BACK REHABILITATION is a significant issue for researchers

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1 916 Fatigue of Abdominal and Paraspinal Muscles During Sustained Loading of the Trunk in the Coronal Plane Kelly Thomas, BAppSc, Raymond Y.W. Lee, PhD ABSTRACT. Thomas K, Lee RYW. Fatigue of abdominal and paraspinal muscles during sustained loading of the trunk in the coronal plane. Arch Phys Med Rehabil 2000;81: Objective: To measure fatigue-related changes in the electromyographic signals of trunk muscles during lateral bend loading, and to determine the effects of load magnitude, gender, and contraction time on these changes. Design: A factorial experimental design. Setting: A research laboratory. Participants: Twenty-four healthy volunteers (12 men, 12 women; mean age SD, yrs). Intervention: Subjects sustained lateral bend loads at 20%, 40%, and 60% of their maximal exertion. Outcome Measures: Median frequency and total power of the power spectrum of the electromyographic signals. Results: Trunk muscles median frequency signals decreased and their total power increased as contraction time increased. These changes became more pronounced as the magnitude of loading increased. Men and women showed a similar rate of median frequency decline, but the total power of signals in men was larger than that in women. All muscles showed signs of fatigue, except that at 20% of maximal exertion only thoracic erector spinae fatigued significantly. Conclusions: Endurance training of paraspinal and abdominal muscles may be useful in preventing low back pain associated with lateral bend loading. The risk of back pain is similar in both genders and is higher with larger load magnitude and longer contraction time. Key Words: Back; Muscles; Biomechanics; Electromyography; Rehabilitation by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation BACK REHABILITATION is a significant issue for researchers and clinicians, because back pain affects 80% of the population at some stage in life. 1,2 The risk of back injuries increases when muscles fatigue as a result of excessive loading, 3 and injuries are frequently related to asymmetrical material handling when the trunk is loaded in the coronal plane. 4 An objective way to assess fatigue is to examine surface electromyographic (EMG) signals of muscles. 5 In this procedure, researchers determine the power spectrum of the signals From Nepean Hospital, Sydney, Australia (Thomas); and School of Physiotherapy, University of Sydney, Sydney, Australia (Lee). This study was carried out at the School of Physiotherapy, Faculty of Health Sciences, The University of Sydney, Sydney, New South Wales, Australia. Ms Thomas was a student of the University of Sydney at the time of the study. Submitted July 27, Accepted in revised form October 20, No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Dr. Raymond Lee, Senior Lecturer, School of Physiotherapy, University of Sydney, Cumberland Campus, East St, Lidcombe, NSW 2141, Australia /00/ $3.00/0 doi: /apmr and compute their median frequency (MF) and total power (TP) of the spectrum. 6,7 The MF typically shifts toward lower frequencies during sustained muscle contraction. 8,9 Such a shift is related to recruitment of small, fast-fatiguing type II fibers, 5 and to the accumulation of metabolites associated with fatigue. 5,10 The TP of the spectrum increases during sustained contraction, 8,9 indicating recruitment of additional motor units to maintain the required force output. Previous work 5,8-11 showed that spinal level, gender, and load magnitude influenced the rate of muscle fatigue during sagittal loading of the trunk. The lumbar erector spinae resisted fatigue less effectively than the thoracic erector spinae, 5 because the lumbar muscle had more type II muscle fibers. 12,13 The erector spinae of male subjects appeared to fatigue more quickly than that of females, 5,11 owing to the greater percentage of type II fibers in the male muscle. In addition, previous research 8,9 demonstrated that the rate of MF decline increased with increasing force of contraction. At high load level, metabolites accumulated and blood flow diminished to a greater extent. 10 The effect of lateral bend loading on the lumbar spine has not received the same degree of attention as sagittal loading. 14 Two studies 4,15 examined the activation patterns of erector spinae and abdominal muscles during lateral bending of the trunk, but they did not look at the role of these muscles in resisting the fatigue imposed by lateral bend loading. One study 16 reported that while subjects resisted submaximal lateral bend moment in an erect posture, abdominal muscles had greater MF shift than erector spinae muscles. The effects of gender and load magnitude on muscle fatigue during lateral bend loading have not been reported in the literature. In the present study, we attempted to address the deficiencies in the literature. The purposes were to examine changes in the power spectrum of trunk muscles during sustained loading in the coronal plane, and to study the effects of gender, load magnitude, and contraction time on the power spectral changes in the various trunk muscles. The results of our study may help identify factors that affect muscle fatigue and injury risk. METHODS Subjects Twenty-four healthy, right-handed volunteers (12 women, 12 men; mean age SD, yrs; height m; weight kg) agreed to participate in this study, which was approved by the Ethics Committee of the University of Sydney. Ninety percent of the subjects were physical therapy students; the other 10% were massage therapists and physical education instructors. Their daily activities generally involved a moderate amount of trunk bending. Subjects had no history of low back pain that required them to seek treatment or take time off work. They were excluded if they had undergone previous back surgery, had a fracture, dislocation, spondylolisthesis or other structural defects of vertebral structures, neck, thoracic or right shoulder pain, an observable spinal deformity, or neuromuscular or inflammatory disease. Subjects were informed about the experimental proce-

2 MUSCLE FATIGUE DURING LATERAL TRUNK LOADING, Thomas 917 dure and any potential risks before they signed a written consent form. Electromyography Active differential EMG electrodes a were affixed to the skin, which had been thoroughly cleaned with methylated spirits. The detection surface of the electrode consisted of two parallel silver bars. The electrodes were positioned longitudinally over the muscles, and the distance between the two recording bars was small (2.5cm), a position that substantially reduces the influence of cross-talk in signal detection. 3,6 After EMG signals were collected, they were amplified within the electrodes and were bandpass-filtered between 20 and 450Hz. Because amplification was performed close to where signals were detected, the signals had much less chance to be contaminated by noise. 10 The electrodes had a common mode rejection ratio of 92dB. The signals were converted from analog to digital signals and were acquired by a personal computer at a sampling rate of 1024Hz. The trunk muscles on the left side of the body were examined: thoracic erector spinae (4cm lateral to the T10 spinous process), 6 lumbar erector spinae (3cm lateral to the L3 spinous process), 6 external abdominal oblique (15cm lateral to the umbilicus), internal abdominal oblique (in the triangle bounded by the lateral edge of the rectus sheath, inguinal ligament, and the line joining the anterior superior iliac spine to the umbilicus), 20 and rectus abdominus (3cm lateral to the umbilicus) The reference electrode was positioned over the left ulnar styloid process. Procedure Subjects were requested to stand in an upright position inside a restraint frame that stabilized the pelvis (fig 1). A line was drawn on a mirror placed in front of the subject, and a marker was placed on the sternum. By aligning the image of the marker with the line on the mirror, the subject was able to maintain the upright position throughout the test. Each subject s maximal isometric lateral bend exertion was determined before the test by having the subject pull up on a handlebar attached to a floor-mounted load transducer located on the right side of the body. Subjects were allowed three trials of 3-second duration, with their best performance noted as their maximal exertion. Three seconds was sufficient to reach a maximum, but was not so strenuous that it caused fatigue. 21 Subjects performed three 2-minute trials of sustained loading at 20%, 40%, and 60% of their maximal lateral bend exertion. Loading was achieved by means of a seatbelt positioned over the right shoulder and connected to dead weights. Researchers collected EMG signals from the beginning to the end of the loading. The order of testing at different load levels was randomized. Rest of 5 minutes was allowed between trials. Muscle function recovered sufficiently after such rest periods. 22,23 The test was stopped if the subject could not maintain the upright posture, if he/she showed any signs of distress or discomfort, or if the subject requested it. Reliability Eight subjects (4 women, 4 men) performed the tests on two separate occasions (1 to 2 days apart) to determine the reliability of the EMG measurement. The second set of tests was conducted at approximately the same time of day under similar testing conditions. Data Analysis The raw EMG signals were processed using a fast Fourier transformation algorithm to obtain the power spectrum. The Fig 1. The experimental set up. Subject stood upright inside a restraint frame while a lateral bend load was applied. MF and TP of the spectrum were computed immediately on loading, 1 minute after loading, and at the end of the 2-minute contraction. All computations were performed using MATLAB for Windows Version 4.2. b Analysis of covariance (ANCOVA) was used to examine the effects of gender, load magnitude, and contraction time on the MF and TP of various muscles. Because initial data analysis revealed significant variations among the initial MF and TP values, these initial values were treated as covariate to account for the differences in the outset. To examine the repeatability of the measurements of MP and TP, Pearsons s product-moment correlation coefficients were computed to reveal any correlation between the measurements on the 2 separate days. Statistical Package for the Social Sciences (SPSS), version 6.1.2, c was used for the statistical analysis, with the statistical significance set at 5% ( p.05). RESULTS Reliability The average correlation coefficients for the MF and TP on the 2 test days were found to be 0.61 and 0.79, respectively. The measurements were considered to be repeatable. Maximal Isometric Exertion The men were able to sustain more weight than the women (mean SD). Maximal isometric exertion for male subjects

3 918 MUSCLE FATIGUE DURING LATERAL TRUNK LOADING, Thomas spinae showed significant MF decrease; at higher loads, all trunk muscles exhibited significant decreases (fig 2). Total Power The ANCOVA revealed that as loads were sustained, TP increased significantly ( p.05) in all muscles (fig 3). Men exhibited greater TP increases than women ( p.05). The TP increased with increase in load magnitude ( p.05), and the greatest increases occurred at 60% of maximal exertion (fig 3). At all load levels, the TP of the signals generated by the abdominal muscles was much greater than that generated by the Fig 2. The mean median frequency of the power spectrum of the EMG signals of the trunk muscles for (A) 20%, (B) 40%, and (C) 60% of maximal lateral bend exertion. LES, lumbar erector spinae; TES, thoracic erector spinae; RA, rectus abdominus; EO, external abdominal oblique; IO, internal abdominal oblique. was N; for female subjects, it was N. The mean maximal exertion of all subjects was N. Median Frequency The ANCOVA revealed no significant gender difference in MF ( p.05). MF declined as the contraction time increased ( p.05). Load magnitude influenced the change in MF significantly ( p.05), and the MF decline at 20% of maximal exertion was much less than at 40% and 60% (fig 2). Furthermore, at 20% of maximal exertion, only the thoracic erector Fig 3. The total power of the EMG signals of the trunk muscles for (A) 20%, (B) 40%, and (C) 60% of maximal lateral bend exertion. LES, lumbar erector spinae; TES, thoracic erector spinae; RA, rectus abdominus; EO, external abdominal oblique; IO, internal abdominal oblique.

4 MUSCLE FATIGUE DURING LATERAL TRUNK LOADING, Thomas 919 erector spinae (fig 3). However, no significant differences existed in the rate of TP increase among the various muscles ( p.05). DISCUSSION The present study showed that isometric lateral bend exertion decreased the MF and increased the TP of the EMG signals power spectrum, indicating fatigue in the various trunk muscles. 5,6,24 The rate of fatigue increased with increase in load magnitude. These findings are similar to those reported in earlier studies that examined sagittal loading. 23,25-27 Gender had no effect on the MF, but men exhibited greater TP increase than women. The latter finding was related to the fact that the maximal lateral bend exertion of the men was much higher than that of the women. Thus, the men were recruiting more motor units to sustain the higher level of force output. At low load magnitude, only thoracic erector spinae was found to show signs of fatigue, as reflected in the MF decline, while at higher load levels, all muscles exhibited fatigue. This finding suggests that muscles fatigue to different extents at different load levels. Cholewicki and McGill 28 documented that the abdominal muscles had larger moment arms than the erector spinae in generating lateral bend moment. Additional research 4,18 found that abdominal muscles were preferentially recruited during lateral bending. This observation is consistent with the present finding that the TP measurements of the abdominal muscles were significantly larger than those of the erector spinae. The present study carries explicit clinical implications. First, all trunk muscles are susceptible to fatigue, and endurance training of these muscles may play a significant role in reducing the risk of low back pain. Assessing the endurance of the paraspinal and abdominal muscles may help the clinician identify subjects who are prone to back pain. Such assessment should be made at different load levels. Women and men fatigue at about the same rate, but men can recruit more motor units to sustain higher load level. This implies that men are more efficient in handling loads, ie, they can sustain higher load with no increase in the level of fatigue. The present experiment also showed that as the magnitude of load increased and the contraction time increased, the trunk muscles showed more fatigue. The increase in fatigue could be associated with an increased risk of low back pain. 3 In the present study, we examined fatigue only in muscles on the contralateral side, because previous studies 16,29 showed that in sustaining a unilateral load in the coronal plane, muscles on the ipsilateral side did not generally show signs of fatigue, although muscles were recruited bilaterally. This finding suggested that examination of the ipsilateral muscles might not provide additional insight. The present study was conducted in young, healthy subjects whose occupations involved a moderate amount of trunk bending and manual handling. It may not be appropriate to generalize the present experimental results to other populations such as trained manual workers or older subjects. Another limitation of the present study is that data were collected for only 2 minutes, because at 60% of maximal exertion, subjects were not able to sustain the load any longer. In a workplace situation, small loads may be sustained longer. Further research is required to address this limitation. During prolonged contractions at low load level, muscle fatigue may operate to a different extent. CONCLUSIONS All trunk muscles are susceptible to fatigue during isometric lateral bend loading. Gender did not appear to have an effect on the rate of muscle fatigue, but contraction time and load magnitude influenced fatigue significantly. The research suggested that endurance training of the trunk muscles may help reduce the risk of low back pain associated with loading in the coronal plane. Fatigue of trunk muscles during prolonged contraction at low load level should be examined in future research. References 1. Waddell G. A new clinical model for the treatment of low-back pain. Spine 1987;12: Andersson GBJ. Epidemiologic aspects on low-back pain in industry. Spine 1981;6: Roy SH. The role of muscle fatigue in low back pain. In: D Orazio B, editor. Back pain rehabilitation. Boston: Andover; Lavender SA, Chen I-H, Trafimow J, Andersson G. The effects of lateral trunk bending on muscle recruitments when resisting nonsagittally symmetric bending moments. Spine 1995;20: Mannion AF, Dolan P. Electromyographic median frequency changes during isometric contraction of the back extensors to fatigue. Spine 1994;19: Dolan P, Mannion AF, Adams MA. Fatigue of the erector spinae muscles: a quantitative assessment using frequency banding of the surface electromyography signal. Spine 1995;20: Solomonow M, Baten C, Smit J, Barata R, Hermens H, D Ambrosia R, et al. Electromyogram power spectra frequencies associated with motor unit recruitment strategies. J Appl Physiol 1990;68: Roy SH, De Luca CJ, Casavant DA. Lumbar muscle fatigue and chronic lower back pain. Spine 1989;14: Seidel H, Beyer H, Brauer D. Electromyographic evaluation of back muscle fatigue with repeated sustained contractions of different strengths. Eur J Appl Physiol 1987;56: De Luca CJ. The use of surface electromyography in biomechanics. J Appl Biomech 1997;13: Nicolaisen T, Jorgensen K. Trunk strength, back muscle endurance and low-back trouble. Scand J Rehabil Med 1985;17: Sirca A, Kostevc V. The fibre composition of thoracic and lumbar paravertebral muscles in man. J Anat 1985;41: Bagnall KM, Ford DM, McFadden KD, Greenhill BJ, Raso VJ. The histochemical composition of human vertebral muscle. Spine 1984;9: Bogduk N, Twomey LT. Clinical anatomy of the lumbar spine. 2nd ed. Melbourne: Churchill Livingstone; Zetterberg C, Andersson GBJ, Schultz AB. The activity of individual trunk muscles during heavy physical loading. Spine 1987;12: Potvin JR, O Brien PR. Trunk muscle co-contraction increases during fatiguing, isometric, lateral bend exertions: possible implications for spine stability. Spine 1998;23: McGill SM. Electromyographic activity of the abdominal and low back musculature during the generation of isometric and dynamic axial trunk torque: implications for lumbar mechanics. J Orthop Res 1991;9: McGill SM. A myoelectrically based dynamic three-dimensional model to predict loads on lumbar spine tissues during lateral loading. J Biomech 1992;25: McGill SM, Juker D, Kropf P. Appropriately placed surface EMG electrodes reflect deep muscle activity (psoas, quadratus lumborum, abdominal wall) in the lumbar spine. J Biomech 1996;29: Basmajian JV, De Luca CJ. Muscles alive: their functions revealed by electromyography. 5th ed. Baltimore: Williams & Wilkins; Dolan P, Adams MA. The relationship between EMG activity and extensor moment generation in the erector spinae muscles during bending and lifting activities. J Biomech 1993;26:

5 920 MUSCLE FATIGUE DURING LATERAL TRUNK LOADING, Thomas 22. Kuorinka I. Restitution of EMG spectrum after muscular fatigue. Eur J Appl Physiol 1998;57: Merletti RM, Sabbahi MA, De Luca CJ. Median frequency of the myoelectric signal: effects of muscle ischaemia and cooling. Eur J Appl Physiol 1984;52: Ng JK-F, Richardson CA. Reliability of electromyographic power spectral analysis of back muscle endurance in healthy subjects. Arch Phys Med Rehabil 1996;77: Merletti RM, Knaflitz M, De Luca CJ. Myoelectric manifestations of fatigue in voluntary and electrically elicited contractions. J Appl Physiol 1990;69: De Luca CJ, Sabbahi MA, Roy SH. Median frequency of the myoelectric signal: effects of hand dominance. Eur J Appl Physiol 1986;55: Mannion AF, Dolan P. Relationship between myoelectric mechanical manifestations of fatigue in the quadriceps femoris muscle group. Eur J Appl Physiol 1996;74: Cholewicki J, McGill SM. Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain. Clin Biomech 1996;11: Kim SH, Chung MK. Effect of posture, weight and frequency on trunk muscular activity and fatigue during repetitive lifting tasks. Ergonomics 1995;38: Suppliers a. Model DE 02-3H; DelSys, Inc, PO Box 15734, Boston, MA b. The Math Works, Inc, 3 Apple Hill Dr, Natick, MA c. SPSS, Inc, 444 North Michigan Ave, Chicago, IL

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