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RIGINAL ARTICLE

ffect of Isometric Upper-Extremity Exercises on thectivation of Core Stabilizing Muscles

ami P. Tarnanen, MSc, Jari J. Ylinen, MD, PhD, Kirsti M. Siekkinen, PT, Esko A. Mälkiä, PhD,
annu J. Kautiainen, BA, Arja H. Häkkinen, PhD
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ABSTRACT. Tarnanen SP, Ylinen JJ, Siekkinen KM,älkiä EA, Kautiainen HJ, Häkkinen AH. Effect of isometric

pper-extremity exercises on the activation of core stabilizinguscles. Arch Phys Med Rehabil 2008;89:513-21.

Objective: To evaluate whether isometric exercises for thepper extremities could sufficiently activate core stabilizinguscles to increase muscle strength.Design: Cross-sectional study.Setting: Department of physical medicine and rehabilitation

t a Finnish hospital.Participants: Healthy adult women (N�20).Interventions: Not applicable.Main Outcome Measures: Peak isometric strength of the

ack and abdominal muscles was measured and relative loading intest exercises was evaluated by surface electromyography.Results: The rectus abdominis and obliquus externus abdo-inis were activated to the greatest degree in a bilateral shoul-

er extension exercise and the average surface electromyo-raphic activity was 114% and 101% compared with themplitude elicited during the maximal isometric trunk flexionxercise. Horizontal shoulder extension elicited the greatestctivation of the longissimus and multifidus muscles. In thisxercise, the activity levels of the left side multifidus andongissimus muscles were 84% and 69%, respectively, com-ared with the level of activity elicited during trunk extension.Conclusions: Of all the exercises studied, bilaterally per-

ormed isometric shoulder extension and unilaterally per-ormed horizontal shoulder extension elicited the greatest lev-ls of activation of the trunk musculature. Thus, it can bessumed that these exercises elicit sufficient levels of contrac-ion of the trunk muscles for the development of their endur-nce and strength characteristics in rehabilitation.

Key Words: Abdominal muscles; Back; Electromyography;xercise, isometric; Rehabilitation.© 2008 by the American Congress of Rehabilitation Medi-

ine and the American Academy of Physical Medicine andehabilitation

HERAPEUTIC EXERCISE is an effective method for thereduction of pain and functional impairment resulting from

he pain, as well as of absences from employment in personsuffering from chronic, nonspecific low back pain (LBP).1-3

From the Department of Health Sciences, University of Jyväskylä, Jyväskylä,inland (Tarnanen, Siekkinen, Mälkiä, Häkkinen); Department of Physical Medicinend Rehabilitation, Jyväskylä Central Hospital, Jyväskylä, Finland (Ylinen, Häkki-en); and Medcare, Aänekoski, Finland (Kautiainen).No commercial party having a direct financial interest in the results of the research

upporting this article has or will confer a benefit upon the authors or upon anyrganization with which the authors are associated.Reprint requests to Sami P. Tarnanen, MSc, Dept of Physical and Rehabilitationedicine, Jyväskylä Central Hospital, Keskussairaalantie 19, Jyväskylä, Finland

0620, e-mail: [email protected].

s0003-9993/08/8903-00083$34.00/0doi:10.1016/j.apmr.2007.08.160

here is also evidence that the performance capacity of LBPatients can be improved with exercise.4 Further, exercise haseen included as a central component in several recommenda-ions for the conservative treatment of LBP.5,6

However, on the basis of a systematic literature review andreatment recommendations, it is still not possible to determinehat kinds of exercises should be performed and how they

hould be prescribed, because the interventions used in theifferent studies have differed greatly. There have also beenaws in the description of how the exercises included in aumber of studies were actually performed.7-9

Many functional and structural changes of the back aressociated with LBP. In comparison with healthy people, thextension and flexion strength levels of LBP patients are di-inished and the extension to flexion strength ratio is de-

reased.10,11 Diminished muscle strength levels have also beenbserved in the lower limbs.12 In addition to the musculartrength characteristics, the muscular endurance of the trunkxtensor muscles has also been proven to be diminished.13,14

The reaction times of the trunk muscles of LBP patients,hich are associated with coordination and movement control,ave been shown to be increased and control of lumbar spineosition, postural and positional awareness, as well as psy-homotor speed, have been noticed to be diminished. Further,he degree and speed of postural sway is greater and theecruitment patterns of the muscles have been shown to beltered.15-21

In relation to anatomic structures, the cross-sectional area ofhe muscles in the lumbar spine region was found to be de-reased and there was also shown to be a decrease in theelative numbers of type I muscle fibers.22,23 Collagen synthe-is in LBP patients has been shown to be decreased comparedith healthy people24; however, the cause-effect relationshipetween these changes and back pain is still unclear.Strength training aims to achieve structural, neural, hor-onal, and metabolic changes in the musculoskeletal and neu-

omuscular systems in order to improve the functional andtructural changes associated with LBP. These changes can bebserved, for example, as hypertrophy of the muscles andonnective tissues, improvements in muscle activation levels,ncreased energy stores, and as increased activity of enzymesnd then, ultimately, as an improvement in performance capac-ty.25,26 With the selection of correct and appropriate exercise,t is possible to focus these changes on the desired structures.n order to achieve a training effect, exercises should be chal-enging in terms of their intensity and volume, and they shouldlso be performed for a sufficiently long period of time.26

In the determination of the appropriate intensity level of thexercises used in rehabilitation, it is often possible to use theeneral recommendations that have been created for healthyeople who are just commencing exercise programs. In thearly, familiarization stage of an exercise program, exerciseoads should be at decreased levels. However, after this initialhase, the load should be gradually increased to a level that is
ufficient to result in changes in the body’s tissues. For exam-
Arch Phys Med Rehabil Vol 89, March 2008

mail to: [email protected].

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514 UPPER-EXTREMITY EXERCISES FOR TRUNK MUSCLES, Tarnanen

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le, to achieve changes in muscle endurance characteristics, aufficiently high level of resistance for the designed exerciseshould be 50% to 70% of 1 repetition maximum (1-RM).27

imilarly, for the improvement of muscle strength characteris-ics, the resistance level should be 60% to 80% of 1-RM.

The activity level of the muscles during the performance ofhe exercises can be estimated using surface electromyography.n the studies that have looked at the activation of the trunkusculature, the resistance used in the exercises has been the

ubject’s own body weight or the load has been directed at therunk musculature through movements of the upper and lowerimbs.28-34 The effect of an unstable base of support on theurface electromyographic amplitude levels of the trunk mus-ulature has also been reported in various studies.35-38

In earlier studies involving surface electromyography, themplitudes recorded during many of the studied exercises wereoticeably less than 50% of the maximal amplitude levels.hus, these exercises may not be sufficiently intense to develop

he strength characteristics of the trunk musculature.29,31,32,34

he effects on trunk muscle activity during the performance ofpper-limb exercises have been studied earlier by Arokoski29,31

nd Behm37 and colleagues, but either a standard resistanceevel was used or the load was not specified. The aim of thisurrent study was to determine, with surface electromyogra-hy, if upper-extremity exercises are able to load the coretabilizing muscles sufficiently to increase muscle strength.

METHODSThe study was performed in the biomechanics laboratory in

he Department of Physical and Rehabilitation Medicine atyväskylä Central Hospital. The research project received thepproval of the local ethics committee. We provided informa-ion to all subjects on how the study would be conducted andhat it would include. All subjects signed a consent form prior

o participation in the study.

Table 1: Background Information of the Study Subjects

Characteristics Mean � SD

Age (y) 38.1�7.2Weight (kg) 62.9�6.3Height (cm) 165.2�6.7BMI (kg/m2) 23�2Physical activity level (METS)* 3�1

bbreviations: BMI, body mass index; SD, standard deviation.A time-weighted intensity average metabolic equivalent (METS)evel (work, commuting, leisure time physical activity, controlledxercises).

Table 2: Elec

Muscle Electrode

Rectus abdominis46 1cm above the navelfrom the mid-line

Obliquus externus abdominis46 Just below the curvat

Longissimus54 3cm laterally from theMultifidus54,55 2cm laterally from the

rch Phys Med Rehabil Vol 89, March 2008

articipantsFor this study, we recruited 20 women, aged 20 to 45 years.

ubjects were secretarial and nursing staff as well as doctors atyväskylä Central Hospital. The background information of theubjects is shown in table 1. Subjects were excluded if they hadny neuromuscular, orthopedic, or cardiorespiratory problemshat prevented the type of physical exertion required for thistudy. The physical activity level of all subjects was deter-ined using a written questionnaire. From the responses, a

ime-weighted intensity average metabolic equivalent level wasalculated for each subject’s daily active time (work, commut-ng, leisure time physical activity, controlled exercises) usinghe MetPro software program.a

urface Electromyography RecordingWe used an 8-channel ME3000P8 surface electromyographb

or measurements. Raw electromyographic data were recordedsing a sampling frequency of 1000Hz and band-pass filteredsing a band-width of 8 to 500Hz (Butterworth). A differentialmplifier was used to strengthen and filter the measured signal,s well as for the dampening of noise, with a common modeejection ratio greater than 110dB, a root mean square of noiseess than 1.6�V, and an amplification level of 412. The am-lifier’s feed impedance was greater than 10G�. The surfacelectromyographic signal was changed into a digital formatith a 12-bit analog-to-digital converter, after which it was

aved on a computer for analysis. The raw electromyographicignal was rectified and averaged. The average amplitude levelin �V) of every exercise was calculated as the average of eachf the analysis period’s data segments (100ms). A 4-secondime period was selected for analysis at the point where thelectric activity level was at its greatest in each exercise.

Round, single use Ag/AgCl surface electrodesc were used.he skin at the electrode attachment sites was shaved, cleanedith sandpaper, and then wiped with alcohol in order to de-

rease skin impedance. The pairs of electrodes were positionednto the rectus abdominis, obliquus externus abdominis, lon-issimus, and multifidus muscles on both sides of the body inhe direction of the muscle fibers (table 2). The distance be-ween the mid-points of the electrodes was 25mm. The refer-nce electrodes, to which a preamplifier was attached, wereositioned in the area of the iliac spine. After the attachment ofhe electrodes, a period of 15 minutes was allowed to passefore the commencement of measurements.

easurementsIn the first phase, we determined individualized adjustments

or each subject in the isometric strength measurement framend they had the opportunity to practice the exercises until

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515UPPER-EXTREMITY EXERCISES FOR TRUNK MUSCLES, Tarnanen

erformance technique was correct. The average time betweenhis familiarization and the actual measurement phase was 8ays (range, 4–17d). On the second test occasion the subjectserformed 3 submaximal warm-up trials for each exercise.eference exercises were performed first and then the upper-

imb exercises. Both were performed in a random order. A resteriod of at least 1 minute was allowed between each effort andhere was a 5-minute rest period between the reference andpper-limb exercises.39,40

Reference exercises. Isometric exercises of the trunk werehe exercises to which we compared the actual exercises beingtudied (fig 1). They were performed in a standing position inhe measurement frame with the feet positioned side by sidend 20cm apart. For the trunk flexion (exercise 1) and exten-ion (exercise 4), the subjects were supported in the measure-ent frame with a 10cm wide pelvic support and a 5cm wide

elt at the level of the greater trochanter. An additional 10cmide support was used at the midpoint of the thighs. A harnessas fastened around the shoulders and chest just below the

lavicles and this was attached to a strain-gauge dynamometerd

ith a metal lock. For the lateral flexion to the right (exercise 2)nd left (exercise 3), the subject was positioned sideways inelation to the measuring frame so that the upper edge of theushioned plate, which was attached to the dynamometer, wasositioned just below the level of the acromion process. Fixa-ion was also provided at the level of the pelvis and thighs. Thepper limbs were held folded at the front of the chest and theubjects set their gaze at a fixed point at the level of the eyes.fter the command to commence each exercise, the subjectserformed a maximal effort for approximately a 5-seconderiod. Two maximal efforts were performed. If the measuredtrength level increased more than 10% from the first effort, 1dditional effort was performed. Repeatability of isometrictrength tests has been evaluated elsewhere.41

Upper-limb exercises. Isometric exercises of the upperimbs were performed with the lower limbs set in a stridingosition in which the heel of the left foot was in the same lines the toes of the right foot (fig 2). The lateral distance between

ig 1. Maximal isometric force measurement of the trunk. (A) Exernd (C) exercise 4: extension.

he feet was 20cm. The left knee was in slight flexion and the u

elvis was fixed as in the reference exercises. The right elbowas at 90° flexion. The upper arm was held against the side of

he body in the right shoulder extension (exercise 5) and flexionexercise 6). In the right shoulder horizontal extension (exer-ise 7) and flexion (exercise 8), the shoulder was held in 90° ofbduction so that the arm was in the horizontal plane. Duringhese exercises, the test subject held on to a handle that wasttached to the dynamometer. Bilateral shoulder extension (ex-rcise 9) was performed with the upper arms held in 45° ofexion. The angle of the upper arm was determined with anlectronic inclinometer.e In this exercise, each subject grippedhe bar that was attached to the dynamometer, with a palmown grip with the hands 20cm apart. Subjects were instructedot to move their trunk. Two maximal efforts of each exerciseere performed and the effort with a higher strength value was

hosen for analysis. The relative loading of the trunk musclesas determined by comparing the ratio of surface electromyo-raphic amplitude during maximal isometric effort of the up-er-limb exercises to the amplitude elicited during maximalsometric efforts of the trunk flexion and extension exercises.

tatistical AnalysisThe results are presented as averages and standard devia-

ions. We used repeated measures analysis of variance in theomparisons of the surface electromyographic activity of eachuscle, which were elicited during the different exercises.aired t tests were used in the comparison of the force valuesnd surface electromyographic activities of the exercises in theifferent shoulder positions (shoulder extension, shoulder hor-zontal extension and shoulder flexion, shoulder horizontalexion) and between the sides. No adjustment was made forultiple testing, but this information can be obtained by mul-

iplying the actual P value by the number of comparisonsade. Statistical analyses were performed using the SPSS

tatistical analysis software program.f

RESULTSThe mean maximal isometric force results of the trunk and

: flexion; (B) exercises 2 and 3: lateral flexion to the right and left;

pper-limb exercises are shown in table 3. The force values


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roduced in upper-limb exercises 5 and 6 were greater than inxercises 7, 8, and 9 (P�.001). Individual differences in forceroduction were the smallest in exercise 6 and the greatest inxercise 7.

The mean surface electromyographic amplitude of the rectusbdominis during exercise 1 was greater on both sides than thectivity levels during the most upper-limb exercises (P�.031)fig 3). However, in exercise 9 the mean activity level of theectus abdominis was 114% compared with that of referencexercise 1.

The activation of the obliquus externus abdominis was at itsreatest in reference exercise 1 on both sides and in exercise 2nd 3 on the side of the lateral flexion. The mean surfacelectromyographic amplitude during reference exercise 1 wasreater than those during the upper-limb exercises (P�.007),xcept for the amplitude levels during exercises 8 and 9 on theeft side and during exercises 6, 8, and 9 on the right side (fig). On the left side, the surface electromyographic amplitude ofhe obliquus externus abdominis was, on average, 82% of thatlicited during trunk flexion in exercise 8 and 99% in exercise 9.he figures for the right side were 74% in exercise 6, 85% inxercise 8, and 102% in exercise 9. In exercise 8, surface

ig 2. Maximal isometric force measurement of the upper limb wiexion; (B) exercises 7 and 8: shoulder horizontal extension and fle

Table 3: Force Production Values for Different Exercises of theTrunk and Upper Limbs

Isometric Force Mean � SD (N) Range (N)

1. Trunk flexion 297�71 142–4292. Trunk lateral flexion to the right 363�88 190–5093. Trunk lateral flexion to the left 360�84 209–4764. Trunk extension 354�98 174–5115. Shoulder extension 234�63 138–3476. Shoulder flexion 207�35 149–2587. Shoulder horizontal extension 156�51 85–2708. Shoulder horizontal flexion 168�31 104–245

f9. Bilateral shoulder extension 174�27 131–250


lectromyographic amplitude was greater than in exercise 6 onoth the left and right sides (P�.036). The surface electromyo-raphic amplitude on the right side in exercise 6 and on the leftide in exercise 7 were greater compared with the contralateralides for both exercises (P�.017).

The surface electromyographic amplitude of the right andeft longissimus during reference exercise 4 was significantlyreater (P�.001) than that elicited during the upper-limb ex-rcises (fig 5). Of all the upper-limb exercises, the greatestevel of surface electromyographic amplitude was produceduring exercise 7, in which it was 69% of the amplitude levelbserved during exercise 4 on the left side and 53% on the rightide. In exercises 5 and 7, the surface electromyographic am-litudes of the longissimus were significantly greater on the leftide of the trunk compared with the contralateral sideP�.007).

The surface electromyographic amplitudes of both the rightnd the left multifidus muscles during reference exercise 4ere greater compared with the upper-limb exercises (P�.033)

fig 6). In regard to the upper-limb exercises, the multifidususcles were activated to the greatest degree in exercise 7. The

urface electromyographic amplitude was 84% of the activityevel of the exercise 4 on the left side and 52% on the right side.he amplitude level of the multifidus was greater on the leftide in exercises 5 and 7 compared with the contralateral sideP�.005).

Activation during reference exercises 2 and 3 was signifi-antly greater on the side of the lateral flexion in all musclesP�.001).

DISCUSSIONOf all exercises included in this study, the greatest surface

lectromyographic amplitudes of the rectus abdominis andbliquus externus abdominis were elicited in bilateral shoulderxtension (exercise 9). This was surprising because the refer-nce trunk exercises were supposed to produce the best acti-ation of the trunk muscles. The reason may be that the arm

e pelvis supported. (A) Exercises 5 and 6: shoulder extension andand (C) exercise 9: bilateral shoulder extension.

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o produce sufficient torque to load the trunk muscles to areater degree compared with the trunk flexion exercise.Shoulder horizontal extension (exercise 7) activated the lon-

issimus and multifidus muscles on contralateral side to thereatest degree. The surface electromyographic amplitude lev-ls of the back and abdominal muscles were greater in thosexercises using longer lever arms. Assuming that the trunkuscles attained their maximal levels of activity during the

eference exercises, then, a sufficient intensity for the devel-pment of muscular strength (�60%) was attained for each

9. Bilateral shoulder extension

8. Shoulder horizontal flexion

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5. Shoulder extension

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ig 3. The averaged surfacelectromyographic amplitudeEMG) of the left and right rec-us abdominis muscles duringaximal isometric efforts of

he different study exercises.

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uscle group in at least 1 maximally performed upper-limbxercise.

In many earlier studies, researchers normalized results byomparing the surface electromyographic amplitude eliciteduring the exercises with that produced during the assumedaximal voluntary contraction exercises. Direct conclusions

bout the suitability of certain exercises for the development ofhe performance capacity of muscles have been made on theasis of normalized values.29,33,36 However, this may be mis-eading in relation to the clinical interpretation of the results.

EMG Rectus abdominis (μV)

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Fig 4. The averaged surface elec-tromyographic amplitude of theleft and right obliquus externusabdominis muscles during maxi-

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irst, the interpretations are affected by the fact that maximalurface electromyographic amplitude levels may not have beenttained during the selected exercises or positions. Maximalrunk strength and muscle activity is not produced in the neutralosition.42-44 Second, large individual variations in the valueseed to be considered when using averages, even with normal-zed values. Exercises do not activate the muscles at the sameelative level for all subjects. In the interpretation of activity

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ig 6. The averaged surfacelectromyographic amplitudef the left and right multifidususcles during maximal iso-
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tudy exercises.


evels, it is also important to remember that the force- electro-yographic activity ratio is not linear.45 Thus, the relative

ctivity levels reported for the exercise efforts only act as auide in the planning of exercise programs.The comparability of the surface electromyographic values

btained from the reference and upper-limb exercises wasmproved by the performance of all the exercises isometricallyn a standing position. In this way, problems with the interpre-

issimus (μV)

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Fig 5. The averaged surfaceelectromyographic ampli-tude of the left and rightlongissimus muscles duringmaximal isometric efforts ofthe different study exer-cises.

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ation of surface electromyographic amplitude associated withhe different types of muscle contractions, as well as those dueo the effect of different measurement positions on the surfacelectromyographic signals, could be avoided, and we also ruledut problems with the standardization of the velocity. Theariation in the actual performance of the exercises was min-mized by strict standardization of the positions of the subjectsuring the exercises and by providing instructions in the correcterformance of the exercises, as well as by practicing theovements before the actual measurement events. Also, per-

orming the measurements with maximal effort was central tohe comparability of the results between the test exercises. Inarlier studies that have evaluated surface electromyography ofrunk muscles, and in which external loads were used, a stan-ard load has been used as the resistance.29,31,37 Using atandard load can lead to problems because the load may be tooight for the stronger subjects and too heavy for weaker sub-ects. Further, according to surface electromyography results,he loading in general in the previous studies has been too low.

With surface electromyography, it has been shown that it isossible to study the activity of the superficial muscles. How-ver, possible cross-talk from other muscles that are not beingtudied needs to be taken into account in the interpretation ofata. The obliquus internus and, possibly, the transversus ab-ominis to some degree, may probably have an effect on theevel of activity of the flat obliquus externus muscles becausehey are located underneath it.46 It has been observed that bothhe obliquus and transversus abdominis muscles are activeuring rotational exercises of the trunk.47 This has significancen the interpretation of the results, especially in relation to thenilaterally performed exercises, which result in loads beingirected at the trunk in a rotational direction. The obliquusxternus and internus muscles on the same side produce forcen different directions during rotational exercises, and this mayeduce the observed activity difference between the left andight sides. The difference between sides could be greater onlyf the activity of the obliquus externus is examined. There isonflicting information on the possibilities of using surfacelectromyography to study the activity of the multifidus. In thetudy by Arokoski et al,29 the results achieved with wire andurface electromyography were compared with each other.ccording to the study, surface electromyography is a validethod to study the activity of the multifidus. Stokes et al48

tated that measurement of the multifidus with surface electro-yography provides a better representation of the activity of

he longissimus muscle. However, the research methods usedn these studies differed from each other and so additionalesearch is required in order to form a definitive conclusionbout the use of surface electromyography in the measurementf the activity of the multifidus. The depth to which a wirelectrode is positioned in a muscle can also have an effect onhe different interpretations, because the multifidus possessesiffering functions with the superficial part producing move-ent while the deeper part controls intersegmental move-ent.49

In the present study the subjects had an opportunity toractice each exercise only in 1 testing session. Several ses-ions would have probably improved the ability to produceaximal efforts, but the same can also be said of the reference

xercises. The positioning of the lower limbs also probablyffected the results. The striding position of the lower limbssed during the upper-limb exercises provided additional sup-
ort and allowed greater force producing capacity of the trunk a
usculature, which was observed during the guidance of thexercises by the researchers. In addition, it must be remem-ered that the activity levels achieved during the studied exer-ises are the maximal levels achieved during these exercises.hen several repetitions are performed, it is not possible toaintain the same level of activity. Thus, the average level ofuscle activity achieved during exercise sessions will be lower

han the achieved maximal results. This margin needs to beonsidered when evaluating exercise intensity.

In earlier surface electromyography�related research stud-es, only one of the upper-limb exercises was the same as in theurrent study; Arokoski et al31 found that the average surfacelectromyographic activity of the rectus abdominis was at theame level that we found in bilateral shoulder extension (ex-rcise 9), but the activity of the obliquus externus abdominisas clearly lower. The difference in the results can be ex-lained by the fact that the subjects in the study by Arokoski31

tood with their feet side by side and their upper arms held in0° of flexion during the performance of the movements. Thus,he prerequisites for force production were probably less fa-orable.It is important to consider the safety of the exercises in-

luded in the rehabilitation programs of LBP patients, as wells any background functional and structural changes on whichhe exercise program is aiming to have a positive effect.50 It isossible to increase exercise safety by the performance of thexercises with the lumbar spine in a neutral position. In thisosition, the structures of the spine are better able to withstandhe loads directed at it, because they are more optimally dis-ributed between the vertebral disks and the facet joints.51

The exercises should be directed at the muscles that improvetability of the lumbar spine. No single group of muscles haset been shown to be more important in regards to the stabilityf the lumbar spine than other muscles.52,53 Efforts should beade to include, in exercise programs, the types of exerciseshere the load can be directed as effectively as possible to all

he muscles of the trunk.

tudy LimitationsWhen applying the results in practice, 2 things need to be

aken into account. First, the upper-limb exercises resulted inigher levels of activity than the reference exercises did ineveral subjects. Thus, the assumption that maximal muscularctivation was produced during the reference exercises was notorrect. Second, we used pelvic support during the measure-ent. If the exercises are performed without support, the ac-

ivity levels of the muscles are likely to be noticeably lower.elvic support makes exercise more nonfunctional, but at theame time it makes exercise easier to perform. With pelvicupport, it is possible to perform exercise effectively andafely, even if the patient has impaired postural control.

CONCLUSIONSOf all the exercises included in this study, the activity level

f the abdominal muscles during the performance of the bilat-ral shoulder extension exercise was close to that eliciteduring trunk flexion. In turn, the activity level of the backuscles, especially on the left side of the trunk during the

erformance of horizontal shoulder extension, was close to thatroduced during trunk extension. It can be assumed that theoads required for the development of the strength and endur-
nce characteristics of the back and abdominal musculature can

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