core muscle strengthening on a - jrnlappliedresearch.com · of lower back pain.8even in...

Vol. 5, No. 3, 2005 • The Journal of Applied Research460

Core Muscle Strengthening on aPortable Abdominal MachineJerrold S. Petrofsky, PhD*Maria Cuneo, DPT*Russell Dial, DPT*Amy Morris, DPT† Ashley K. Pawley* Jennifer J. Hill, BST†

*Department of Physical Therapy, Loma Linda University, Loma Linda, California†Department of Physical Therapy, Azusa Pacific University, Azusa, California

formed in an abdominal crunch for therectus abdominis muscles and 4.4 timeshigher for the oblique muscles with theabdominal machine compared withabdominal crunches. These differenceswere significant (P<0.01). Thus, theCross Crunch machine provided a reli-able form of exercise for the abdominaland oblique muscles with one simplemovement and can be performed in theseated position.

INTRODUCTIONIt is well established that abdominaland lower back strength is important inpreventing lower back and soft-tissueinjuries during lifting tasks.1,2 For exam-ple, Iwai et al3 showed that musclestrength was directly related to disabili-ty level and lower back pain in youngindividuals such as collegiate wrestlers.While the same relationship does notseem to hold for young children,4 inyoung and older adults, paraspinal andabdominal muscle dysfunction firstleads to abnormal muscle activity dur-ing exercise5 and later to reduced mus-cle strength and eventual backinjuries.6,7

Certainly, lower back strength andabdominal strength are not the only fac-tors that contribute to lower back

KEY WO R D S : e x e r c i s e, e x e r t i o n ,w o r k , posture

ABSTRACTTwenty-nine men and women who werebetween 17 and 49 years of age wereexamined to evaluate the effectivenessof a new abdominal exercise device, theCross Crunch machine (Savvier, LP,Carlsbad, CA), on muscle use duringabdominal flexion exercises. Subjectswere a mixture of fit and sedentary indi-viduals. They were engaged in exerciseon the Cross Crunch machine in theseated position, and the exercise wascompared with supine conventionalcrunches without the machine. The mus-cles studied were the rectus abdominisand the external oblique muscles.

To evaluate the use of the muscles,electromyogram (EMG) electrodes wereplaced on the right and left rectus abdo-minis muscles and the right and leftexternal oblique muscles. The relation-ship between workload on the portableabdominal machine and EMG of therectus abdominis and oblique muscleswas linear. At the heaviest workload,work accomplished (duration x EMGamplitude) with the Cross Crunch was1.9 times higher than the work per-

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The Journal of Applied Research • Vol. 5, No. 3, 2005 461

injury. In a recent publication, backextensor strength in smokers was lessthan in nonsmokers with the same levelof lower back pain.8 Even in well-trained athletes such as gymnasts, lowback pain can be a limiting factor intheir lives.9 In addition, abdominal exer-cise can be important in rehabilitationfor improving bowl and bladder func-tion.10

Once lower back pain and injuriesoccur, it is tempting to use surgical inter-vention such as lumbar fusion to resolvechronic pain.11 Surgery is also often per-formed for curvature of the spine, evenin young soldiers with back pain.12 Butsurgical techniques are often verypainful, costly, with poor prognosis, andthe results are no different than whatcan be seen in good therapeutic exerciseprograms.13 However, even if surgery hasbeen performed, proper lower back andabdominal strengthening is important inthe rehabilitation after surgery.14

Perhaps one reason why athletesdevelop a high proportion of lower backpain is that many types of athletic train-ing do not train muscles evenly on bothsides of the abdominal area and acrossthe lower back. For example, when mus-cle fatigue and fatigue-related biochemi-cal changes were assessed during cycliclifting tasks, imbalances existed acrossthe spinal cord causing a torque at thedisk if one side of the body was fatiguedmore than the other.15,16 This associationbetween muscle fatigue and lower backinjury or pain has been observed bymultiple investigators.16,17

The most common abdominal exer-cise is a partial curl. The exercise can beperformed supine with the feet on thefloor and 12″to 18″ apart, with the knees

flexed.18 Abdominals are flexed to liftthe shoulders and head off the floor toan angle of 30o. The arms may becrossed at the chest or the level of thehead with the fingers next to the ears. Apartial curl is considered the closest tothe ideal position to maximize the use ofthe abdominal muscles.19 Another posi-tion is with the knees flexed to 90o withthe feet on the wall, this will emphasizethe rectus abdominis muscles.20 Catstretches can also be used for the lowerback muscles, and a variety of exercisetechniques are used for the oblique muscles.

In an effort to exercise the abdomi-nal muscles, many machines have beendeveloped for use in athletic training21

and after abdominal surgery.22

Abdominal exercise can also be per-formed in the seated position by a vari-ety of different machines.21,23 Butwhether an abdominal exercise is doneon a machine, in a seated position, or onthe floor, there is variation in muscleactivity.19,24 Substitution of other musclesis common and hinders training of theabdominal and lower back muscles.25

Ultimately, back extensor and abdomi-nal strength are still considered the goldstandard for analyzing a predispositionto back injuries.26-29 As such, it is impor-tant to understand which muscles areused and to what extent the exercise isaccomplishing these goals. This analysisof muscle use can be performed with thesurface electromyogram (EMG).30

The EMG, when measured by sur-face electrodes above an active muscle,represents an interference pattern thatprovides the summation of activity ofthe underlying muscle fibers.31 Theamplitude of the surface EMG is gener-

Table 1. General Characteristics of Subjects

Age (years) Weight (kg) Height (cm) Mean ± SD 38.3 ± 13.4 73.3 ± 15.3 167.3 ± 11.8



ally related to the tension developed inmuscle.31,32 Therefore, the EMG providesa useful measure in assessing both theextent of muscle activity and musclefatigue.33-36 There is a linear relationshipbetween the amplitude of EMG andtension in muscle during brief contrac-tions.31,33,37 However, the absolute ampli-tude of the EMG varies from day to day.Variation has been attributed to the typeof electrode (needle or surface) or thesize or position of the electrodes.38,39

However, the EMG provides an accu-rate and reliable index of muscle use if itis normalized in terms of the maximumEMG during a maximum muscle con-traction on a given day.

Recently we observed considerablemuscle training and balance trainingbilaterally across the abdominal muscleswith a small portable exercise device

called the 6 Second Abs machine(Savvier, LP, Carlsbad, CA).19,24,40 Thisstudy describes the evaluation of a dif-ferent machine called the Cross Crunch(Savvier, LP) which is somewhat similarin function to the 6 Second Abs machinein that resistance is provided throughlatex bands; however, the Cross Crunchis more compact and is specificallydesigned to perform a side torso twistingcrunch that is performed in the seatedposition. This is particularly importantfor people who are overweight or olderand, due to deconditioning, would find itimpossible to exercise on the floor.Seated exercises could encourage activi-ty in this population.

SUBJECTSTwenty-nine subjects (14 men, 15women) participated in these experi-ments. This population was chosen basedon power analysis of the variance toprovide reliability in the analysis. Theirages ranged between 17 to 49 years old.The general characteristics of the sub-jects are shown in Table 1. All experi-mental protocols and procedures wereapproved by the Human ReviewCommittee at Azusa Pacific University,and all subjects signed a statement ofinformed consent acknowledging thatthey are fully aware of the purposes andprocedures of the project.

METHODSExercise DeviceThe exercise machine was a commercialexercise device that was produced bySavvier, LP, of Carlsbad, California(Figure 1). The device consisted of a rec-tangular plastic frame with an elasticband on the inside to adjust resistance.Resistance could be increased in 3 dif-ferent stages so that it became increas-ingly more difficult to compress therectangle (30, 50, or 75 lbs) (13.6, 22.7, or34 kg). The upper part of the rectanglewas placed in the subject’s hands with

Figure 1. Exercise device used in these studies.



the base of the machine on the seat tothe left or right of the subject (Figure 1).The device was then compressed fully,thereby exercising the oblique muscles.

EMGThe EMG was recorded through 2 bipo-lar vinyl adhesive electrodes (silver, sil-ver-chloride) with an active surface areaof 0.5 cm2. One electrode was placedover the belly of the active muscle. Thesecond electrode was placed 2 cm distalto the active electrode. EMG was ampli-fied using a 4-channel amplifier, whosefrequency response was flat from DC to1000 Hz. The common mode rejectionratio of the amplifier was greater than120 dB. The EMG was then digitized at1000 samples/second by a Biopac 16-bitanalog to digital converter (Biopac Inc.,Santa Barbara, CA) and displayed and

stored on a computer for later analysis.The amplitude of EMG was assessed bydigitizing and half wave rectifying theraw data and calculating the mean volt-age of rectified EMG.

Measurement of Strength of theAbdominal MusclesIsometric strength of the abdominalmuscles was measured in the seatedposition. To accomplish this, subjects satwith the hips at an angle of 90˚. A modi-fied exercise device with strain gaugeswas used to assess maximum strength.Strength was measured in the forward,left, and right rotation directions (Figure2). The strain gauge was linear from 0 to200 kg of force. The output of the trans-

Figure 3. Oblique side-bending exercise.

Figure 2. Strength measurement during thestudy for oblique muscles.

15Petrofsky-vol5no3 1/26/70 6:44 PM 63


ducer was amplified with a strain gaugeconditioner amplifier with a gain of 1000and digitized in a Biopac 16-bit analogto digital converter and displayed andstored on a computer. The output wasstored and analyzed as the averagestrength over the middle of a 3-secondcontraction. Strength was measured byan isometric contraction for 3 seconds.At least 1 minute was allowed betweeneach of 3 contractions to allow forrecovery.

ProceduresFirst, EMG was assessed during a maxi-mum effort as well as strength duringflexion facing forward and while thetrunk was rotated to the left and right asdescribed in the Methods section. Next,EMG was assessed over the left andright rectus abdominis and left and rightexternal oblique muscles during the fol-lowing exercises:

The abdominal machine was placedon the side of the subject with the baseon the chair. The arms were placed onthe handles so the body of the subjectwas rotated to the left to perform theoblique side-bending exercise (Figure 3).The machine was depressed fully over a2- to 3-second period. The arms were

then reversed to the right-hand side andthe exercise repeated.

A standard crunch was performedwhile laying supine on the floor with thehip and knees bent to approximately 45ºand the arms folded and crossed overthe chest. No device was used and thesubject lifted his or her shoulders highenough for his or her scapulae to clearthe floor.

Finally, for 4 sets of resistive bands,the force to depress the bands in themachine was tested to verify the tensionand linearity of the machines.

Each exercise was performed over a3-minute period while EMG was meas-ured. All exercises were repeated in part1 at 3 loads (30, 50, and 75 lbs or 13.6,22.7, and 34 kg).

Data AnalysisStatistical analysis involved the calcula-tions of mean standard deviations, analy-sis of variance (ANOVA), and t-tests.The level of significance was P<0.05.EMG was analyzed over 3-second peri-ods through the digitized EMG data.

RESULTSMuscle strength during flexion of theabdominal muscles in the forward facing

Table 2. Strength of the Subjects

Flexion Right Rotation Left RotationStrength (kg)Mean ± SD 65.8 ± 12.5 48.5 ± 8.0 43.9 ± 9.1

Table 3. Variability in Resistance of Exercise Device to Fully Depress the Machine (kg) by BandColor

Band ColorTrial Yellow Orange Red1 14.5 18.3 42.12 15.8 27.6 32.33 22.4 26.2 34.44 19.8 23.8 41.6Mean ± SD 18.1 ± 3.6 24.0 ± 4.1 37.6 ± 5.0



direction and during right and left rota-tion exercises is shown in Table 2. Table2 displays the mean ± the respectivestandard deviation for the group of sub-jects. Muscle strength was greatest forflexion and lowest for the right and leftrotations.

The force necessary to compress theabdominal exercise machine fullyagainst the 4 different sets of resistancebands that were tested is shown in Table3.

The average force to depress theCross Crunch machine to the fullestextent for the 3 colors of resistancebands was 18.1, 24.0, and 37.6 kg. Thiscorresponded to a load of 39, 52, and 82lbs for the 3 bands. For the group of sub-jects as a whole, the maximum resistanceof the device was less than the maxi-mum strength of the subjects, but forsome subjects, the maximum resistanceof the device did exceed the maximumstrength of the subjects. The averagestrength of the men for right rotation,for example, was 51.1 kg, a force more

than adequate to depress the machinefully, whereas for the women, maximumstrength was 35.6 kg, making it impossi-ble for some women to fully depress themachine. However, the data from thesewomen were averaged in the results tothe fullest extent that they could depressthe machine.

The EMG activity during flexion ofthe abdominal muscles with use of theabdominal machine was linearly relatedto the workload. For example as shownin Figure 4, for facing left, as the resist-ance was increased by changing thebands, so did the EMG activity of themuscles. But the variation from one sub-ject to the next was large as shown bythe large standard deviations in this fig-ure. Therefore, EMG, rather then beingexpressed as an absolute value, was nor-malized in the remainder of the analysisas a percent of the maximum muscleactivity during the maximum strengthdeterminations. This allowed for differ-ences in EMG activity between oneindividual and another.

Figure 4. The relationship between load and EMG activity (raw EMG) for the group of subjects ±the SD for left bending abdominal exercise. The muscle shown is the left oblique muscle.



The average normalized muscleactivity in the left oblique, right oblique,left rectus abdominis, and right rectusabdominis are shown in Figure 5 duringexercise. Each panel shows muscle activ-

ity from one of the four muscles duringleft and right side-bending exercise andstandard abdominal crunches. The great-est muscle activity was during exerciseagainst the red band. Here, as shown in

Figure 5. Illustrated here is the average muscle activity of the left and right oblique and left andright rectus abdominis muscles during each of the three exercises. Each panel shows EMG activ-ity expressed as a percentage of the EMG during the maximum strength determinations. Datafrom the (A) left oblique muscles, (B) the right oblique muscles, (C) left rectus abdominis mus-cles, and (D) the right rectus abdominis muscles. The three exercises are left and right abdomi-nal exercises in the seated position compared with standard abdominal crunches lying supineon the floor. All data are the mean result for the entire group ± the respective SD. Data areshown for the yellow, orange, and red bands for the left and right muscle groups (left and rightbars).



Figure 5, muscle activity was still onethird higher for the rectus abdoministhan that seen for standard crunches. Forthe red band, the greatest difference inEMG activity between crunches and useof the exercise machine was in theoblique muscles. Activity for the left andright oblique muscles was 49% ± 11%and 34% ± 12 % of maximum EMGactivity during work on the machineagainst the highest load for rotation tothe left, whereas during abdominal

crunches, it was 19% ± 7% and 26% ±12% of the maximum muscle. Thusoblique activity was more than twice ashigh as that seen during standardcrunches. Even for exercise against themiddle load band (orange band), rectusand oblique activity was similar to thestandard crunches. For the yellow band,muscle activity was about half that ofcrunches.

Not only was the EMG activityhigher, but the duration of the exercise



was longer using the exercise machinecompared with abdominal crunches. Theaverage duration of the exercise for allsubjects was 4.04 ± 1.92 seconds for theleft rotation exercises, 3.92 ± 1.52 sec-

onds for the right rotation exercises, and2.04 ± 0.92 seconds for the abdominalcrunches. Thus, with a significantlylonger duration of the exercise on themachine versus standard abdominal

Figure 6. Illustrated here is the work during exercise for the (A) left oblique muscles, (B) rightoblique muscles, (C) left rectus abdominis muscles, and (D) right rectus abdominis muscles forexercise at the heaviest work load during left and right rotational exercises (all using the exercisemachine) compared with supine abdominal crunches. Work is calculated as the product ofmuscle activity ¥ the duration of the exercise. Since muscle activity is normalized to each per-son, it is in relative units. Data are shown for the yellow, orange, and red bands for the left andright muscle groups (left and right bars).



crunches, work would also be greater forthe machine on a per crunch basis. Bymultiplying the product of time byEMG-derived muscle activity, a workindex was computed. This work indexdata are shown in Figure 6.

When work was computed for theleft oblique (Figure 6A), right oblique(Figure 6B), left rectus abdominis(Figure 6C), and right rectus abdominis

(Figure 6D) muscles for exercise rotat-ing left and right with the exercisemachine versus supine crunches, theresults show that work was much higherfor any given exercise than supinecrunches. As can be seen in this figure,the total work for exercise against thered band was 4.0 times higher on theCross Crunch machine than for standardcrunches. For the oblique muscles, workwas 4.42 times greater during side exer-



cises versus standard crunches. For therectus abdominis muscles, work was 1.7times greater. A similar relationship wasseen with the other bands, but with alighter load, work was proportionallyless. For the orange band, work for theoblique muscles was still 3 times greaterthan for standard crunches, while for therectus abdominis, it was approximatelyequal to standard crunches. Total workof all the muscles was more than twiceas high on the abdominal crunchmachine versus standard crunches. Forthe yellow resistance band, work wasabout equal for the oblique musclesbetween the two types of exercise and20% less for the rectus abdominis dur-ing Cross Crunch exercise comparedwith standard crunches.

DISCUSSIONLower back pain is a major problem inthe United States today.1,2 This is truenot only of older individuals but youngadults as well.4 There is a strong correla-tion between weak abdominal muscleactivity and back injuries,6,7 but even inwell-trained gymnasts, lower back paincan be a limiting factor.9

Numerous abdominal machineshave been developed to prevent orreduce back pain.21 Some machines havethe subjects in the seated position whileothers mimic standard crunches in thesupine position.21,23 However, substitu-tion of muscles during use of thesemachines is common and hinders trueabdominal strengthening of the rectusabdominis and oblique muscles.25 In thisinvestigation, a new lightweight machinewas evaluated, the Cross Crunch, whichwas ergonomically designed to performcrunches in the seated position.

Figure 6 shows that with the CrossCrunch machine, the work accomplishedby the rectus abdominis and obliquemuscles was up to 4 times higher thanstandard abdominal crunches. Threeworkloads were examined here: 30, 50,

and 75 lbs (13.6, 22.7, and 34 kg).Obviously, when doing a standardabdominal crunch from the floor, theonly load being lifted is the weight ofthe body. In this study, subjects wereable to work against a progressive loadby changing the bands. This allowed fora progressive workout regimen toincrease strength and endurance in theabdominal muscles. When exercise wasaccomplished against the greatest resist-ance (red band), work was much greaterthan could be accomplished in anabdominal crunch. Here, 10 side crunch-es against the red band was equivalentto 40 standard crunches, or with theorange band, 10 crunches would beequivalent to 20 standard crunches. Buteven for the weakest load, the work wassimilar to standard floor crunches. Thisis significant in that people with disabili-ties, the obese or poorly conditionedpeople who normally could not do floorcrunches, may accomplish work in theseated position with the same or morework than standard floor crunches. Plus,the exercise, unlike abdominal crunches,exercised both the rectus abdominis andoblique muscles in one single exercise,whereas crunches condition principallythe rectus abdominis muscles.

In summary, this type of exercise forrectus abdominis and external obliquemuscles provides more work than stan-dard abdominal crunches. It can beaccomplished from the seated position,and at the work levels seen here, it canprovide improved strength, flatter andfirmer abdominal muscles, and greaterstability for sitting, standing, and reaching.32

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