374
Effect of Fatigue on Torque Output and Electromyographic Measures of Trunk Muscles During Isometric Axial Rotation Joseph K.-F. Ng, PhD, Mohamad Parnianpour, PhD, Carolyn A. Richardson, PhD, Vaughan Kippers, PhD ABSTRACT. Ng JK-F, Parnianpour M, Richardson CA, Kippers V. Effect of fatigue on torque output and electromyographic measures of trunk muscles during isometric axial rotation. Arch Phys Med Rehabil 2003;84:374-81.
© 2003 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation
Objectives: To examine the changes in torque output resulting from fatigue, as well as changes in electromyographic measures of trunk muscles during isometric axial rotation and to compare these changes between directions of axial rotation. Design: Subjects performed fatiguing right and left isometric axial rotation of the trunk at 80% of maximum voluntary contraction while standing upright. Setting: A rehabilitation center. Participants: Twenty-three men with no history of back pain. Interventions: Not applicable. Main Outcome Measures: Surface electromyographic signals were recorded from 6 trunk muscles bilaterally. The primary torque in the transverse plane and the coupling torques in sagittal and coronal planes were also measured. Results: During the fatiguing axial rotation contraction, coupling torques of both sagittal and coronal planes were slightly decreased and no difference was found between directions of axial rotation. Decreasing median frequency and an increase in electromyographic amplitude were also found in trunk muscles with different degrees of changes in individual muscles. There were significant differences (P⬍.05) between right and left axial rotation exertions in median frequency slope of external oblique, internal oblique, latissimus dorsi, and iliocostalis lumborum muscles, but no such difference was found in median frequency slope of rectus abdominis and multifidus muscles. This could be attributed to different functional roles among the muscles. Similar differences (P⬍.05) between right and left axial rotation in median frequency slope were also detected in the electromyographic amplitude slope of the trunk muscles. Coefficient of variation of the torque output and electromyographic activation in most of the trunk muscles increased during the fatigue process. Conclusion: The changing coupling torque, different fatigue rate, and activation changes of trunk muscles, as well as the increase in variability during fatiguing axial rotation exertion, could affect the internal loading and stability of the spine; this needs to be further quantified in future studies. Key Words: Abdominal muscles; Back; Electromyography; Fatigue; Isometric contraction; Rehabilitation.
USTAINED POSTURE IN A twisted position has been S implicated in back pain incidence in workers. A trend of higher risk of pain was shown in workers who spent more time
From the Departments of Physiotherapy (Ng, Richardson) and of Anatomical Sciences (Ng, Kippers), University of Queensland, Australia; and Department of Mechanical Engineering, Sharif University of Technology, Iran (Parnianpour). Supported by the Dorothy Hopkins Award for Clinical Study and the Manipulative Therapists Special Group of Queensland, Australia. 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 author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Joseph K.-F. Ng, Dept of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Hong Kong, e-mail:
[email protected]. 0003-9993/03/8403-7267$30.00/0 doi:10.1053/apmr.2003.50008
Arch Phys Med Rehabil Vol 84, March 2003
1-4
in trunk axial rotation.4 Fatigue inevitably occurs when certain postures are maintained for prolonged periods of time. Fatigue of the trunk muscles may have a significant bearing on spinal injuries that commonly occur in daily activities. Muscle fatigue is usually defined as the failure of the muscle to maintain the target force (ie, the mechanical failure point of the muscle).5 However, before this failure point is reached, the muscle is already fatigued.6-8 Fatigue, from this perspective, is an ongoing process that begins when a muscle starts to contract. In a sustained contraction, biochemical, physiologic, and related electromyographic changes begin to show even when the force of the contraction is maintained.7 It has been argued that muscle fatigue may be better defined as a time-dependent process with regard to physiologic and biochemical changes.7,8 Fatigue can affect the neural control of the spine. In dynamic fatiguing sagittal activities, the flexion and extension range (primary plane) decrease over time.9-11 However, there is an increase in range of movement for other planes—lateral flexion9,10 and axial rotation.9,11 The increased range in the secondary planes of motion has been explained by the diminished control and coordination of the neuromuscular system during fatigue.9 This diminished neural control may indicate that spinal stability can be compromised and may result in back injuries. The loss of neural control could be caused by an impairment of input from the muscle spindles of paraspinal muscles, which is important for the position sense of the spine.12 Accordingly, fatigue of back muscles may affect the position sense of the trunk. It has been shown that muscle fatigue can induce impairment in the sensation of position change of the lumbar spine.13 This decreased ability to sense a change in position would affect the neuromuscular system in its role of maintaining spinal position and equilibrium. Furthermore, fatigue could delay the reaction time of the muscle in response to a sudden load, so that injury to the spine may occur in the event of unexpected perturbation in load or position of the joint.14 Axial rotation of the trunk is a complex exertion involving many trunk muscles. Each trunk muscle has a main functional role as the prime mover, antagonist, or stabilizer. By comparing the activity of each muscle between right and left isometric axial rotation, functional role has been shown to differ among individual abdominal and back muscles.15 Changes in trunk muscle activity during fatiguing isometric axial rotation have been investigated only in recent years.16,17 No studies have been conducted to compared the fatigue changes of trunk muscles between directions of axial rotation. Torque production in sagittal and coronal planes has also been observed during contractions in axial rotation15; however, the effect of fatigue on changes of this kinetic coupling is still not known.
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
To understand the neural control of the trunk during fatigue, it is important to monitor the torque output in 3 planes. Electromyographic activity of trunk muscles and kinetic coupling of the torque have been shown to be affected by exertion levels during axial rotation.15 A clinically important observation is that back pain patients exhibited different activation strategies in some abdominal muscles when compared with healthy subjects during isometric axial rotation at exertion levels greater than 50% of maximum voluntary contraction (MVC).18 In addition, higher fatigue rates of back muscles in back pain patients were found during back extension exertion at 80% of MVC.19 Therefore, in this study, we investigated the fatigue changes of torque output, as well as electromyographic measures of trunk muscles during isometric axial rotation exertion at 80% of MVC, and compared these changes between directions of axial rotation. METHODS Participants Twenty-three healthy men without history of back pain were recruited for this study. The mean ⫾ standard deviation (SD) of age, height, and weight were 30.2⫾7.9 years, 1.75⫾0.07m, 68.1⫾10.3kg, respectively. All subjects were right-handed. The study was approved by the Medical Research Ethics Committee of The University of Queensland, and all subjects gave written informed consent to participate. Equipment A triaxial dynamometer, B200 Isostation,a was used to measure the torque produced by the trunk about the 3 planes of the body in 6 directions. These included the primary torque in the transverse plane (right/left axial rotation) and the coupling torque in the sagittal plane (flexion, extension) and in the coronal plane (right/left lateral flexion). Electromyographic signals from both abdominal and back muscles were recorded by surface electrodes and were amplified, band-pass filtered at 5 to 500Hz, and sampled at 1000Hz. The torque data and electromyographic signals were collected with the data acquisition system AMLAB II workstation.b Electrodes Surface electrodes were placed over 3 pairs of abdominal muscles and 3 pairs of back muscles. Before the recording electrodes were placed, the skin was shaved, cleaned with alcohol, and abraded with fine sandpaper. Skin resistance of less than 5k⍀ was considered acceptable. To have the optimal pickup of the electromyographic signals, the electrodes were placed in parallel with the muscle fiber orientation. The electrodes for the rectus abdominis muscle were placed 1cm above the umbilicus and 2cm lateral to the midline. For the external oblique muscle, 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.20 For the internal oblique muscle, electrodes were placed 1cm medial to the anterior superior iliac spine (ASIS) and beneath a line joining both ASIS.20 McGill et al21 found that surface electrode positions similar to those used in our study for external and internal oblique muscles recorded most of the signals from those muscles. The electrodes for the latissimus dorsi muscle were placed over the muscle belly at the T12 level and along a line connecting the most superior point of the posterior axillary fold and the S2 spinous process. The T12 level was selected to avoid the pressure of the thoracic pad on the electrode.22 For the iliocostalis lumborum muscle, the electrodes were placed at
375
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.23 For the multifidus muscle, the electrodes were placed at the L5 level and aligned parallel to the line between the PSIS and the L1-2 interspinous space.23 This electrode position for the multifidus has produced findings similar to those recorded with intramuscular electrodes.24 Experimental Procedure Subjects stood erect with the L5-S1 interspinous space aligned with the flexion and extension axis of the B200 Isostation. Their pelvis and lower legs were stabilized by the pelvic restraint, as well as by thigh and knee straps. The torso was fixed by the chest restraint and thoracic pad, according to the instruction manual. In preparation for the isometric testing, the machine was mechanically locked in 3 planes. The tasks included maximum isometric contractions in 3 planes and 2 fatigue tests at 80% of MVC in both right and left axial rotation, with maximum contractions and fatigue tests each arranged in a randomized order. The subjects were asked to fold their arms across their chest, hold them above the chest restraint, and maintain that position during all exertions. To lessen the chance of injury, subjects were instructed to avoid any sudden movements during the exertions. MVCs in flexion, extension, lateral flexion to both sides, and axial rotation to both sides were measured for 5 seconds with a 2-minute rest between trials. Visual feedback and verbal encouragement were given to ensure that the subject gave his maximal effort. Subjects were given a 15-minute rest after the maximal exertion testing. Reference exertion levels for 80% of MVC for both right and left axial rotation were then computed and displayed as a reference line on the visual feedback monitor. There were another 2 tolerance lines set in the feedback monitor at ⫾10% of the reference exertion level. The subjects were asked to maintain the contraction at the reference line and between the 2 tolerance lines for as long as possible (fig 1). The fatigue test ended when the subject could not continue, or until the initial torque was reduced by more than 10% after repeated encouragement.25 To quantify the subjective feeling of the fatiguing contraction, the subject was asked to rate his perceived exertion, using Borg’s Ratings of Perceived Exertion (RPE) Scale26 (15 grades, range 6 –20) immediately after the exertions. Subjects were given a 15-minute rest between the 2 fatigue tests. Previous studies27,28 have shown that electromyographic parameters recover within 15 minutes after fatigue. Each subject attended 2 sessions—a familiarization session at least 3 days before testing and the actual testing session. The procedure at the familiarization session was similar to that in the testing session, except there was no placement of surface electrodes. The familiarization session was intended to minimize the learning effect and to allow the subject to become familiar with the equipment and the testing procedure. Triaxial torque data and the electromyographic activity in the trunk muscles were collected during the testing session. Data Analysis For the measurement of fatigue changes, 2 seconds of coupling torque and electromyographic data at every 10th percentile of the endurance time were extracted. Means of the coupling torque values were computed. Frequency and amplitude analyses were applied to the extracted electromyographic signals. For the frequency analysis, the signals were analyzed with fast Fourier transform, and the median frequency was computed. The median frequency is the frequency that divides the power spectrum into 2 regions with equal power; it has been proven to be a valid measure of the spectral compression Arch Phys Med Rehabil Vol 84, March 2003
376
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
Fig 1. Subject performing isometric axial rotation at 80% of MVC with the real-time feedback from a visual monitor.
associated with muscle fatigue.29 To depict the electromyographic amplitude changes during fatigue, root mean square (RMS) values were computed. To compare results between subjects, the torque values as well as electromyographic amplitude data of individual muscles during the fatigue process were normalized with respect to the maximum torque values and electromyographic amplitude data acquired during the maximum exertions in 3 planes. For the quantification of the maximum exertions, the torque data were inspected visually, and a stable region with 3 seconds of data was selected.22 In addition, the simultaneous 3-second electromyographic amplitude data were extracted for further analysis. The baseline torque and electromyographic values were subtracted from the values recorded during exertions. The baseline electromyographic data were the minimum values observed in any exertion, whereas the baseline torque values Arch Phys Med Rehabil Vol 84, March 2003
were those obtained during relaxed standing in the B200 Isostation. Least squares linear regression analysis was applied to the coupling torque and electromyographic data. The average slope values of the regression of torque, median frequency, and RMS values for the whole period of fatiguing exertion were computed. To depict the coupling torque and electromyographic measures at the early part of the contraction, means and SDs of coupling torque and electromyographic data at the initial period (5%) of the endurance time were calculated. Statistical Analysis Paired t tests were performed for the endurance time, sagittal and lateral flexion coupling torques, and electromyographic data of individual trunk muscles to evaluate the difference between directions of axial rotation. Coefficients of variation
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
377
(CVs) were computed for the torque and electromyographic data during the fatigue process. The change of CV during fatigue was also determined by regression analysis. The relation between the median frequency slope data and the endurance time was examined with the Pearson product-moment correlation coefficient. Statistical significance was set at .05. RESULTS Endurance Time and Score for RPE The average endurance time for right axial rotation was 45⫾25 seconds; for left axial rotation, it was 49⫾21 seconds. No significant difference was found between directions of axial rotation for the endurance time. The mean score for RPE was 18⫾1 (Borg RPE Scale maximum score, 20) for both directions of axial rotation. Torque Values At the initial period (5% of the endurance time) of sustained axial rotation at 80% of MVC, coupling torques in the other planes were found— 0% to 2% of MVC of flexion coupling torque and 24% to 30% of MVC ipsilateral lateral flexion coupling torque (fig 2). During the fatiguing contraction, both sagittal and lateral flexion (ipsilateral) coupling torques decreased (fig 3). No difference between directions was found in the rate of decrease for these coupling torques. There was a significant increase in variability in torque, not only in the intended axial rotation torque, but also for coronal plane coupling torque (table 1). Electromyographic Measures The electromyographic activity of the abdominal and back muscles at the initial period (5% of the endurance time) of the fatiguing contraction is shown in figure 2. Higher levels of activation were found in the internal oblique, latissimus dorsi, and iliocostalis lumborum muscles on the ipsilateral side as well as in the external oblique muscle on the contralateral side. The median frequency slope decreased in all muscles with different degrees of decline for individual muscles. There were significant differences (P⬍.05) between right and left fatiguing axial rotation exertions in the decline of median frequency of bilateral external oblique, internal oblique, latissimus dorsi, and iliocostalis lumborum muscles (fig 4). On the other hand, no differences were found in the median frequency slope in bilateral rectus abdominis and multifidus muscles between right and left axial rotation exertions. Similar differences (P⬍.05) between right and left axial rotation were found in the RMS slope, though the electromyographic activation in all the muscles showed an increase (fig 5). Significant increases in electromyographic activation variability were found in most of the abdominal and back muscles during fatiguing right and left axial rotation (table 1). There was significant correlation between mean median frequency slope of 3 pairs of abdominal muscles and endurance time in axial rotation (r⫽.60, P⬍.001). However, no relation was found (r⫽.16, Pⱖ.05) between mean median frequency slope of 3 pairs back muscles and endurance time.
Fig 2. Electromyographic (EMG) activation (RMS values, mean and SD) of (A) abdominal and (B) back muscles at the initial period (5% of the endurance time) of right and left fatiguing axial rotation (Rot) exertions. (C) Coupling torques in sagittal and coronal planes are also shown. Abbreviations: R, right; L, left; RA, rectus abdominis; EO, external oblique; IO, internal oblique; LD, latissimus dorsi; IL, iliocostalis lumborum; MU, multifidus; F/E, flexion/extension; LF, lateral flexion. Significant differences between right and left axial rotation are indicated (**P<.01).
DISCUSSION
those researchers examined the endurance in isometric axial rotation exertion in only 1 direction. Average endurance time for sustaining axial rotation contraction at 80% of MVC in our study was 45 to 49 seconds. This is in accord with previous studies in which endurance times decreased as higher torque levels were maintained. For isometric axial rotation at 40% of MVC, an average endurance time of 163 seconds was found in a group of men and women.16 For 60% of MVC axial rotation exertion, Kumar and Narayan17 found mean endurance times of 102 seconds for men and 113 seconds for women.
Endurance Time Similar endurance time was shown between directions of axial rotation exertion in the present study even though all the subjects were right-hand dominant. It is not possible to compare our results with those of previous studies16,17 because
Torque Measurement A new finding from our study relates to the changes of coupling torque in other planes during the fatiguing axial rotation exertion. The coupled sagittal and lateral flexion (ipsilateral) torques both decreased slightly despite the primary Arch Phys Med Rehabil Vol 84, March 2003
378
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
Fig 3. Changes in coupling torques (mean and SD) in sagittal and coronal planes during fatiguing right and left axial rotation (Rot) exertions. (Lateral flexion coupling torque values to right and left are designated as positive and negative, respectively. The positive slope values for left lateral flexion coupling torque means that the coupling torque is decreasing to the neutral, ie, without coupling torque.)
torque being maintained in the transverse plane. It seems that neural strategies adapt to the required task demand, which can affect the coordinated trunk recruitment during fatiguing axial rotation exertions. The change in coupling of the torque may be
Fig 4. Electromyographic frequency (median frequency, mean and SD) slope values of (A) abdominal and (B) back muscles during fatiguing right and left axial rotation exertions. Significant differences between right and left axial rotation are indicated (*P<.05; **P<.01).
related to the changing recruitment patterns, and to the cocontraction patterns of the trunk muscles. In addition, we found that the contraction became less stable near the end of fatiguing
Table 1: Changes in the CV for Torque and Electromyographic Activation During Fatiguing Right and Left Axial Rotation Exertions Right Axial Rotation CV Slope
Torque Axial rotation Flexion/extension Lateral flexion Electromyographic activation R rectus abdominis L rectus abdominis R external oblique L external oblique R internal oblique L internal oblique R latissimus dorsi L latissimus dorsi R iliocostalis lumborum L iliocostalis lumborum R multifidus L multifidus
†
0.05 58.31 4.26† 0.73* 0.66† 0.39† 0.41† 0.72† 0.73 0.52† 0.50† 0.46* 1.48* 0.58* 0.69†
Abbreviations: R, right; L, left; CI, confidence interval. * P⬍.05. † P⬍.01.
Arch Phys Med Rehabil Vol 84, March 2003
Left Axial Rotation
95% CI
CV Slope
95% CI
0.03–0.07 ⫺20.23 to 136.85 3.40–5.12
0.04† 91.69 4.75†
0.01–0.07 ⫺54.20 to 237.59 3.65–5.84
0.12–1.33 0.35–0.98 0.24–0.54 0.25–0.57 0.59–0.86 ⫺0.07 to 1.52 0.32–0.73 0.22–0.78 0.13–0.79 0.11–2.85 0.04–1.12 0.23–1.16
0.96† 0.74† 0.37† 0.37† 1.35† 0.87† 0.89 0.57 0.55† 0.46† 1.31† 0.63†
0.72–1.19 0.52–0.95 0.25–0.50 0.20–0.53 0.63–2.07 0.60–1.15 ⫺0.16 to 1.94 ⫺0.11 to 1.25 0.24–0.85 0.25–0.67 1.06–1.55 0.29–0.96
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
Fig 5. Electromyographic amplitude (RMS, mean and SD) slope values of (A) abdominal and (B) back muscles during fatiguing right and left axial rotation exertions. Significant differences between right and left axial rotation are indicated (*P<.05; **P<.01).
task. This was shown by an increase in the CV of the torque output, not only in axial rotation but also in the coronal plane. The lack of smoothness may be attributed to the increased variability of activation of most of the trunk muscles we studied. It would be of great interest to determine the stability of the spine during fatiguing exertion. During the 80% of MVC axial rotation exertion, the primary component of generated torque is kept invariant (with ⫾10% tolerance limit of the target torque). Hence, the equilibrium requirement in the transverse plane remains approximately the same. However, the redundant system of muscles may achieve the goal of performing 80% of MVC by different recruitment patterns. It may be that this alteration of muscle recruitment affects both the coupling torque and spine stability. The stiffness analysis, such as those done by Cholewicki and McGill30 and Kiefer et al,31 would be warranted in future studies to determine whether the altered recruitment has a compensatory or deleterious effect on spinal stability. Electromyographic Findings This study is the first to compare the fatigue changes of trunk muscles between right and left isometric axial rotation exertions. No difference in median frequency slope between right and left axial rotation was found in the rectus abdominis and multifidus muscles. However, such differences were found in the external oblique, internal oblique, latissimus dorsi, and iliocostalis lumborum muscles. These differences between directions were also seen in the electromyographic activity data at the initial period of the contraction. The different behavior of
379
rectus abdominis and multifidus muscles compared with other trunk muscles could be explained by their main functional role as stabilizers during axial rotation.15 As shown in the present study and in others,15,32,33 a number of abdominal and back muscles are recruited during axial rotation contraction. This can be attributed to the fact that the agonists involved in axial rotation produce torque in other directions.34 To maintain trunk stability, cocontractions of other muscles are essential. The magnitude of cocontraction is higher in axial rotation than in lateral flexion, flexion, or extension.33,35,36 With respect to these observations, fatigue in most of the abdominal and back muscles is inevitable. We observed a decrease in median frequency in all of the abdominal and back muscles. It is interesting to note that median frequency decline was also shown in the muscles with low activity. One reason may be related to the oxygen level in the muscle. Significant reduction in oxygenation of the back muscles has been shown during contraction at low exertion levels.37,38 An increase in electromyographic activity was also evident in the abdominal and back muscles during the fatiguing exertion. The pattern involved an increase in activation, which was higher in the internal oblique, latissimus dorsi, and iliocostalis lumborum muscles when they functioned in their agonist rather than in their antagonist role. A similar pattern of median frequency decline was also found in these muscles. O’Brien and Potvin16 found a greater increase in electromyographic amplitude in agonists than in antagonists during axial rotation exertion. The median frequency decline in the trunk muscles during sustained axial rotation exertion may result from the spectral compression associated with a gradual increase of the lowerfrequency content of the surface electromyographic signals. Decrease in muscle fiber conduction velocity has been the major factor offered as an explanation of this spectral compression to lower frequency,7,39,40 and this is because of changes in pH and potassium ion concentration.41 Other factors that could affect the spectral compression include changes in the recruitment and firing rate as well as the synchronization of motor units, but these factors are still inconclusive.7,39,40 The rate of the frequency shift or compression, which is commonly expressed as the rate of decline in median frequency, is regarded as the fatigue rate of the muscle.19,25,42-45 There is a relation between median frequency decline of the back muscles and the endurance time during back extension.25,43,46-48 Interestingly, this relationship was also shown in axial rotation, although many muscles are involved during the exertion. In this study, we found a correlation between mean median frequency slope of the abdominal muscles and endurance time. Of particular interest was the finding that the median frequency decline of the external oblique muscle was greater (more negative median frequency slope values) when the muscle acted as an antagonist than when the muscle functioned as an agonist. Similar observations have been made, although in neither study16,17 was an explanation offered for this phenomenon. This greater decline differed from that of other trunk muscles during axial rotation, in which the muscle acting as the agonist usually demonstrated a greater median frequency slope than that when the muscle was working in its antagonist role (eg, latissimus dorsi). It may be that the role of the external oblique muscle is more complicated than just that of a prime mover or an antagonist.15 In addition, the electrode placement in the present study (on external oblique) may only represent the activity pattern of part of the muscle, because varied activity in different regions of external oblique has been shown in axial rotation.49,50 Arch Phys Med Rehabil Vol 84, March 2003
380
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
The different fatigue rates, as well as changes in activation of the abdominal and back muscles found during the fatiguing axial rotation, may have important implications for the stability of the spine. It would be expected that recruitment of the motor units and changes in the firing rate of individual muscles may not be the same because of factors such as muscle fiber composition, the levels of contraction, and the functional role of the muscle during axial rotation exertion. The different fatigue rates of trunk muscles may pose risks to the spine because of a disturbance of force balance. In addition, changes in activation of trunk muscles, such as shear and compression force,10,51 could affect the internal loading of the spine. We found increased variability of the activation of most of the trunk muscles during fatigue. The recruitment may be altered to a pattern that is perhaps less stable, thereby compromising the stiffness of spine. This possibility is supported by the parallel finding that variability of the torque output also increased. It is only speculative to relate the less coordinated muscle performance (less smooth and stable axial torque output as the net result of trunk muscle recruitment) to a less stable spine as the risk factors that may be present at the end of the fatiguing tasks. It is supposed that before fatigue, the muscle recruitment can generate the desired torque with more attention being given to the stability requirements of the spine. As fatigue continues, the MVC declines.52 Hence, the decline in strength increases the relative difficulty of the task. Therefore, the loss of smoothness may result from the increasing percentage of MVC that this task represents as the subject continues the endurance test. As muscle fatigue occurs, the continuation of task performance becomes more paramount than the quality of the performance and the stability. The neural control system may choose to accept, or face the risk of, exposing the spine to a more injury-prone loading condition at a lower stability level. This speculative model requires verification and experimental validation. Limitations of Study To control the confounding variable of gender differences in the muscle fiber composition and the endurance capacity of back muscles,53 only men were recruited in this study. Further studies with women participating are needed to confirm our results. It must be acknowledged that crosstalk may affect the interpretation of the electromyographic data, for example, the electromyographic findings in the external oblique muscle could be contaminated by the myoelectric signals from the underlying internal oblique muscle. Davis and Mirka50 compared the electromyographic activity of abdominal obliques recorded by fine-wire and surface electrodes in 1 subject during axial rotation and found that the internal oblique activity could be collected by surface electrodes over the antagonistic external oblique. CONCLUSION Changes of coupling torques in sagittal and coronal planes were observed although the primary torque was maintained during fatiguing axial rotation exertions. Different fatigue rate and activation changes were also found in abdominal and back muscles. During the fatigue process, there were increases in the variability of torque output and the activation of the trunk muscles. This could affect the coordination of the performance and may influence spinal stability. It would be important to include the torque and electromyographic fatigue changes in future studies to increase our understanding of the effects on the internal loading and the stability of the spine. Arch Phys Med Rehabil Vol 84, March 2003
Acknowledgments: We thank the staff of the Workers’ Compensation Board of Queensland for their invaluable assistance during the whole data collection process. References 1. Burdorf A, Govaert G, Elders L. Postural load and back pain of workers in the manufacturing of prefabricated concrete elements. Ergonomics 1991;34:909-18. 2. Punnett L, Fine LJ, Keyserling WM, Herrin GD, Chaffin DB. Back disorders and nonneutral trunk postures of automobile assembly workers. Scand J Work Environ Health 1991;17:337-46. 3. Burdorf A, Naaktgeboren B, de Groot HC. Occupational risk factors for low back pain among sedentary workers. J Occup Med 1993;35:1213-20. 4. Hoogendoorn WE, Bongers PM, de Vet HC, et al. Flexion and rotation of the trunk and lifting at work are risk factors for low back pain. Results of a prospective cohort study. Spine 2000;25: 3087-92. 5. Edwards RH. Human muscle function and fatigue. In: Porter R, Whelan J, editors. Human muscle fatigue: physiological mechanisms. Ciba Foundation Symposium No. 82. London: Pitman Medical; 1981. p 1-18. 6. Bigland-Ritchie B, Woods JJ. Changes in muscle contractile properties and neural control during human muscular fatigue. Muscle Nerve 1984;7:691-9. 7. De Luca CJ. Myoelectrical manifestations of localized muscular fatigue in humans. Crit Rev Biomed Eng 1984;11:251-79. 8. De Luca CJ. Use of the surface EMG signal for performance evaluation of back muscles. Muscle Nerve 1993;16:210-6. 9. Parnianpour M, Nordin M, Kahanovitz N, Frankel V. The triaxial coupling of torque generation of trunk muscles during isometric exertions and the effect of fatiguing isoinertial movements on the motor output and movement patterns. Spine 1988;13:982-92. 10. Marras WS, Granata KP. Changes in trunk dynamics and spine loading during repeated trunk exertions. Spine 1997;22:2564-70. 11. van Diee¨ n JH, van der Burg P, Raaijmakers TA, Toussaint HM. Effects of repetitive lifting on kinematics: inadequate anticipatory control or adaptive changes? J Mot Behav 1998;30:20-32. 12. Brumagne S, Lysens R, Swinnen S, Verschueren S. Effect of paraspinal muscle vibration on position sense of the lumbosacral spine. Spine 1999;24:1328-31. 13. Taimela S, Kankaanpa¨ a¨ M, Luoto S. The effect of lumbar fatigue on the ability to sense a change in lumbar position. A controlled study. Spine 1999;24:1322-7. 14. Wilder DG, Aleksiev AR, Magnusson ML, Pope MH, Spratt KF, Goel VK. Muscular response to sudden load. A tool to evaluate fatigue and rehabilitation. Spine 1996;21:2628-39. 15. Ng JK, Parnianpour M, Richardson CA, Kippers V. Functional roles of abdominal and back muscles during isometric axial rotation of the trunk. J Orthop Res 2001;19:463-71. 16. O’Brien PR, Potvin JR. Fatigue-related EMG responses of trunk muscles to a prolonged, isometric twist exertion. Clin Biomech (Bristol, Avon) 1997;12:306-13. 17. Kumar S, Narayan Y. Spectral parameters of trunk muscles during fatiguing isometric axial rotation in neutral posture. J Electromyogr Kinesiol 1998;8:257-67. 18. Ng JK, Richardson CA, Parnianpour M, Kippers V. EMG activity of trunk muscles and torque output during isometric axial rotation exertion: a comparison between back pain patients and matched controls. J Orthop Res 2002;20:112-21. 19. Roy SH, DeLuca CJ, Casavant DA. Lumbar muscle fatigue and chronic lower back pain. Spine 1989;14:992-1001. 20. Ng JK, Kippers V, Richardson CA. Muscle fibre orientation of abdominal muscles and suggested surface EMG electrode positions. Electromyogr Clin Neurophysiol 1998;38:51-8. 21. McGill S, 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: 1503-7. 22. Tan JC, Parnianpour M, Nordin M, Hofer H, Willems B. Isometric maximal and submaximal trunk extension at different flexed positions in standing. Triaxial torque output and EMG. Spine 1993; 18:2480-90.
FATIGUE AND TRUNK AXIAL ROTATION EXERTION, Ng
23. De Foa JL, Forrest W, Biedermann HJ. Muscle fibre direction of longissimus, iliocostalis and multifidus: landmark-derived reference lines. J Anat 1989;163:243-7. 24. Arokoski JP, Kankaanpa¨ a¨ M, Valta T, et al. Back and hip extensor muscle function during therapeutic exercises. Arch Phys Med Rehabil 1999;80:842-50. 25. van Diee¨ n JH, Oude Vrielink HH, Housheer AF, Lo¨ tters FB, Toussaint HM. Trunk extensor endurance and its relationship to electromyogram parameters. Eur J Appl Physiol 1993;66:388-96. 26. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982;14:377-81. 27. Hara T. Evaluation of recovery from local muscle fatigue by voluntary test contractions. J Hum Ergol (Tokyo) 1980;9:35-46. 28. Kuorinka I. Restitution of EMG spectrum after muscular fatigue. Eur J Appl Physiol 1988;57:311-5. 29. Stulen FB, DeLuca CJ. Frequency parameters of the myoelectric signal as a measure of muscle conduction velocity. IEEE Trans Biomed Eng 1981;28:515-23. 30. Cholewicki J, McGill SM. Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain. Clin Biomech (Bristol, Avon) 1996;11:1-15. 31. Kiefer A, Shirazi-Adl A, Parnianpour M. Stability of the human spine in neutral postures. Eur Spine J 1997;6:45-53. 32. 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;9:91-103. 33. Marras WS, Granata KP. A biomechanical assessment and model of axial twisting in the thoracolumbar spine. Spine 1995;20:144051. 34. Dumas GA, Poulin MJ, Roy B, Gagnon M, Jovanovic M. Orientation and moment arms of some trunk muscles. Spine 1991;16: 293-303. 35. Thelen DG, Schultz AB, Ashton-Miller JA. Co-contraction of lumbar muscles during the development of time-varying triaxial moments. J Orthop Res 1995;13:390-8. 36. Marras WS, Granata KP. Spine loading during trunk lateral bending motions. J Biomech 1997;30:697-703. 37. Jensen BR, Jørgensen K, Hargens AR, Nielsen PK, Nicolaisen T. Physiological response to submaximal isometric contractions of the paravertebral muscles. Spine 1999;24:2332-8. 38. McGill SM, Hughson RL, Parks K. Lumbar erector spinae oxygenation during prolonged contractions: implications for prolonged work. Ergonomics 2000;43:486-93. 39. Ha¨ gg GM. Interpretation of EMG spectral alterations and alteration indexes at sustained contraction. J Appl Physiol 1992;73: 1211-7. 40. DeLuca CJ. The use of surface electromyography in biomechanics. J Appl Biomech 1997;13:135-63.
381
41. Juel C. Muscle action potential propagation velocity changes during activity. Muscle Nerve 1988;11:714-9. 42. Biedermann HJ, Shanks GL, Forrest WJ, Inglis J. Power spectrum analyses of electromyographic activity. Discriminators in the differential assessment of patients with chronic low-back pain. Spine 1991;16:1179-84. 43. Mannion AF, Dolan P. Electromyographic median frequency changes during isometric contraction of the back extensors to fatigue. Spine 1994;19:1223-9. 44. Roy SH, DeLuca CJ, Emley M, Buijs RJ. Spectral electromyographic assessment of back muscles in patients with low back pain undergoing rehabilitation. Spine 1995;20:38-48. 45. Ng JK, Richardson CA, Jull GA. Electromyographic amplitude and frequency changes in the iliocostalis lumborum and multifidus muscles during a trunk holding test. Phys Ther 1997;77:954-61. 46. Mannion AF, Connolly B, Wood K, Dolan P. The use of surface EMG power spectral analysis in the evaluation of back muscle function. J Rehabil Res Dev 1997;34:427-39. 47. Sparto PJ, Parnianpour M, Reinsel TE, Simon S. Spectral and temporal responses of trunk extensor electromyography to an isometric endurance test. Spine 1997;22:418-26. 48. Kankaanpa¨ a¨ M, Taimela S, Laaksonen D, Ha¨ nninen O, Airaksinen O. Back and hip extensor fatigability in chronic low back pain patients and controls. Arch Phys Med Rehabil 1998;79: 412-7. 49. Mirka G, Kelaher D, Baker A, Harrison A, Davis J. Selective activation of the external oblique musculature during axial torque production. Clin Biomech (Bristol, Avon) 1997;12:172-80. 50. Davis JR, Mirka GA. Transverse-contour modeling of trunk muscle-distributed forces and spinal loads during lifting and twisting. Spine 2000;25:180-9. 51. Sparto PJ, Parnianpour M, Marras WS, Granata KP, Reinsel TE, Simon S. Neuromuscular trunk performance and spinal loading during a fatiguing isometric trunk extension with varying torque requirements. J Spinal Disord 1997;10:145-56. 52. Sparto PJ, Parnianpour M. An electromyography-assisted model to estimate trunk muscle forces during fatiguing repetitive trunk exertions. J Spinal Disord 1999;12:509-18. 53. Ng JK, Richardson CA, Kippers V, Parnianpour M. Relationship between muscle fiber composition and functional capacity of back muscles in healthy subjects and patients with back pain. J Orthop Sports Phys Ther 1998;27:389-402. Suppliers a. Isotechnologies Inc, 328 Elizabeth Brady Rd, Hillsborough, NC 27278. b. Associative Measurement Pty Ltd, 112 Talavera Rd, North Ryde, NSW 2113, Australia.
Arch Phys Med Rehabil Vol 84, March 2003