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Paper text:
many workers who perform 
 mmh face a high risk of work - related low back pain and impairment1 .
ergonomic approaches for reducing the incidence of 
 mmh - related injuries have involved engineering design / redesign to fit the task to worker 
 capabilities2 .
lower lifting origin heights increase 
 trunk flexion , and asymmetric starting and ending points increase twist and lateral flexion 
 motions8,9,10 . to address trunk function
, biomechanical studies generally measure trunk muscle activity 
 using surface electromyography ( emg ) .
mmh studies typically focus on measurements of the 
 lumbar erector spinae extensor muscle , because trunk extensor tasks are a serious risk for 
 low back pain13 .
several previous studies 
 reported that trunk extensor muscle activity correlated linearly with trunk extension 
 strength4 , 14 , 15 .
these studies suggested 
 that increased trunk bending posture required increasing extensor activity of the trunk 
 muscles .
others found that nonsagittal trunk moments can influence spine loading8 , 16,17,18 ; 
 the activity of trunk muscles increased compared to a sagittal symmetric origin
. therefore , 
 trunk function research has reported on the impact of the load destination position and 
 asymmetric posture when a worker lifts a load . for a lowering task ,
one study investigated 
 trunk muscle emg activation in subjects with and without chronic low back pain19 .
the objectives of this study were to investigate the 
 differences in trunk muscle activity in lifting and lowering tasks at various heights .
the experiment was conducted on 30 current students ( 13 males , 17 females ; mean age 20.40  
 0.65 ) at s university ( republic of korea ) , who had no medical history of injury , 
 musculoskeletal disorders , cardiovascular , or neurological conditions .
their average heights 
 and weights were 168.40  6.93  cm and 59.70  7.07  kg , respectively .
all subjects were fully 
 informed about the procedures and aims of the current experiment , and the study protocol was 
 approved by our institutional review board . to measure muscle activity resulting from repetitive lifting and lowering tasks
, this study 
 used an wireless zerowire system ( aurion , milan , italy ) .
analog signals collected from 6 
 wireless channels of a surface emg were converted into digital signals , and data were 
 analyzed using myoresearch xp master 1.06 ( noraxon system inc . , scottsdale , az , usa ) with a 
 personal computer .
the raw emg signals were amplified with a gain of 1,000 and frequency 
 bandwidth of 20450  hz .
muscle activity was assessed by attaching electrodes ( type sx230 
 data log ii , biometrics ltd . ,
gwent , uk ) to both sides of the abdominal external oblique 
 muscle ( eo ) , rectus abdominis muscle ( ra ) , and lumbar erector spinae muscles ( es ) .
the 
 electrode placement for es was 30  mm lateral to the l3 spinous process .
the electrodes for 
 the eo were placed on the mid - axial line between the 10th rib and the anterior superior 
 iliac spine , and for the ra , over the muscle belly , about 30  mm lateral to the abdominal 
 midline and above the umbilicus .
the design of this experiment was cross - sectional and a one - group repeated test .
the weight 
 of the load ( 60  40  20  cm ) was fixed at 10  kg and 6  kg for males and females , 
 respectively .
six asymmetrical ( 90 counter - clockwise , sagittal plane ) lifting and lowering 
 tasks were performed 3 times at 3 heights .
the ranges of lifting tasks were from ankle to 
 knee height ( ak ) , from ankle to shoulder height ( as ) , and from knee to shoulder height ( ks ) . 

the lowering tasks were from knee to ankle height ( ka ) , from shoulder to ankle height ( sa ) , 
 and from shoulder to knee height ( sk ) .
the subjects 
 performed both lifting and lowering tasks at their self - selected speeds . before execution of 
 the tasks ,
the subject was introduced to the general technique for squat lifting ( straight 
 back , bent knees ) . for statistical comparisons , muscle activities were averaged over the 3 trials .
one - way 
 analysis of variance ( anova ) was used to determine the main effects of the heights on each 
 muscle and task .
least significant difference ( lsd ) was used for post hoc comparisons of 
 muscle activity at various heights . an independent t - test
statistical 
 analyses were performed using spss software ( ver . 12.0 for windows ; spss inc . , chicago , il , 
 usa ) , and statistical significance was set at p < 0.05 .
the mean and standard deviations of muscle activities for heights and tasks are presented 
 in table 1table 1.comparison of muscle activities ( mean  sd ,  mv ) according to the heights and 
 taskslt .
ak : ankle to knee height , as : ankle to shoulder height , 
 ks : knee to shoulder height , ka : knee to ankle height , sa : shoulder to ankle height , 
 sk : shoulder to knee height .
* comparison between lifting and lowering task ( p < 
 0.05 ) .. for bilateral es , there was a significant main effect of height on lifting 
 task ( p < 0.05 ) . according to the post hoc analysis ,
the muscle activity of the bilateral 
 es was significantly lower in ks ( left : 26.86  10.70 ; right : 23.06  9.10 ) , versus ak 
 ( left : 34.82  16.15 ; right : 33.07  12.61 ) and as ( left : 37.44  17.91 ; right : 35.03  
 15.37 ) in the lifting task ( p < 0.05 ) .
however , there was no significant difference in eo 
 or ra ( p > 0.05 ) in the lifting task . for the lowering task ,
the muscle activity was 
 statistically significantly different in the left es ( ka : 32.14  15.26 ; sa : 33.69  14.89 ; 
 sk : 25.28  10.59 ) , right es ( ka : 29.35  12.40 ; sa : 30.36  12.04 ;
sk : 21.07  7.89 ) , and 
 left eo ( ka : 9.72  3.22 ; sa : 12.86  5.57 ; sk : 11.95  4.39 ) ( p < 0.05 ) .
the post hoc 
 analysis showed that the emg activity of bilateral es was significantly lower with sk than 
 with ka or sa ( p < 0.05 ) .
ak : ankle to knee height , as : ankle to shoulder height , 
 ks : knee to shoulder height , ka : knee to ankle height , sa : shoulder to ankle height , 
 sk : shoulder to knee height .
there was no significant difference in the right eo , bilateral es , or ra between the tasks 
 ( p > 0.05 ) .
however , the left eo activity in the ak ( lifting task ) was significantly 
 higher compared with that of the ka ( lowering task ) ( p < 0.05 ) .
this study showed that the muscle activity of es in the trunk range ( ks and sk ) was lower 
 than that of the other ranges for lifting and lowering tasks . on the other hand , eo activity 
 was higher in the lifting task compared with that in the lowering task in the lower range 
 ( ak vs. ka ) .
several biomechanical studies have reported that large moments were created in 
 the trunk area during lifting tasks . in particular , the lumbar area between l5 and s1 had 
 the potential to incur the greatest moment in lifting20 , 21 .
the muscle fibers 
 sampled by the emg electrode are assumed to be a reasonable approximation for the activity 
 of the entire muscle body .
emg and muscle force are considered to be related linearly22 . in this study , the task height increased 
 the muscle activity of es not only in the lower range but also in the higher range , as 
 compared with the trunk area .
reported that trunk extensor muscle activity 
 increased ~25% for a maximal extension task performed at more flexed postures ( 40 ) in 
 comparison with upright postures14 .
thus , 
 the decreased muscle activity of the es indicates that the trunk area is the most effective 
 territory for lifting and lowering , with similar trends for lifting tasks reported 
 previously4 . in the current study
, there 
 was no significant difference in muscle activities of the eo or ra for the various heights . 

the es is assisted by additional musculoskeletal mechanisms to provide sufficient extensor 
 moment when lifting heavy weights23 . 

intra - abdominal pressure and partitioned muscles are included in the mechanisms for 
 additional moments from other structures .
thus , differential recruitment of the eo and ra muscles might have been unnecessary for the 
 various heights with the weight used .
. however , there have been few studies 
 examining the psychophysical data and movement velocity of differences between lifting and 
 lowering3 , 24 , 25 .
lee investigated the 
 minimal acceptable handling time interval in which subjects could perform lifting and 
 lowering task at various weights ( 10 , 15 , 20  kg ) without becoming strained , uncomfortable , 
 tired , weakened , overheated , or out - of - breath , during 4-hour endurance handling period using 
 a psychophysical methodology24 .
the 
 minimum handling time interval for each lifting task was significantly higher than that of 
 its corresponding lowering task .
on the other hand , heavier loads produced larger minimum 
 acceptable handling time intervals .
potvin demonstrated that decreased maximum loads were 
 acceptable during lifting tasks compared with lowering tasks3 .
in contrast , the present study demonstrated that the muscle 
 activities of the es , ra , and right eo were not significantly different in lifting and 
 lowering . in physics
, power is the rate of doing work , equivalent to an amount of energy 
 consumed per unit time . in this study ,
the subjects performed both lifting and lowering 
 tasks at their self - selected speeds .
thus , it is 
 assumed that the workers thought the lifting task was harder for the lighter weight because 
 of movement velocity . in this study ,
the left eo activity was lower in the lowering task 
 compared with that in the lifting task at the lower range ( ak vs. ka ) .
the left eo 
 decelerated trunk rotation as an antagonist muscle during the counter - clockwise task .
the 
 function of the antagonist muscle was assumed to be relatively unnecessary during the ka 
 lowering task because this position appeared to show soft tissue pressure .
this is 
 apparently the first report to demonstrate differences in trunk muscle activity in lifting 
 and lowering tasks at various heights . in conclusion ,
the results show that the optimal range for the task was at trunk height , not only for 
 lifting but also for lowering tasks .
additionally , there was no significant difference in es 
 between the lifting and lowering tasks , which could be applied to job design in actual 
 workplaces .
however , this study was limited by the use of a relatively light weight for 
 safety reasons and by failure to measure the velocities of the tasks .
[ purpose ] biomechanical data for manual material handling are important for appropriate 
 engineering design . 
 the goal of this study was to investigate differences in trunk muscle 
 activity in lifting and lowering tasks at various heights . 
 [ subjects and methods ] thirty 
 healthy , young adult subjects performed 6 asymmetrical lifting and lowering tasks at 
 various heights . 
 trunk muscle activity of the abdominal external oblique muscle ( eo ) , 
 rectus abdominis muscle ( ra ) , and lumbar erector spinae muscles ( es ) were recorded using 
 surface electromyography ( emg ) . 
 [ results ] the emg activities of the bilateral es differed 
 significantly among heights 
 . the left eo activity in the ankle to knee lifting task was 
 significantly increased compared with that of the knee to ankle lowering task . 
 however , 
 there were no significant differences in the right eo , bilateral es , or ra between lifting 
 and lowering tasks . 
 [ conclusion ] the results show that the optimal range for manual 
 material handling was at trunk height , not only for lifting but also for lowering 
 tasks .