Joseph Carolyn Richardson

Backexercises addressing deep muscles with a prime stabilising role such as multifidus are important in the rehabilitation of patients with low back pain. Electromyography of erector spinae and multifidus of 18 healthy subjects was investigated during prone arch and two isometric back extension exercises; trunk holding and leg holding. When compared with prone arch, both erector spinae and multifidus were recruited at a high level during trunk holding (76-79 per cent) and leg holding (66-68 per cent). Relative activity of erector spinae and multifidus was similar in different loading conditions and this implied they were working together asa single functional unit. Further studies are needed to investigate the possibility of selective recruitment of individual lumbar muscles in back exercises.

[Ng JK-F and Richardson CA: EMG study of erector spinae and multifidus in two isometric back extension exercises. Australian Journal of Physiotherapy 40: 115-121]

Key words: Back; Electromyography; Exercise; Muscle Contraction

JK-F Ng ProfDipPhty, PrCSpinalManipTher, MPhtyStis a part-time tutor and research assistant in the Department of Physiotherapy, The University of Queensland. CA Richardson BPhty(Honsl, PhD is a senior lecturer in the Department of Physiotherapy, The University of Queensland. Correspondence: Dr Carolyn Richardson, Department of Physiotherapy, The University of Queensland, St Lucia, Qld 4072.

ORIGINAL ARTICLE

EMG study of erector spinae and multifidus in two isometric back extension exercises

.p hysioth. erapistshave long .been using back exercise therapy in the rehabilitation of patients

suffering from low back pain. The design and prescription of most appropriate exercises, aiming at an enhancement of the function of back muscles in the prevention and treatment of spinal disorders, is of Utmost importance.

In the past, the back extensor muscles have usually been considered as a single functional unit (Floyd and Silver 1955). Recent studies have demonstrated functional differences between parts of the synergistic back muscle group - the larger superficial

. back muscles, erector spinae and the smaller deep back muscles, multifidus (Jonsson 1970, Vink et aI1987).

It was proposed by Panjabi et al (1989) that the intersegmental spinal muscles playa greater role in maintaining spinal stability than the superficial muscle groups. The superficial muscles, namely longissimus thoracis and iliocostalis lumborum, with their larger size and longer lever arm, are better designed to produce movement and to counterbalance external loads. In contrast, with close proximity to the lumbar spine, intersegmental muscles, namely multifidus, are ideally suited to enhancing spinal stability of this region (panjabi et al 1989). In addition, the role of stabilising the lumbar spine during rotation of trunk has been

at:t:iibutedto multifidus. Its function is considered to be the elimination of the unwanted flexion moment generated by abdominal obliques as they rotate the trunk (Macintosh and Bogduk 1986). The difference in histochemical fibre types, with more slow twitch fibres in multifidus than in erector spinae (Sirca and Kostevc 1985), may also suggest that multifidus has a greater role in spinal stability.

Traditionally, the treatment of low back pain has included strengthening exercises for the back extensor muscles. Prone arch or their variations, prone trunk and leg lifting exercises, have been shown to decrease back pain (Manniche et alI991). Exercises such as this may well increase the ability to produce gross trunk movements, but may not be specific enough when the function of individual muscles within the lumbar extensor group are considered. Therefore, identifying and designing exercises for these particular muscles would allow physiotherapists to more effectively focus their rehabilitative and preventive exercise programmes. In designing back exercises, several factors need to be considered. These include type of muscle work, range of movement of the exercise and whether the trunk or lower limbs (or both) are used as the resistance force.

For the type of muscle work, the use of isometric training has been

From page 115 advocated by Jull and Richardson (1994) as an ideal means of improving the recruitment of those back muscles capable of enhancing spinal stability. In addition, the protective co-contraction pattern of the trunk muscles may be achieved through the development of the isometric holding capacity of these muscles in early rehabilitation exercises Gull and Richardson 1994).

It is generally accepted that isometric back extension exercises performed in neutral position are less likely to provoke further pain and disability since they involve less joint movement. It has been shown that prone arch extension exercise produces high stresses on the lumbar discs which are loaded in this extreme position (Nachemson and Elfstrom 1970), and may present a problem for the patient suffering from apophyseal joint dysfunction Gackson and Brown 1983). Hence, this type of hyperextended position should be avoided (White and Panjabi 1978) and a position that keeps the spine in a more neutral alignment is preferable when back extension exercises are performed (Lindh 1989).

Trunk or lower limbs can be used as the resistance force in back extension exercises. Grieve (1988) has previously advocated the use of the leg holding for training back extensors to improve spinal stabilisation. This prone leg holding position was also adopted by Haigh (1989) who found a significant increase in endurance of back extensors after the training programme.

To date, only a few researchers have investigated the activity level of back muscles in the prone position, but these investigations have mainly concentrated on prone arch back extenSion exercise Gonsson 1970; Pauly 1966, Valencia and Munro 1985). No study has investigated the extent of recruitment of erector spinae and multifidus during isometric back extension in the neutral prone position. The aims of the present study were to examine, using electromyography (EMG), levels of muscle activity of erector spinae and multifidus during trunk and leg holding in healthy

ORIGINAL ARTICLE

figure 1. Testing in the prone arch position.

subjects and to determine which isometric back extension exercise is most suitable for the training of individual muscles of the back extensor group.

Methods Subjects Eighteen healthy male subjects aged between 18 and 27 years were studied. In order to determine the effect of the length of body parts on the level of recruitment of erector spinae and multifidus during trunk and leg holding, subjects' trunk and leg lengths were measured (William and Marfell­Jones 1991). The mean and standard deviation of the age, weight and anthropometric measurements were as follows: age,20.3 ± 2.1 years; weight, 66.2± 9.7kg; height, 176.7 ± 7.6CIil; trunk length, 88.3 ± 3Acm; leg length, 88.3 ± 6.2ctn. No subjects had any history of back pain that interfered with activities of daily living Or had any spinal Surgery, neuromuscular or . musculoskeletal disorders. Any subJect with tightness of erector spinae, iliopsoas, rectus femoris, iliotibial tract

or hamstrings was excluded, as this tightness may have resulted in altered patterns of muscular recruitment .. Ganda and Schmid 1980). In additIon, subjects were excluded if they were assessed as having a marked postural deviation, as this may indicate trunk muscular imbalance (Sahrmann 1988). Each subject gave written consent to participate in the study, which was approved by the Medical Research Ethics Committee of The University of Queensland.

Electromyography Skin preparation of electrode sites involved shaving, cleaning with alcohol, and the skin puncturing technique described by Anderson and Champion (1988). Inter-electrode resistance was checked using an AC impedance meter and a skin resistance oflessthan 5k.Q was considered acceptable (Gilmore and Meyers 1983). Bipolar silver/silver chloride surface electrodes (Medi-Trace, Graphic Controls Canada Ltd) were placed longitudinally to the belly of the right erector spinae at the level of L1; and to the right multifidus just lateral

Figure 2. Testing in the trunk holding position.

to L5 (Roy et al1989). Inter-electrode distance was 28mm. The earth electr~de was placed on the right acrOInlon process. In order to investigate the influence of distant myoelectric activity (cross talk) recorded in surface EMG m·er erector spinae and multifidus, a pilot study was performed using the branched electrode technique which has been suggested to reduce cross talk (Koh and Grabiner 1993). It was found that the influence of cross talk from erector spinae on the multifidus EMG recordings was minimal.

Signal processing The EMG signal was passed through a pre-amplifier (Medelec PA63) to an amplifier/filter (Medelec AA6MKIII) then to a computer screen for realtime viewing of the raw EMG signals of each muscle. Amplifier gains were adjUsted for each subject. The filter bandwidth was set at 0.8-800Hz. The signals were sampled at a rate of 2500 samples per second with an on-line computer system, recorded and stored for later off-line analysis.

ORIGINAL ARTICLE

Testing procedure Two trials in each of the prone arch, trunk holding and leg holding positions were performed hy each subject in a random order and were as follows: 1. Prone arch. The subject lay

prone with both hands under the forehead. It involved gradual hyperextension of the upper trunk and both legs until their vertical height failed to increase (Figure 1).

2. Trunk holding. The subject lay prone over two couches with one couch supporting the upper body and the other supporting the pelvis and legs. The subject was positioned to ensure that both anterior superior iliac spines (ASIS) Were on the edge of the couch that supported the pelvis and legs. The lower body WaS stabilised bystraps over the hips, knees and ankles. The horizontal position of the upper torso in relation to the legs was adjusted by a goniometer. Subjects were . instructed to place their hands under the forehead with elbows

out to the sides. The couch supporting the head and trunk was then lowered. A marker on a vertical stand was placed over T7 level to provide feedback of the position to be maintained during the isometric horizontal hold. This horizontal position was monitored throughout the contractions (Figure 2).

3. Leg holding. The procedure was similar to trunk holding except that in this manoeuvre the upper body was fixed and the legs held against gravity. The upper torso was secured by straps over thoracic levels and the marker was placed over the back of the knees (Figure 3).

Each contraction was sustained for five seconds and a three-minute rest was given between trials.

Data analysis In the past, different methods have been utilised in the analysis ofEMG signals, including rectification, averaging or integration of rectified signals and the derivation of root mean square (rms) values. Rms EMG, dependent on the area, number and firing rate of the motor unit action potential trains, was advocated by Basmajian and De Luca (1985) as the most appropriate method of signal analysis. In addition, a linear relationship has been demonstrated between rms EMG and torque of erector spinae at different exertion levels during trunk extension in standing position (Tan et aI1993). Consequently, rms EMG was the method of data analysis employed in this study.

A waveform editing programme was used to identify and isolate the third second of the five-second raw EMG data recorded in the testing procedure. Once extracted, a conversion programme was used to convert the one second of raw signal to rms data. For prone arch, the trial with highest rmsEMGvalues was selected for normalisation procedure since it was the maximum effort that was demonstrated by the subject in the two ..

111 trials. For the two isometric back extension exercises, trunk holding and leg holding, means of the rms EMG values for each of the two trials were calculated and then used for subsequent analysis. To enable comparison between

positions and muscles, the rms EMG data of erector spinae and multifidus during trunk and leg holding were expressed as a percentage of that recorded during prone arch as it has been shown in previous studies G onsson 1970, Pauly 1966) that the electrical activity of erector spinae and multifidus was maximal in prone arch.

Results Repeatability Repeatability between trials for each position and muscle was examined by analysis ofvariance (ANOVA). The root mean square errors (VMSE) and coefficients of variation (CV) were calculated from the ANOV A. Repeatability was considered acceptable for rms EMG of the two muscles in trunk and leg holding with CVs of 4.59 - 10.56 per cent (Table 1).

Entire sample results Means and standard deviations of the normalised EMG activity of the two muscles were calculated and are illustrated graphically for both trunk and leg holding in Figure 4. Two-way ANOVAs revealed that there were significant differences between the normalised EMG data recorded in trunk and leg holding for erector spin~e (F(1.I7i: 5.98,p<0.05) and multIfidus 1/'(1.17)= 12.11,p<0.005).

Grouped subject results During trunk and leg holding position, leverage and hence resistance force may be considered as a potential confounding variable. It has been demonstrated that under submaximal conditions, changes in the muscle activation pattern of erector spinae and multifidus are possible (Dieen et al 1993). In order to establish whether the change of loading would produce a change in the level of activation of

ORIGINAL ARTICLE

figure 3. Testing in the leg holding position.

erector spinae and multifidus, subjects were grouped according to their ratio of trunk to leg length. Consequently, subjects with trunks longer than the legs (ratio of trunk to leg length> 1) were designated in Group 1, while subjects with legs longer than their trunks (ratio of trunk to leg length < 1) were in Group 2. When grouped in this manner, Groups 1 and 2 consisted of eight subjects each. There were two subjects with identical trunk and leg

length (ratio of trunk to leg length = 1) and no grouping was assigned, since the sample size was too small.

EMG ratio of erector spinae compared to multifidus were calculated for the trunk holding and leg holding in the two groups. To compare the ratio values recorded between muscles within groups, Duncan's multiple range tests were applied and no significant difference was found (Table 2).

ORIGINAL ARTICLE

Figure 4. Means an~ standard de~i~tions of the entire sample of the normalised EMS activity of erector spmae and multifidus during trunk holding and leg holding.

Discussion To date, information relating to relative activity of erector spinae and multifidus in isometric contractions or dynamic activities is lacking. One of the few studies to investigate both erector spinae and multifidus in different positions and movements

found that the two muscles contracted together and powerfully during prone arch and other activities (pauly 1966). These findings were later supported by Jonsson (1970) who found marked increases in activity of the two muscles in leg lifting or prone arch. Since no normalisation of data was performed in these studies, the relative contribution

of erector spinae and multifidus to these movements could not be quantified.

In the present study, erector spinae and. multifidus showed greater activity durmg prone arch than during either of the two isometric back extension ~xercises. Even so, a high activity level, m the range of 76-79 per cent in trunk hold~g and 66-68 per cent in leg holding, was recorded in these two muscles. This high level of activation would be sufficient to have training effects on the back extensor muscles eithe~ in strength or endurance, ' espeCIally when submaximal exercise intensity is only allowed in the early stages of rehabilitation of low back injury Gull 1990). With the spine in a neutral position, these exercises produce less stresses on lumbar discs and are therefore preferable to the conventional back exercise, prone arch.

E~ect:0r spinae was significandy more act::lve m trunk holding (76 per cent) ~at.t in leg holding (66 per cent). A SImIlar pattern was also observed in multifidus, being 79 per cent for trunk holding and 68 per cent for leg holding. This phenomenon could be explain~d by the difference in loading of the dIfferent body parts, being greater for the upper body than the lower limbs. From a summary of cadaver studies on body segment

From Page 119 parameters (Le Veau 1992), trunk and both upper limbs constitute about 69 per cent of the total body weight, while the legs account for the remaining 31 per cent. Due to the difference in loading, erector spinae and multifidus thus show greater activity during trunk holding, this being significantly higher than the activity in leg holdi~.:

Although the load of the upper torso is approximately two times greater than the lower limbs, the activity of erector spinae and multifidus are only about 15 per cent greater in trunk holding than in leg holding. This disproportionate increase in EMG activity in comparison with the load the muscles had to hold against gravity may be due to the different functional roles and mechanical advantage of the back muscles involved in the two isometric back extension exercises. It is possible that erector spinae and

multifidus are capable of different roles, yet still have a primary functional role, such as prime mover or synergist during the back extension exercises. In trunk holding, both erector spinae and multifidus function as prime movers working against the weight of the trunk and both upper limbs. While in leg ho1din3, the two m:uscles .function to stabilise the pelvis wlth theIr attachments to the ilia and sacrum. This provides a stable base for the working of gluteus maximus and hamstrings which are responsible for holding the legs in the prone position. Recent work by Valencia and Munro (1985), also found maximal activity in both SIdes of multifidus in unilateral hip extension from the prone position and this increase in activity of multifidus was believed to stabilise the pelvis during the leg lifting. .

Another explanation to consider is that of the lever arms of the muscles and the resistance. In trunk holding, the centre of gravity of the upper torso is located at approximately the TIl vertebra (LeVeau 1992) whereas the insertion site of erector spinae go as far as the Tl level (Macintosh and Bogduk 1987). Erector spinae are, therefore, well disposed to act as extensors of the

ORIGINAl ARTIClE

trunk. In leg holding, since the centre of gravity of the lower limb is located above the knee joint (Le Veau 1992), erector spinae and multifidus are not working ina position of good mechanical advantage as the moment produced by the lower body is augmented by the great distance between the pelvis and the knee.

It must be acknowledged that the electrode position used in the present study may not reflect the whole activity level of erect or spinae and multifidus, since the fascicles of the two muscles span multiple vertebral levels. Although rms EMG values ate ~orrelated well to the torque generated In erector spinae in standing position (Tan et aI1993), further work is required to analyse the relationship between EMG data and muscular force for isometric trunk extension in the prone position, especially for the deep back muscles.

The other major finding of this study was the relative extent of recruitment of erector spinae and multifidus in the two isometric back extension exercises. Examination of the grouped subject EMG data reveals no significant difference in the EMG ratio of erector spinae to multifidus during trunk and leg holding when comparing between those subjects with a longer trunk (Group 1) and those with a longer leg (Group 2). The activity of the two

, muscles is similar, in spite of changes of loading condition. It may be concluded from the data that erector spinae and multifidus are probably working as a single functional unit in the exercises of trunk and leg holding. In order to establish the effect of different loading conditions on the relative activation of the individual back muscles, resistance force other than the trunk and leg weight in prone position remain to be explored.

In patients with low back pain, recent findings suggested that fatigue is more pronounced in multifidus than in erector spinae (Biedermann et al 1991). It may be possible that the other back extensor synergist, erector spinae, which contribute most of the extensor torque during trunk extension (Bogduk

et al 1992), can be substituted for the recruitment of multifidus in general back extension exercises. Therefore, general exercises may not be useful to activate individual muscles within the synergistic back extensor group selectively and appropriately, especially in patients with low baCk pain. In clinical practice, Grieve (1988) advocated that segmental muscle may be isolated using specific positions such as sitting and submaximal resistance. Therefore, investigations need to be continued in order to establish if exercises can be designed to allow selective recruitment of individual lumbar extenSOrs, especially deep back muscles such as multifidus in normal subjects and patients with low back pain.

Conclusion This study has contributed to the knowledge of the functions of erector spinae and multifidus in two isometric back extension exercises. During trunk and leg holding, it was found that erector spinae and multifidus were recruited at a high level compared with that in prone arch position. In particular, the relative activity of the two muscles waS similar in different loading conditions. Based on these findings, it is proposed that both erector spinae and multifidus act as a single functional unit in the two isometric back extension exercises. Furthermore, it is apparent from the data derived from this investigation that, while high loading may be an ideal way in which to enhance the strength of the back extensor muscles as a whole, an additional training model is required to selectively recruit individual muscles within the back extensors group, especially muscles with a greater stability role such as multifidus.

Acknowledgements The authors would like to thank the subjects who took part in this study; Mr Bang Bm (Electronics Engineer) for assistance with technical matters; and Mr Ivor Horton (Statistician) for advice concerning statistical analysis.

References Anderson AB and Champion SHS (1988): Skin

preparation for surface EMG. Proceeding~ of the Bicentennial Conference of the Australian Physiotherapy Association. Canberra, ACT, pp. 11-29.

Basmajian ]V and De Luca CJ (1985): Muscles Alive, Their Functions Revealed by Electromyography. (5th ed.) Baltimore: Williams and Wilkins, pp. 65-100.

Biedermann HJ, Shanks GL, Forrest WJ and Inglis J (1991): Power spectrum analyses of electromyo~phic activity. Discriminators in the differential assessment of patients with chronic low-back pain. Spine 16: 1179-.1184.

Bogduk N, MacintoshJE and Pearcy MJ (1992): A universal model of the lumbar back muscles in the upright position. Spine 17: 897-911.

DieenJH van, Oude Vrielink lIHE and Toussaint HM (1993): An investigation into the relevance of the pattern of temporal activation with respect to erector spinae muscle endurance. European Journal of Applied Physiology 66.: 70-75. .

FloydWF and Silver PHS (1955): The function of the erectores spinae muscles in certain movements and postures in man. Journal of Physiology 129: 184-20J.

Gilmore KL and Meyers JE (1983): Using surface electromyography in physiotherapy research. Australian Journal of Physiotherapy 29: 3-9.

Grieve GP (1988): Common Vertebral Joint Problems. (2nd ed.) Edinburgh: Churchill Livingstone, pp. 613-628.

Haigh S (1989): Training isometric endurance of the trunk extensors in a clinical environment. Proceedings of the 6th .Biennial Conferen!!e of the Manipulative Therapists Association of Austr:ilia. Adelaide, South Australia, pp. 99-103.

Jackson CP and Brown MD (1983): Analysis of current approaches and a practical guide to prescription of exercise. Clinical Orthopaedics and Related Research 179: 46-54.

o RIG I N ALAR TIC LE

Janda V and Schmid HJA (1980): Muscles ~s a pathogenic factor in back pain. Pro!!eedings of the 4th Conference of the International Federation of Orthopaedic Manipulative Therapists. Christchurch, New Zealand, pp. 1~23.

Jonsson B (1970): The functions of individual muscles in the lumbar part of the spinae muscle. Electromyography 10: 5-2l.

Jull GA (1990): Aspects of conservative therapy for the acutely painful lumbar disc. Journal of Manual Medicine 5: 84-88.

Jull GAand Richardson CA (1994): Rehabilitation of active stabilisation of the lumbar spine. In Twomey LT and Taylor JR (Eds): Physical Therapy of the Low Back. (2nd ed.) New York: Churchill Livingstone (in press), pp. 251-273.

Koh 1J and Grabiner MD (1993): Evaluation of methods to minimize cross talk in surface electromyography. Journal ofBiomechanics26 (Supplement 1): 151-157.

LeVeau BF (1992): Williams and Lissner's Biomechanics of Human Motion. (3rd ed.) Philadelphia: W.B. Saunders, pp. 297-307.

Lindh M (1989): Biomechanics of the lumbar spine. In Nordin M and Frankel VB (Eds): Bask Biomechanics of the Musculoskeletal System. (2nd ed.) Philadelphia: Lea and Febiger, pp. 183-207.

Macintosh JE and Bogduk N (1986): The biomechanics of the lumbar multifidus. ClmicalBiomechanics I: 205-213.

Macintosh JE and Bogduk N (1987): The morphology of the lumbar erector spinae. Spine 12: 658-668.

Manniche C, Lundberg E, Christensen I, Bentzen Land Hesselsele G (1991): Intensive dynamic ba!!k exer!!ises for chronic low back pain: a clinical trial. Pain 47: 53-63.

Nachemson AL and Elfstrom G (1970): Intravital dynamic pressure measurements in lumbar discs. A study of common movements, maneuvers and exercises. Scandinavian Journal of Rehabilitation Medicine (Supplement) 1: 1-40.

Panjabi M, Abumi K, DuranceauJ and OxIand T (19.89): Spinal stability and intersegmental musCle forces. A biomechanical model. Spine 14: 194-200.

Pauly JE (1966): An electromyography analysis of certain movements and exercises: I. Some deep muscles of the back. Anatomical Record 155: 223-234.

Roy SH, De Luca CJ and Casavant DA (1989): Lumbar muscle fatigue and chronic lower back pain. Spine l4: 99.2-1001.

Sahrmann SA (1988): Adult posturing. In Kraus SL (Ed.): TMJ DisorderS: Management of the Craniomandibular Complex. New York: Churchill Livingstone, pp. 295-309..

Sirca A and Kostevc V (1985): The fibre type composition of thoracic and lumbar paravertebral muscles in man. Journal of Anatomy 141: 131-137.

TanJC, Parnianpour M, NordinM, Hofer H and Willems B (1993): Isometric maximal and sribmaximal trunk~nsion atdifferentflexed positions in standing. Triaxial torque output and EMG. Spine 18: 2480-2490.

Valencia FP and Munro RR (1985): An electron'lyographic study of the lumbar multifidus in man. Electromyography and Clinical Neurophysiology 25: 205-221.

VinkP, van derVeide EAand VerboutAJ (1987): A functional subdivision of the lumbar extensor musculature. Recruitment patterns and force-RA-EMG relationships under isometric conditions. Electromyography and Clinical Neurophysiology 27: 517-525.

White AA and Panjabi MM (1978): Clinical Biomechanics of the Spine. Philadelphia:J.B. Lippincott, pp. 277-344.

William DR and Marfell-Jones MJ (1991): Kinanthropometry. In MacDougall JD, Wenger HA and Green HJ (~ds): Physiological Testing of the HI~h­Performance Athlete. (2nd ed.) ChampaIgn: Human Kinetics Books, pp. 223-308.