recent work has demonstrated 
 the importance of the abdominal muscles in ensuring sufficient spine stability to prevent 
 buckling and enhance function2 , 3 .
the curl - up , performed as an abdominal exercise , has been 
 shown to produce reasonable levels of activity in the rectus abdominis ( ra ) muscle while 
 minimizing the resultant spine load and has been incorporated into several low back fitness 
 programs4 .
the use of unstable surfaces underneath the subject for stability training of the injured 
 low back is becoming more popular4 .
swiss 
 balls have been incorporated into strength - training regimens and can reportedly be used to 
 more effectively train the musculoskeletal system . performing strength exercises on swiss 
 balls has been advocated based on the belief that a labile surface will provide a greater 
 challenge to the trunk musculature , increase the dynamic balance of the user , and possibly 
 train users to stabilize their spines to prevent and treat injury5 .
studies have been conducted to document the loads imposed 
 on the spine during various abdominal exercises6 , but the effect of unstable surfaces has not been examined .
we 
 believe that there is a clinical need to understand the effects of using unstable surfaces 
 to challenge the muscular system during the curl - up exercise .
recent evidence regarding the conservative management of low back pain suggests that the 
 restoration of neuromuscular control in the transverse abdominis ( tra ) , together with 
 minimal contraction of other superficial oblique , internal , and external abdominal muscles , 
 is essential for effective treatment during the early stages of rehabilitation7 , 8 . 

previous studies have demonstrated that performance of the abdominal hollowing exercise in 
 particular is far more effective than performance of general core - stabilizing techniques in 
 improving the cross - sectional area of the tra9 , 10 .
the abdominal hollowing exercise is 
 designed to emphasize deep local muscle activity while minimizing the activity of the more 
 superficial global muscles .
although many studies on the effect of the abdominal hollowing 
 exercise in rehabilitation programs have been performed , none have examined the combined 
 effect of the abdominal hollowing exercise and curl - up exercise on an unstable surface .
therefore , the purpose of this study was to investigate the effects of the abdominal 
 hollowing exercise on trunk muscle activity during the curl - up exercise on an unstable 
 surface by measuring electromyography ( emg ) activity .
fourteen healthy subjects ( nine males , five females ) volunteered to participate in this 
 study ( table 1table 1.summary of anthropometric characteristics and neck disability index of the study 
 participantscharacteristiccontrol
group(n = 13)mnp group(n = 14)gender ( n , male)66age ( years ) , mean ( sd)20.6  1.620.6  1.5height ( cm ) , mean ( sd)167 .  6.9168.0  8.0weight ( kg ) , mean ( sd)60.7  10.761.0  12.4neck disability index ( ndi ) ( % ) , mean ( sd)3.3  2.616.9  7.1*mnp : mild neck pain ; sd : standard deviation .
all subjects were in good health and reported no history of neurological 
 and/or respiratory diseases .
none of them had undergone institutional care for any 
 pathological back conditions during the preceding year .
* significant difference between the two 
 groups ( p < 0.05 ) emg data were collected using a biopac mp150wsw data acquisition system ( biopac systems , 
 inc . ,
surface electrode pairs were oriented along the line of action of the 
 underlying muscle fibers on the right side : ra , centered 2  cm lateral and caudal to the 
 umbilicus ; tra , centered 2  cm cephalad to the pubic bone , just lateral to the midline , and 
 parallel to the superior pubic ramus ; external oblique ( eo ) , centered over the tip of the 
 eighth rib diagonally ; and internal oblique ( io ) , centered 2  cm proximal to the midpoint 
 from the anterior superior iliac spine ( asis ) to the symphysis pubis diagonally11 .
the maximum voluntary isometric contraction ( mvic ) of each muscle was 
 measured using the maneuver suggested by kendall et al.12 for normalization .
the subjects were instructed on how to perform the curl - up and abdominal hollowing 
 exercises by the investigator .
all subjects were instructed to place their hands on their 
 abdomen for tactile feedback during both exercises . for the abdominal hollowing ,
the 
 subjects were instructed to draw the lower part of the abdomen up and in toward the spine 
 without moving the trunk or pelvis . a pressure biofeedback unit ( chattanooga group , inc .
, 
 tn , usa ) was used to ensure that the subjects were performing the exercises correctly .
a 
 pressure cuff unit was placed under their lumbar spine and inflated to 40 mmhg before the 
 exercises were performed13 .
when the 
 subject performed the hollowing correctly , the pressure either stayed at 40 mmhg or 
 decreased , and the subjects felt their abdomen hollow .
the subjects held the trunk and 
 pelvis in that position for 5  s while continuing to breathe normally1 .
the first task 
 was to perform a traditional curl - up exercise on a flat floor with the hips and knees flexed 
 to 90 and legs supported on a platform .
the subject s hands were placed by the sides of 
 their body , and only the head and shoulders were elevated from the flat floor .
the next two 
 tasks varied based on the type of unstable surface . for the second task
, the subjects 
 assumed the supine position on a flat floor with the hips and knees flexed to 90 and legs 
 supported on a gym ball .
ball inflation was checked between subjects to ensure that the 
 diameter remained at 70  cm prior to each test . for the third task
, the subjects assumed the 
 supine position on a flat floor with the hips and knees flexed to 90 and legs supported on 
 a gym ball . only the head and shoulders
were then elevated from the flat floor with 
 performance of the abdominal hollowing exercise during the curl - up exercise .
the subjects 
 held each contraction for 5  s and then returned to the resting position .
each subject had 
 1  min of rest between each trial and 5  min of rest between the tasks .
repeated - measures anova with post hoc bonferroni 
 correction was used to determine the trunk muscle activation during the three exercises ( p 
 < 0.05 ) .
the mean emg amplitudes of the abdominal muscles during the three curl - up exercises are 
 presented in table 2table 2.mean relative muscle activity ( % mvic ) and standard deviation of the various 
 abdominal muscles during three different curl - up exercisesmusclesstable surfaceunstable surfaceunstable surfacewith abdominalhollowing exerciserectus abdominis*41.6  14.649.3  15.026.2  10.5external oblique43.6  18.648.2  12.849.8  19.8internal oblique*42.1  18.047.2  16.662.6  21.2transverse abdominis*19.2  7.438.4  13.556.5  20.4*p < 0.05 .
the emg activities of the ra and eo trunk muscles during the curl - up exercise 
 were significantly higher on an unstable surface than on a stable surface .
the emg activity 
 of the ra was lower when combined with the abdominal hollowing exercise on an unstable 
 surface than that when performing the curl - up exercise on both a stable and unstable 
 surface .
the emg activities of the tra and io were higher when combined with the abdominal 
 hollowing exercise on an unstable surface than those during the curl - up exercise on both a 
 stable and unstable surface .
the present study showed that the combination of the abdominal hollowing exercise and 
 curl - up exercise on an unstable surface was associated with higher emg activities of the tra 
 and io and lower emg activity of the ra than during the curl - up exercise on both stable and 
 unstable surfaces .
the emg activities of the ra and eo were significantly higher on an 
 unstable surface than on a stable surface during the curl - up exercise .
several studies have suggested that the swiss ball may be helpful in increasing trunk 
 muscle activity for stabilization of the lumbar spine during rehabilitation exercises5 . during the curl - up exercise on the swiss 
 ball ,
the ra and eo activity may undergo more physical demand , as evidenced by the fact that 
 the swiss ball created more perturbations in our study .
contraction of superficial trunk 
 muscles such as the ra and eo without local muscle contraction may lead to compressive 
 loading and shearing forces during the curl - up exercise , which may induce stress on the 
 inner tissues during the entire range of motion and serve as a major source of spinal pain . 

although the curl - up exercise on an unstable surface induced higher emg activities of the ra 
 and eo , a reverse effect may be seen in individuals with spinal instability .
thus , 
 clinicians who apply an unstable surface to increase the trunk muscle activity must consider 
 the provocation that patients endure . in this study ,
the emg activities of the tra and io were higher during the combination of 
 the abdominal hollowing exercise and curl - up exercise on an unstable surface .
conversely , 
 the emg activity of the ra decreased during the combination of the abdominal hollowing 
 exercise and curl - up exercise on an unstable surface .
the abdominal hollowing exercise was 
 developed for neuromuscular retraining and kinesthetic awareness of the tra14 .
our results showed that abdominal 
 hollowing exercise resulted in higher emg activity of local muscles than of global muscles . 

the local muscle system involves the tra and io , which provide dynamic stability to each 
 spinal segment15 .
the findings of the 
 present study suggest that the abdominal hollowing exercise as a therapeutic option may be 
 used to preferentially load certain local muscle groups during the curl - up exercise on an 
 unstable surface , and that these effects may be favorably produced in individuals with low 
 back pain .
first , our results can not be generalized to 
 other populations because all the subjects who participated in the study were healthy young 
 individuals .
second , surface emg was used to monitor muscle activity , leaving the 
 possibility of crosstalk from adjacent muscles .
this study provides empirical evidence that the curl - up exercise in combination with the 
 abdominal hollowing exercise is more useful for enhancing abdominal muscle activity than is 
 the curl - up exercise on an unstable surface .
this finding offers clinical insight into the 
 additive effect of the abdominal hollowing exercise in selectively stimulating local muscles 
 and suggests that it may be used as an alternative core stabilization technique for the 
 management of patients with low back pain .
these findings should be considered when 
 selecting rehabilitation exercises for neuromuscular retraining of the abdominal 
 muscles .
 [ purpose ] the purpose of this study was to investigate the effects of the abdominal 
 hollowing exercise on trunk muscle activity during the curl - up exercise on an unstable 
 surface by measuring electromyography ( emg ) activity . 
 [ subjects ] fourteen young healthy 
 adults ( nine male , five female ) voluntarily participated in this study . [ methods ] each 
 subject was asked to perform a curl - up exercise on two supporting surfaces ( stable and 
 unstable surfaces ) combined with the abdominal hollowing exercise on an unstable surface . 
 
 the muscle activities of the rectus abdominis ( ra ) , external oblique ( eo ) , internal 
 oblique ( io ) , and transverse abdominis ( tra ) were measured using surface emg during 
 performance of the curl - up exercise . 
 [ results ] the emg activity of the ra and eo was 
 significantly higher on an unstable surface than on a stable surface during the curl - up 
 exercise . 
 the emg activities of the tra and io were greater in combination with the 
 abdominal hollowing exercise on an unstable surface than during the curl - up exercise on 
 both a stable and unstable surface . 
 [ conclusion ] these findings suggest that the local 
 trunk muscle activity during the curl - up exercise is more strongly affected by combination 
 with the abdominal hollowing exercise than by performance on an unstable supporting 
 surface . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION