in recent years , core training has been widely studied since it has been considered a pivotal issue in health , rehabilitation and sports performance ( hibbs et al . , 2008 ) . however , the definition of the core varies with the interpretation of the literature ( hibbs and thompson , 2008 ) . anatomically , the core region has been described as the area bounded by the abdominal muscles in the front , by paraspinal and gluteal muscles in the back , by diaphragm on the top and by pelvic floor and girdle musculature at the bottom ( richardson et al . , 1999 ) . the core represents the connection between lower and upper limbs and should be considered as a functional unit in which different muscles interact , even if not located in the thoraco - lumbar region ( such as shoulders and pelvic muscles ) . however , literature concerning core training sometimes fails to distinguish between concepts of core stability and core strength . faries and greenwood ( in hibbs and thompson , 2008 ) formulated the following clear definitions : core stability refers to the ability to stabilize the spine as a result of muscle activity , while core strength refers to the ability of muscles contractions to produce and transfer force as a result of muscle activity . since strength and motor control are complementary qualities , the core training programmes can target mainly , but not exclusively , at muscle strengthening and/or motor control of core musculature . motor control training seems to require low intensity stabilization exercises focused on efficient integration of low threshold recruitment of local and global muscle systems . conversely , core strength training seems to require high intensity and overload training of the global muscle system . vezina and hubley - kozey ( 2000 ) suggested that core stability programmes should include muscle activation below 25% of maximum voluntary contraction ( mvc ) , while core strength training should include activation higher than 60% of mvc to result in strength benefits . the available evidence suggests that to adequately train the core muscles in athletes , strength and conditioning specialists should focus on implementing multi - joint full body exercises , rather than core - specific exercises ( martuscello et al . , 2013 ) . exercises involving the full body linkage such as plank exercises , have been advocated to the capacity of transmitting force through the body linkage ( schoenfeld et al . , 2014 ) . training with labile systems has been documented to offer unique opportunities for linkage training challenges ( mcgill et al . , 2015 ) . several studies examined core muscle activation during the execution of various exercises on stable and unstable surfaces ( for a review see : behm et al . , 2010 ) . the use of unstable surfaces contacting the subject s feet or hands is becoming popular in strength training . instability can be obtained through the use of many devices and techniques including , but not limited to , unstable platforms such as bosu or swiss balls . more recently , suspension training systems have been added to the list of instability training devices . in suspension training , lower or upper limbs many core directed exercises are designed with such a device , creating a wide variety of challenges . these exercises consist of multi - planar and multijoint movements , and are executed with complex techniques . it is important to quantify the muscle contraction intensity since it is a key factor in establishing training effects induced by this sort of exercises . although considerable research has examined more traditional means of instability training ( behm and drinkwater , 2010 ) , little previous research has evaluated the effects of suspension training on muscle activation . in particular , some studies focused on core - directed exercises ( atkins , 2014 ; byrne et al . , 2014 ; czaprowski et al . , 2014 ; mok et al . , 2014 ; snarr and esco , 2014 ) , whereas others investigated the effect of the application of suspension system on core muscle activity in push exercises ( calatayud et al . , 2014 further investigation of these exercise approaches is needed to understand their influence on muscle activation and joint load levels . the primary purpose of this study therefore , was to examine differences in core muscle activation across four full - body linkage exercises using a suspension training system . these exercises were chosen from a spectrum of whole body linkage exercises focused on the anterior core musculature executed in instable conditions , including a roll - out , bodysaw , pike , and knee - up . although the selected exercises were mainly focused on anterior slings , we wanted to provide a comprehensive view of core muscle activation by monitoring rectus abdominis , internal and external oblique , and paraspinal muscles . it was hypothesized that significant differences would be found in core muscles among exercises . the second aim of the study was to determine which of these exercises would reach the threshold of 60% of mvc , expected to be high enough to increase muscle strength . it was hypothesized that the four exercises would elicit muscle activity in excess of 60% of mvc in the rectus abdominis , i.e. the muscle on which the main focus was put considering the selected exercises . seventeen healthy participants were recruited ( age 27.32.4 years , body height 1725 cm , body mass 69.29.3 kg ) . all participants were physically active , declaring three practice sessions per week of resistance training . inclusion criteria for study participation were as follows : no past or present neurological or musculoskeletal trunk or limb pathology , no cardiorespiratory disease , no history of abdominal , shoulder or back surgery , and no psychological problems . participants were instructed to refrain from performing strenuous physical activity in the 24 hours preceding all experimental sessions . the study was previously approved by the research ethics committee of the department of medical sciences , university of turin . the surface electromyographic ( emg ) signals were obtained from six trunk muscles with concentric bipolar electrodes ( code , spes medica , battipaglia , italy ) . before the placement of the electrodes , the skin was slightly abraded with adhesive paste and cleaned with water in accordance to seniam recommendation for skin preparation ( hermens et al . , 2000 ) . the electrodes were placed according to the instructions described in previous methodological works ( beretta piccoli et al . , 2014 ; boccia and rainoldi , 2014 )  lower rectus abdominis : on the lower part of the rectus abdominis , 3 cm lateral to the midline ; upper rectus abdominis : on the upper part of the rectus abdominis , 3 cm lateral to the midline ; external oblique : 14 cm lateral to the umbilicus , above the anterior superior iliac spine ( asis ) ; internal oblique : 2 cm lower with respect to the most prominent point of the asis , just medial and superior to the inguinal ligament ; lower erector spinae : 2 cm lateral to the l5-s1 ; upper erector spinae : 6 cm lateral to the l1-l2 . the electrodes were placed only on the left ( randomly chosen ) side of the body ; the reference electrode was positioned on the wrist . the signal of a biaxial electrogoniometer ( sg 150 , biometrics ltd , gwent , uk ) positioned at the level of the shoulders ( for the roll - out and bodysaw ) or the hips ( for the pike and knee - tuck ) , depending on which joint was more involved during the exercise , was used as a trigger to highlight exercise repetitions . the emg signals were synchronized with the electrogoniometer signal , amplified ( emg - usb , ot bioelettronica , torino , italy ) , sampled at 2048 hz , bandpass filtered ( 3-db bandwidth , 10 - 450 hz , 12 db / oct slope on each side ) , and converted to digital data by a 12-bit a / d converter . samples were visualized during acquisition and then stored in a personal computer using ot biolab software ( version 1.8 , ot bioelettronica , torino , italy ) for further analysis . the participants recruited were instructed with regard to the correct technique of suspension exercise and the mvc procedure during the first experimental session conducted one week before the measurement session . the participants were asked to refrain from physical activity 24 hours before the measurements . during the measurement session , participants performed 4 exercises with the use of suspension straps ( trx suspension trainer ; fitness anywhere lcc , san francisco , ca , usa ) in random order . the exercises were selected based on a previous study ( behm and drinkwater , 2010 ) that indicated them as important in developing core strength . at the beginning of the measurement session , three mvc exercises were performed twice for 5 s , with 2 min rest between them . the following standardized exercises ( ng et al . , 2002 ) were used to activate maximally the trunk muscles ( figure 1 ) : upper rectus abdominis ( ura ) and lower rectus abdominis ( lra ) : body supine with hips and knees flexed 90 , with feet locked . participants flexed the trunk ( i.e. crunch execution ) against resistance at the level of the shoulders;external oblique ( eo ) and internal oblique ( io ) : side - lying with the hip at the edge of the bench and feet locked by a second operator . participants performed side - bend exercise against resistance at the level of the shoulder;lower erector spinae ( les ) and upper erector spinae ( ues ) : prone position with asis at the edge of the bench and feet locked by a second operator . upper rectus abdominis ( ura ) and lower rectus abdominis ( lra ) : body supine with hips and knees flexed 90 , with feet locked . participants flexed the trunk ( i.e. crunch execution ) against resistance at the level of the shoulders ; external oblique ( eo ) and internal oblique ( io ) : side - lying with the hip at the edge of the bench and feet locked by a second operator . participants performed side - bend exercise against resistance at the level of the shoulder ; lower erector spinae ( les ) and upper erector spinae ( ues ) : prone position with asis at the edge of the bench and feet locked by a second operator . standardized exercises used to maximally activate trunk muscles : lower rectus abdominis and upper rectus abdominis ( left ) ; internal oblique and external oblique ( middle ) ; lower erector spinae and upper erector spinae ( right ) . participants were required to achieve a range of motion with the correct technique execution and to maintain a neutral position of the spine and pelvis in each exercise . a certified strength and conditioning coach monitored the exercise performance to ensure that the exercise was properly executed considering its technique . each exercise was repeated three times and lasted 6 s. a metronome set at 30 beats per minute was used to ensure proper timing ( with 4 beats for each repetition ) : 2 s from the initial position to the final position ( concentric phase ) ; 2 s of maintenance ( isometric phase ) ; and 2 s returning to the starting position ( eccentric phase ) . the exercises were performed with 3 min of rest in - between to allow complete recovery . the random order of the exercises allowed to mitigate the effects of cumulative fatigue on emg estimates . the following exercises were used ( figure 2 ) : roll - out : participants assumed an inclined standing position while placing each hand on the strap handles , with elbows and wrists placed below the shoulders , arms perpendicular to the floor and shoulders flexed approximately 45 ; they then performed a shoulder flexion moving the hands forward;bodysaw : participants assumed a prone position , they placed elbows below the shoulders , both forearms touching the floor , while placing each foot on the strap handle ; participants then flexed the shoulders and extended the elbows pushing the body backwards;pike : participants assumed a push - up position with the feet in strap handles , then they flexed hips to approximately 90 , while keeping the knees fully extended;knee - tuck : participants assumed a push - up position while placing each foot in the strap handle , then they flexed both hips and knees to approximately 90 , bringing the knees forward . roll - out : participants assumed an inclined standing position while placing each hand on the strap handles , with elbows and wrists placed below the shoulders , arms perpendicular to the floor and shoulders flexed approximately 45 ; they then performed a shoulder flexion moving the hands forward ; bodysaw : participants assumed a prone position , they placed elbows below the shoulders , both forearms touching the floor , while placing each foot on the strap handle ; participants then flexed the shoulders and extended the elbows pushing the body backwards ; pike : participants assumed a push - up position with the feet in strap handles , then they flexed hips to approximately 90 , while keeping the knees fully extended ; knee - tuck : participants assumed a push - up position while placing each foot in the strap handle , then they flexed both hips and knees to approximately 90 , bringing the knees forward . initial and final positions of each exercise : 1 ) roll - out ; 2 ) bodysaw ; 3 ) pike ; 4 ) knee - tuck . the average rectified value ( arv ) of emg signals was computed off - line with numerical algorithms using non - overlapping signal epochs of 0.5 s ( hibbs et al . , 2011 ) . the mean value of arv over the two repetitions was calculated for each muscle and normalized with respect to the maximum arv obtained during the correspondent mvc . the normality assumption of the data was evaluated with the shapiro - wilk test ; homoscedasticity and autocorrelation of the variables were assessed using the breusch - pagan and durbin - watson tests . the differences between exercises ( pike  bodysaw  knee - tuck  roll - out ) and between muscles ( lra  ura  eo  io  les  ues ) were compared with the 2-way analysis of variance ( anova ) . for the purpose of this report statistical analyses were conducted using the r statistical package ( version 3.0.3 , r core team , foundation for statistical computing , vienna , austria ) . all participants managed to complete each exercise trial and thus , were included in the data analysis . figure 3 shows the box plots of the activation values ( % of mvc ) of each muscle during the four exercises . muscle activation ( median , ir ) figure 3each box plot shows the muscle activation ( as percentage of maximum voluntary contraction ) during exercise . table 1muscle activation ( median , ir ) expressed as percentage values of electromyographic amplitude normalized to maximum voluntary contraction . results of the two - way anova after tukey multiple comparisons are reported as symbols ; p < 0.01.lower rectus abdominisupper rectus abdominisexternal obliqueinternal obliquelower erector spinaeupper erector spinaepike57 ( 36 ) 41 ( 48 ) 55 ( 21)23 ( 20)12 ( 7)9 ( 4)bodysaw100 ( 42 )  
  
 57 ( 52)59 ( 33)32 ( 20)4 ( 3)8 ( 6)knee - tuck54 ( 50 )  
  44 ( 41 ) 42 ( 7 ) 18 ( 26)8 ( 5)6 ( 5)roll - out140 ( 89 ) 
  67 ( 78 )  
 71 ( 44 ) 40 ( 31)9 ( 5)11 ( 6)indicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the pikeindicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the bodysawindicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the knee - tuckindicates statistically significant difference betweenthe indicated exercise ( explained in row ) with respect to the roll - out each box plot shows the muscle activation ( as percentage of maximum voluntary contraction ) during exercise . muscle activation ( median , ir ) expressed as percentage values of electromyographic amplitude normalized to maximum voluntary contraction . results of the two - way anova after tukey multiple comparisons are reported as symbols ; p < 0.01 . indicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the pike indicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the bodysaw indicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the knee - tuck indicates statistically significant difference between the indicated exercise ( explained in row ) with respect to the roll - out the normalized 
 lra activity was 140% ( ir , 89% ) of mvc during the roll - out , 100% ( ir , 
 42% ) of mvc during the bodysaw , 57% ( ir , 36% ) of mvc during the pike 
 and 54% ( ir , 50% ) of mvc during the knee - tuck . the normalized lra 
 values were significantly higher ( p < 0.01 ) 
 during the roll - out and bodysaw compared to the pike and knee - tuck . the roll - out exercise showed significantly greater activation 
 ( p < 0.01 ) than the bodysaw . the 
 normalized ura activity was 67% ( ir , 78% ) of mvc during the roll - out , 
 57% ( ir , 52% ) of mvc during the bodysaw , 41% ( ir , 48% ) of mvc during 
 the pike and 44% ( ir , 41% ) of mvc during the knee - tuck . the 
 normalized ura values were significantly higher ( p 
 < 0.01 ) during the roll - out compared to the pike and 
 knee - tuck . the normalized eo activity was 71% ( ir , 44% ) of mvc 
 during the roll - out , 59% ( ir , 33% ) of mvc during the bodysaw , 55% 
 ( ir , 21% ) of mvc during the pike and 42% ( ir , 7% ) of mvc during the 
 knee - tuck . the normalized eo values were significantly higher 
 ( p < 0.01 ) during the roll - out compared to 
 the knee - tuck . the normalized io activity was 40% ( ir , 31% ) of 
 mvc during the roll - out , 32% ( ir , 20% ) of mvc during the bodysaw , 23% 
 ( ir , 20% ) of mvc during the pike and 18% ( ir , 26% ) of mvc during the 
 knee - tuck . during all exercises the normalized io values were not 
 significantly higher ( p < 0.01 ) . the 
 normalized les activity was 9% ( ir , 5% ) of mvc during the roll - out , 
 4% ( ir , 3% ) of mvc during the bodysaw , 12% ( ir , 7% ) of mvc during the 
 pike and 8% ( ir , 5% ) of mvc during the knee - tuck . during all exercises the normalized les values were not significantly higher 
 ( p < 0.01 ) . the normalized ues 
 activity was 11% ( ir , 6% ) of mvc during the roll - out , 8% ( ir , 6% ) of 
 mvc during the bodysaw , 9% ( ir , 4% ) of mvc during the pike and 6% 
 ( ir , 5% ) of mvc during the knee - tuck . during all exercises the 
 normalized ues values were not significantly higher 
 ( p < 0.01 ) . table 2 shows the estimate 
 ( difference of means ) at 95% of the confidence interval after tukey 
 multiple comparisons ; in this case only  exercise 
 factor  was considered . estimate 
 at 95% of the confidence interval after tukey multiple comparisons 
 with the  exercise factor  considered . the estimate 
 shows the difference of means ( % of maximum voluntary contraction ) . indicates the statistical significance of the adjusted p - value . the roll - out exercise showed significantly ( p < 0.01 ) higher 
 activation compared to the bodysaw ( 16% , ci 8 - 23% ) , pike ( 26% , ci 
 18 - 33% ) and knee - tuck ( 29% , ci 21 - 37% ) . pike and knee - tuck exercises 
 showed significantly higher activation compared to the bodysaw of 10% 
 ( 28% ) and 13% ( 6 - 21% ) . suspension training has become 
 increasingly popular as a training tool . despite this popularity , relatively little research exists on the effects of such training on 
 muscle activation magnitude . the first objective of the study was to 
 investigate the activation differences of four exercises ( roll - out , 
 bodysaw , pike and knee - tuck ) to better characterize suspension 
 training . our findings indicate that suspension exercises could be an 
 effective strategy to reach high to very high activation of abdominal 
 muscles such as the rectus abdominis and external oblique . to 
 facilitate comparisons between exercises and previous studies , we 
 categorized muscle activation into four levels according to previous 
 studies , with < 21% as low , 2140% as moderate , 
 4160% as high , and > 60% as very high ( escamilla et al . , 
 2010 ) . exercises used in the present study provide a range of 
 medium to high intensity exercises through which participants or 
 athletes can progress during a training or rehabilitation programme 
 ( blanchard and 
 glasgow , 2014 ) ( figure 2 ) . roll - out exercise 
 was the most challenging for core musculature , followed by bodysaw , 
 pike and knee - tuck exercises ( table 2 ) . the roll - out showed 
 the highest activation of rectus abdominis and oblique muscles 
 compared to other exercises . although lra showed much greater 
 activation in roll - out and bodysaw compared to pike and knee - tuck 
 exercises , the other muscles showed smaller differences . these 
 findings could suggest that in the exercises characterized by 
 shoulder flexion ( such as roll - out and bodysaw ) , the increased 
 requirement of core stability was reflected more by the lower rectus 
 abdominis . according to vezina and hubley - kozey 
 ( 2000 ) , the exercises that generate muscle activity greater 
 than 60% of mvc might be more conducive to developing muscular 
 strength . the rectus abdominis ( both parts ) and eo reached activation 
 higher than 60% of mvc ( or very close to that threshold , 55% ) in the 
 roll - out and bodysaw ; consequently these exercises can be considered 
 suitable for strength training of these muscles . although in the 
 knee - tuck and pike , the rectus abdominis and eo did not reach the 
 threshold of 60% , they presented high activation levels ( 41 - 60% mvc ) . while strengthening of the core is important , an activation level 
 below 60% might be beneficial in increasing muscle endurance within 
 the core . since the core muscles are primarily composed of type i 
 fibres ( haggmark and thorstensson , 
 1979 ) , muscular endurance should also be a major concern when 
 designing strength and conditioning programmes ( vezina and hubley - kozey , 
 2000 ) . due to large demand for muscle activation , all the 
 proposed exercises might be appropriate for extremely fit individuals 
 in the latter stages of a progressive abdominal strengthening or 
 rehabilitation programme . this is an expected 
 result as all exercises focused on anterior abdominal wall muscles . this finding confirms that in the herein selected whole - body linkage 
 exercises , the activation of core muscles can be mainly focused on 
 abdominal muscles while keeping the paraspinal muscles involved with 
 low intensity . although no direct comparison can be made 
 between the selected suspension exercises compared to previously 
 reported similar exercises , it is possible to highlight the following 
 differences . we can compare only the activation of the rectus 
 abdominis , since for oblique muscles we used a different 
 normalization exercise than the other three studies . plank exercises 
 are frequently included in spine stabilization programmes as a means 
 of improving motor control for spine stabilization . when plank 
 exercises are performed on stable or unstable support surfaces , the 
 reported activation level of the rectus abdominis and eo ranges from 
 low to moderate ( garcia - vaquero et al . , 2012 ) . when executed in suspension condition , rectus abdominis muscles also 
 showed moderate activation ( byrne and bishop , 2014 ) . only 
 when the planks were performed with a similar technique ( instability 
 on lower limb and shoulder flexion ) was the activation similar to 
 that reported here , which was very high for the rectus abdominis 
 ( mcgill and 
 andersen , 2015 ) . therefore , we can assume that our exercises 
 were more challenging than an isometric plank in a stable 
 condition . in the roll - out , we found very high activation of 
 lra ( 140% ) and ura ( 67% ) . these levels were higher than previously 
 reported values obtained during the execution of the roll - out with the 
 swiss - ball ( about 50 - 60% for rectus abdominis ) ( escamilla and lewis , 2010 ; 
 marshall and 
 desai , 2010 ) and similar to the values reported with the use 
 of the power wheel , being very high for ura ( 76% ) and lra ( 81% ) 
 ( escamilla et 
 al . , 2006 ) . in the pike , we found high activation of lra ( 57% ) 
 and ura ( 41% ) . the values reported for the pike executed with the 
 swiss ball ( escamilla and lewis , 2010 ) and 
 power wheel ( escamilla and babb , 2006 ) were 
 similar for ura ( swiss ball 47% ; power wheel 41% ) and lra ( swiss ball 
 55% ; power wheel 53% ) . in the knee - tuck , we observed high activation 
 of lra ( 54% ) and ura ( 44% ) . otherwise , the values reported for the 
 knee - tuck executed with the swiss ball ( escamilla and lewis , 2010 ) and 
 power wheel ( escamilla and babb , 2006 ) were 
 lower for both ura ( swiss ball 32% ; power wheel 41% ) and lra ( swiss 
 ball 35% ; power wheel 45% ) . our findings suggest that the two 
 parts of the rectus abdominis can be activated differently according 
 to the needs of the motor task ( kibler et al . , 2006 ) . this 
 finding could be explained by the possibility to ( voluntary or 
 involuntary ) modulate the activation ratio between rectus abdominis 
 parts in order to achieve the best control of the core region . this 
 could be justified by the metameric innervation of rectus abdominis 
 muscles ( duchateau et al . , 1988 ) , although this issue is still controversial ( monfort - panego et al . , 2009 ) . however , lra muscles were generally more active than ura because of 
 confounding methodological factors . mvcs of the lra and ura in fact 
 were estimated by a standardized exercise to activate maximally the 
 trunk muscles : it could be argued that the same exercise fully 
 activated ura whereas it failed to fully activate lra . hence , the emg 
 amplitude recorded during mvc was not the maximum achievable . consequently , throughout experimental exercises , lra seemed 
 relatively more active than ura because its reference value of mvc 
 was underestimated . , arv estimates of 
 emg signals exceeded the mvc reference values ( arv higher than 100% ) . this inconsistency might be due to incomplete activation during mvc 
 ( as in the case of the lower rectus abdominis ) and other confounding 
 factors related to emg technique ( relative shift of muscle belly with 
 respect to electrodes occurring in dynamic tasks and different 
 activation between isometric and dynamic tasks , among others ) . as widely reported , variability of muscular activation between 
 participants was high . this suggests that performing these exercises , 
 some individuals might produce more or less activation than the 
 average activity indicated here . although 17 individuals participated 
 in this research , the differences in their fitness level and exercise 
 experience could have affected the performance of the exercises and 
 the resulting activation levels . crosstalk between muscles was 
 minimized by using an innovative detection system based on 
 concentric - ring electrodes which had been reported as having higher 
 spatial selectivity compared to the traditional detection systems and 
 reducing the problem of crosstalk from nearby muscles ( farina and cescon , 
 2001 ) . findings from this study , based on 
 electromyographic analysis , showed that roll - out exercise was the 
 most challenging . moreover , roll - out and bodysaw exercises executed in 
 suspension activated the rectus abdominis and external oblique 
 muscles at intensities higher than , or very close to , 60% of the 
 maximum voluntary contraction . based on these findings , we can assume 
 that roll - out and bodysaw exercises can be used to adequately 
 strengthen the antero - lateral , superficial aspect of the core region , 
 and thus they can be considered core strength exercises . these 
 findings appear to have particular relevance for well - trained 
 individuals given the high demand imposed by these exercises .
abstracta quantitative observational laboratory study was conducted to characterize and classify core training exercises executed in a suspension modality on the base of muscle activation . in a prospective single - group repeated measures design 
 , seventeen active male participants performed four suspension exercises typically associated with core training ( roll - out , bodysaw , pike and knee - tuck ) . 
 surface electromyographic signals were recorded from lower and upper parts of rectus abdominis , external oblique , internal oblique , lower and upper parts of erector spinae muscles using concentric bipolar electrodes . the average rectified values of electromyographic signals were normalized with respect to individual maximum voluntary isometric contraction of each muscle . 
 roll - out exercise showed the highest activation of rectus abdominis and oblique muscles compared to the other exercises . 
 the rectus abdominis and external oblique reached an activation higher than 60% of the maximal voluntary contraction ( or very close to that threshold , 55% ) in roll - out and bodysaw exercises . 
 findings from this study allow the selection of suspension core training exercises on the basis of quantitative information about the activation of muscles of interest . 
 roll - out and bodysaw exercises can be considered as suitable for strength training of rectus abdominis and external oblique muscles .