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CLINICAL SCIENCES: Clinically Relevant

EMG activity normalization for trunk muscles in subjects with and without back pain


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Medicine & Science in Sports & Exercise: July 2002 - Volume 34 - Issue 7 - p 1082-1086
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Normalization of electromyographic (EMG) activity is a procedure that is commonly used for comparing myoelectric activity of different muscles or different individuals. It is important to point out that there are concerns in the limitations of using normalization strategies in EMG data (2,8,24,27,29). Subcutaneous fat, skin impedance, and electrode placement are some candidate sources of intermuscular and intersubject variability that make these comparisons challenging otherwise (10,14,23). Differences in subcutaneous tissue thickness alone could contribute up to 81.2% of the variance in EMG amplitude measurement (6).

One normalization procedure is to express EMG amplitude relative to muscle-specific maximal values, which are typically recorded during maximal isometric contractions. The normalization procedure does not consider the nonlinear relationship between muscular activation and muscular force, nor does it compensate for differences in muscle length or contractile velocity. However, expressing the amplitude of muscular activity relative to a maximal or submaximal value is the preferred means by which comparisons between muscles or individuals are made.

When maximal isometric contractions are utilized to obtain a value for maximal muscular activation, the optimal mechanical conditions to elicit maximum muscle action should be considered. In muscles that cross joints of the upper and lower extremities, this is relatively straightforward compared with the muscles of the trunk which operate in many degrees of freedom. To find a single maximal value in trunk muscles, some researchers have recorded maximal EMG amplitude from contractions in several directions (e.g., 9,12,15). Because these data have not been reported in detail, the prime action for recording maximal EMG amplitude has not been established. Therefore, it was the purpose of the present experiment to examine the EMG amplitude of six bilateral trunk muscles during maximal isometric contractions in three cardinal planes and to determine the appropriate action that should be used to obtain the best representation of maximal activation for EMG normalization applications in healthy subjects and back-pain patients.



Forty-three male subjects were recruited for this study. The study was approved by the Medical Research Ethics Committee of The University of Queensland, and all subjects gave their written informed consent to participate. Healthy subjects (N = 28) did not recall any history of back pain, and their mean (± SD) age, height, and weight were 30.2 ± 7.6 yr, 1.7 ± 0.1 m, and 67.2 ± 10.1 kg, respectively.

Fifteen back-pain patients (age = 27.9 ± 6.7 yr, height = 1.8 ± 0.1 m, weight = 74.4 ± 9.5 kg) were selected based on their back pain (a) of insidious or nontraumatic onset; (b) of at least 12 months duration; (c) of severity that required either treatment, sick leave, or bed rest; and (d) of a nature that is either episodic with at least one episode of back pain each year or semicontinuous with periods of greater or lesser pain. Subjects were excluded if their pain was caused by neoplasm, infection, or neuromuscular disease, or if they had previous spinal surgery. As it was important to avoid pain during the testing, only patients with minimal or no pain at the time of testing were recruited. The level of pain at the time of testing was rated at 1.1 ± 0.9 (range 0–2.8) using a 10-cm visual analog scale (7). The mean duration of back pain of the patient group was 6.1 ± 3.9 yr (range 1.1–15.5).


A dynamometer, B200 Isostation (Isotechnologies, Hillsborough, NC) was used to measure the torque produced by the trunk about the three planes of the body in six directions. EMG signals from both abdominal and back muscles were amplified, band-pass filtered at 5–500 Hz, and sampled at 1000 Hz. The torque data and EMG signals were collected with a data acquisition system, AMLAB II workstation (Associative Measurement, Sydney, Australia).

Electrode placement.

Surface electrodes were placed over three pairs of abdominal muscles and three pairs of back muscles, and the positions were described in details in previous studies (15,17,18). The electrodes for rectus abdominis were placed 1 cm above the umbilicus and 2 cm lateral to the midline. For the external oblique, electrodes were placed just below the rib cage and along a line connecting the most inferior point of the costal margin and the contralateral pubic tubercle. For the internal oblique, electrodes were placed 1 cm medial to the anterior superior iliac spine (ASIS) and beneath a line joining both ASISs. The electrodes for latissimus dorsi were placed over the muscle belly at T12 level and along a line connecting the most superior point of the posterior axillary fold and the S2 spinous process. For the iliocostalis lumborum, the electrodes were placed at the L2 level and aligned parallel to the line between the posterior superior iliac spine (PSIS) and the lateral border of the muscle at the 12th rib. For the multifidus, the electrodes were placed at the L5 level and aligned parallel to the line between the PSIS and the L1–2 interspinous space.

Experimental procedures.

The subject was positioned in erect standing with the L5–S1 interspinous space aligned with the flexion/extension axis of the B200 Isostation. The pelvis and lower legs of the subjects were stabilized by the pelvic restraint, as well as thigh and knee straps. The upper torso was fixed by the chest restraint and thoracic pad as tight as possible so as not to affect normal breathing. The subjects were asked to fold their arms across their chest, hold their arms above the chest restraint, and maintain this position during all contractions. To prepare for the isometric testing, the machine was mechanically locked in three planes.

Maximum voluntary contractions for one trial in flexion, extension, lateral flexion to both sides, and axial rotation to both sides were measured for 5 s with a 2-min rest between trials. The six maximum contractions were arranged in a randomized order. Before each contraction, the subject was given at least one practice trial for warm-up. Visual feedback and verbal encouragement were given to ensure the maximal effort was produced by the subject. To decrease the chance of injury, subjects were instructed to avoid any jerky contractions.

Each subject attended two sessions: a familiarization and a testing session. The familiarization session was included to minimize the learning effect and allow the subject to gain some knowledge of the equipment and testing procedure (without placement of surface electrodes). The familiarization session was held at least 3 d before the testing session. Data of the torque produced and the EMG activity in the trunk muscles during different contractions were collected in the testing session.

For the test-retest reliability of the EMG measurement, 12 subjects (10 healthy subjects and 2 back-pain patients) were asked to return at least 3 d after the first testing session. The testing procedure in the two testing sessions was the same. The electrode positions were reproduced in the retest by recording on a template in the first session.

Data analysis.

The duration of the EMG data to be analyzed was identified based on the stable 3-s data of measured torque (25). Root mean square values of the EMG data were calculated to quantify the amplitude of EMG signals. EMG data of individual muscles were normalized with respect to the maximum EMG values acquired during the maximum contractions in three planes. Analysis of variance (ANOVA) with repeated measures design was used to examine the difference in EMG data of the 12 trunk muscles in six directions between healthy subjects and back-pain patients. To compare the activity level of each individual trunk muscle among the six contractions, repeated measures ANOVA of a general linear model with simple contrast was applied to the data. The reliability for all the measurements was examined using intraclass correlation coefficients (ICC) with a one-way ANOVA (22).


The mean normalized EMG data for the six trunk muscles for the normalization protocol in healthy subjects and back-pain patients are presented in Table 1 and 2, respectively. There were no differences in normalized EMG activity between healthy and back-pain groups (F = 2.927, P = 0.095) with no significant interactions for group by muscle (F = 1.157, P = 0.315) and for group by direction (F = 0.844, P = 0.520).

Mean normalized EMG data (%) of the trunk muscles during maximal contraction in three planes in healthy subjects (N = 28).
Mean normalized EMG data (%) of the trunk muscles during maximal contraction in three planes in back pain patients (N = 15).

For both healthy subjects and back-pain patients, significant main effects for direction of contraction on EMG activity were found for all the trunk muscles (all F-values ≥ 16.275, all P-values ≤ 0.000). For rectus abdominis, external oblique, internal oblique, and multifidus, the one single contraction direction that produced maximum EMG signals was significantly different from the other five directions (all F-values ≥ 6.467, all P-values ≤ 0.025). Rectus abdominis demonstrated maximal activity in trunk flexion, external oblique in lateral flexion, internal oblique in axial rotation, and multifidus in extension. For latissimus dorsi and iliocostalis lumborum, the two directions with highest EMG activity were significantly different from the remaining four directions (all F-values ≥ 22.640, all P-values ≤ 0.000). Latissimus dorsi demonstrated maximal activity during contractions in lateral flexion (2/3 of the subjects) and axial rotation (1/3 of the subjects), iliocostalis lumborum during contractions in extension (2/3 of the subjects), and lateral flexion (1/3 of the subjects).

Excellent reliability (ICC ≥ 0.90) was found in seven muscles: left rectus abdominis (ICC = 0.96), left external oblique (ICC = 0.94), right latissimus dorsi (ICC = 0.93), left latissimus dorsi (ICC = 0.92), right iliocostalis lumborum (ICC = 0.90), as well as right and left multifidus (ICC = 0.93). Good reliability (ICC = 0.75–0.89) was demonstrated in the remaining five muscles: right rectus abdominis (ICC = 0.89), right external oblique (ICC = 0.87), right internal oblique (ICC = 0.77), left internal oblique (ICC = 0.75), and left iliocostalis lumborum (ICC = 0.89).


The present study is a detailed report to describe the trunk muscle activity during maximal contractions in three cardinal planes for subjects with and without back pain. In both healthy subjects and back-pain patients, maximum activation was found in contraction in a single direction in rectus abdominis, external oblique, internal oblique, and multifidus (rectus abdominis, flexion; external oblique, lateral flexion; internal oblique, axial rotation; and multifidus extension). For latissimus dorsi and iliocostalis lumborum, maximum activity was found in two directions (latissimus dorsi, lateral flexion and axial rotation; and iliocostalis lumborum, extension and lateral flexion).

Functional anatomists consider rectus abdominis as a trunk flexor, so it was expected that maximum contraction in flexion would produce maximum activation. On the other hand, abdominal oblique muscles are usually attributed to be involved in flexion, lateral flexion, and axial rotation because of their large attachment area. It has been demonstrated that there are differences in activation between external and internal oblique in trunk exertions (13,15,26). The present study also supported this observation because the direction of contraction that recruited the muscle most was not the same for external oblique and internal oblique.

For the internal oblique, maximum activity was found during axial rotation contraction. Thelen et al. (26) found maximal internal oblique activity during axial rotation contraction in two thirds of their subjects. For the external oblique, maximal activation was found in lateral flexion contraction in the present study. Previous studies have reported varied findings. Zetterberg et al. (30) demonstrated that external oblique activity was higher in lateral flexion than in a flexion contraction. On the contrary, Thelen et al. (26) found that flexion was the direction for maximal external oblique activity in seven of the nine subjects tested. This may be attributed to the differences in electrode placement between the studies. Schultz et al. (20) as well as Ashton-Miller and Schultz (1) found that the medial part of external oblique was maximally activated during a flexion contraction. Although for the lateral part of external oblique, highest activity was found during attempted lateral flexion. Through biomechanical analysis, Dumas et al. (3) demonstrated that anterior portion of external oblique produce larger moment in flexion, whereas greater moment in lateral flexion was found in the posterior portion of external oblique.

Regarding the back muscles, maximal activity was found during lateral flexion and axial rotation contractions in latissimus dorsi. For iliocostalis lumborum, greatest activity was demonstrated during extension and lateral flexion contractions. Although for multifidus, maximal activity was found in extension contraction. In a previous study, Thelen et al. (26) found that eight of the nine subjects showed highest activity in latissimus dorsi during axial rotation contraction and maximal activity of iliocostalis lumborum and multifidus in isometric extension. Although iliocostalis lumborum and multifidus are usually described as back extensors, it was found that iliocostalis lumborum was also involved in lateral flexion. Iliocostalis lumborum has been suggested to be recruited in lateral flexion and at high force levels of extension (28). Evidence of functional difference between iliocostalis lumborum and multifidus has been shown in previous studies (15,16).

Among the back muscles, latissimus dorsi and iliocostalis lumborum were the muscles with more than one identified direction for maximum activation. This demonstrates the complexity of back muscle activation during contraction in various directions. In biomechanical modeling, the trunk muscles are capable of generating muscle moment in different planes (4). As a result, there will be several muscles that can participate in a contraction. To meet the demand, the neural control system can choose among the muscles that can contribute to an action. This affords freedom to the central nervous system (CNS) to recruit various muscles. It has also been suggested that the CNS adjust the motor unit activation within muscle segment so as to maximize the contraction and meet the functional demands of the task (19). In vivo, the choice of muscles being selected may depend on the individuals’ differences in muscle strength and motor-skill level. Granata et al. (5) found that there was a difference in activity in some trunk muscles during lifting between experienced workers and nonexperienced subjects.

Although the mean value of EMG activation during contraction in a certain direction was significantly greater than that in other directions, it should not be interpreted that the same pattern of activation was demonstrated in all subjects. In few subjects, external oblique was found to be at maximum levels in the flexion direction. More variability was noted in internal oblique with maximal activity levels shown in flexion or lateral flexion. It has to be acknowledged that there were other maneuvers or movements that may activate the trunk muscles greater than that in contractions in a planar direction. For instance, a recent study has provided evidence that higher muscle activity could be produced in combined exertions (e.g., flexion and axial rotation) than exertion in a single plane (e.g., flexion) (21).

For back-pain patients with current pain, maximal isometric contractions as performed in the present study may not be suitable, and other normalization strategies should be considered. In a recent study, Marras and Davis (11) used a regression equation for the normalization of EMG data. The subjects required only to perform submaximal contractions and the maximum torque was predicted by the subject’s anthropometric measurements. The predicted maximal torque (strength) and submaximal torque and EMG recording were used to estimate the maximum activity of muscles for normalization purposes, assuming a linear relationship. They identified the direction of contraction (torque) that had the highest relationship with EMG activities of each muscle. This direction determined the direction to be chosen for estimation of maximum EMG activation in each muscle.

In conclusion, maximum contraction in at least six directions of the three cardinal planes is needed to normalize all the trunk muscles examined in the present study. Due to individual neural control preferences, the maximum activation of latissimus dorsi and iliocostalis lumborum muscles may require consideration of maximum contractions in two distinct contraction. Recent evidence may indicate the need to add combined contractions to these six planar exertions to ensure capturing the maximum activation of trunk muscles.

This study was supported financially by the Dorothy Hopkins Award for Clinical Study and the research support grant of Manipulative Therapists Special Group of Queensland, Australia. The authors are most grateful to the staff of the Workers’ Compensation Board of Queensland for their invaluable assistance during the whole data-collection process.

Address for correspondence: Joseph K.-F. Ng, Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong; E-mail: [email protected]


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