Electromyography of trunk muscles in isometric graded axial rotation

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1 Journal of Electromyography and Kinesiology 12 (2002) Electromyography of trunk muscles in isometric graded axial rotation Shrawan Kumar *, Yogesh Narayan, Doug Garand Department of Physical Therapy, University of Alberta, Edmonton, Alberta, Canada T6G 2G4 Received 15 February 2001; received in revised form 13 June 2001; accepted 15 June 2001 Abstract This study was conducted to determine the pattern, magnitude, and phasic inter-relationship of the trunk muscles in maximal isometric and graded isometric axial rotational contractions and compare them with those previously observed from the same subjects in the same experimental session in dynamic conditions. In 50 normal young healthy subjects (27 male and 23 female), after a suitable skin preparation, bipolar silver silver chloride recessed pregelled surface electrodes were placed on external oblique, internal oblique, rectus abdominis, pectoralis major, latissimus dorsi, erector spinae at T 10 and L 3 levels bilaterally with 2 cm interelectrode distance. EMG signals from grounded subjects were suitably preamplified and amplified by a fully isolated system. These subjects were stabilized in an upright-seated posture in the Axial Rotation Tester (AROT), which was placed in isometric mode for force and rotation output from the AROT. The 14 channels of EMG, the force and the rotation were sampled at 1 khz. The subjects initially registered their isometric maximal voluntary contraction (MVC) on both sides which was used for reference and then performed their 25%, 50% and 75% of MVC bilaterally in an isometric mode in a random order. The EMG magnitude, the slope of the rise of the EMG, and the phasic interrelationship of muscles were analyzed. The results showed that female sample generated only 65% of torque of their male counterparts. There were no significant differences between the male and the female samples in the EMG variables. Exertions to the left and to the right were not significantly different from each other for the measured variables. However, the magnitude contribution of the muscles and the slope of rise of EMG were significantly different in two directions (p 0.001). The phasic interrelationship of the external obliques, the latissimus dorsi and the erector spinae were different from other muscles (p 0.01). With the increasing grades of contraction the latissimus dorsi and the external obliques increased their magnitude significantly whereas that of the erectores spinae underwent a decrease in proportionate terms (but not in absolute magnitude) suggesting their role as stabilizers but not as rotators Elsevier Science Ltd. All rights reserved. Keywords: Axial rotation; Trunk twisting; Low back pain; EMG; Trunk muscles; Graded contraction 1. Introduction Epidemiologically, trunk rotation has been associated with over 60% of low back injuries [1]); and, it has been ascribed as the sole factor in causation of back injuries in 33% of cases [2]. In spite of such significance trunk rotation remains a sparsely studied area. Many aspects of trunk rotation remain unclear and information unexplored. Basmajian [3] reported antagonistic activity of deep lumbar muscles during axial rotation. An occasional activity in ipsilateral multifidi and rotators * Corresponding author. Tel.: ; fax: address: shrawan.kumar@ualberta.ca (S. Kumar). during axial rotation was reported by Carlsoo [4] and Morris et al. [5]. In another study, Pope et al. [6] demonstrated that antagonistic activity during isometric axial rotation was much higher than reported previously. Furthermore, the authors reported little difference between the activities of the left and right pairs of many muscles studied. They reported the greatest magnitude of EMG activity in the erector spinae in isometric axial rotational activities. McGill [7] studied EMG of the external and internal obliques, the rectus abdominis, the latissimus dorsi, the upper and lower erector spinae in maximal isometric and isokinetic axial rotation. He reported a dominant role played by the ipsilateral latissimus dorsi and the contralateral external obliques in these activities. He also linked the upper erector spinae with torque generation /02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 318 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Marras and Mirka [8] reported significant response of the trunk muscles to trunk angular velocity, trunk torque level and trunk posture. Kumar [9] and Kumar et al. [10] reported the phasic and magnitude relationship of the external and internal obliques, the rectus abdominis, the pectoralis major, the latissimus dorsi and the erector spinae at T 10 and L 3 levels in an unresisted trunk axial rotation at a normal velocity. These studies were designed to determine the mechanism of the initiation, sustenance and the execution of axial rotation. These authors described the spinal neutral zone, the magnitude of the axial rotation, the phasic relationship of the muscles and their relative contribution in unresisted axial rotation. Kumar [9] and Kumar et al. [10] found that there was a differential in onset timing of different muscles; the ipsilateral thoracic erector spinae and the contralateral external obliques firing roughly 54 ms before the actual rotation of the trunk began. Due to the variability of these timings they inferred a general pattern rather than a rigid time relationship. The axial rotation, in their work, appeared to be initiated and sustained by the external oblique, the thoracic erector spinae and the latissimus dorsi. Further, a recoil of the connective tissues stretched during axial rotation controlled by the original agonist muscles, which caused the initial rotation, achieved the restoration of the neutral posture. Davis and Mirka [11] in their transverse contour modeling of trunk muscles used EMG from different segments of the broad muscle to develop a multivector solution. They reported such an effort reduces the calculated shear forces at intersegmental levels. Despite these works, it is unclear whether the process of axial rotation has a similar phasic and magnitude relationship in the isometric and the dynamic modes. It is also unclear if the magnitude of the force exertion alters the recruitment of muscles in temporal and magnitude domains. Is there a differential contribution from certain muscles, such as the agonists, as the magnitude of force exertion increases? In order to answer these questions an experiment was designed in which bilateral graded isometric axial rotation was studied. 2. Materials and methods 2.1. Experimental sample Fifty normal young adults (27 male and 23 female) volunteered for this study. A low back pain episode within the past year requiring one-week absence from work, other musculoskeletal or neuromuscular disorders, spinal and abdominal surgery were chosen as exclusion criteria. Ethics approval was granted and informed consent was signed by all subjects. The anthropometric details of subjects are provided in Table Tasks These subjects were asked to assume their neutral position in an upright sitting position appropriately stabilized in the Axial Rotation Tester (AROT). After staying in this posture for a few seconds the subjects were asked to exert isometrically with visual feedback to maintain force level for a 5 s period at 25%, 50% and 75% of their previously measured isometric MVC, in left and right directions. The combinations of the force levels and the direction of the exertions were fully randomized. After each exertion the subjects were allowed a minimum of 2 min of rest before exerting for the next random combination condition generated by the computer Equipment Axial rotation tester The Axial Rotation Tester (AROT) (Fig. 1) was specially designed to study axial rotation with minimal flexion/extension at the hip or in the torso [12,13]. It did not restrict movement of the shoulder or affect the shape changes of the thorax. The AROT (Fig. 1) consisted of a rigid metal frame mounted on a metal base plate. Inside the frame, mounted on the base plate, was an adjustable chair that could be slid back and forth and adjusted vertically. It had Velcro belts to stabilize the seated subjects. The backrest of the chair was sawn off to allow room for electrode placement and freedom to rotate. Directly above the chair, supported by a long bar, was an adjustable shoulder harness mounted on a circular plate. This plate, in turn, was attached to a spring-loaded rod sliding within a sleeve with a locking screw to position it rigidly at any chosen position. The rod could rotate when the positioned subject underwent axial rotation. This rotation was measured by a high precision potentiometer. The potentiometer was mounted on a support plate beside the rod. The rod and the potentiometer were coupled through a set of gears. Mounted on the crossbar was a dial gauge to read off the extent of rotation bilaterally Torque measuring system The force exerted during a graded axial rotation was measured by an Intechnology load cell (I #500) and a force monitor equipped with a signal conditioner (Prototype Design Ltd., Ann Arbor, MI). The circular plate of the AROT was attached to an immovable object by means of an airplane steel cable with an intervening load cell. The torque output was continually displayed on a visual scale in the subject s sight and also fed to the computer via the data acquisition card. A product of the force reading and the lever arm length (distance between the center of the spring loaded shoulder harness rod and the point of attachment of the shoulder harness on the shoulder of the subject) provided the torque values.

3 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Table 1 Statistic Male (n=27) Female (n=23) Age Height Weight Age Height Weight (yr) (cm) (kg) (yr) (cm) (kg) Mean SD Minimum Maximum Fig. 1. Axial Rotation Tester (AROT) EMG system The EMG system consisted of the surface electrodes, the electrode cables, the preamplifiers and the amplifiers. The silver silver chloride surface electrodes of 1 cm diameter with recessed pre-gelled elements (HP ) were used with an inter-electrode distance of 2 cm. These electrodes were connected to fully isolated preamplifiers by means of short cables and tip plugs. The low noise and low non-linearity preamplifiers were specially made and had a common mode rejection ratio of 130 db and a wide bandwidth. Those preamplifiers fed to a low power, high accuracy instrumentation amplifier designed for signal conditioning and amplification. The amplifier system was run off an internal charged battery. The amplifier had AC coupled inputs with a single pole RC filter with a cutoff frequency of 8 Hz. The EMG signals were full wave rectified and linear envelope detected Computer system The outputs of the load cell, the AROT potentiometer and the EMG amplifiers were sampled at 1 khz and fed to a MetraByte DAS 20 A to D board. Such converted digital signals were stored in the hard disc of a 486 with a tape backup for archival purposes Experimental procedures The subjects were weighed and measured for their height. Their ages were also recorded. Fourteen pairs of disposable pre-gelled surface electrodes (HP ) were applied to the subjects at an interelectrode distance of 2 cm after suitable preparation of the skin with an alcohol acetone mixture. These electrodes were placed on the erector spinae leveled with the spinous processes of the T 10 and L 3 vertebrae bilaterally 4 cm lateral to the tips of the spinous processes. Surface electrodes were

4 320 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) also applied to the left and right latissimus dorsi at the superior lateral aspect where the muscle bands converge. On the ventral side, the surface electrodes were applied bilaterally to the pectoralis major (8 cm medial to acromium and 8 cm inferior where bands of pectoralis major converge), rectus abdominis 4 cm above umbilicus and 4 cm lateral to the midline bilaterally, the external obliques (8 cm inferior to the costal edge on lateral aspect), and the internal oblique (in the area of external oblique aponeuroses to minimize overlap with the external oblique). A ground electrode was applied to the anterosuperior iliac spine. The subjects were wired to an isolated preamplifier system described before to provide onsite amplification. Subsequent to the application of the electrodes the subjects were asked to undergo axial rotation bilaterally to ensure minimal cross talk between the external and internal obliques. In its presence the electrodes were repositioned to eliminate or reduce the cross talk to a minimum. The signals from the external and internal obliques were also subjected to a cross correlation analysis to further detect any cross talk. The subjects were then seated in the chair of the axial rotation tester. The seat was adjusted for height, to have comfortably resting feet and knee at 90. The seat was then aligned with the axial rotation harness, which was lowered onto the subject s shoulders and fastened. The subject were stabilized in this seated position hip down using four Velcro straps at the hip, distal thigh, proximal shin, and ankle. First, maximum voluntary contractions (MVC) were determined by having the subjects perform their MVC in axial rotation in both directions in a neutral posture without jerking, according to the American Industrial Hygiene Association protocol [14]. They were instructed to slowly build their maximal force in the first 2 s and hold the contraction at maximal level for another 3 s, at which time the trial was terminated. For the graded isometric contractions the subjects were given a marked target on the display and were asked to reach that level within the first 2 s and maintain their contraction at that level for another 3 s before terminating the trial. Since the moment arm of the exertion was fixed, the variation in the force level was taken as an index of the exertion. the subject to initiate the contraction. After 5 s from the first beep the computer beeped again signaling the subject to relax. The data were acquired from the 14 EMG channels, load cell and the potentiometer of the AROT. Subsequent to each data collection period of 7 s the third module of the software produced an instant plot of all channels on the computer screen for quality control. Trials which deviated from the protocol, and trials with spurious or noisy signals were discarded. After appropriate correction the trial was repeated for clean signals and exertions according to the predescribed protocol Data analysis The previously collected (appropriate and clean) data were loaded into the hard drive of the computer and windowing was done for analysis (Fig. 2). As soon as the torque began to rise from the baseline or reached baseline (within 1% of the peak torque of the condition) the computer software automatically drew a vertical line marking the start and end of the cycle. A smoothing routine (11 point linear smoothing repeated twice) was used to smooth the signals to reliably interpret the pattern. Using the previously marked start position as the point of reference, the onset and all other timings were measured with respect to it. RMS EMG was recorded using 25 ms time constant. Any low level EMG activity of any muscle prior to a steep rise was designated anticipation and the point where a sudden and sustained increase in the EMG began was designated as the onset. The times of anticipation and onset of all 14 EMG channels with respect to the torque onset (start of the cycle) were 2.5. Data acquisition Data were acquired using modular software designed for the project. In the first module, the subject data were entered and the data files created in the computer. At this stage the computer also generated a random sequence of all submaximal graded experimental conditions. The second module ran the data acquisition program, collected the data and wrote them to the pre-created files. The sampling rate was set at 1 khz and recording duration was set for 7 s. One second into the data acquisition window the computer generated a first beep signaling Fig. 2. Activity windowing of superimposed plot of torque and EMG to determine phasic relationships: A anticipation and O onset of EMG.

5 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Table 2 Normalized torque and normalized total EMG output of all trunk muscles combined during graded isometric axial rotation 25% Max 50% Max 75% Max 100% Max Normalized Normalized Normalized Normalized Normalized Normalized Normalized Normalized torque total EMG torque total EMG torque total EMG torque total EMG (%) (%) (%) (%) (%) (%) (%) (%) Male: Effort towards Left Right Female: Effort towards Left Right determined. For this determination, each channel of the EMG was plotted with the torque trace and task cycle duration individually (Fig. 2). The relevant sections were amplified in case of any ambiguity. Using the cursor, the points of beginning of anticipation (low level EMG activity above baseline) and a clear onset (a sharp rise in EMG trace) were marked and stored in the memory. The duration between the beginning of the anticipation and the onset (a sharp rise in EMG trace) was designated as the period of anticipation. Using the start time previously determined as the reference, the computer calculated the specific times of these events with respect to the cycle onset for deciphering phasic interrelationships. The software automatically joined these points to delineate the onset and anticipation segments. From these lines the slopes ( rise/ t) of the curves were calculated for these segments. The software also performed the linear envelope detection of all EMG channels from which it identified the maximum EMG (peak) and average EMG scores of all channels. These scores were normalized against the corresponding values from MVC. In addition, time of the peak EMG, the EMG per second (EMG percent magnitude time time) and the (area under the EMG trace) were also calculated by the software. Finally, the software divided the chosen activity into segments of 10% of the task cycle and measured the magnitude of EMG and time product; and the normalized amplitude of EMG at that percent of task cycle using this analysis software. A statistical analysis of these EMG parameters was carried out using the SPSS to calculate the descriptive statistics, analysis of variance, and Pearson s product moment correlation between the torque and EMG. A regression analysis was also carried out to determine the predictability of the torque from the EMG measurements. 3. Results 3.1. Torque and total EMG output pattern over the task cycle At MVC the male sample produced a maximal isometric torque of Nm and 97.5 Nm for axial rotation to the left and right respectively. These values for the female sample were 61.8 Nm and 64.7 Nm respectively. For the MVC the maximal values of both the torque and EMG were achieved for around 50% of the task cycle. Peak and average torques were highly correlated (r=0.9; p 0.001). The normalized torque and EMG values are presented in Table Magnitude analysis Fig. 3 The mean normalized peak EMG values for the graded axial rotation from the neutral to the left and from the neutral to the right for both males and females are presented in Table 3. The maximum EMG activity among the males was recorded in the ipsilateral latissimus dorsi. In the females, however, the greatest EMG activity was recorded from the contralateral pectoralis. Among the males the next highest level of activity was recorded from the contralateral external obliques. Conversely, in females, after the pectoralis were the ipsilateral latissimus dorsi and the contralateral external obliques. The total normalized EMG output of the channels as measured by the product of voltage and time reveals a descending order of magnitude contribution among the males of the ipsilateral latissimus dorsi, contralateral external oblique, contralateral pectoralis and ipsilateral lumbar erector spinae (Fig. 4 and Table 4). The normalized scores of different channels were significantly different from each other among both the males and females (p 0.01). The ANOVA for the activity specific normalized

6 322 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Fig. 3. The magnitude contribution of different muscles in graded contractions in male samples in percent area. EMG magnitude values revealed that there were no differences between the genders. While the axial rotation to the left was significantly different from that to the right (EMG magnitude, slope of EMG rise, phasic relationship (p 0.01), there were no differences between the left and right sides when the agonists were compared with the agonists and the antagonists were compared with the antagonists. The significant main effects showed that different grades of contraction were significantly different from each other (Fig. 5). Similarly, different muscles were significantly different from each other except the rectus abdominis. There were significant twoway interactions between the gender and muscle (p 0.001) implying that males and females use a different strategy for axially rotating the trunk. Whereas males used contralateral external oblique, ipsilateral internal oblique and latissimus dorsi the females used contralateral pectoralis in this experimental set-up to execute axial rotation. A significant two-way interaction between the percent effort and muscles (p 0.001) indicated that with increasing grades of contraction, agonist activity increases disproportionately. Similarly with changing direction of effort, the muscle changes roles, providing a significant two-way interaction between direction of effort and muscles (p 0.001). A Scheffe s post hoc analysis revealed differences between the muscles shown in Fig. 6. The peak EMG was strongly correlated with the average EMG and the EMG area (v=0.96 to 0.99; p 0.001). Both the peak and the aver- Table 3 The mean normalized peak EMG values of individual muscles during graded isometric contraction in left axial rotation in male and female samples, in percent of contraction of the same muscle during MVC Muscles Gender Male Female 25% Max 50% max 75% Max 100% 25% Max 50% Max 75% Max 100% Max EMG Max EMG Max EMG Max EMG Max EMG Max EMG Max EMG Max EMG Left external oblique Right external oblique Left internal oblique Right internal oblique Left rectus abdominis Right rectus abdominis Left pectoralis Right pectoralis Left latissimus dorsi Right latissimus dorsi Left lumbar erector L Right lumbar erector L3 Left thoracic erector Right thoracic erector

7 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Fig. 4. Phasic relationship of torso muscles with respect to generated torque in 25% MVC in a male subject. age EMG were significantly correlated with both peak and average torques (p 0.001). With the regression analysis it was revealed that each of the muscle s EMG magnitudes had a significant correlation with maximum torque (p 0.01). However, the percent variability accounted for never rose above 32% but generally stayed below 20%. In a stepwise forward regression the predictability increased to above 80% (p 0.001) and the variables in the regression equation to be entered were the contralateral pectoralis, external oblique and ipsilateral latissimus dorsi and erector spinae at the T 10 area Phasic relationship The mean anticipation and onset timing of all the channels for the male as well as female samples are presented in Tables 5 and 6. Even in the prime movers for the directional axial rotation a varying and inconsistent relationship between the anticipation and onset time of all the individual EMG channels was found. The axial rotation is initiated by the contralateral external obliques, and ipsilateral internal obliques, latissimus dorsi, and erector spinae muscles (more at the thoracic than at the lumbar level). Among the males in the axial rotation to the left, the right external oblique fired between 20 to 66 ms before torque generation. However, the left internal obliques fired from 49 s before to 20 s after torque application. The left latissimus dorsi began firing between 8 and 72 s before torque onset. The left erector spinae at the lumbar level began its activity from 26 ms to 46 ms prior to torque and at the thoracic level the firing of the left erector spinae began between 42 and 75 ms before torque onset. However, the onsets of most of these muscles were generally delayed until after the torque had already begun to be applied, except at the left lumbar and thoracic erector spinae in some grades of contraction. These did not show a consistent pattern with increasing levels of effort. A similar variable pattern was found in the males, in the axial rotation from the neutral to the right, as well the females in both directions. Looking at the time of peak activity of different muscles in these graded axial rotations, the ANOVA revealed a significant main effect for the grade of the effort (p 0.001) and the different muscles studied (p 0.001). There were significant two-way interactions between the gender and the effort grade (p 0.048) implying that the peak magnitude in two genders occurred at different times. Similarly the significant twoway interaction between the direction of effort and muscles involved (p 0.001) indicated changing roles of agonism and antagonism between muscles. A follow up Scheffe s post hoc analysis revealed that each effort grade level was significantly different from all other (p 0.001). The peak activity of the latissimus dorsi and the erector spinae muscles occurred sooner than all other muscles. The differences between all other muscles were not consistent Slopes of the EMG traces The slopes of the EMG channels had identical patterns among males and females, although the magnitudes of slopes were much steeper among the men compared to the women. For slopes the patterns were quite consistent in both genders as well as in both directions of effort (0 to the left; and 0 to the right). The magnitudes of the slopes are presented in Table 7. Invariably the slopes of the prime movers were significantly greater than those of the antagonists and the stabilizing muscles (p 0.001). 4. Discussion Most of the studies which have investigated axial rotation have looked at static contraction [6,13,15 17]

8 324 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Table 4 The mean normalized EMG area (5.s) of individual muscles during graded isometric contractions to the left in axial rotation in male and female samples Muscles GENDER Male Female 25% Max 50% Max 75% Max 100% 25% Max 50% Max 75% Max 100% Area EMG Area EMG Area EMG Area EMG Area EMG Area EMG Area EMG Area EMG Left external oblique Right external oblique Left internal oblique Right internal oblique Left rectus abdominis Right rectus abdominis Left pectoralis Right pectoralis Left latissimus dorsi Right latissimus dorsi Left lumbar erector L Right lumbar erector L3 Left thoracic erector Right thoracic erector Fig. 5. The relationship between total EMG output and torque in different graded contractions in axial rotation to the left in male samples. or maximal isometric and maximal isokinetic exertions [7,8,12]. Given the frequency with which submaximal rotation is associated with low back injuries/pain, it is a matter of interest to investigate submaximal activities and compare them with maximal effort. A significant difference in the recruitment pattern, magnitude imbalance, load sharing, and temporal pattern of the peak muscular activity for the muscles involved may provide additional insight into the muscle activity pattern and thereby the control and execution of axial rotation. However, the results of this study suggest that the pattern of magnitude development, relative timing and recruitment patterns during different grades of contraction for these muscles were not significantly different from each other. Clearly the EMG magnitude differences of these muscles between the different grades of contraction were largely proportional to the grade of effort. However, an examination of the timing of different EMG parameters reveals a general inconsistency between the different muscles in any contraction grade and the same muscles in different grades of efforts. It may be that a fixed grade and an isometric contraction may have influenced such behavior. Since no motion occurred, slight differences in timing of recruitment or peaking was of no particular consequence. In contrast, even in an unresisted normal velocity motion execution the phasic relationships show a very consistent pattern of timing [10]. In a situation where motion occurs, a coordinated effort between the agonists, antagonists and stabilizers will be of considerably greater significance. Another significant difference observed in this study compared to that of Kumar et al. [10] was the role of the pectoralis muscles. In an unresisted rotation of the trunk, the pectoralis muscles did not play a significant role. This is particularly interesting because the data for that study and the current study were obtained in the same session from the same subjects without moving electrodes. In the current study, in

9 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Fig. 6. Post hoc differences between the tested muscles. this set of isometric experiments the pectoralis function was strong, particularly in women. In contrast, the male sample used this muscle to a much lower extent. Thus it appears that the male subjects retained the recruitment of torso muscles similar to what they used in unresisted activity. It is likely that the female sample may have done the same if the experimental set-up was different. Due to the shoulder harness of the AROT, rotary torque could be applied either by pushing against the harness using the shoulder as the primary force generator or twisting the trunk holding the shoulders rigid, so even though the force was applied through the shoulders it was generated at the trunk level. When the latter strategy was used the latissimus dorsi and external obliques acted as a force couple with the spine serving as the fulcrum. It is unclear why females chose a strategy of force generation different from that of males in this experimental condition. It is speculated that lower strength and greater flexibility of the female trunk may have contributed to this observed phenomenon somewhat. The EMG magnitude of different muscles in graded axial rotation revealed an interesting pattern. Whereas the contralateral external obliques and ipsilateral latissimus dorsi continued to contribute the same percentage of total EMG output in different grades of contraction in both genders, some of the other muscles revealed a varying pattern. With increasing grade of contraction the EMG magnitude of the rectus abdominis kept on rising proportionally and that of the erector spinae kept on progressively declining, among both males and females. However, the EMG magnitudes of these muscles were significantly different between males and females (p 0.01). It would thus appear that the erector spinae may primarily be playing the role of stabilizers contributing only secondarily to the twisting activity (more at the thoracic level than at the lumbar level). In contrast, the latissimus dorsi and external oblique are the primary rotators of the trunk. This will further contribute to the logic that once the stabilization of the trunk has been achieved greater rotary torques do not proportionally increase the demand on stabilizers. The agonists were the same corresponding muscles on

10 326 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Table 5 The mean anticipation time of individual muscles during graded contractions to the left in axial rotation in male and female samples Muscles Gender Male Female 25% Max 50% Max 75% Max 100% 25% Max 50% Max 75% Max 100% Antic (ms) Antic (ms) Antic (ms) Antic (ms) Antic (ms) Antic (ms) Antic (ms) Antic (ms) Left external oblique Right external oblique Left internal oblique Right internal oblique Left rectus abdominis Right rectus abdominis Left pectoralis Right pectoralis Left latissimus dorsi Right latissimus dorsi Left lumbar erector L Right lumbar erector L3 Left thoracic erector Right thoracic erector Table 6 The mean onset time of individual muscles during graded contractions to the left in axial rotation in male and female samples Muscles Gender Male Female 25% Max 50% Max 75% Max 100% 25% Max 50% Max 75% Max 100% Onset (ms) Onset (ms) Onset (ms) Onset (ms) Onset (ms) Onset (ms) Onset (ms) Onset (ms) Left external oblique Right external oblique Left internal oblique Right internal oblique Left rectus abdominis Right rectus abdominis Left pectoralis Right pectoralis Left latissimus dorsi Right latissimus dorsi Left lumbar erector T Right lumbar erector T3 Left thoracic erector Right thoracic erector the left and right sides, and the EMG pattern was the same between the two genders. Interestingly, the patterns of antagonists and stabilizers were different. There was no consistent magnitude relationship between different contraction grades. At 25% MVC the EMG of the antagonists and stabilizers registered approximately 60% and with the MVC their magnitude reached 100%. The magnitude of torque developed in the male and

11 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) Table 7 The mean magnitude of slopes of different EMG channels during graded contractions to the left on axial rotation Muscles Gender Male Female 25% Max 50% Max 75% Max 100% 25% Max 50% Max 75% Max 100% Slope (µv/s) Slope (µv/s) Slope (µv/s) Slope (µv/s) Slope (µv/s) Slope (µv/s) Slope (µv/s) Slope (µv/s) Left external oblique Right external oblique Left internal oblique Right internal oblique Left rectus abdominis Right rectus abdominis Left pectoralis Right pectoralis Left latissimus dorsi Right latissimus dorsi Left lumbar erector L Right lumbar erector L3 Left thoracic erector Right thoracic erector female sample showed the same relationship of 100:65, which has normally been reported for other activities and rotational activities by several authors [12,18 20]. From the results of this study it is apparent that though the pattern and magnitude of muscle recruitment in isometric rotation was significantly different from those in dynamic activity, they did not vary between different grades of contraction. Thus a repeated submaximal contraction in the axial rotation may not be interpreted as an aberrant muscle behavior. 5. Conclusion The pattern of the EMG magnitude development, the relative timing of muscle activity and the recruitment pattern of the muscles in isometric graded contractions were significantly different from those of unrestricted dynamic axial rotation (p 0.01). However, the said variables between the various grades of isometric contraction did not differ significantly. In isometric graded axial rotation contraction, the magnitudes of the muscle EMG were proportional to the grades. The timing of the recruitment of the different muscles in the isometric graded contractions were variable due to a lack of motion. Due to the experimental set-up with the shoulder harness an unexpected and variable force was applied for rotation by the female sample using the shoulder. With an increase in the grade of the isometric contraction the relative contribution of the erector spinae decreased, implying a stabilizer role for it. From the results of this study in comparison to the dynamic study, it is fair to say that the trunk muscle loads between the two activities are significantly different. References [1] Manning DP, Mitchell RG, Blanchfield LP. Body movements and events contributing to accidental and non-accidental back injuries. Spine 1984;9: [2] Schaffer H. Back injuries associated with lifting. In: Bulletin Washington, DC: Department of Labor, Bureau of Labor Statistics, 1982:1 20. [3] Basmajian JV. In: Muscles alive: The functions revealed by electromyography. Baltimore, MD: Williams and Wilkins, 1978: [4] Carlsoo S. The static muscle load in different work postures: An electromygraphic study. Ergonomics 1961;4: [5] Morris JM, Benner G, Lucas JB. An electromyographic study of the intrinsic muscles of the back in man. J Anat 1962;96: [6] Pope MH, Svensonn M, Andersson GBJ, Broman H, Zetterberg C. The role of prerotation of the trunk in axial twisting efforts. Spine 1987;12: [7] McGill S. 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: [8] Marras W, Mirka M. A comprehensive evaluation of trunk response to asymmetric trunk motion. Spine 1992;17: [9] Kumar S. An electromyographic study of unresisted trunk rotation with normal velocity among normal males. In: Proceedings of IEEE. EMBS, 1993 Oct 28 31; San Diego, CA, [10] Kumar S, Narayan Y, Zedka M. An electromyographic study of

12 328 S. Kumar et al. / Journal of Electromyography and Kinesiology 12 (2002) unrestricted trunk rotation with normal velocity among healthy subjects. Spine 1996;21: [11] Davis JR, Mirka GA. Transverse contour modeling of trunk muscle-distributed forces and spinal loads during lifting and twisting. Spine 2000;25: [12] Kumar S. Isolated planar trunk strength measurement in normals. Part III: Results and database. Int J Ind Ergonomics 1996;17: [13] Kumar S, Narayan Y. Spectral parameters of trunk muscles during fatiguing isometric axial rotation in neutral posture. J Electromyogr Kinesiol 1998;8: [14] Chaffin DB. Ergonomic guide for the assessment of human strength. Am Ind Hyg Assoc J 1975;36: [15] Kumar S, Dufresne RM, Garand D. Effect of posture on back strength. Int J Ind Ergonomics 1991;7: [16] Kumar S, Narayan Y, Stein RB, Snijders C. Muscle fatigue in axial rotation of the trunk. Int J Ind Ergon 2000;28: [17] Pope MH, Andersson GBJ, Broman H, Svensson M, Zetterberg C. Electromyographic studies of the lumbar trunk musculature during the development of axial torques. J Orthop Res 1986;4: [18] Chaffin DB, Herrin GD, Keyserling WM. Pre-employment strength testing. J Occup Med 1978;20: [19] Kroemer KHE, Marras WS. Evaluation of maximal and submaximal static muscle exertions. Human Factors 1981;23: [20] Ayoub MM. Lifting capacity of workers. J Human Ecol 1977;6: Shrawan Kumar is currently a Professor in Physical Therapy in the Faculty of Rehabilitation Medicine and in the Division of Neuroscience, Faculty of Medicine. He joined the Faculty of Rehabilitation Medicine in 1977 and rose to the rank of Full Professor in Dr. Kumar holds B.Sc. (Biology and Chemistry), and M.Sc. (Zoology) degrees from the University of Allahabad, India and a Ph.D. (Human Biology) degree from the University of Surrey, UK. Following his Ph.D., he did his post-doctoral work at Trinity College, Dublin in Engineering, and worked as a Research Associate at the University of Toronto in the Department of Physical Medicine and Rehabilitation. For his lifetime work, Dr. Kumar was recognized by the University of Surrey, UK by the award of a D.Sc. degree in Dr. Kumar was invited as a Visiting Professor for the year at the University of Michigan, Department of Industrial Engineering. he was a McCalla Professor Dr. Kumar has over 250 scientific peer-reviewed publications and works in the area of musculoskeletal injury causation/prevention with special emphasis on low-back pain. He has edited/authored seven books/monographs. He currently holds a grant from NSERC. His work has been supproted in the past, in addition to the above, by MRC, WCB and NRC. He has supervised or is supervising 10 M.Sc. students, 3 Ph.D. students and 2 post-doctoral students. He is Editor of the International Journal of Industrial Ergonomics, Consulting Editor of Ergonomics, Advisory Editor of Spine, and Assistant Editor of the Transactions of Rehabilitation Engineering. He serves as a reviewer of several other international peer-reviewed journals. He also acts as a grant reviewer for NSERC, MRC, Alberta Occupational Health and Safety, and B.C. Research. Yogesh Narayan obtained his B.Sc. in electrical engineering from the University of Alberta, Canada. He specializes in signal processing and software design/development. After graduating he worked on research projects in biomedical engineering at the Grey Nuns Hospital, Edmonton, Alberta, Canada. Currently, he is a research engineer with the Ergonomics Research Laboratory, at the University of Alberta, and a professional member of APEGGA (The Association of Professional Engineers, Geologists and Geophysicists of Alberta).