EMG spectral characteristics of spinal muscles during isometric axial rotation

Journal of Electromyography and Kinesiology 9 (1999) 21–37

EMG spectral characteristics of spinal muscles during isometric axial rotation Shrawan Kumar *, Yogesh Narayan Department of Physical Therapy, University of Alberta, 3–75 Corbett Hall, Edmonton, Alberta, T6G 2G4, Canada Received 13 November 1997; received in revised form 23 February 1998; accepted 18 March 1998

Abstract The objective of this study was to determine the frequency profile, median frequency (MF) and mean power frequency (MPF) of trunk muscles in an isometric graded maximal voluntary contraction (MVC) in isometric axial trunk rotation from a neutral upright seated posture. Twelve young healthy subjects (seven males, five females) were instrumented with surface electrodes on their external obliques, internal obliques, rectus abdominis, pectoralis, latissimus dorsi and erector spinae at T10 and L3 levels bilaterally. These subjects were stabilized in seated posture in an axial rotation tester (AROT) and asked to perform a graded isometric contraction of their maximal value to both right and left directions from a neutral posture within a period of 10 s. EMG from all 14 channels were sampled at 1 kHz at 10% intervals of MVC from 10% to MVC. These samples were subjected to fast Fourier transform analysis. The frequency profile plots demonstrated the power of muscles involved in agonistic and antagonistic activity. However, the frequency composition showed little difference between them. The MF was higher in agonists of the same muscle. The MPF was always higher than MF. Both values were generally insignificantly different between different levels of contraction. However, with increasing level of contraction there was increase in power.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Trunk muscles; Spectral parameters; EMG; Frequency profile

1. Introduction Frequency-domain parameters of myoelectric signals reveal aspects of motor function that are not possible to discern by the use of amplitude of the same signals. It is for this reason that the frequency characteristics of several muscles have been studied and reported in the literature [1–6]. These studies have revealed that the power spectral characteristics or their relationship with independent factors such as level of contraction are not uniform across muscles. Different muscles seem to have different pattern as well as band width, which also are reported to be affected by the proportion of different fibre types in the muscle in question. Several studies have reported that the mean power frequency (MPF) of the power spectrum increases with the level of contraction as it rises from a fractional value to

* Corresponding author. Tel.: ⫹ 1-403-492-5979; fax: ⫹ 1-403492-1626.

the maximum voluntary contraction (MVC) [7–10]. A similar pattern of behaviour is reported for median frequency as well [11]. Solomonow et al. [6], in an experiment using orderly stimulation of cat gastrocnemius, found a linear increase in the median frequency. On the contrary, other studies have shown that there was no relationship between the median frequency and the magnitude of contraction [12–14]. Similarly, it has been reported the MPF of the power spectrum was unrelated to the magnitude of force [4,15]. However, Bilodeau et al. [1], in their experiment with triceps brachii and anconeus with graded contractions at the levels of 10, 20, 40, 60, 80 and 100% MVC, found that the MPF of the anconeus increased significantly up to 60% of MVC. The authors reported no significant change in MPF of the triceps brachii across different levels of contraction. They later reported that the ramp and step contractions affected the MPF and MF differently in triceps brachii and anconeous, although there was no statistically significant difference between them [2]. Yet another factor that is reported to have a significant

1050-6411/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 0 5 0 - 6 4 1 1 ( 9 8 ) 0 0 0 1 6 - 9

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effect on the spectral parameters of the EMG signals and its response to contraction level is the proportion of fibre types present in any given muscle. Fast twitch muscle type (Type II) is known to be phasic fibre responding to the need for higher force production. Therefore, these are recruited when higher tension is evoked [16]. Their gradual recruitment with increasing force level [17] is suggested to be the reason for the increase in MPF with increasing level of contraction as observed and reported by some workers [8,18]. The MPF of the EMG signals is related to the fibre composition of the muscle [19,20]. However, triceps brachii (65% fast twitch) and anconeus (65% slow twitch) responded in a manner exactly opposite [1] to that expected from the foregoing logic with increasing force level. This creates difficulty in explaining the results. The authors suggested that a differential thickness of skin and intervening tissues between the electrodes and the muscles would have had different filtering characteristics, manifesting this result. Whether the foregoing was the cause of the effects observed remains to be established beyond doubt. Another study reported that up to 10% of variation in MPF was random variation [21]. Even this, however, does not explain the reported results of the foregoing study. Most of the studies conducted and reported have been on muscles of the upper and lower extremities, presumably due to the ease of access. In view of the fact that even in the extremities different muscles show different spectral characteristics and varying behaviour due to force of contraction and fibre type composition, there is little which can be extrapolated to the muscles of the trunk. Since the motor behaviour of these muscles may have a role in and explain aspects of back injuries and pain, a spectral profiling of spinal muscles becomes useful. Since trunk twisting has been reported by several authors to be highly associated with back injuries [22– 24,38], it was chosen as the activity for the study. It is with this purpose that the spectral characteristics of the erector spinae, latissimus dorsi, external obliques, internal obliques, rectus abdominis and pectoralis major were studied and are reported here. The objective of this study was to describe the frequency profile of trunk muscles in a standardized gradual and graded isometric maximal voluntary contraction in trunk axial rotation from a neutral posture. Furthermore, another aim was to determine the median and mean power frequencies of these trunk muscles and compare them at different levels of contraction between agonists and antagonists. 2. Materials and methods 2.1. Subjects Data from 12 normal young and asymptomatic subjects was recorded. The experimental sample consisted

of seven males (mean age 25.1 yr, standard deviation 5.3 yr; mean weight 69.1 kg, standard deviation 5.8 kg; and mean height 176.2 cm, standard deviation 5.2 cm) and five females (mean age 21.6 yr, standard deviation 2.8 yr; mean weight 57.9 kg, standard deviation 11.4 kg; and mean height 166.1 cm, standard deviation 9.9 cm). These subjects were screened for neuromuscular and musculoskeletal disorders, and any spinal or abdominal surgery. The subjects were informed about the objectives and procedures of the study and they signed an informed consent. 2.2. Equipment 2.2.1. Axial rotation tester (AROT) The axial rotation tester (AROT) was a device specially designed and fabricated to obtain reliable, repeatable and standardized axial twisting of the human trunk, preventing any flexion or extension. As axial rotation is a motion coupled with lateral flexion, the device permitted free lateral flexion thus providing a floating axis for rotation. Because the device tests subjects in seated posture, it also eliminates any contribution of the hips and lower extremities in twisting of the trunk. The device is described in its entirety by Kumar [25]. Briefly, it is designed to stabilize lower extremities hip down in a seated posture and the shoulders such that the vertical spinal axis is aligned to the device’s rotation axis. Such stabilization allows free twisting motion to occur only in the thoracolumbar region with accompanying lateral flexion. The AROT was equipped with a load cell and a precision potentiometer to provide continuous output of the twisting force applied and the angular motion achieved. The output of the load cell was divided into two; one was fed to the data collection system and the other was fed to a digital display device for feedback to the subject. 2.2.2. EMG system The EMG system consisted of surface electrodes, electrode cables, preamplifiers and amplifiers. Silver–silver chloride circular surface electrodes of 1 cm diameter and recessed pregelled elements (HP 144445) were used with inter-electrode distance of 2 cm. These electrodes were connected to 16-channel, fully isolated, low-noise amplifiers. These amplifiers had low non-linearity, high common mode rejection ratio (130 dB) and a wide bandwidth (25 MHz). These preamplifiers fed to a lowpower, 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 bandpass filter with a low cut-off frequency at 8 Hz and high cut-off at 500 Hz. The preamplifiers and amplifiers were built by Measurement Systems, Inc., Ann Arbor, Michigan.

S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

2.2.3. Controller and A/D board The outputs of the AROT load cell, precision potentiometer and EMG amplifiers were fed to a MetraByte DAS 20 A/D board. These signals were sampled at 1 kHz. The sampled signals were stored in the hard disk of a 486 computer with a tape backup (Colorado Memory Systems Inc.) for further analysis and interpretation.

2.3. Experimental procedures

The subjects were weighed and measured for their height. Their age was also recorded. These subjects were instrumented with 14 pairs of disposable, pregelled, surface electrodes (HP 144445) at an inter-electrode distance of 2 cm after suitable preparation of the skin with an alcohol–acetone mixture. These electrodes were placed on erector spinae levelled with spinous processes of T10 and L3 vertebrae bilaterally, 4 cm lateral to the tips of the spinous processes. Surface electrodes were also applied to the left and right latissimus dorsi. On the ventral side, surface electrodes were applied bilaterally to the pectoralis major, rectus abdominis, external oblique and the internal oblique (in the area of external oblique aponeuroses to minimize overlap with it). A ground electrode was applied to anterosuperior iliac spine. Prepared subjects were seated in the chair of the axial rotation tester. The seat was adjusted for height so that the subjects were seated comfortably resting their feet, with the knee at 90° angle. The seat was then aligned with the axial rotation tester harness, which was lowered on the subjects’ shoulders and fastened. The subjects were stabilized in this upright neutral posture, seated position hip down, by using four velcro straps at the hip, distal thigh, proximal shin and ankle. The circular disc above the shoulder harness was attached to an immovable object by means of an airplane cable with the load cell in its path but leaving no slack. The subjects were then asked to attempt maximal voluntary contraction (MVC) in torso rotation by applying a force through their shoulders on the harness, which was locked in position through the airplane cable and maintained the isometric condition. The rotation was attempted such that force gradually increased up to MVC within a period of 10 s. Since the trial was started after the data acquisition began and was terminated before the end of the sampling period, the total duration of contraction lasted 5 to 7 s from start to finish. In order to obtain graded contraction the subjects were provided with visual feedback. The subjects were required to practise this activity several times before the day of the experiment for them to become familiar with the process and be able to produce consistent torque with no motion of the trunk.

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2.4. Data acquisition Data were acquired using a custom-designed modular software for the project. It allowed input of subject data and created data files. Subsequently it acquired the data according to predetermined variables (e.g., sampling rate, duration, etc.). All 14 channels were sampled at 1 kHz for a window of 10 s with the entire trial lasting 5 to 7 s. From the latter, segments of 0.4 s (0.2 s before reaching the desired force and 0.2 s after the force was reached) were obtained for data extraction, frequency profiling and further analyses. 2.5. Data analysis Previously collected data were loaded into the computer memory. The EMG samples of all 14 channels (erector spinae at T12 and L3 levels, latissimus dorsi, pectoralis major, rectus abdominis, external and internal obliques bilaterally) were processed in the time domain at levels of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% contraction. The power spectra were calculated from the raw signals by means of Welch’s method [26]. The method involves sectioning the record and averaging modified periodograms of the sections. Thus the sampled signals were divided into three equal segments of 256 points in length, processed through a type 1 Welch window and then subjected to power spectral analysis. The window had a tapered shape which attenuated the end points; therefore, a fractional overlap of 0.715 was used to marginally recover the samples at the end points. The overlapping also allowed the variance in the estimation to be reduced, while maintaining a desired spectral resolution and dependency between segments. For each of the sampling periods, the 401 points were divided into three equidistant sliding and overlapping segments of 256 points. From each segment the average value was subtracted for DC removal and then a Welch window applied to it. Subsequently the power spectrum of the segment was calculated. Since Fast Fourier transfrom (FFT) assumes signal stationarity, its non-stationarity was also checked. For the final power spectrum the average of the three segment spectra was taken. The latter was then smoothed with linear polynomial smoothing using seven point segments and repeating once. From these power spectra the frequency profile of the muscle was obtained. The frequency values for each of the channels at each of the task grades were plotted against the power spectral density. 3. Results 3.1. Torque In their maximal voluntary contraction for axial rotation to the left and right, the male sample generated

Female

Male

0→left 0→right 0→left 0→right 0→left 0→right 0→left 0→right

22.4 21.5 11.0 10.5 2.2 0.7 3.4 ⫺0.2

10.3 5.9 3.7 3.7 2.3 1.7 2.9 1.9

31.2 28.5 14.1 14.5 1.5 1.3 2.9 0.3

M

M

SD

20%

10%

Direction Grades of contraction

M—mean; SD—standard deviation.

Angular deviation (deg.)

Male

Torque (N m)

Female

Gender

Variable

11.8 6.7 4.1 4.5 1.9 1.2 2.9 1.5

SD 41.8 38.2 17.4 21.3 0.9 2.1 2.6 0.8

M

30%

14.3 11.7 5.9 6.3 1.7 1.2 3.0 1.7

SD 53.1 48.3 22.1 26.3 0.3 2.8 2.3 1.1

M

40%

17.0 14.9 9.7 8.7 1.5 1.5 3.1 1.7

SD 58.6 54.1 28.1 33.7 0.3 2.8 1.5 1.6

M

50%

23.3 17.8 11.2 9.4 1.3 2.2 2.7 1.4

SD 70.9 68.0 34.9 40.1 ⫺0.2 3.3 0.8 1.9

M

60%

Table 1 Mean torque (N m) and angular deviation (deg.) from the neutral posture during maximal and graded axial rotation

30.8 19.6 11.9 10.7 1.2 2.3 2.0 1.4

SD 82.9 78.7 40.1 46.0 ⫺0.6 3.8 0.4 2.2

M

70%

34.8 22.7 13.7 12.2 1.2 2.7 1.8 1.4

SD

97.5 92.3 46.1 53.6 ⫺1.0 4.2 0.0 2.6

M

80%

42.4 31.0 15.8 15.3 1.4 3.2 1.7 1.5

SD

111.9 104.7 52.2 60.8 ⫺1.6 4.6 ⫺0.2 2.8

M

90%

47.1 34.0 18.6 17.1 1.7 3.5 1.6 1.4

SD

122.7 115.4 58.7 67.9 ⫺2.1 5.1 ⫺0.5 3.5

M

100%

52.7 37.0 20.3 18.6 1.9 3.7 1.6 1.5

SD

24 S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

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Fig. 1. Power spectra of left and right external obliques in a gradual contraction to maximal voluntary isometric contraction in attempted rightward axial rotation of the trunk from a neutral posture.

a mean torque of 122.7 N m and 115.4 N m, respectively. The female sample, on the other hand, generated mean MVC torques of 58.7 N m and 67.9 N m, respectively. Thus females generated torques which were close to half of those of males. The details are presented in

Table 1. The deviation of the torso in the axial plane was less than 5°, thus staying very close to the neutral position.

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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

Fig. 2. Power spectra of left and right internal obliques in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward axial rotation of the trunk from a neutral posture.

3.2. Frequency profile The frequency–power plots of the 14 muscles of a sample subject for graded levels of contraction are presented in Figs. 1–7. The power of primary agonist muscles was always considerably higher than those of the respective antagonistic muscles. This was obvious by

comparing contralateral external obliques with ipsilateral external obliques; and ipsilateral latissimus dorsi, internal obliques and erector spinae with their respective contralateral counterparts. These plots also clearly demonstrate little change in the median frequency with increasing grades of contraction, except an increase in their power.

S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

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Fig. 3. Power spectra of left and right rectus abdominis in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward rotation of the trunk from a neutral posture.

In antagonistic internal oblique, past 75% MVC, there were two frequencies with greater power than the rest. It should also be pointed out that, for this muscle, power was quite low. For ipsilateral rectus abdominis such deviation started much sooner, perhaps around 30% MVC. The ipsilateral pectoralis had very little power but a wide range of frequencies from 0 to 500 Hz seemed to be present. Contrary to this, the contralateral pectoralis

demonstrated high power level and a considerably narrower band of frequencies, the majority of these under 150 Hz. In erector spinae, the ipsilateral muscles demonstrated a wider frequency band (8 –250 Hz compared with 8 –150 Hz in contralateral muscles) as well as higher power (30 nW) than contralateral (1.2 nW). The erector spinae at thoracic level had similar power as that at the lumbar level (30 nW).

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Fig. 4. Power spectra of left and right pectoralis in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward rotation of the trunk from a neutral posture.

3.3. Mean median frequency The mean median frequencies of the 14 muscles (external and internal obliques, rectus abdominis, pectoralis major, latissimus dorsi, erector spinae at 10th thoracic and 3rd lumbar vertebral levels bilaterally) are presented in Tables 2 and 3 for all grade levels and maximum voluntary contraction, for both left and right

axial rotations, and for both male and female samples. The mean median frequencies were always higher in agonist muscles as compared with antagonists and stabilizers at all levels of contraction (for example, at MVC, the mean median frequencies (Hz) were: contralateral external obliques 75 vs. 51, ipsilateral latissimus dorsi 76 vs. 42, internal oblique 61 vs. 52, erector spinae at L3 55 vs. 46 and erector spinae at T10 65 vs. 52 among

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Fig. 5. Power spectra of left and right latissimus dorsi in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward rotation of the trunk from a neutral posture.

males, and similar values among females, see Tables 2 and 3). Generally, different grades of contractions did not evoke significantly different median frequency (Tables 2 and 3). For example, the median frequencies (Hz) at 10% and MVC were 64 and 75, 52 and 61, 41 and 51, 39 and 52, 72 and 76, 33 and 42, 50 and 55, 33 and 46, 55 and 65, 33 and 52 for left external obliques,

left internal obliques, right external obliques, right internal obliques, left latissimus dorsi, right latissimus dorsi, left erector spinae at L3, right erector spinae at L3, left erector spinae at T10 and right erector spinae at T10, respectively. In erector spinae the median frequency of the EMG signals of the ipsilateral and contralateral muscles were generally within 15 Hz of each other, rang-

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Fig. 6. Power spectra of left and right erector spinae at T10 in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward rotation of the trunk from a neutral posture.

ing between 44 and 68 Hz. The pectoralis muscle, however, had more variable median frequency in the range of 48–108 Hz and 30–60 Hz for ipsilateral and contralateral muscles, respectively, among males. In females, they were even more variable, ranging between 40 and 113 Hz for ipsilateral muscles and between 38 and 61 Hz

for contralateral muscle. Among female subjects the rectus abdominis was found to be more variable than among male subjects (Tables 2 and 3).

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Fig. 7. Power spectra of left and right erector spinae at L3 in a gradual contraction to maximal voluntary isometric contraction in an attempted rightward rotation of the trunk from a neutral posture.

3.4. Mean power frequency The mean power frequency for all 14 muscles for graded and maximal voluntary contraction during axial rotation to left and right are presented in Tables 4 and 5 for male and female samples, respectively. The mean power frequencies for all muscles in both genders were found to be approximately 20 Hz higher than the corre-

sponding median frequency. The pattern of the mean power frequency was similar to that observed for median frequency, being higher for the agonists compared with the antagonists except for erector spinae at thoracic as well as lumbar levels (for instance, in neutral to leftward isometric axial rotation among males, contralateral versus ipsilateral external obliques—99 vs. 60, 111 vs. 58, 107 vs. 67, and 100 vs. 58 Hz for 25%, 50%, 75% and

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Table 2 Mean median frequencies (Hz) of individual muscles during maximal and graded contractions in left and right axial rotations among males Muscles

10% M

20% SD

M

30% SD

M

40% SD

Neutral to left grade of contraction (% MVC) LEO 64 31 66 29 69 28 LIO 52 15 55 15 60 14 REO 41 10 41 10 43 10 RIO 39 21 45 26 44 27 LRA 46 34 53 38 64 36 RRA 32 11 37 14 51 29 LP 48 57 49 59 52 64 RP 45 16 44 16 44 16 LLD 72 20 78 19 83 17 RLD 33 8 37 7 37 7 50 11 52 11 52 8 LL3 33 9 39 10 45 14 RL3 55 15 56 13 58 13 LT10 33 11 39 10 45 21 RT10 Neutral to right grade of contraction (% MVC) LEO 42 9 44 8 45 7 LIO 32 19 33 19 31 17 REO 52 21 60 17 73 20 RIO 52 32 55 27 59 25 LRA 45 30 41 29 47 24 RRA 58 38 55 45 51 28 LP 38 23 30 10 35 11 RP 56 36 63 37 64 38 LLD 36 4 36 4 37 4 RLD 89 33 88 23 98 29 46 18 41 14 37 11 LL3 48 12 49 10 47 10 RL3 52 34 43 17 46 18 LT10 51 11 52 14 55 16 RT10

50%

60%

70%

80%

90%

100%

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

82 63 47 44 75 57 63 52 85 37 54 47 60 45

24 15 9 15 39 17 71 23 15 5 9 13 13 8

74 61 50 33 69 56 54 46 79 37 50 45 63 44

25 18 12 14 27 25 56 14 26 7 9 12 16 13

70 65 51 42 82 63 76 47 83 38 47 46 66 57

22 16 13 12 23 19 59 14 28 4 7 6 17 15

71 69 50 46 84 67 89 46 83 39 45 50 68 51

18 13 13 16 29 21 47 8 24 4 8 13 16 11

71 69 51 46 85 73 108 46 80 47 49 56 73 57

19 13 11 18 33 23 61 6 19 9 11 13 19 6

74 68 51 54 74 70 81 50 74 40 55 49 70 49

25 18 13 16 36 26 66 10 21 8 14 7 20 12

75 61 51 52 80 80 78 60 76 42 55 46 65 52

23 11 9 14 34 17 46 16 15 8 14 18 15 20

46 39 80 61 63 68 48 86 40 92 47 49 56 61

7 15 25 24 14 36 23 51 5 6 11 11 21 14

44 47 71 63 71 75 48 68 40 83 45 46 55 59

7 20 14 20 17 22 26 23 4 19 15 10 20 14

46 49 65 69 59 71 45 82 38 84 40 49 49 64

11 18 14 28 24 32 27 64 4 14 13 11 18 16

50 52 68 64 72 84 53 100 39 83 51 52 54 64

10 20 24 14 19 35 15 70 6 10 14 10 14 15

54 50 85 74 63 84 52 107 40 86 55 49 57 65

12 23 34 17 19 35 13 74 5 11 24 10 21 14

46 50 80 76 71 81 51 93 42 86 47 52 46 66

11 24 26 12 26 23 16 64 4 12 10 8 8 13

50 52 68 79 62 88 59 87 40 88 49 60 44 68

11 9 24 15 31 38 12 59 8 10 10 19 10 12

M—mean; SD—standard deviation; LEO—left external oblique; LIO—left internal oblique; REO—right external oblique; RIO—right internal oblique; LRA—left rectus abdominis; RRA—right rectus abdominis; LP—left pectoralis; RP—right pectoralis; LLD—left latissimus dorsi; RLD— right latissimus dorsi; LL3—left erector spinae at L3; RL3—right erector spinae at L3; LT10—left erector spinae at T10; RT10—right erector spinae at T10.

MVC respectively; ipsilateral versus contralateral latissimus dorsi—87 vs. 59, 96 vs. 59, 98 vs. 65, and 90 vs. 57 Hz for 25%, 50%, 75% and MVC respectively; ipsilateral versus contralateral internal obliques—76 vs. 52, 78 vs. 47, 83 vs. 57, and 80 vs. 67 Hz for 25%, 50%, 75% and MVC respectively; and ipsilateral versus contralateral erector spinae at thoracic level—73 vs. 76, 80 vs. 79, 88 vs. 90, and 78 vs. 83 Hz for 25%, 50%, 75% and MVC respectively). Similar patterns were found for other conditions (neutral to right) among males, and both activities among females (Tables 4 and 5). However, between the grades of contractions, the mean power frequency was more variable than the mean median frequency with a spread of approximately 10 Hz except in pectoralis, where the maximum spread was 33 Hz in males and 63 Hz in females.

4. Discussion Axial rotation is an asymmetric and mechanically complex activity which involves some muscles in agonistic role (contralateral external oblique, ipsilateral internal oblique, latissimus dorsi and erector spinae) and their contralateral counterparts in antagonistic role [27]. However, significant and strong EMG muscle activity in contralateral muscles has been reported as well [28]. A significant activity among muscles such as erector spinae and rectus abdominis, which do not have their fibres oriented in the direction of motion, could possibly be serving the function of stabilizing the spine. Because of the incongruence of the fibre orientation with the axis of motion, their contraction cannot be contributing efficiently to the spinal mechanics even in a stabilizing role. However, ipsilateral external obliques and contrala-

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Table 3 Mean median frequencies (Hz) of individual muscles during maximal and graded contractions in left and right axial rotations among females Muscles

10% M

20% SD

M

30% SD

M

40% SD

Neutral to left grade of contraction (% MVC) LEO 63 34 74 47 72 43 LIO 47 17 48 19 48 19 REO 49 15 47 13 48 13 RIO 30 8 29 6 27 6 LRA 66 21 80 22 70 19 RRA 59 32 66 17 60 27 LP 101 66 113 70 85 70 RP 43 18 43 17 44 17 LLD 102 35 105 34 98 32 RLD 33 14 36 11 36 10 46 9 45 8 47 8 LL3 39 4 41 4 40 6 RL3 54 12 55 11 58 12 LT10 45 19 52 9 45 8 RT10 Neutral to right grade of contraction (% MVC) LEO 39 4 40 4 39 3 LIO 37 17 37 18 38 19 REO 70 37 70 38 84 38 RIO 49 33 52 22 48 12 LRA 48 25 47 23 57 14 RRA 59 34 61 36 63 27 LP 39 15 39 13 38 13 RP 78 64 77 67 85 71 LLD 35 7 35 7 36 6 RLD 85 24 86 22 83 19 41 6 40 5 41 4 LL3 48 10 46 9 46 9 RL3 50 34 49 32 51 24 LT10 53 6 52 6 53 7 RT10

50%

60%

70%

80%

90%

100%

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

68 53 49 28 70 59 88 41 92 39 47 42 59 45

28 25 15 6 19 28 77 15 28 8 7 6 12 13

67 56 49 27 72 60 83 52 92 41 47 41 62 50

28 23 17 7 16 27 74 14 28 8 6 7 12 16

69 59 49 29 77 60 65 52 96 41 47 41 63 49

32 17 17 9 19 25 80 11 24 12 5 9 10 16

73 63 49 31 78 66 67 53 98 39 46 38 64 43

36 10 18 9 14 15 85 6 18 18 5 11 12 13

78 61 51 32 80 73 87 59 95 41 48 37 63 45

36 8 16 9 5 14 87 5 22 18 6 15 12 20

84 67 52 45 75 78 104 61 99 38 49 48 65 57

47 13 10 6 12 11 68 17 13 13 9 8 17 12

71 67 49 46 83 78 44 56 91 38 52 48 66 51

17 6 10 8 14 24 17 9 7 9 8 8 7 12

41 38 81 48 62 58 38 83 38 81 41 45 43 61

4 19 37 8 21 24 14 55 6 17 3 7 20 11

44 38 77 52 65 59 44 97 41 81 40 46 42 66

8 18 38 8 24 27 8 57 4 12 4 7 19 8

47 39 81 56 67 59 48 103 42 80 40 49 43 66

9 20 37 8 22 25 12 61 5 7 5 9 16 12

47 38 86 65 77 67 47 96 40 87 43 52 48 66

6 20 43 11 19 15 16 53 9 12 6 12 8 15

47 34 86 66 77 80 48 77 39 89 45 50 55 77

6 18 42 8 18 26 11 61 9 12 6 10 11 13

45 34 87 66 75 78 43 58 37 87 50 49 49 76

7 16 41 8 17 14 10 44 4 14 8 11 10 12

48 40 63 64 79 71 48 40 40 80 55 53 47 66

2 14 25 8 10 30 14 18 3 8 7 11 9 11

M—mean; SD—standard deviation; LEO—left external oblique; LIO—left internal oblique; REO—right external oblique; RIO—right internal oblique; LRA—left rectus abdominis; RRA—right rectus abdominis; LP—left pectoralis; RP—right pectoralis; LLD—left latissimus dorsi; RLD— right latissimus dorsi; LL3—left erector spinae at L3; RL3—right erector spinae at L3; LT10—left erector spinae at T10; RT10—right erector spinae at T10.

teral internal obliques have their fibre orientation better aligned in the direction of the motion and thereby are optimally suited to act as antagonists in eccentric mode of contraction. Hence, in order to provide spinal stability, the mechanically inefficient contraction of nonaligned muscles is likely to be at a significantly higher level than what would be necessary had they been aligned. It is, therefore, surmized that these muscles will also be contracting at a high level, evoking significant recruitment of their motor units in addition to eccentric contraction of the antagonistic obliques. Spectral parameters, e.g., median frequency and mean power frequency, are dependent on the conduction velocity [29–31] and the conduction velocity increases with the diameter of fibres. Thus it stands to reason that as a muscle shortens in contraction, it will increase its diameter, thereby increasing conduction velocity [4,32]. The increased conduction velocity will manifest itself in higher values of median frequency and mean power fre-

quency. Such was the case in the results obtained in this experiment for the agonistic muscles. However, the impact of the conduction velocity alone on the MF and MPF is not expected to be large [32]. A contracted and shortened muscle requires higher rates of stimuli to produce high tension compared with a longer muscle [33]. In addition, to produce and/or maintain a force, an additional excitation is required [34,35]. Also in a shortened agonist muscle one will need a greater number of active muscle fibres to generate the desired force due to the maximized cross-bridging between actin and myosin filaments. Finally, the firing rate is dependent on the mechanical properties of the muscle [36]. In a contracted muscle there is a decrease in dynamic stiffness [33] and damping [37]. Such changes would require considerably higher firing rates to generate and produce a force. All of the foregoing phenomena are expected to have a combined effect on the median frequency and mean power frequency.

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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

Table 4 Mean power frequency (Hz) of individual muscles during maximal and graded contractions in left and right axial rotations among males Muscles

10% M

20% SD

M

30% SD

M

40% SD

Neutral to left grade of contraction (% MVC) LEO 96 31 100 30 106 28 LIO 70 11 74 9 78 8 REO 50 10 53 9 54 10 RIO 49 23 52 26 53 26 LRA 69 40 74 38 87 35 RRA 50 11 58 18 68 26 LP 84 56 89 59 86 61 RP 62 18 59 18 60 18 LLD 89 20 95 16 101 12 RLD 56 14 59 19 61 23 64 12 66 12 68 11 LL3 57 8 65 17 71 19 RL3 70 14 72 14 75 15 LT10 66 17 74 22 80 29 RT10 Neutral to right grade of contraction (% MVC) LEO 51 10 52 10 54 8 LIO 45 18 45 17 46 16 REO 87 22 96 18 110 16 RIO 68 31 73 31 76 26 LRA 62 32 60 24 66 20 RRA 79 42 77 42 78 29 LP 56 36 48 11 51 16 RP 98 46 105 43 113 33 LLD 59 17 58 14 59 14 RLD 102 29 105 25 110 27 72 28 70 25 70 17 LL3 62 13 63 12 63 13 RL3 82 40 76 26 84 24 LT10 69 16 69 15 71 14 RT10

50%

60%

70%

80%

90%

100%

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

115 80 57 54 98 77 98 68 102 62 68 75 79 79

26 8 9 16 35 18 67 25 10 17 8 19 15 20

111 78 58 47 99 75 102 66 96 59 65 71 80 79

25 11 11 13 20 25 46 14 16 13 9 14 18 17

105 80 59 53 104 84 120 66 98 62 62 74 84 90

25 9 11 15 20 24 46 13 18 14 11 13 17 24

105 82 58 57 103 89 132 65 99 64 63 78 86 89

20 5 11 18 26 26 46 11 16 13 10 20 13 20

106 85 58 58 105 92 144 64 96 65 66 85 88 96

17 9 10 17 30 25 51 10 14 11 10 21 18 18

98 85 57 65 99 91 108 65 91 59 66 76 86 86

22 10 12 14 33 20 59 9 14 9 10 11 16 16

100 80 58 67 103 98 98 72 90 57 66 71 78 83

26 5 9 15 31 13 56 19 10 12 9 19 11 27

55 53 116 76 84 95 65 131 65 111 81 68 95 76

9 17 27 23 12 30 26 45 20 15 16 16 27 13

55 58 113 73 92 100 63 122 65 99 77 63 92 73

11 21 19 25 16 25 29 31 16 15 20 13 27 13

56 60 103 78 78 95 61 114 61 97 68 62 84 76

12 20 21 28 25 31 29 56 13 13 16 11 25 14

59 64 102 77 87 107 69 134 61 98 82 65 91 77

9 24 29 14 15 27 21 63 12 11 16 11 17 14

60 59 107 84 83 101 69 137 61 99 82 63 91 81

10 25 33 14 14 28 22 66 12 10 23 10 25 11

57 64 104 87 85 99 63 122 66 101 74 66 81 83

12 27 30 16 22 21 17 57 13 7 10 11 8 13

58 66 93 88 80 102 75 111 61 105 72 70 73 81

12 9 27 17 25 35 18 63 25 10 11 18 17 9

M—mean; SD—standard deviation; LEO—left external oblique; LIO—left internal oblique; REO—right external oblique; RIO—right internal oblique; LRA—left rectus abdominis; RRA—right rectus abdominis; LP—left pectoralis; RP—right pectoralis; LLD—left latissimus dorsi; RLD— right latissimus dorsi; LL3—left erector spinae at L3; RL3—right erector spinae at L3; LT10—left erector spinae at T10; RT10—right erector spinae at T10.

It would appear that in an asymmetric isometric contraction, while the agonist muscles contract, the antagonistic and stabilizer muscles will also be contracting. A considerably lower value for MF and MPF among nonaligned antagonistic and stabilizer would tend to indicate that the force production and accompanying level of contraction would probably be significantly lower. However, reports of strong antagonistic muscle activity in axial rotation tend to confuse the picture. The latter especially true when it is not possible to differentiate between the forces produced by agonists and antagonists separately. It is, however, likely that the antagonist oblique muscles may undergo a small amount of lengthening while undergoing eccentric contraction and may not be responsible for the production of similar magnitudes of forces as agonsits. However, since these are likely to provide a significant component of the antagonistic force, they too demonstrated high MF and MPF. In a study of power spectra of elbow extensors com-

paring ramp and step isometric contractions, the authors reported a relatively gradual and steady increase in the MF with the level of ramp contraction, and rather stable and similar values for various levels of contraction for the step contraction [2]. In the experiment reported here the trend of the values did not correspond with those of Biladeau et al. [2]. In most cases (erector spinae at both levels, latissimus dorsi and external obliques bilaterally) the MF had insignificantly different values for all levels of contraction. In some abdominal muscles (generally internal oblique) the value of MF for 25% level of contraction was lower than the rest of the other values. However, a clearcut trend did not emerge. These stable values of MF for a ramp contraction in trunk muscles are clearly different from what was reported in elbow muscles. The significance of such an observation remains unclear. However, the trunk, being a large part of the body controlled by spinal muscles which are generally responsible for gross motion, may have large

S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

35

Table 5 Mean power frequency (Hz) of individual muscles during maximal and graded contractions in left and right axial rotations among females Muscles

10% M

20% SD

M

30% SD

M

40% SD

Neutral to left grade of contraction (% MVC) LEO 94 35 105 48 104 46 LIO 72 28 72 30 74 32 REO 61 21 59 18 60 18 RIO 44 9 41 8 39 7 LRA 97 22 107 28 97 22 RRA 81 30 88 20 81 25 LP 124 63 135 64 117 59 RP 66 30 68 30 67 26 LLD 116 31 119 31 116 31 RLD 56 21 56 20 58 20 60 14 60 12 60 11 LL3 61 10 63 10 58 11 RL3 71 18 72 16 73 15 LT10 87 24 91 14 85 17 RT10 Neutral to right grade of contraction (% MVC) LEO 46 6 47 6 48 5 LIO 48 20 48 20 51 21 REO 104 39 101 40 114 47 RIO 69 38 72 31 69 25 LRA 68 29 69 25 76 16 RRA 76 37 78 39 84 29 LP 50 11 49 10 50 10 RP 106 63 106 66 115 65 LLD 52 11 50 11 51 10 RLD 111 36 110 34 108 30 56 11 56 10 58 9 LL3 64 13 64 12 67 12 RL3 78 41 75 42 78 39 LT10 74 12 75 9 77 8 RT10

50%

60%

70%

80%

90%

100%

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

101 75 61 39 95 80 111 62 112 62 60 57 75 81

41 38 19 7 23 28 69 17 27 21 9 14 13 24

98 78 61 40 94 80 105 66 114 66 60 56 75 82

42 39 20 7 22 28 67 16 22 23 9 14 11 26

100 82 62 41 98 83 98 65 115 69 61 58 76 82

43 34 19 7 22 24 66 14 16 25 9 14 12 29

104 87 63 44 100 89 104 66 116 66 62 59 77 76

43 28 19 4 16 18 69 12 13 29 10 12 14 23

107 87 66 49 101 94 119 71 114 62 66 61 76 73

43 24 17 4 9 16 79 12 17 23 11 20 14 31

109 89 66 63 100 97 124 71 115 61 67 73 79 85

46 14 16 9 10 13 73 19 12 13 11 15 17 28

91 80 60 57 106 95 79 63 106 56 65 67 80 76

26 9 14 11 12 15 34 14 12 9 10 9 11 17

50 51 113 69 78 81 53 121 58 105 61 68 78 80

5 21 42 19 18 18 12 54 14 25 8 12 34 8

52 49 110 71 84 82 56 132 64 102 62 70 82 82

7 22 37 13 22 21 15 61 16 19 11 15 36 9

53 48 114 73 89 84 57 136 65 102 64 71 83 82

8 23 36 10 21 19 17 66 17 14 14 16 36 13

55 47 123 76 96 93 58 132 62 104 67 72 90 82

6 23 46 7 18 23 19 64 16 16 10 18 29 13

56 45 122 79 95 98 57 111 58 103 69 72 94 87

7 21 49 6 17 33 18 61 16 13 6 15 22 11

56 45 119 79 93 94 55 96 56 101 74 69 85 90

9 20 47 10 10 9 13 53 7 19 14 13 27 13

54 56 87 74 97 89 57 77 50 95 75 73 75 83

3 14 34 4 11 18 10 31 5 11 11 13 16 14

M—mean; SD—standard deviation; LEO—left external oblique; LIO—left internal oblique; REO—right external oblique; RIO—right internal oblique; LRA—left rectus abdominis; RRA—right rectus abdominis; LP—left pectoralis; RP—right pectoralis; LLD—left latissimus dorsi; RLD— right latissimus dorsi; LL3—left erector spinae at L3; RL3—right erector spinae at L3; LT10—left erector spinae at T10; RT10—right erector spinae at T10.

motor unit territory. Due to the inefficient mechanical mileu and need for a stronger contraction, motor unit recruitment may not be as important a process in generation of greater force as the firing rate. Furthermore, spinal muscles do have a significant proportion of type I postural muscle fibres, which also reduces the need as well the chance for increasing recruitment to be a dominant process in force production. A more plausible explanation for the wide variation in MF and MPF of the pectoralis muscle lies in variable contraction of the muscle. As these muscles were not one of the primary movers they varied their contraction during the course of contraction, phasing in and out at perhaps strategic time as may have been perceived by other contracting muscles for producing the required force magnitude. 5. Conclusions This study reports the frequency profile of the trunk muscles during a standardized flexion/extension free

axial rotation. The latter demonstrates that with increasing force the frequency composition changes little but the magnitude of power increases considerably. A consistent significant difference in the magnitudes of median frequency and mean power frequency was found. Therefore, in axial rotation, for the trunk muscles the median frequency and mean power frequency should not be equated. Both the median and mean power frequencies for the agonist muscles were significantly higher than those found for antagonistic muscles, except for ipsilateral external oblique and contralateral internal obliques. Orientation of these muscles was in the direction of motion and they were optimally positioned for a significant antagonistic activity, thereby demonstrating a high level of MF and MPF. With the isometric trial conditions the results indicate that the agonistic muscles had undergone shortening, producing more force than the antagonistic muscle, which may have lengthened slightly. However, this was not the case for ipsilateral external obliques and contralateral internal obliques, which may have been contracting eccentrically.

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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37

The spectral parameters of the prime movers and stabilizers demonstrated a steady pattern, whereas the secondary muscles such as pectoralis were variable.

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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology 9 (1999) 21–37 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 1982. Dr Kumar holds BSc (biology and chemistry) and MSc (zoology) degrees from the University of Allahabad, India, and a PhD (human biology) degree from the University of Surrey, U.K. Following his PhD 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 life-time work, Dr Kumar was recognized by the University of Surrey, U.K. by the award of a DSc degree in 1994. Dr Kumar was invited as a Visiting Professor for the year 1983–84 at the University of Michigan, Department of Industrial Engineering. He was a McCalla Professor 1984–85. Dr Kumar has over 200 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

37

supported in the past, in addition to the above, by MRC, WCB and NRC. He has supervised or is supervising 10 MSc students, three PhD students, and two 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 for several other international peerreviewed journals. He also acts as a grant reviewer for NSERC, MRC, Alberta Occupational Health and Safety, and BC Research. Yogesh Narayan obtained his BSc in electrical/electronics engineering from the University of Alberta, Canada. He specialized in digital signal processing and microprocessorbased systems design. After graduating, he worked on research projects in biomedical engineering, at the Grey Nuns Hospital, Edmonton, Alberta, Canada. Currently, he is a Research Assistant for Dr Shrawan Kumar at the University of Alberta.