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RIGINAL ARTICLE

ransversus Abdominis and Obliquus Internus Activity Duringilates Exercises: Measurement With Ultrasound Scanning

rit Endleman, MSc, MCSP, Duncan J. Critchley, MSc, MCSP

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ABSTRACT. Endleman I, Critchley DJ. Transversus abdo-inis and obliquus internus activity during Pilates exercises:easurement with ultrasound scanning. Arch Phys Med Reha-

il 2008;89:2205-12.

Objective: To assess activity of transversus abdominisTrA) and obliquus internus abdominis (OI) muscles duringlassical Pilates exercises performed correctly and incorrectly,nd with or without equipment.

Design: Repeated-measures descriptive study.Setting: Pilates studio.Participants: A volunteer sample of women (n�18) anden (n�8), mean age � SD (43�14y), with more than 6onths classical Pilates training and no back pain or other

ondition likely to influence abdominal muscle activity.Interventions: Participants performed Pilates imprint, hun-

reds A and B, roll-up, and leg-circle exercises on a mat. Theundreds exercise was also performed on a reformer (slidinglatform). Mat imprint and hundreds exercises were instructedo be performed correctly (with abdominal drawing-in) orncorrectly (without drawing-in).

Main Outcome Measure: Thickness of TrA and OI middlebers measured with ultrasound imaging.Results: TrA thickness increased during the mat imprint, hun-

reds A, hundreds B, leg-circle, and roll-up exercises (all P�.001)ompared with resting. OI thickness increased during the matmprint, hundreds A, hundreds B, leg-circle (all P�.001), andoll-up exercises (P�.002) compared with resting. TrA thicknessuring reformer hundreds B was greater than during mat hundreds

(P�.011); OI thicknesses were not different for this compari-on. During incorrect imprint, neither TrA or OI thicknesses wereifferent to resting. TrA and OI muscle thicknesses were moder-tely correlated (R�.410; P�.001).

Conclusions: This study provides the first evidence that aelection of classic Pilates exercises activates TrA and OI. Usef the reformer exercise machine can result in greater TrActivation in some exercises. TrA and OI did not functionndependently during these exercises. Research into the train-ng effects of Pilates or in patient populations can be under-aken using ultrasonography in submaximal exercises.

Key Words: Abdominal muscles; Exercise; Rehabilitation;ltrasonography.© 2008 by the American Congress of Rehabilitation Medi-

ine and the American Academy of Physical Medicine andehabilitation

From the School of Human Health and Performance, University College LondonEndleman); Academic Department of Physiotherapy, King’s College London,Critchley), London, UK.

No commercial party having a direct financial interest in the results of the researchupporting this article has or will confer a benefit on the authors or on any organi-ation with which the authors are associated.

Reprint requests to Duncan J. Critchley, MSc, MCSP, Academic Dept of Physio-herapy, King’s College London, Guy’s Campus, London, SE1 6RT, UK, e-mail:[email protected].

0003-9993/08/8911-00932$34.00/0doi:10.1016/j.apmr.2008.04.025

HE EXERCISE REGIMEN known as pilates, founded onthe teachings of Joseph Pilates (1880–1967), has become

popular choice for people seeking conditioning and rehabil-tation; devotees in the United States increased 1000-fold be-ween 1990 and 2000.1 Pilates advocates claim the exercisesnvolve activation of the transversus and obliquus internusbdominal muscles thought to stabilize the lumbar spine; henceilates has application in lumbo-pelvic pain rehabilitation andance, sport, or athletic conditioning. This claim has had min-mal formal investigation.2

There has been considerable recent interest in neuromuscularontrol of spinal motion in rehabilitation and conditioning.odels of trunk muscle function developed from Bergmark’s3

lassification of local muscles, such as TrA,4,5 attaching to thepine and global muscles, such as middle fibers of OI,3 span-ing the pelvis to ribs, stress the importance of TrA as a spinaltabilizer, and are influential in contemporary physical therapynd conditioning practice.4,6-10 Others, such as McGill etl,11-13 do not employ this classification, and suggest all trunkuscles have a role in spinal stability.Classic Pilates aims to improve health and well being with

xercises that emphasize abdominal and low back muscletrengthening while maintaining good posture and body align-ent.14 Exercises are mat-based or use equipment, such as the

eformer (fig 1; and see Methods: Materials), as assistivend/or resistive exercise tools.14 It is claimed that Pilates ex-rcises activate TrA and OI muscles15,16; consequently, Pilatess advocated as a progression from spinal stabilization physicalherapy (involving TrA retraining) as maintenance or preven-ative exercises for people with back pain.7 To date, no pub-ished studies have evaluated TrA or OI muscle activity duringilates exercises.All classic Pilates exercises involve the imprint action, pull-

ng the navel toward the spine. Pilates believed pulling in thebdomen in this way protected the spine.14 The imprint actionppears to be similar to the low abdominal drawing-in exercisesed to activate TrA in spinal stabilization training.10 It isecognized that many patients find low abdominal drawing-inifficult to learn even when taught by expert physical thera-ists.10 With the widespread popularity of Pilates, there isariation in teaching standards, and there are differences inmphasis of the importance of the imprint action.14-16 It is notnown whether it is necessary to maintain the imprint actionuring exercises, according to classic Pilates principles, inrder to activate TrA and OI muscles most effectively.Ultrasound imaging has been used as a noninvasive tool foreasuring abdominal muscle thickness change during respira-

List of Abbreviations

ANOVA analysis of varianceEMG electromyographicICC intraclass correlation coefficientsMVC maximum voluntary contractionOI obliquus internus abdominis
TrA transversus abdominis
Arch Phys Med Rehabil Vol 89, November 2008

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2206 ABDOMINAL MUSCLE ACTIVITY IN PILATES, Endleman

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ion, abdominal muscle exercises, and gait.17-21 Thicknesshange in TrA and OI has been shown to have good correlationith EMG activity up to 30% to 50% MVC,22,23 suggesting

hat thickness change is a valid measure of activity in theseuscles at these levels of effort. Unfortunately, the relationship

n obliquus externus abdominis is much weaker, and ultra-ound-measured thickness change cannot be confidently useds a gauge of activity in this muscle.22,24

The aims of the investigation were to evaluate the followingypotheses with ultrasound measurements of muscle thickness:1) Pilates exercises performed by Pilates-trained healthy per-ons would result in greater activity in TrA and OI muscleshan at rest. (2) Performing the exercises correctly, followinglassic Pilates principles of maintaining the imprint action, orrawn-in abdominal wall, would result in greater TrA and OIuscle activity than performing the same exercises incorrectly.

3) Using the Pilates reformera equipment would result inreater TrA and OI muscle activity than when performing theame exercise on mats.

In addition, we evaluated the correlation of TrA and middlebers of OI muscle activity during a range of exercises aimedt promoting trunk stability in order to investigate the degree ofunctional independence of these muscles.

METHODS

articipantsAfter we obtained ethical approval from the University Col-

ege London Research Ethics Committee and written informedonsent, 8 men and 18 women who had attended supervisedlassical Pilates sessions at least once a week for the previousmonths were recruited from a Pilates studio. Potential par-

icipants were excluded if they were under 18 years old, hadpinal or abdominal surgery at any time, had low back pain inhe last 2 years, had visible scoliosis, had neuromuscular dis-rders, or were pregnant.

aterialsA portable ultrasound scannerb was used in B mode with a

.5-MHz linear head. A Pilates reformer, a sliding platform onracks set in a frame, was used for the equipment-based exer-

Fig 1. (A) Reformer hundreds

ise (see fig 1). Participants can lie, sit, or stand on the slidingao

rch Phys Med Rehabil Vol 89, November 2008

latform, which is free to move, thus providing a unidirection-lly unstable base of support. Exercises are performed byoving the platform against spring resistance.

roceduresEach participant’s demographic data, height, and weight

ere recorded. All exercises were initially demonstrated by theesearcher, a physical therapist who is also an experiencedilates instructor. Exercises were conducted in random orderver a 50-minute period to minimize possible fatigue effects.ll testing took place in the studio of Pilates International,ighgate, London, UK.

ltrasonographyThe ultrasound machine was calibrated prior to data collection

y comparing with metal phantoms of known dimensions. Allubjects had their antero-lateral abdominal muscles imaged in aupine position initially, to familiarize the researcher with thearticipant’s muscle topography and fascial landmarks that coulde used to improve measurement reliability (fig 2). All stored

ig 2. Example ultrasound image of antero-lateral abdominal wall (A)

nd (B) reformer hundreds B.

t rest and (B) during imprint. OE, obliquus externus abdominis; OI,bliquus internus abdominis; TrA, transversus abdominis.

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2207ABDOMINAL MUSCLE ACTIVITY IN PILATES, Endleman

mages for measurement purposes were made with the transducerrientated antero-laterally with the midpoint of the transducerositioned midway between the iliac crest and the lowest rib alonghe anterior axillary line of the left abdomen. At this point, theiddle fibers of TrA can be imaged simultaneously with theiddle fibers of OI.21 Care was taken to position the transducer

erpendicularly to the abdominal wall at all times for optimalmage clarity and increased accuracy of readings. Static viewsere stored using the freeze facility of the scanner timed to the endf exhalation17; the thickness of muscles was measured using theutomatic calipers software of the scanner. All ultrasound scansere taken with the scanner head in the same position, using skin

andmarks or an indelible pen to minimize repositioning error.hicknesses of TrA and OI muscles were measured in restingupine position, during the following exercises, then repeated inesting supine position to assess short-term intratester reliability.

xercisesMuscle thickness ultrasound measurements were taken dur-

ng the imprint (fig 3) abdominal draw-in action, the startingosture for all other classic Pilates exercises. After consultation

Fig 3. Mat imprint.

Fig 4. (A) Mat hundreds A an

ith experienced classic Pilates instructors, a representative setf 3 classic Pilates mat exercises was chosen: the hundreds (fig), roll-up (fig 5), and leg-circle (fig 6). The exercises chosenmploy the Pilates principles of breathing control and abdom-nal muscle control. They are included in every classical Pilateslass and can be modified and progressed. The 3 exercisesrovided a variety of different ways of loading the abdominaluscles, such as through a raised leg (leg-circle), raised legs

hundreds A), raised legs and head (hundreds B), raised armsroll-up A), and raised arms and trunk (roll-up B). Roll-up andundreds are dynamic exercises, so measurements were takent 2 stages of each of these exercises, described, for example,s hundreds A and hundreds B (see fig 4). The imprint actionnd hundreds exercises were performed with both correct andncorrect instructions (fig 7). The hundreds exercises wereerformed on the reformer (see fig 1) as well as on a mat.Although all participants were experienced students of clas-

ic Pilates and might be expected to know how to perform thexercises correctly, a standardized teaching protocol was fol-owed for each exercise. Teaching correctly performed exer-ises emphasized imprinting, or maintaining a drawn-in abdo-en during exercises, while breathing freely with a mobile rib

age. The phrases scoop the belly or navel to spine are useduring exercises to reinforce this teaching. When teaching thencorrect exercises, participants were not corrected if theirntero-lateral abdomen rose away from their spine. Participantsho drew in their abdomen during incorrect exercises were

nstructed to push the abdominal muscles out and allow them toise (see fig 7). This was also demonstrated to them by thexaminer. All teaching was conducted by a single Pilatesnstructor to ensure consistency.

ata AnalysisRepeated resting thickness measurements were compared

ith the methods described by Rankin and Stokes25 usingland and Altman plots and ICC. The correlation coefficient

R2) indicates the extent to which the pairs of numbers for thesevariables lie on a straight line. Distributions were checked to

nsure that parametric assumptions were met. Muscle thick-esses of TrA and OI in all conditions were compared sepa-ately with repeated-measures 1-way ANOVAs in order toompare the effect of the 4 exercises with rest, between cor-ectly and incorrectly performed exercises, and between exer-

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ise with and without equipment. In order to assess whetherrA and OI thicknesses were related, Pearson’s correlationetween the 2 muscle thicknesses was tested, and lines of bestt for 8 possible relationships were compared. Statistical sig-ificance was accepted at P�.05. Data were analyzed usingPSS 12.0.c All data are presented as means � SDs unlesstherwise stated.

RESULTS

emographic DataParticipants had a mean � SD age of 43�14 year old, weight

f 62.3�9.1kg, 1.70�0.74m tall, 21.7�2.8kg/m2 body mass in-ex and had practiced Pilates for 8.2�7.2 years, ranging from 6onths to 25 years.

epeatabilityVisual inspection of Bland and Altman plots suggested no

ystematic differences between the 2 ultrasound measurementsor either resting muscle. ICCs were .82 and .91 for TrA andI, respectively. These results suggest that there is good short-

Fig 5. (A) Roll-up

Fig 6. Leg-circle.


erm intraoperator repeatability for either resting muscle thick-ess measurement using ultrasound imaging.

omparisons of Muscle Thickness During Exercisend RestBoth muscles increased in thickness during all correctly

erformed exercises compared with at rest (hypothesis 1).here were significant differences between all exercises in bothrA (P�.001) and OI (P�.001) thickness, assessed witheasures repeated 1-way ANOVAs (tables 1, 2; fig 8).

omparisons of Muscle Thickness During Exerciseserformed Correctly and IncorrectlyThe effects of performing the exercises incorrectly varied

etween the 2 muscles and between different exercises. Themprint exercise performed correctly resulted in significantlyreater thickness than when the exercise was performed incor-ectly in both TrA and OI. During the imprint exercise per-ormed incorrectly, neither TrA nor OI was thicker than at resthypothesis 2) (see tables 1, 2; see fig 8).

There were no differences in TrA thickness between cor-ectly and incorrectly performed mat hundreds A or mat hun-reds B exercises, and both correctly and incorrectly performedat hundreds A and mat hundreds B exercises resulted in

ignificantly greater TrA thickness than at rest. During the matundreds A exercise, there was no difference in OI thicknessetween correctly and incorrectly performed exercises; duringhe mat hundreds B exercise, OI muscle thickness was greater

nd (B) roll-up B.

Fig 7. (A) Correct imprint and (B) incorrect imprint.

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hen this exercise was performed incorrectly (hypothesis 2)see tables 1, 2; see fig 8).

omparisons of Muscle Thickness During Exercises Withnd Without EquipmentDuring the hundreds A exercise, TrA muscle thickness was

he same regardless of whether the exercise took place on a matr on the reformer. During the hundreds B exercise, TrA washicker when exercising with the reformer. Performing theundreds A and hundreds B exercises on the reformer made noifference to OI muscle thickness compared with the samexercises performed on a mat, although the muscle was close toeing significantly thicker during the hundreds A exercise onhe reformer (P�.56; hypothesis 3) (see tables 1, 2; see fig 8).

It was observed post hoc that the more demanding exercises—theat hundreds A, the mat hundreds B, the leg-circle, and the roll-up B

xercises—were associated with greater muscle thickness com-ared with the imprint or roll-up A exercises (see tables 1, 2;ee fig 8).

Pearson’s correlation between all TrA and OI muscle thick-esses R2 was equal to .410 (P�.001). A power relationshiprovided the closest line of best fit (F�66.73; P�0.001), butas only a little better than a linear relationship (F�62.58;�0.001) (fig 9).

DISCUSSIONThe main finding was that both TrA and OI muscles were

ignificantly thicker during all correctly performed Pilates ex-rcises investigated compared with resting supine. TrA and OIuscle thickness change has been shown to correlate withMG muscular activity up to moderate levels of effort,22,23

uggesting that this thickness increase indicates muscle activ-ty, possibly in order to help stabilize or protect the spine.10 The

Pilates exercises investigated therefore activate the deeperbdominal muscles. This has been frequently claimed but notlearly demonstrated prior to this study.

The imprint action, taught when first learning Pilates and theasis of all other classic Pilates exercises, showed a differencen muscle thickness according to whether the exercise waserformed correctly or not. During higher effort exercises, suchs hundreds A and B, there were no significant differencesetween exercises performed correctly and incorrectly. How-ver, participants were all experienced Pilates practitioners,nd it may have been hard for them to unlearn the exercises,articularly in complex movements such as the hundreds oreg-circle exercises, components of which may have been per-ormed unconsciously. Some participants reported finding itifficult to perform the exercises incorrectly because they wereo accustomed to drawing in their abdomens. Further studiesould investigate the activity of the abdominal muscles duringilates exercises among naive or patient populations.Using the reformer equipment resulted in thicker TrA during

he higher effort hundreds B exercise, but no difference duringhe lower effort hundreds A, and no differences in OI. Theundreds exercise involves lying supine on the sliding board ofhe reformer, which may provide external lumbo-pelvic stabi-ization and thus minimize the effects of the extra resistancend challenge to balance provided by the equipment.

Some models of spinal stability propose independent func-ions for TrA and OI.3,10 The moderate correlation betweenhese 2 muscles found in the present study does not support thisiew. There is controversy regarding whether isolated TrAuscle exercises are desirable or practical in programs aimed

t enhancing muscular stabilization of the spine.10,11,13,26,27 In
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f 26 participants during the imprint action correctly performedaccording to an experienced classic Pilates instructor andhysical therapist), suggesting that an isolated TrA contractions hard to achieve even in a highly trained population. Beithnd Harrison28 argue that because of the anatomy of the com-ined aponeurosis and their reflex connections, the abdominaluscles cannot function independently. These results support

hat view.The use of ultrasound to measure abdominal muscle thick-

ess as a proxy of muscle activity is becoming increasinglyopular.17-21,29,30 Non-Pilates exercises designed to activate thetabilizing abdominal muscles have been demonstrated to in-rease the thickness of TrA and OI,9,19,29,31 findings similar tohose of this study. Repeated measurement of the thicknessrom the same portion of abdominal muscle requires accuratedentification of muscle architecture and careful repositioningetween scans to ensure that the same portion of the abdominalall is measured each time; however, reliability was investi-ated and demonstrated to be very good. Similar very good-o-excellent intrarater reliability (ICC) has been reported inther studies.9,23,32 Thickness change and EMG signal haveeen demonstrated to correlate well in both TrA and OI at lowo moderate levels of effort.22,23 Hodges et al22 concluded thathe relationship was weaker during efforts greater than 20% to0% MVC from results of a small sample (n�3), whereascMeeken et al23 demonstrated a good relationship to 50%VC in a larger group (n�9). Data from both these studies22,23

uggest muscle thickness increase tends to be smaller for theame increase in EMG signal during higher efforts. Ultrasoundeasurement of thickness may therefore have underestimated

ifferences between more strenuous exercises so that differ-nces may exist but were not detected between, for example,orrect and incorrect hundreds B. The percentage of MVC ofransversus and OI muscles during these exercises is unknown,ut comparison with similar exercises from other studies sug-ests that most were less than 30% and unlikely to havexceeded 50%; during the dying bug exercise (crook lying,oth arms and both legs raised from plinth), similar to theundreds B exercise, oblique abdominal muscle EMG activityas never greater than 25% MVC33; in a curl-up exercise,

imilar to the present roll-up B, OI EMG activity was reporteds 42% of MVC.34 In the present study, during more strenuousxercises such as hundreds A and B and roll-up B, musclesere thicker than during lower effort exercises such as the

mprint action or roll-up A, increasing confidence in the valid-

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ig 8. TrA and OI thickness measurement (mm) during differentilates exercises.

ity of this method of measuring of muscle activity. However,P NO

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urther studies should investigate the relationship of abdominaluscle thickness and EMG activity during different exercises

nd with larger sample sizes.21

The present study measured muscle thickness during 3 of 34ossible classic Pilates exercises. The exercises were chosenfter consultation with experienced instructors and are believedo be representative of the conditioning system. Further re-earch could compare other exercises. Exercises such as leg-ircle and roll-up are dynamic but were measured in a station-ry position in the present study. Video recording real-timeltrasound can be used to measure muscle thickness duringynamic exercises,20,23 which may give more valid findings.ltrasound scanning is a safe, reliable, and practical tool that

ould be used to evaluate change in both the TrA and OIuscle thickness after Pilates or other training programs if

ubmaximal exercises are investigated.

tudy LimitationsBecause of the weaker relationship between muscle thick-

ess and EMG activity at higher percentages of MVC, lessonfidence can be placed in the results of more strenuousxercises.

CONCLUSIONSA significant increase in thickness, representing muscle ac-

ivity, was demonstrated in both TrA and OI during 3 Pilatesxercises performed according to classic Pilates principles.uscle thicknesses during the imprint action performed incor-

ectly are no greater than at rest. Further research is needed tonvestigate other exercises, the longer-term effects of perform-ng Pilates exercises, and patient populations such as those withow back or pelvic pain.

Acknowledgments: We thank Bruce Lynn, PhD, and Jeroenergmann, MSc, for statistical advice, Elizabeth Proud for editorialssistance, Heather Sampson, Martin Sampson, and the staff at Pilatesnternational, Highgate, London, UK, for loan of facilities, equipment,nd guidance regarding exercise choice, and all participants.

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Fig 9. Correlation of TrA and OI thickness with lines of best fit.

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Suppliers. Pilates International, Unit 1, Broadbent Close, 20/22 Highgate High

St, London, N6 5JG UK.. Aloka 55D-900; Aloka Co Ltd, 6-22-1, Mure, Mitaka-shi, Tokyo,

181-8622 Japan.
. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.

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