups were 0.57 ( 0.10 ) kg - m / bw and 0.39 ( 0.15 ) 
 kg - m / bw , that of rotation to the right was 0.37 ( 0.12 ) kg - m / bw and 0.30 ( 0.10 ) kg - m / bw , 
 and that of rotation to left and 0.43 ( 0.12 ) kg - m / bw 0.27 ( 0.11 ) kg - m / bw , respectively . the control group had greater absolute muscular strength than the training group in 
 extension and rotation to the left and right . however , there were no significant differences 
 between the groups . in the control group , extensor and rotator muscular strength had slightly decreased after 
 six weeks without training . however , the results were not significantly different ( table 3table 3.the values of muscular strength each group at pre - and post - trainingcontrol group ( n=5)training group ( n=7)pre trainingpost trainingpre trainingpost trainingextension ( kg - m / bw)0.57 ( 0.10)0.54 ( 0.12)0.39 ( 0.15)0.59 ( 0.14)*rotation to right ( kg - m / bw)0.37 ( 0.12)0.35 ( 0.12)0.30 ( 0.10)0.41 ( 0.08)*rotation to left ( kg - m / bw)0.43 ( 0.12)0.38 ( 0.04)0.27 ( 0.11)0.38 ( 0.08)*the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant differences ( p<0.05 ) . ) . in the training group , muscular strength improved after six weeks training . the absolute extensor muscular strength was 0.39 ( 0.15 ) kg - m / bw before training , and it 
 increased to 0.59 ( 0.14 ) kg - m / bw after six weeks of s.e.t ( p<0.05 ) ( table 3 ) . the results showed muscular strength of rotation to the 
 right was 0.30 ( 0.10 ) kg - m / bw before training , and it increased significantly to 0.41 
 ( 0.08 ) kg - m / bw after six weeks of training ( p<0.05 ) . the muscular strength of rotation 
 to the left was 0.27 ( 0.11 ) kg - m / bw before training , and it increased significantly to 0.38 
 ( 0.08 ) kg - m / bw ( p<0.05 ) after 6 weeks of training ( table 3 ) . the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant differences ( p<0.05 ) . comparison of the slope of the median frequency between the control and training group 
 showed no significant difference at baseline for all the muscles measured . in the control 
 group , the slope of the median frequency for all muscle sites showed no improvement after 
 six weeks . the slope of median frequency in the training group showed a significant improvement on the 
 left side of the multifidus after training ( the slopes at baseline and post - training were 
 0.11 and 0.08 , respectively ) ( p<0.05 ) ( table 4table 4.median frequency comparison of the values of each group at pre - and 
 post - trainingslope of mf on each muscle sitecontrol group ( n=5)training group ( n=7)pre trainingpost trainingpre trainingpost trainingl - es0.04 ( 0.02)0.04 ( 0.02)0.06 ( 0.01)0.05 ( 0.01)r - es0.04 ( 0.01)0.05 ( 0.03)0.04 ( 0.01)0.05 ( 0.02)l - mf0.06 ( 0.03)0.06 ( 0.03)0.11 ( 0.01)0.08 ( 0.01)*r - mf0.06 ( 0.04)0.05 ( 0.04)0.11 ( 0.02)0.08 ( 0.01)the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant differences ( p<0.05 ) . ( units :  hz / s ) l - es : left side 
 erector spinae ; r - es : right side erector spinae ; l - mf : left side multifidus ; r - mf : 
 right side multifidus ) . however , the other muscle sites did not show any significant changes after 
 training . the wilcoxon rank test was used to compare the pre - and post - training values of each 
 group , and * significant differences ( p<0.05 ) . ( units :  hz / s ) l - es : left side 
 erector spinae ; r - es : right side erector spinae ; l - mf : left side multifidus ; r - mf : 
 right side multifidus that study recruited 12 lbp baseball players , and 
 carried out three stages of exercise training . in the first stage , the second stage was the trunk 
 muscular strengthening course , and the third stage was stabilization exercise training on 
 slings . pain intensity was significantly reduced after s.e.t , but it was not apparent at 
 trunk muscular strengthening stage28 . that is , the s.e.t was more effective at reducing pain than the trunk muscular strengthening 
 course for lbp patients . chang et al . also investigated the effects of six weeks of s.e.t on 
 lbp patients29 . they found that the 
 disability level , functional fitness and trunk proprioception improved after six weeks of 
 training . the studies 
 of chang and chu et al . both demonstrate that six weeks of s.e.t is effective at reducing 
 the pain intensity and disability levels of lbp patients . however , these two studies did not 
 measure the pain intensity and disability level simultaneously28 , 29 , and they did 
 not utilize a control group . in the present study , there was a significant improvement in muscular strength and 
 endurance after six weeks training , a result which is similar to those of previous studies . bronfort et al . compared muscular strength among three different types of exercise mode . they found that trunk strengthening exercise combined with spinal manipulation and trunk 
 strengthening exercise combined with non - steroid anti - inflammatory drugs elicited greater 
 improvements than spinal manipulation combined with the trunk stretching exercise32 . they also found a significant improvement 
 in trunk muscular strength at the 11th week . according to a previous study18 , a significant increase in the activation 
 of the rectus abdominis was elicited by exercising on a swiss ball indicating that exercise 
 on an unstable surface had increased the difficulty of exercise and the muscular activation . chok et al . measured the maintenance time of trunk extensor endurance as the training 
 outcome . in their study , sub - acute lbp patients were randomly assigned to a control group or 
 a experiment group . the experimental group attended exercise training for six weeks , but the 
 control group did not . after the 
 exercise training , the maintenance time of trunk extensor endurance was not a significantly 
 different between the two groups , despite the experimental group having better maintenance 
 time of trunk extensor endurance than the control group at weeks 3 and 633 . in contrast , bronfort et al . found that 
 subjects in the trunk muscular strength exercise groups showed greater improvements in back 
 extensor endurance than the other two groups at week 11 , but not at week 532 . the slope of the median frequency indicates neuromuscular fatigue and a positive slope 
 indicates the absence of neuromuscular fatigue ; a negative slope indicates the presence of 
 neuromuscular fatigue , and a more negative slope indicates a higher level of neuromuscular 
 fatigue . the present study found that the slope of the median frequency of the multifidus 
 muscles was a more negative value at baseline than after training . this result means the 
 multifidus muscles had a lower level of neuromuscular fatigue in the endurance test after 
 training . in the study of oddsson et al.16 , the lbp group carried out trunk extensor endurance tests at 40% 
 and 80% mvc in a standing position . their results of that study show the slope of the median 
 frequency at 40% mvc ranged from 0.03 to 0.26 ( hz / s ) , and the median value was 0.035 
 ( hz / s ) , indicating neuromuscular fatigue did not accumulate . in that study , subjects 
 performed 40% mvc , as in our study ( about 52% mvc ) , but the testing position in that study 
 was standing , which differed from our study . to the best of our knowledge , most studies 
 which have measured trunk extensor endurance as an exercise outcome have used the 
 maintenance time rather than the slope of the median frequency . few studies have used the 
 slope of median frequency to compare the difference between healthy subjects and the lbp 
 subjects . our present study , used the slope of the median frequency , and found there was a 
 significantly more negative slope for the left multifidus at baseline in pre - training for 
 the training group , indicating a higher level of neuromuscular fatigue at the pre - training 
 measurement than at the post - training measurement . the training group had a border - line 
 significantly higher level of neuromuscular fatigue at the pre training measurement for the 
 right multifidus relative to the post - training measurement . twelve patients participated in this 
 study , and there were randomly allocated to the training group and the control group , with 
 seven subjects in the training group and five subjects in the control group . a trail with 
 80% power and a level of significance of 5% ( two tailed ) was calculated to require 
 approximately 20 patients in the training group to detect significant differences between 
 pre- and post - training . another limitation of this study was that the training lasted for 
 only 6 weeks . this may not have been long enough time to improve local muscular endurance . 
 in our results , the slope of the median frequency showed a significant improvement for the 
 left side multifidus but not for the other back muscles . prolonged exercise training may 
 lead to greater improvements in muscular endurance of the other back muscles . the exercise program 
 should be prolonged for chronic lbp patients to try to achieve greater improvement . the 
 slope of the median frequency for objectively measures neuromuscular fatigue and should be 
 considered as a parameter for assessing the training effects .