Journal of Electromyography and Kinesiology 8 (1998) 257–267
Spectral parameters of trunk muscles during fatiguing isometric
axial rotation in neutral posture
Shrawan Kumar *, Yogesh Narayan
Department of Physical Therapy, University of Alberta, Edmonton, Alberta, Canada T6G 2G4
Abstract
Axial rotation of the trunk is commonly associated with back injury and pain. However, the behaviour of trunk muscles in axial
rotation is poorly understood. The objective of this study was to measure spectral parameters from the EMG of erector spinae at
T10 and L3 levels, latissimus dorsi, external and internal oblique, rectus abdominis and pectoralis major muscles bilaterally in a
standardized repeatable axial rotation at 60% MVC to fatigue. Twelve young and healthy subjects were recruited after screening
for musculoskeletal disorders. Surface electrodes were applied to the named seven trunk muscles bilaterally. Subjects were seated
in the device called Axial Rotation Tester and stabilized such that they could rotate only their thoracolumbar spine. Other motions
were prevented. Subjects held 60% of their MVC for a period of 2 min. Samples (2.1 s) were obtained at every 10 s interval at a
sampling frequency of 1 KHz. Samples were subjected to FFT analysis. The total power and the median frequencies were analyzed.
The median frequency for different muscles were different (p ⬍ 0.001). The slopes of decline of the median frequencies of the
agonists were different for different muscles (p ⬍ 0.001). This differential fatiguing rate could conceivably create a force imbalance
potentiating back injury.  1998 Elsevier Science Ltd. All rights reserved.
Keywords: Fatigue; Trunk muscles; Median frequency; Trunk rotation; EMG; Isometric contraction
1. Introduction
One predominant mechanical factor which has been
linked to the low-back pain is trunk rotation [7–9,17,26–
29]. Manning et al. [17], in a carefully controlled epidemiological study, demonstrated that rotation or twisting
of the trunk was involved in 11.4% of accidental back
injuries and 49% of insidious back pain, and was the
third most common body movement associated with
low-back pain. Schaffer [27] reported that of all injuries
which occur during lifting, 33% were attributable to
rotation as a single factor. However, a quantitative
measurement and understanding of the behaviour of
trunk muscles during sustained rotational stress is lacking.
An antagonistic activity of deep lumbar trunk muscles
during axial rotation has been reported [2]. Occasional
activity in the ipsilateral multifidi and rotatores during
axial rotation has also been reported [22]. A similar
* Corresponding author.
1050-6411/98/$19.00  1998 Elsevier Science Ltd. All rights reserved.
PII: S 1 0 5 0 - 6 4 1 1 ( 9 8 ) 0 0 0 1 2 - 1
occasional activity was described on both sides in
another study [5]. High level of activity on the contralateral side has been found when positions of lateral bend
and rotation were combined with an external load [1].
The agonistic myoelectric activity of the ipsilateral
internal oblique and contralateral external oblique were
approximately proportional to the magnitude of the axial
torque produced in an isometric sub-maximal contraction [25]. The agonistic and antagonistic EMG activity
of abdominal (external and internal obliques and rectus
abdominis) and spinal (latissimus dorsi, upper and lower
erectores spinae) muscles in isometric and isokinetic
maximal voluntary contraction has been studied [18].
This study, however, investigated the EMG magnitude
relationships of the spinal muscles, and spectral parameters were not reported.
Kumar [14], in a study of unresisted controlled axial
rotation, investigated the activation pattern of spinal
muscles and found that contralateral external oblique,
ipsilateral erectores spinae and latissimus dorsi became
active before other muscles. These agonistic muscles
fired up to 300 ms earlier than some of the antagonistic
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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
muscles in an upright seated unresisted axial rotation.
The agonistic muscles were responsible for 65% of the
total EMG, whereas antagonistic muscles were responsible for 35%. This study did not investigate or report
on the spectral parameters of spinal muscles in axial
rotation.
Evaluation of muscle fatigue is of central significance
to back injury causation. Myoelectric manifestation of
muscle fatigue may provide information to allow the
possibility to predict the impending time of mechanical
fatigue. It has been well documented that the amplitude
parameters, such as average rectified EMG value or
RMS value, show a substantial initial increase, and only
when mechanical fatigue is approached do they begin to
decrease [3,4,21]. Furthermore, the power spectral density function of the myoelectric signal undergoes frequency shift and compression during sustained contractions [3,6,13,19,20,23,24]. Such changes in the
electromyogram are universally accepted as manifestations of localized muscle fatigue. The most frequent
parameters used to study muscle fatigue have been
median frequency, mean frequency and zero crossings
[6,10,12,20]. The spectral content of the myoelectric signal depends on (a) the number of active motor units
whose electrical activity is picked by electrodes, (b) their
firing rates, (c) the position of the active muscle fibres
with respect to electrodes, and (d) the conduction velocity [3]. The spectral shift and compression during prolonged contraction is reported to be due to reduction in
conduction velocity, and a variation in spatial distribution of the active muscle fibres [3,6,11,19,20].
By computing the regression line over progressively
longer periods, the probability of fatigue was predicted
[11]. It was shown that the percentage change of spectral
variables was greater than those of conduction velocity
[19]. This study found that if the muscle was fatiguing
slowly, these variables showed an almost linear decrease
with time that could be described by a least-square
regression line. If the muscle was fatiguing rapidly it
showed a curvilinear pattern. It was argued that in these
cases the definition of a fatigue index was no longer
apparent and advocated use of different criteria [20]. The
authors suggested a possible approach to consider indices defined by individual variables as projections. They
discussed the representation of muscle fatigue by time
course of a specific variable for which a descriptor of
its decrease or increase may be defined.
Though both amplitude and spectral parameters have
been advocated as measures of fatigue, in a comparative
study on trapezius it was concluded that estimates of
fatigue based on amplitude parameters were such that
they could jeopardize both the calibration and the estimations of muscle load (obscuring assessment of
fatigue) [23,26]. To obtain indications of muscle fatigue,
the authors recommended that the amplitude measure-
ments should not be used without simultaneous calculation of spectral parameters.
In this study the objective was to use a spectral parameter (median frequency) normalizing in time against
the total change in that specific muscle as advocated by
Merletti et al. [20] to measure fatigue in time of the truncal muscles, both on dorsal and abdominal sides. A
second objective of the study was to determine the progression of fatigue during isometric contraction in neutral posture.
2. Materials and methods
2.1. Subjects
Twelve young normal subjects (seven male and five
female) volunteered for the study. None of the subjects
had back pain in the preceding 12 month period requiring absence from work for a week. The subjects were
also screened for musculoskeletal and neuromuscular
disorders. Spinal and abdominal surgery were used as
exclusion criteria. All subjects were informed about the
purpose and procedure of the experiment and willingly
signed the informed consent form. The demographic
details of the experimental sample are provided in
Table 1.
2.2. Equipment
2.2.1. Axial rotation tester (AROT)
AROT was specially designed to study isolated axial
rotation with minimal flexion/extension at hip or in torso
[15]. It also did not restrict movement of shoulder or
shape changes of thorax. The AROT (Fig. 1) consisted
of a rigid metal frame mounted on a metal base plate
measuring 80 × 140 cm. The frame was 180 cm high
and was cross-braced by a tension wire. Inside the frame
mounted on the base plate was an adjustable chair which
could be slid back and forth and adjusted vertically. The
backrest of the chair was sawed 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 to
read off the extent of bilateral rotation.
The seat of the AROT was mounted on a metal frame
which could be raised or lowered by a jack underneath.
Four nylon and velcro straps stabilized the hip, distal
S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
259
Table 1
Demographic data of the experimental sample
Gender
Male (N = 7)
Variable
Age (years)
Mean
25.1
Standard deviation 5.3
Minimum
18.0
Maximum
34.0
Female (N = 5)
Height(cm)
Weight (kg)
Age (years)
Height (cm)
Weight (kg)
176.2
5.2
168.0
185.5
69.1
5.8
63.5
81.1
21.6
2.8
18.0
26.0
166.1
9.9
153.5
181.0
57.9
11.4
45.0
73.5
short-cable and tip plug. The four, 4-channel fully isolated, low noise preamplifiers were specially made with
two stages of gain ( × 10 and × 100). These preamplifiers
had low non-linearity, high common mode rejection ratio
(130 dB) and a wide bandwidth (25 MHz). These 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 single pole RC filter with a cut-off frequency
at 8 Hz. The preamplifiers and amplifiers were built by
Measurement Systems, Michigan.
2.2.3. Controller and A to D board
The outputs of the AROT potentiometer and EMG
amplifiers were fed to a MetraByte DAS 20 A to D
board. This analog digital board was capable of sampling
at up to 100 kHz. Such converted digital signals were
stored in the hard disc of a 486 with a tape backup
(Colorado Memory Systems) for archival purposes.
Fig. 1.
Schematics of axial rotation tester.
thigh, proximal tibia and ankles. The shoulder harness
was adjustable in both height and width to accommodate
subjects of different dimensions. Once they were snugly
fitted at the shoulders, additional velcro straps which
went around the front, back and side were applied to
secure placement and minimize motion. At the top of
the shoulder harness there was a circular plate with a
groove in its rim for a resistance cable. The latter was
rigidly attached above with a spring-loaded shaft to raise
or lower the shoulder harness assembly and fix rigidly
in vertical direction allowing free rotation. To this shaft
a precision potentiometer with one output to a computer
and another to a dial was coupled through gear wheels.
2.2.2. EMG system
The EMG system consisted of surface electrodes,
electrode cables, preamplifiers, and amplifiers. Silver–
silver chloride surface electrodes of 1 cm diameter with
recessed pre-gelled elements (HP 144445) were used
with inter-electrode distance of 2 cm. These electrodes
were connected to 4-channel pre-amplifiers by means of
2.3. Experimental procedures
The subjects were weighed and measured for their
heights. Their age was also recorded. These subjects had
applied to them 14 pairs of disposable pre-gelled surface
electrodes (HP 144445) at an interelectrode distance of 2
cm after suitable preparation of the skin with an alcohol–
acetone mixture. These electrodes were placed on erector
spinae levels 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 aponeuroses to minimize
overlap with the external oblique). A ground electrode
was applied to anterosuperior iliac spine. The subjects
were wired to an isolated preamplifier system to provide
on-site amplification.
Such prepared subjects were seated in the chair of the
axial rotation tester. The seat was adjusted for height to
have comfortably resting feet and knees at 90°. The seat
was then aligned with the axial rotation harness, which
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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
was lowered on the subjects’ shoulders and fastened.
The subjects were stabilized in this seated position hip
down 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 leaving no slack. The subjects were then asked to
attempt torso rotation to the left applying force through
shoulders on the harness which was locked in position
through the air plane cable. The rotation was attempted
such to apply force without jerking and building it to
60% MVC (previously recorded in the same posture and
the same activity) within a couple of seconds and then
maintaining the force for a period of 2 min or till they
could no longer hold at the force level.
2.4. Data acquisition
Data were acquired using custom-designed modular
software for the project. It allowed entering 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 period of 2.1 s at the start and at every 10 s interval
for a period of 120 s.
2.5. Data analysis
For data analysis, previously collected data were
loaded back in the computer. The entire task duration
Fig. 2.
was divided into 10 s time intervals. The durations of
all 14 channels were divided in 10% segment of task
cycle. The sample of 2.1 s at the beginning of each of
the 10 s segments and one at the end of the trial period
were compared with 10 percentile task values. Where
they did not fall on the exact time, the nearest sample
was substituted. Once these data were selected, they
were written to precreated files in the computer memory
by the software and the remaining data were discarded.
The bilateral EMG of erector spinae at T12 and L3 levels,
latissimus dorsi, pectoralis major, rectus abdominis,
external and internal obliques were then processed in frequency domain. The power spectra were calculated from
the raw signals using Welch’s method [30]. The method
involves sectioning the record and averaging modified
periodograms of the sections. Thus the sampled signals
were divided into three equal segments of 2048 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 0.987 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 2100 points were divided into
three equidistant sliding and overlapping segments of
2048 points. From each segment the average value was
subtracted for DC removal and then a Welch window
Total power of ventral muscles in males.
S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Fig. 3.
261
Total power of dorsal muscles in males.
applied to it. Subsequently the power spectrum of the
segment was calculated. Since FFT assumes signal stationarity its nonstationarity was checked. For the final
power spectrum the average of the three segment spectra
was taken. The latter was then smoothed using linear
polynomial smoothing using 7 point segments and
repeating once. From these power spectra the median
frequencies and the slope of their decline were obtained.
The median frequency values for each of the channels
at each of the tasks 10 percentile values were plotted
and the slope of their decline was calculated by fitting
regression lines (best fit) to the data. These slope values
were then subjected to analysis of variance to determine
significant differences between these slopes.
3. Results
The mean duration for which the isometric axial
rotation trials could be held by male and female subjects
were 102.05 s and 113.13 s, respectively. The duration
of trial and interval between experimental samples
allowed in most cases data samples either at the 10 percentile points of the task cycle or very close to the task
percentile value. Out of a total 132 batches of data only
seven were displaced by 3 s or less.
As the attempted rotation was to the left, the left latissimus dorsi, left erector spinae at T12 and L3 levels were
primarily responsible for rotational torque on the dorsal
side; and right external oblique and left internal oblique
on the ventral side. Since the rotation was resisted by
the shoulder harness, force was also applied directly to
it to exert rotational torque. The latter involved pectoralis major in concentric mode on the right and probably in eccentric mode on the left. Such a contribution
of these muscles is presented in Figs. 2 and 3 for males,
and Figs. 4 and 5 for females. Among females, relative
reductions in overall power in left latissimus dorsi in
comparison to left thoracic erector spinae and also of the
right external oblique were clear differences from those
of men.
The median frequency plots for male and female subjects are presented in Figs. 6 and 7, respectively. A significant drop in median frequency of all muscles tested
with progression of time was clear. The left latissimus
dorsi and thoracic erector spinae demonstrated highest
frequencies and greatest drop with progression of time
among dorsal muscles. Among ventral muscles the highest value of median frequency as well as greatest drop
was seen in left pectoralis followed by the left rectus
abdominis. Most of the median frequencies recorded for
dorsal or abdominal muscles ranged between 40 and 80
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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Fig. 4.
Total power of ventral muscles in females.
Hz for both males and females. An analysis of variance
for gender difference in median frequency of the power
spectra of the corresponding muscles was non significant. However, there were significant differences
between the median frequencies of different muscles (p
⬍ 0.001). A multiple range testing with Scheffe test set
at 0.05 alpha level revealed muscles with significantly
different median frequencies which are depicted in
Fig. 8.
The maximal median frequency for each muscle was
invariably recorded in the first 10% of the task cycle and
the minimal value though ranged from 60% to 100% of
task cycle was most frequently encountered between
80% and 100% of the task cycle. There was a progressive decline in the median frequency from the beginning
to the end. The most precipitous drop was in the value
of the median frequency occurring within the first 10%
of the task cycle. Subsequent to the first 10% of the cycle
the magnitude of the drop progressively decreased. The
total percent drop in the median frequency of the
muscles in two genders is presented in Table 2. The
regression line which fitted the data points was described
by the following expression.
MF =
bo + b1 −1/2z2
e
b3√2⌸
where
MF = median frequency
bo = magnitude
b1 = magnitude
b2 = central point
b3 = spread
z2 = constant given by the expression
Trial − b2
b3
These regression lines were significant fit for the data
with significant correlation, especially for agonist
muscles (r = 0.51–0.78; p ⬍ 0.01). The overall slope of
decline of the median frequency of different muscles
were significantly different (p ⬍ 0.001) and varied from
− 0.56 to − 4.02 Hz per 1% of the task cycle among
males and from − 0.77 to − 5.36 Hz per 1% of the task
cycle in females. Generally the slope of the decline of
the agonist muscles were consistent and smoother with
lower peak values than those of others. The slopes of the
total task cycle of muscles among men were significantly
different from those of women (p ⬍ 0.02). The slope of
the decline of the median frequency in different parts of
the task cycles were different. Thus, the task cycle was
divided in three segments: (a) 0–40%, (b) 41–70%, and
(c) 71–100%. The overall and segmental slopes are
presented in Table 3. The segmental slopes between the
S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Fig. 5.
263
Total power of dorsal muscles in females.
two genders were not significantly different. However,
the different muscles clearly demonstrated significantly
different segmental slope (p ⬍ 0.001).
4. Discussion
The pattern of the total power during the course of
exertion reveals the muscle behaviour. For the rotational
movement, the ipsilateral latissimus dorsi and internal
oblique and contralateral external oblique will form a
force couple with the spinal axis serving as the fulcrum.
As such their activity and power are expected to be high.
Total power plots (Figs. 2 and 3) clearly reveal this. In
addition, to assist the action and maintain stability of the
spinal column, the erector spinae are also involved. The
erector spinae at the thoracic level, because they are not
aligned in the direction of the desired motion, can contribute to the effort, but do not serve as the prime mover.
With the passage of time during the trial the mean torque
dropped from 77 N-m to 40 N-m among men and from
45 N-m to 24 N-m among women. The total power of
the dorsal muscles significantly and precipitously
dropped by 40% of the task cycle. On the ventral side,
the right external oblique demonstrated highest power
of all abdominal muscles, followed by the left internal
oblique. Both these muscles are expected to play agonist
roles in axial rotation. The power of these muscles also
dropped significantly by 50% of the task cycle. Maintaining the force to exert axial rotation at that stage
appears to have shifted from the agonists to the right
pectoralis which was positioned for such a role. It is for
this reason that a significant rise in its total power at
50% of the task cycle is clear (Fig. 2). Beyond this time
a compensatory rise and fall between the total power of
agonists and right pectoralis points to their compensatory
action in this experimental set-up. Such a behaviour is
clearly unique to the conditions of this experiment, as
not all situations will provide means of supplementing
axial rotational forces through shoulder contact. In this
sense the action of right pectoralis may act as a confounding factor. However, the latter will be the case only
in quantitative assessment of specific values. In qualitative and pattern assessment the impact of this variable
will be less impactful.
From the total power plots, it is clear that females
have evoked a somewhat different pattern in axial
rotation compared to males. Whereas the differences on
dorsal side are less dissimilar, the clear difference is on
the ventral side. Among the dorsal muscles, the left thoracic erector spinae appears to play a greater role than
that of the left latissimus dorsi, though both play a major
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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Fig. 6.
Median frequency plot of male subjects.
role. While the left latissimus dorsi drops precipitously
by 40% of the cycle, the left thoracic erector spinae
drops significantly by 60% of the cycle. It would seem
to indicate that among females, latissimus dorsi may not
be as strong a muscle as it is in their male counterparts.
A striking difference among females from those males
was in lack of power in right external oblique. To exert
and maintain the rotary force they evoked their right pectoralis which acted in a manner to form a force couple
with the dorsal muscles. Such an observation lends support to the argument that females are considerably
weaker in axial rotation [16], which may be due to the
weakness of the abdominal muscles and latissimus dorsi.
Similar to their male counterpart, they could maintain a
stable rotary torque only up to 40–60% of the task cycle,
beyond which it dropped significantly, presumably due
to localized muscle fatigue.
A lack of difference between the slope of decline in
median frequency for males and females indicates that
there is no gender difference in pattern of fatigue. However, a significant difference between the slopes of
decline of the median frequency of different trunk
muscles (p ⬍ 0.001) clearly indicates that there may be
a differential fatiguing rate of different muscles. Axial
rotation is an activity in which muscles on the opposite
side of the fulcrum are in balance with each other in
an asymmetric and stressful activity. The progression of
fatigue will result in synchronization of motor units [6],
resulting in jerky contraction. A differential fatigue rate
may reduce the force provided by different muscles differently. The latter may lead to a varying degree of uncoordination. Furthermore, the decline in force output of
different muscles may not be proportional. Thus a breakdown of orderly recruitment of motor units and coordinated and balanced force output of different muscles can
be disrupted. The latter can conceivably cause a sudden
and large load on already loaded and stretched soft
tissues, even through micromotions of jerky contractions. The foregoing could load the tissues beyond their
tolerance, precipitating soft tissue injuries of the back.
Such a mechanism is likely to be more hazardous in
asymmetric activities involving rotation as compared to
symmetric activity due to the power differential of the
involved muscles.
As far as the observation in the current experiment of
comparatively shallower slopes of agonists and larger
values of slopes of muscles not primarily involved in
causing axial rotation, two factors deserve mention.
First, the agonist muscles are the prime muscles responsible for the activity and tend to maintain their action
throughout the duration. This may necessitate orderly
reduction in force output due to progressive and slow
S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Fig. 7.
Fig. 8.
265
Median frequency plot of female subjects.
Muscles with significantly different median frequencies.
tiring. Thus, it is unlikely to undergo extreme variation
during the course of contraction. Secondly, those
muscles which are not primarily involved in the contraction have the opportunity to phase in and out and contract in a less orderly manner. As seen in the current
experiment, the left pectoralis muscles have had a high
value of median frequency and dropping almost exponentially right from the start. It could also be possible
that initially they contracted eccentrically, then relaxed
or contracted concentrically to a variable extent. Some
of these factors are speculative as they were not verified
in the experiment. However, extremely low total power
of the left pectoralis, but large and precipitous drop in
Table 2
Total percent decline in median frequency of the trunk muscles at the fatigue termination of activity
Male
Female
Left
Right
Left
Right
EO
IO
RA
P
LD
TES
LES
40
21
35
27
33
14
24
21
24
26
34
38
47
13
55
20
24
18
27
23
18
10
23
33
12
9
19
22
EO = External oblique; IO = internal oblique; RA = rectus abdominis; P = pectoralis; LD = latissimus dorsi; TES = thoracic erector spinae; LES
= lumbar erector spinae.
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S. Kumar, Y. Narayan / Journal of Electromyography and Kinesiology8 (1998) 257–267
Table 3
Overall and segmental slopes (Hz percent of the task cycle) of the median frequencies during axial rotation of males and females
Gender
Male
Overall
CHAN
LEO
LIO
LL3
LLD
LP
LRA
LT10
REO
RIO
RL3
RLD
RP
RRA
RT10
−
−
−
−
−
−
−
−
−
−
−
−
−
−
2.64
0.92
0.61
1.18
4.02
1.25
1.13
0.84
0.94
0.56
0.64
0.74
1.44
0.97
Female
Phase 1
Phase 2
Phase 3
1.34
1.08
0.68
0.42
0.21
2.52
0.96
0.73
1.27
3.47
0.36
0.43
0.43
3.17
− 1.54
− 0.70
0.91
0.05
− 3.80
− 1.72
0.50
0.66
0.27
0.35
0.15
0.74
2.34
2.49
− 8.35
− 3.83
− 2.30
− 3.70
− 13.00
− 4.89
− 3.16
− 2.78
− 1.96
0.73
− 1.76
− 2.32
− 5.43
0.98
−
−
−
−
−
−
−
−
−
−
Overall
−
−
−
−
−
−
−
−
−
−
−
−
−
−
2.21
1.07
0.97
1.91
5.36
2.41
1.17
0.96
0.77
0.97
1.10
0.92
2.92
1.18
Phase 1
− 4.77
− 3.37
− 2.16
− 3.72
− 11.24
− 5.35
− 3.26
− 2.36
− 2.39
− 2.84
− 1.60
− 1.69
− 6.46
− 3.00
Phase 2
−
−
−
−
−
−
−
−
−
−
−
−
−
−
3.05
0.71
0.95
1.89
4.27
0.91
1.39
0.17
0.62
0.17
0.82
1.28
3.93
1.51
Phase 3
0.80
− 0.21
0.52
− 0.21
− 0.88
− 1.04
0.66
− 0.44
− 0.08
0.61
− 0.28
− 0.26
3.14
− 0.31
Overall = entire task cycle.
Phase 1 = first 40% of the task cycle.
Phase 2 = 41–70% of the task cycle.
Phase 3 = 71–100% of the task cycle.
LEO = left external oblique.
LIO = left internal oblique.
LL3 = left erector spinae at 3rd lumbar vertebral level.
LLD = left latissimus dorsi.
LP = left pectoralis.
LRA = left rectus abdominis.
LT10 = left erector spinae at 10th thoracic vertebral level.
REO = right external oblique.
R10 = right internal oblique.
RL3 = right erector spinae at 3rd lumbar vertebral level.
RLD = right latissimus dorsi.
RP = right pectoralis.
RRA = right rectus abdominis.
RT10 = right erector spinae at 10th thoracic vertebral level.
median frequency within the first 10% of the task cycle
may corroborate the foregoing argument. Thus a lack of
serious consequence of the contraction/relaxation of
other muscles in this experimental activity for generating
rotary torque allowed them the flexibility of varied
action. Finally, since the rotary force required and maintained was 60% of the MVC, a cocontraction of many
muscles was a necessary action. Due to the asymmetric
nature of the task, postural stability may be challenged
and require coordinated contraction of agonists as well
as antagonists. Muscles not required as prime movers, if
they can undergo a sudden change in tone, may have the
potential of playing a role in injury precipitation.
obliques were primarily active. In sustained high level
contraction (60% MVC), the drop in the median frequencies of the prime movers was slow and steady,
whereas those muscles not primarily involved in the task
was more variable. The role of pectoralis muscle in this
study was affected by its ability to exert on the harness
as a complimentary muscle. The median frequencies of
different trunk muscles were significantly different. The
slope of decline of the median frequency of different
muscles were significantly different. However, there was
no significant difference between the two genders in
overall slopes, but some segmental slopes did differ significantly.
5. Conclusions
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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, UK. 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 DSc
degree in 1994. Dr. Kumar was invited as a Visiting Professor for the
year 1983–1984 at the University of Michigan, Department of Industrial
Engineering. He was a McCalla Professor 1984–1985.
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 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 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 BSc in
Electrical/Electronics Engineering from the University of Alberta, Canada. He specialized in
digital signal processing and microprocessor
based 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.