ORIGINAL ARTICLE

Eva A. Andersson · Johnny Nilsson · Zhijia Ma · Alf Thorstensson

Abdominal and hip flexor muscle activationduring various training exercises

Accepted: 12 August 1996

Abstract The purpose of this study was to provide ob-jective information on the involvement of different ab-dominal and hip flexor muscles during various types ofcommon training exercises used in rehabilitation andsport. Six healthy male subjects performed altogether 38different static and dynamic training exercises – trunkand hip flexion sit-ups, with various combinations of legposition and support, and bi- and unilateral leg lifts.Myoelectric activity was recorded with surface electro-des from the rectus abdominis, obliquus externus, ob-liquus internus, rectus femoris, and sartorius musclesand with indwelling fine-wire electrodes from the iliacusmuscle. The mean electromyogram amplitude, normal-ised to the highest observed value, was compared be-tween static and dynamic exercises separately. The hipflexors were highly activated only in exercises involvinghip flexion, either lifting the whole upper body or thelegs. In contrast, the abdominal muscles showed markedactivation both during trunk and hip flexion sit-ups. Inhip flexion sit-ups, flexed and supported legs increasedhip flexor activation, whereas such modifications did notgenerally alter the activation level of the abdominals.Bilateral, but not unilateral, leg lifts required activationof abdominal muscles. In trunk flexion sit-ups an in-creased activation of the abdominal muscles was ob-served with increased flexion angle, whereas the oppositewas true for hip flexion sit-ups. Bilateral leg lifts resultedin higher activity levels than hip flexion sit-ups for theiliacus and sartorius muscles, while the opposite was truefor rectus femoris muscles. These data could serve as abasis for improving the design and specificity of test andtraining exercises.

Key words Abdominal muscles · Electromyography ·Iliopsoas · Leg lifts · Sit-ups

Introduction

Different forms of training exercises for the flexor mus-cles of the trunk and hip are of interest in many sportsand rehabilitation programme, such as those for lowback pain. A strong muscle corset around the lumbarspine could increase stability and prevent excessive loadsin extreme positions. The training exercises most oftenused are of the sit-up type, that is lying supine and raisingpart of, or the whole upper body. Frequently there is adesire to design the exercises so that they engage specificmuscles within the abdominal and the hip flexor synergy.There are many hypotheses about how training exercisesshould be modified to achieve specific effects on certainmuscles, including variations in the range of motion,starting position and fixation for the legs. Objectivetesting of such hypotheses requires reliable quantitativedata from simultaneous recordings of myoelectric ac-tivity from both trunk and hip flexor muscles understandardised and systematically varied conditions.

The area has attracted much research over the years,but most previous information has been obtained inqualitative studies and often only on single muscles[Flint 1965a, LaBan et al. 1965, Linden and Delhez 1986(iliacus); Walters and Partridge 1957, Godfrey et al.1977, Ricci et al. 1981, Ross et al. 1993, McGill 1995(rectus femoris); Floyd and Silver 1950, Walters andPartridge 1957, Partridge and Walters 1959, Sheffield1962, Flint 1965a,b, Flint and Gudgell 1965, Lipex andGutin 1970, Gutin and Lipez 1971, Carman et al. 1972,Girardin 1873, Godfrey et al. 1977, Ekholm et al. 1979,Halpern and Bleck 1979, Noble 1981, Ricci et al. 1981,Miller and Meidoros 1987, McGill 1995, Shirado et al.1995 (rectus abdominis and obliquus externus); Floyd andSilver 1950, Walters and Partridge 1957, Partridge andWalters 1959, Sheffield 1962, Carman et al. 1972, Millerand Meidoros 1987, Ross et al. 1993, McGill 1995 (ob-liquus internus)].

Differing results in the literature may be due to dis-agreements with respect to nomenclature (e.g. Ekholm

Eur J Appl Physiol (1997) 75: 115 – 123 Springer-Verlag 1997

E.A. Andersson (&) · J. Nilsson · Z. Ma · A. ThorstenssonDepartment of Neuroscience, Karolinska Institute,Department of Sport and Health Science,University College of Physical Education and Sports,Box 5626, S-114 86 Stockholm, Sweden

et al. 1979 versus Flint 1965a,b, Flint and Gudgell 1965,Ricci et al. 1981, Miller and Medeiros 1987, concerningthe use of the term ‘‘curl-up’’, meaning either a shoulderlift sit-up or raising of the whole upper body) or lack ofstandardisation of type, range and speed of motion (e.g.Halpern and Bleck 1979 versus Noble 1981 versus Wal-ters and Partridge 1957, Partridge and Walters 1959, whoclaim abdominal muscle involvement is higher, lower orequal in the trunk compared to hip flexion sit-ups).

Changes in velocity in dynamically performed ex-ercises usually after the muscle activity levels. Yet, only asmall number of the earlier reports has standardized thespeed of movement of the dynamic exercises (Godfreyet al. 1977; Miller and Medeiros 1987; McGill 1995) andin none of these studies were average electromyogramvalues for several subjects presented. For sit-ups, suchaverage values have only been reported in three previousstudies, two static ones comparing two different leg po-sitions (McGill 1995) and three different head positions(Shirado et al. 1995) at one static sit-up angle, and onedynamic sit-up study, testing the effects of a lumbarsupport device (Ross et al. 1993). Furthermore, in staticexercises there is a lack of systematic measurements invarious positions over a range of motion in sit-ups,which are needed to evaluate possible position-specificvariations in muscle activation levels.

The purpose of this study was to describe the meanEMG activity levels for three trunk and three hip flexormuscles during different static and dynamic sit-up andleg lift exercises, with standardised positions, velocityand modifications for the leg. Preliminary account of thepresent data have been given in abstract form (Anders-son et al. 1989, 1994).

Methods

Subjects

Six healthy, habitually active, male subjects participated in thestudy. Their average age, body mass and height were 25 (22–29)years, 75 (65–84) kg and 1.81 (1.76–1.87) m, respectively. The studywas approved by the Ethics Committe of the Karolinska Institute.All the subjects gave their informed consent to participate in thestudy.

Experimental setup

The subjects were placed on a horizontal bench (Fig. 1A). Theyperformed both dynamic and static exercises, either lifting theupper body (sit-ups) or the legs. In all exercises the arms were keptcrossed over the chest. When lifting the upper body the head washeld in a neutral position. All leg lifts were made with straight legs.A rest period of approximately 2 min was given between each se-parate task.

Dynamic exercises

Four different dynamic exercise were studied (Fig. 1B)

1. Trunk flexion situp (TF), with no movement at the hip and withthe lumbar back in contact with the bench

2. Hip flexion situp (HF), i.e. lifting a straight upper body via hipflexion

3. Spontaneous situp (SP), without any specific instruction, car-ried out as a combination of a trunk and a hip flexion situp

4. Leg lift (LL) via hip flexion, with straight knee joints, either bi-or unilaterally, with the pelvis in a neutral position.

Each of exercises (1)–(3) was performed in four different ways:with the legs straight or bent (knees at 90 and hips at 135°) and withor without support for the legs. In total 15 different dynamic taskswere studied. A series of four to five cycles of each task was per-formed, out of which one representative cycle was selected foranalysis. Thus, no estimation of intra-individual variability wasmade.

The movement ranges were from 0 to 30° in TF and 0 to 60° inHF, SP and LL (0° = straight body), respectively. The upper limitfor the movement was set by a manual goniometer. The angles weretaken between the horizontal and a line from the T12-L1 level inTF and the hip joint in HF, SP and LL to the C7 level when liftingthe upper body, and to the lateral malleolus in the leg lift. Theexercises were performed to the rhythm of a metronome set at 1 Hz.The duration of the entire movement, including upward anddownward phases, was 2 s in TF and 4 s in HF, SP and LL,respectively. Thus, all the dynamic exercises were performed with amean velocity of approximately 30° · s–1.

Static exercises

The TF, HF and bi- and unilateral LL were also studied in differentstatic positions. The TF and HF were performed either withstraight or bent supported legs. The static angles were in TF: 10, 20,30° and maximal (approximately 40–45°); and in HF and LL: 10,30 and 60° (angles defined as in dynamic exercises). A total of 23static positions were investigated. To obtain the specific angles inthe static tests a manual goniometer was used. Each static task wasperformed once by every subject.

Movement recordings

Two electrogoniometers were used to record movements (Fig. 1A).One was placed at the trochanter major on the left side formovements at the hip joint and the other under the bench with astring taped to the skin over the spinous process of C7 for move-ments of the upper body. Motions in TF and LL were indicated byeach goniometer separately (Fig. 2), whereas both goniometersresponded to movements in the HF (parallel response, Fig. 2) andSP exercises (sequenced response).

Fig. 1 A Experimental setup. The hip goniometer (a) indicatesmovements at the hip the bench goniometer (b) records both trunkand hip movements. B) The exercises employed and their overallangular excursions: trunk flexion (TF), hip flexion (HF), spontaneoussitup (SP) and leg lift (LL). C) Positions of the surface electrodes onthree abdominal muscles (obliquus externus OE, rectus abdominisRA, obliquus internus OI) and the two hip flexor muscles (sartoriusSA, rectus femoris RF). The site for insertion of the intra-muscularfine-wire electrodes into the iliacus (IL) muscle is also indicated. Forfurther description, see Methods

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Electromyography

The EMG recordings were made on the left side of the body, withsurface electrodes from the hip flexor muscles: sartorius (SA) andrectus femoris (RF) and from the abdominal muscles: rectus ab-dominis (RA), obliquus externus (OE) and obliquus internus (OI)(Fig. 1C). The surface electrodes used were Beckman miniaturesilver/silver chloride, diameter of pick-up area 4 mm, with a fixedinterelectrode distance of 8 mm. The EMG of the hip flexor iliacus(IL) was recorded with indwelling bipolar fine-wire electrodes. Thewires were of stainless steel, 0.22 mm in diameter, teflon-insulatedexcept for 3 mm at the tip, with a 5-mm interelectrode distancewhen hooked to a needle (0.7 × 70 mm). The needle was insertedwith the wires, after local anaesthesia, about 3-cm lateral to thefemoral artery, 1-cm medial to the SA muscle and 1-cm inferior tothe inguinal ligament (Fig. 1C). The position of the wires in ILmuscle was confirmed with ultrasound technique in one subject (cf.Andersson et al. 1989, 1994, 1995, 1996).

All EMG signals were differentially pre-amplified (100 times)close to the site of the electrodes using customized light-weightamplifiers attached to the skin. Signals were then bandpass filtered10–1000 Hz, further amplified (10–50 times) and collected togetherwith the signals from the goniometers on magnetic tape for sub-sequent analogue to digital conversion (sampling frequency 0.5kHz) and computer analysis.

Average EMG amplitude was calculated from the rectified andfiltered EMG signals during the whole movement cycle in dynamicexercises, beginning 200 ms before the start of motion (cf. Fig. 2).Also, the upward and downward phases of the movement wereanalysed separately. In static exercises mean EMG amplitude va-lues were taken during the first 2 s after a stable position had beenattained.

The average EMG amplitude value for each individual musclein each dynamic exercise was expressed as a percentage of thehighest EMG value observed for that muscle in any of the dynamicexercises for each subject. The corresponding procedure was exe-cuted separately for the static tasks. The mean of the percentagevalues and standard error of the mean (SEM) for the six subjectswere calculated both for the individual muscles and for the ab-dominal (RA, OE, OI) and hip flexor synergies (IL, RF, SA).

Statistics

Differences in EMG activity between specific exercises (dynamicand static tasks separately) for each muscle were tested for sig-nificance with a one-way ANOVA, using a repeated measures de-sign with one repeating factor. The post-hoc Newman-Keuls testwas used to detect significant differences in relative EMG levelsbetween specific tasks (P < 0.05).

Results

Representative recordings of movement and EMG pat-terns are shown in Fig. 2 for dynamic TF, HF and ipsi-lateral leg lift (LLli). The general appearance of thegoniometer and EMG curves for each task was similar forall subjects. In Fig. 2 typical variations in EMG are de-monstrated between exercises, individual and groups ofmuscles as well as positions in the range of motion.Generally, a selective activation was seen for the ab-dominals (RA, OE, OI) in TF with straight legs and forthe hip flexors (IL, RF, SA) in ipsilateral LL. The EMGamplitude for all active muscles during an exercise was onaverage about 50% higher in the upward compared to thedownward phase. In HF, the abdominals showed a de-crease in activity with increased degree of flexion, result-ing in a period of low activity in the uppermost position.In TF, the pattern was reversed. This variation in EMGwith position is more systematically accounted for in theseries of experiments with static exercises (cf. below).

Dynamic exercises

Mean activity levels for the six subjects in the differentdynamic exercises are shown in Fig. 3 for each muscle,and in Table 1 for the trunk and the hip flexor synergies,respectively.

In TF, the abdominal muscles were activated to amoderate to high degree, RA showing the highest relativevalue, 58%–70% of the recorded maximum, and OI 52%–54%. The values for OE, 18%–24%, were significantlylower than in any of the other exercises in which theupper body was raised. No significant difference was seenin the EMG levels of any of the abdominal muscles de-pending on leg position or whether the legs were sup-ported or not. The trunk flexor synergy as a wholeshowed an average relative activation of 45%–48%.

The hip flexor muscles demonstrated low activity inall types TF, average values for the hip flexor synergyranging from 9% to 19%. The highest values were pre-sent for IL and SA in TF with bent legs, 15%–18% and20%–30%, respectively.

In dynamic HF, the abdominal synergy demonstratedactivity levels (75%–80%) which were significantly higherthan in TF. The difference was most marked for OE. Nosignificant differences in relative EMG levels for theabdominals were seen between the leg modifications:straight supported (ss), straight unsupported (s) andbent supported (bs). The mean values for these threetasks were: RA 77%–84%, OE 78%–86%, and OI 62%–

Fig. 2 Typical recordings of angular displacements and rectified andfiltered electromyogram (EMG) in dynamic trunk flexion (TF) andhip flexion (HF) exercises with straight and supported legs (ss), andipsilateral leg lift (LLli). The graphs represent angular displacement ofthe bench goniometer (BG) and the hip goniometer (HG), EMG fromRA, OE, OI, IL, RF, and SA. The amplifications and scales on the yaxis for each muscle are the same in the three situations. For otherdefinitions see Fig. 1

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75%. The fourth modification, HF with bent legs and nosupport (b in Fig. 3 and Table 1) is not directly com-parable with the other tasks. It was mechanically im-possible to execute, and resulted instead in a TF to anangle of about 40–45°, with a static period at the mostflexed position of about 1 s. In this task the averagevalue for the trunk flexor synergy was 80%.

The hip flexor synergy showed, in contrast to theabdominals, distinct changes in activity with different legmodifications, supported legs resulting in higher relativelevels. Highest average values were obtained with bs,

80%, 90% and 68% for IL, RF and SA, respectively.Average values for ss, were lower (60%, 66% and 50%)and decreased even further to 49%, 31% and 38%, in s.Statistical significance was present for the differencebetween s and bs for all three hip flexors and between sand ss for RF. In the incomplete b leg task, resulting in amaximal TF and no or minor HF, the activation of IL,RF and SA was low, 29%, 9% and 28%, respectively.The average values for the hip flexor group were sig-nificantly different among all HF tasks: bs (79%), ss(59%), s (39%) and b (23%), respectively.

In SP, where the subjects started to flex the trunk firstand then the hip, the muscle activities showed similarlevels and patterns as in pure HF. There was no sig-nificant differences between corresponding HF and SPtasks for any muscle, except for lower values for the hipflexor synergy with ss legs in SP (40%) compared to inHF (62%). When performing SP with b legs two of thesix subjects succeeded in lifting the whole lumbar backand reaching the stipulated hip angle of 60°.

Fig. 3 Mean electromyogram amplitudes (and SE) in dynamicexercises (six subjects) for each muscle expressed as a percentage ofthe highest value recorded for that particular muscle in any of thedynamic exercises: trunk flexion (TF), hip flexion (HF), spontaneoussitup (SP) and leg lift (LL). The first three exercises were performed infour different ways: with legs straight supported (ss), straightunsupported (s) bent supported (bs) and bent unsupported (b),respectively. A further three types of leg lifts was performed: bilateral(2), ipsilateral (1i) or contralateral (1c). For other definitions see Fig. 1

Table 1 Mean relative electromyogram amplitude values in dy-namic exercises (six subjects) for the two muscle synergies: trunk(rectus abdominis, obliquus extermis, obliquus intermis) and hipflexors (iliacus, rectus femoris, sartorius) in trunk flexion (TF), hip

flexion (HF), spontaneous situp (SP) and leg lift (LL). The TF, HFand SP were performed with legs straight supported (ss), straightunsupported (s) bent supported (bs) and bent unsupported (b) andLL as bilateral (2) ipsilateral (1i) or contralateral (1c) leg lifts

TF HF SP LLss s bs b ss s bs b ss s bs b 2 1i 1c

Trunk Flexor SynergyMean 48 48 45 45 75 80 76 80 71 75 72 77 62 8 8SEM 14 16 11 12 7 2 6 6 5 1 3 4 6 1 2Hip Flexor SynergyMean 9 9 16 19 62 39 79 23 40 32 73 28 76 65 7SEM 2 2 3 4 5 3 6 4 4 3 5 4 6 5 2

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In bilateral LL, the abdominal muscles RA and OEwere highly activated (70% and 65%, respectively),nearly to the levels reached in HF and SP. The OI, onthe other hand, showed a moderate activity level (50%),similar to that in TF. The average value for the ab-dominals in bilateral LL was 62%. No or very low ac-tivity (< 10%) was present for the abdominals if onlyone leg was raised, irrespective of side.

The hip flexors IL and SA were highly activated in bi-and LLli, with EMG values for IL of 86% and 68%, andfor SA of 93% and 71%, respectively. In bilateral LL thevalues for these two muscles were significantly higherthan in the dynamic TF tasks as well as in HF and SPperformed with unsupported legs. The correspondingvalues for RF were clearly lower, 48% and 56%. The RFvalues in LL were similar to those in HF and SP with sslegs. The mean values for the hip flexor synergy was 76%and 65% in bi-LL and LLli, respectively. Negligible ac-tivity was seen in the hip flexor muscles when the con-tralateral leg was lifted.

Static exercises

Mean muscle activity levels in the different static ex-ercises for the six subjects are shown in Fig. 4 for eachmuscle, and in Table 2 for the two flexor synergies.

In TF a similar increase in activity level with anglewas seen for all the abdominal muscles. As in the dy-namic exercises, the activity level for OE was compar-ably low, changing from 7% to 55% of the recordedmaximum from 10° to the maximally flexed angle (about40 to 45°), compared to RA, 24%–75%, and OI, 18%–87%. There were no significant differences between thetwo leg positions (ss and bs).

The hip flexor muscles demonstrated no or low activityin all the static TF exercises. At the maximal TF angle,RF showed an average of 22% in both leg positions.

In HF the trunk flexor synergy showed high activa-tion values at the smallest angle (average 88%–92% at10°) which decreased to 80%–84% at 30° and to 45%–46% at 60°, respectively. The values for each abdominalmuscle at the final angle were significantly lower than atthe initial angle. No significant differences were seenbetween the two different leg positions at correspondingangles. The abdominal activation at 10 and 30° of HFwas significantly higher, both for individual muscles andthe trunk flexor synergy, than in any static task in TF,except at 30° and maximal flexion angle for RA and OI.The absolute level of activation of the abdominal mus-cles was higher in static HF at 10° than during the dy-namic HF, where the abdominals also were performing astatic contraction. The ratios were 1.6:1 (RA), 1.5:1(OE) and 1.3:1 (OI), respectively.

The individual hip flexor muscles behaved differentlywith changes in hip angle. In HF with ss the relativeactivity, from 10 to 60° tended to increase for IL, 46%–63% and for SA, 46%–54%, but decreased for RF, 66%–32%. However, the only significant change was the lowerRF activity at the final angle. The highest hip flexoractivity was seen in the bs, at an angle of 30° in the hip,

Fig. 4 Mean electromyogram amplitudes (and SE) in static exercises(six subjects) for each muscle expressed as a percentage of the highestvalue recorded for that particular muscle in any of the static positions.In trunk flexion (TF) the positions were 10, 20, 30° and maximalflexion angle, and in hip flexion (HF) and leg lifts (LL) 10, 30 and 60°.For abbreviations, see Figs. 1 and 3

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for RF (88%), IL (74%) and SA (71%). The corre-sponding value for the hip flexor synergy (78%) wassignificantly higher compared to all other static situptasks, except for 60° HF with bs. In general, the in-dividual hip flexor activity tended to be higher in bscompared to in ss at corresponding angles, the differencebeing significant only for RF at 60°.

In LL the activation of the abdominals was moder-ate, 38%–47%, and of similar magnitude at differentangles when keeping both legs raised. In contrast, theactivation was absent or low (< 10%) when one leg waslifted, irrespective of side.

Individual hip flexor muscles showed different pat-terns with changing angles also in LL. Typical in bi-LLand LLli was an increased activity with angle both for IL(32%–88%) and for SA (35%–81%). For these twomuscles, the most flexed position in bilateral LL de-monstrated the highest value of all the static exercises.The RF, on the other hand, showed relatively low ac-tivity levels, 31%–42%, irrespective of angle. With thecontralateral leg lifted, the hip flexors were essentiallysilent at all angles.

Discussion

The present results showed a task specificity in the ac-tivation levels of trunk and hip flexor muscles in variousforms of common training exercises used in sports andrehabilitation. Certain modifications led to specific ac-tivation of hip and trunk flexor synergies as well as ofindividual muscles. Results from dynamic exercises wereessentially in agreement with those from correspondingstatic measurements, the latter also allowing for a moredetailed position specific analysis. When comparing re-lative activation levels it has to be remembered that alsoduring dynamic exercises some of the investigatedmuscles actually perform static, stabilising, contractions.It should also be recognised that the EMG level ofmuscles undergoing length changes during dynamic ex-ercises represent a mean of a concentric and an eccentricmuscle action.

The abdominal trunk flexor synergy could be selec-tively activated in dynamic TF situps, particularly theones performed with straight legs. Higher relative EMG

values for the abdominals were reached in spontaneoussitups, but they also involved hip flexor activation. Theseexercises were the only ones studied where the abdom-inal muscles underwent length changes. Both the dy-namic HF situps and bilateral LL, the abdominals actstatically to maintain the trunk segment rigid and sta-bilise the pelvis, respectively. Conversely, a selective in-volvement of the hip flexor synergy was only evident inLLli. No or negligible abdominal activity was seen insingle leg lifts regardless of side, which is in agreementwith earlier findings (de Sousa and Carman 1974; Fur-lani et al. 1972). It would seem that the fact that theother leg is resting straight on the floor is enough tostabilise the pelvis during the lifting of one leg. Duringthis exercise, contribution to the lumbar spine stabilityin the frontal plane is made by the quadratus lumborummuscle and the deep lateral portion of erector spinae onthe opposite side, probably to counteract the lateraltorque produced by psoas on the leg raising side (seeAndersson et al. 1995; 1996). The only situation wherethe hip flexors IL and SA were statically activated was inTF situps performed with bent legs, either with orwithout supported legs. Here the activation of thesemuscles appears to be needed to prevent a backwardtilting of the pelvis.

Within the respective synergies the individual musclesgenerally acted in concert. Notably among the abdom-inals, the relative level of activation of the OE musclewas lower than that of the RA and the OI in TF situps.Among the hip flexors, examples of conspicuous ex-ceptions from a common behaviour were the changes inEMG with hip angle in static bi-LL and LLli. A highactivation of IL and SA with a relatively low involve-ment of RF was achieved in LL at extreme angles.Further, the activity of RF decreased with higher anglesin HF situps with straight legs, in contrast to IL and SA.As has been suggested the different behaviour of the RFmuscle might be related to its additional function as aknee extensor (Carlsoo and Molbech 1966; Carlsoo andFohlin 1969; Fujiwara and Basmajian 1975). Thus, thismuscle cannot be regarded as representative for thewhole hip flexor synergy which has been suggested ear-lier (Walters and Partridge 1957).

Comparing static HF situps and leg lifts, all the ab-dominal muscles showed significantly higher average

Table 2 Mean relative electromyogram amplitude values in staticexercises (six subjects) for the two muscle synergies: trunk (rectusabdominis, obliquus extermis, obliquus intermis) and hip flexors

(iliacus, rectus femoris, sartorius). For definition of angles andabbreviations, see Methods, Fig. 4 and Table 1. mx Maximallyflexed angle

TF HF LLss bs ss bs 2 1i 1c

10 20 30 mx 10 20 30 mx 10 30 60 10 30 60 10 30 60 10 30 60 10 30 60

Trunk Flexor SynergyMean 17 28 52 66 16 28 45 72 88 80 45 92 84 46 38 47 46 7 9 9 9 9 9SEM 3 4 6 6 2 4 6 6 3 4 5 2 3 4 3 3 4 2 1 1 2 2 2Hip Flexor SynergyMean 4 4 6 13 6 8 8 15 53 55 49 61 78 64 39 57 66 36 50 59 4 4 4SEM 1 1 1 3 1 2 1 2 5 5 6 5 6 6 5 6 8 5 4 7 1 1 1

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activity levels at 10° and 30° of flexion in the hip jointwhen raising the whole upper body straight compared tocorresponding angles in bilateral LL. In this range ofmotion our results are in agreement with earlier reportsshowing that the abdominal muscles are markedly lessinvolved in double leg raising compared to exercisesinvolving raising the upper body (Lipez and Gutin 1970;Sheffield 1962). However, at 60° hip angle the activitylevels were similar in the two tasks. This, again, stressesthe importance of taking the position specificity intoconsideration when evaluating exercises.

The present results agree with earlier investigationsreporting moderate to marked activity levels in bilateralLL for RA and OE and somewhat lower for OI (Floydand Silver 1950; Sheffield 1962; Flint 1965a; Flint andGudgell 1965; Carman et al. 1972; de Sousa and Fur-landi 1974). One exception is the study of Partridge andWalters (1959) in which they have reported a very lowabdominal activation in bilateral leg lifts. A possibleexplanation could be that the exercise in their case wascarried out with the pelvis tilted forward. This specula-tion was supported by additional trials in the presentstudy where the pelvic inclination was suddenly changedduring static bilateral LL at 10°. A marked increase inactivity for all abdominal muscles, especially for RA,was seen during a backward pelvic tilt combined with ahypo-lordotic back, and a corresponding distinct de-crease forward pelvic tilt combined with a hyper-lordo-sis. A similar pattern has been shown for the threeabdominal muscles during pelvic tilts in standing(Oddsson and Thorstensson 1990). The opposite patternwas observed for IL and SA, whereas the changes inactivity levels for RF were not so clear.

The abdominal activity increased with static angle inTF situps. This could be due to a decreased musclelength and thus a lower force producing capacity (Inmanet al. 1952; Bigland and Lippold 1954) and an increasedinner resistance of the trunk, e.g. due to elongation ofpassive structures (Inman et al. 1952; Lippold 1952;Bigland and Lippold 1954; Komi and Buskirk 1972).Evidently, these effects surpassed the effects of a de-creased external moment of the gravitational force act-ing on the trunk and a possible increase in muscle leverarm length. The opposite pattern that is a decreasedabdominal activity with angle, was seen in HF situps,and is clearly a consequence of a decreased externalmoment on the upper body due to increased inclination(Inman et al. 1952; Lippold 1952; Bigland and Lippold1954; Komi and Buskirk 1972) since the length of themuscles and their mechanical advantage remained con-stant. In the different static positions during bilateral LLwith the pelvis maintained in a stable, neutral, positionby the abdominals, the activity levels of the abdominalsremained constant. This was true in spite of a change inexternal moment at the hip caused by changes of thelever arm length for the gravitational force acting on thelegs at different angles. Earlier, an absence of activity hasbeen reported for the iliopsoas muscle during the first30° of a ‘‘full situp’’ with straight, but not with bent

unsupported legs (LaBan et al. 1965). This is probablydue to an initial TF which has to be made since the legsare unsupported. During this initial TF the present re-sults showed that IL was activated with bent and notwith straight legs. Similar results were obtained when aspontaneous situp was to be performed.

Although no systematic comparison was made be-tween the different phases of the dynamic exercises, it wasobserved that the EMG values were about 50% higherduring the upward, concentric, compared to the down-ward, eccentric phase for all muscles in the different ex-ercises. The RA and OI showed an even larger differenceduring the TF situps. This is in accordance with earlierfindings of less muscle activation needed to produce acertain force eccentrically than concentrically (Biglandand Lippold 1954; Komi and Buskirk 1972; cf. also Segerand Thorstensson 1994; Cresswell and Thorstensson1994). There have been several reports in the literatureindicating a higher muscle activation in the upwardcompared to the downward phase during ‘‘full situps’’ forthe abdominal muscles and RF (Walters and Partridge1957; Sheffield 1962; Flint 1965a,b; Flint and Gudgell1965; Lipez and Gutin 1970; Gutin and Lipez 1971;Girardin 1973; Godfrey et al. 1977; Ricci et al. 1981),although an objective quantification is generally lacking.

In the present study it was seen that changes in legposition or whether the legs were supported or not, didnot markedly alter the total average activity levels for theabdominal muscles, within each type of situp exercise.This is to be expected since none of the abdominalmuscles passes the hip joint. Our results are in agreementwith previous studies on HF situps reporting equal ac-tivity levels between straight and bent supported legs forRA (Lipez and Flint 1970; Gutin 1965a; McGill 1995)and the two oblique muscles (McGill 1995). Also, anearlier study on TF situps showed equal activity levelsbetween two leg modifications for RA and OE (Ekholmet al. 1979). However, there are also reports on varyingactivity levels with different leg modifications during HFsitups for RA (Walters and Partridge 1957; Flint 1965a;Flint and Gudgell 1965; Lipez and Gutin 1970; Godfreyet al. 1977; Noble 1981) and OI (Noble 1981). The rea-sons for this discrepancy are not clear, but may be thelack of objective quantification of EMG or standardi-sation of the tasks in these studies.

There is a misconception that the involvement of thehip flexors will decrease by flexing the legs in situps (seeGutin and Lipez 1971; Girardin 1973). We found theopposite pattern, i.e. the hip flexor muscles were alwaysengaged to a higher degree with bent than with straightlegs, in all forms of situp tasks, provided that the exercisewas mechanically possible. This was not only seen withthe legs supported in HF and spontaneous situps, butalso with unsupported legs for the two subjects whosucceeded in lifting the whole trunk in all four leg mod-ifications. It has been suggested that the reason for theneed of a higher activation in these situations is likely tobe the decrease in force producing capability caused bythe muscle being shortened (Inman et al. 1952; Bigland

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and Lippold 1954). This pattern has also been shownboth for the vertebral psoas and IL muscles in HF situps(Andersson et al. 1989, 1994, 1995, 1996). In recentpublications, we have also reported data comparing thetwo portions of the iliopsoas muscle (Andersson et al.1995, in press). It was shown that the vertebral psoasmuscle is markedly engaged, whereas IL is quiet or ac-tivated to a low level, during a TF situp at a maximalangle, with the legs flexed 90° at the hip and knees and thelower legs placed on a chair without support, a taskwhich has been considered not to involve the psoasmuscle (Ash and Burnett 1989).

The choice of a specific training exercise should dependon the purpose of the training, which may be selectively toincrease the strength of a certain muscle or muscle groupto regain function after injury or improve performance ina certain sport. Guiding this choice in the latter case couldbe the strength profile among elite competitors in thatparticular event. Higher than normal maximal strengthhas been demonstrated by different categories of eliteathletes in TF, and also in HF for elite gymnasts (An-dersson et al. 1988). An incentive for selecting trainingexercises in a rehabilitation context may be a selectivemuscle weakness, even though it is generally difficult todetermine whether such a weakness is a cause or a con-sequence of the ailment. In low back pain patients of theinsufficiency type, we have observed a lower maximalstrength capacity in HF (Thorstensson et al. 1985). Ex-ercises to improve selectively the strength performancewould be TF situps for weakened abdominal muscles andunilateral LL for hip flexor muscles. A further incentive totrain the hip flexor muscles can be derived from resultsemphasising the role of these muscles, particularly thepsoas and IL, for the stability of the lumbar spine, pelvisand hip, especially in sitting (Andersson et al. 1995, 1996).It is noteworthy that such strength training should beaccompanied by stretching exercises, since a shortenediliopsoas muscle may contribute to an undesired posi-tioning of the pelvis and lumbar spine. The present datamay serve as a guide in making test and training exercisesmore specific and purposeful for various needs.

Acknowledgements The authors want gratefully to acknowledgethe contribution by Dr. Lars Oddsson, Prof. Per-Olof Astrand, Dr.Toshio Moritani, Dr. Anders Angerbjorn, Dr. Helen Grundstromand Photographer Styrbj�orn Bergelt. This study was supported bygrants from the Swedish Work Environment Fund (90-0798) andthe Research Council of the Swedish Sports Federation (27/91).

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