|
Poor neuromuscular endurance of low back musculature has been
related to the potential for developing low back pain (Alaranta
et al., 1995;
Biering-Sørensen, 1984;
Hultman et al., 1993;
Mayer et al., 1995;
Nelson et al., 1995;
Smidt et al., 1983).
Additionally, decreased trunk strength and endurance associated
with a cyclical pattern of deconditioning through pain, avoidance
and inactivity are noted as defining characteristics (Biering-Sørensen,
1984;
Mayer and Gatchel, 1988).
There are numerous potential risk factors for developing back pain
including poor back extensor endurance (Canadian Society for Exercise
Physiology-CSEP, 2004)
and identifying potential risk factors, such as poor lumbar extensor
endurance may be important. The most widely reported fatigue test
in the literature is the Biering-Sørensen test (Moreau et al., 2001).
A modified Biering-Sørensen test to measure back fatigue is currently
in use by the Canadian Society for Exercise Physiology in their
Canadian Physical Fitness and Lifestyle Approach (CPAFLA) testing
(CSEP, 2004).
Administration of the Biering-Sørensen test is inconsistently practiced
in the literature, including differences in arm position, number
of straps (or no straps) and conclusion criteria. These variations
have been grouped together as modified Biering-Sørensen tests (Moreau
et al., 2001).
This test is generally considered safe for both healthy and clinical
populations (Alaranta et al., 1994;
1995;
Biering-Sørensen, 1984;
Moffroid, 1997;
Nordin et al., 1987;
Mannion and Dolan, 1994;
Peltonen et al., 1998).
While forces required to maintain a horizontal position are well
below forces of maximal voluntary isometric activations (MVIA) in
healthy populations (Jørgensen and Nicolaisen, 1986;
Mayer et al., 1995;
Moffroid et al., 1993),
they may rise to as much as 85% of a MVIA in a patient with chronic
low back pain (Hultman et al., 1993).
It has been suggested that performance of maximal activations in
patients with low back pain could compromise safety (Moffroid et
al., 1993).
There is considerable range of mean fatigue times reported for the
Biering-Sørensen test in the literature ranging from 84s to 180s
in healthy males (Biering-Sørensen, 1984;
Jørgensen and Nicolaisen, 1986;
1987;
Hultman et al., 1993;
Kankaanpaa et al., 1998a;
Mannion and Dolan, 1994;
Nicolaisen and Jørgensen, 1985;
Sparto et al., 1997)
and 80s-194s for males with low back pain (Biering-Sørensen, 1984;
Jørgensen and Nicolaisen, 1987;
Hultman et al., 1993;
Nicolaisen and Jørgensen, 1985).
The wide range of fatigue times may be related to the variety of
modified protocols implemented in these studies as well as the degree
of low back disability between individuals. Fatigue has been defined
as a transient decrease in working capacity (Asmussen, 1979),
loss of force output leading to reduced performance (Fitts and Metzger,
1993)
or a decline in the force-generating capacity of the muscle (Degens
and Veerkamp, 1994).
Fatigue also may be experienced during prolonged submaximal intensity
contractions without an apparent decrement in the targeted force.
This type of fatigue may be defined as an acute impairment of performance
that includes an increase in the perceived effort necessary to exert
a desired force and an eventual inability to produce this force
(Enoka and Stuart, 1992).
All these definitions imply that the effects of fatigue can contribute
to the risk factors associated with low back pain. The modified
Biering-Sørensen test as employed by the Canadian Society for Exercise
Physiology, Canadian Physical Fitness and Lifestyle Approach testing
attempts to ensure standardization of testing and thus a valid assessment
of back health (Albert et al., 2001).
Although the modified Biering-Sørensen test is generally considered
a measure of low back function measuring overall lower back fatigue,
activity of the biceps femoris and hip extensors have been argued
to substantially contribute to fatigue times (Kankaanpaa et al.,
1998B).
Significant correlation has been observed between Biering-Sørensen
fatigue times and EMG median frequency slopes of the biceps femoris
(Moffroid et al., 1994;
Moffroid, 1997).
It would seem that more than just the erector spinae are involved
in back fatigue.
Ng et al. , 1997
demonstrated that the multifidus has more activity than the iliocostalis
lumborum during Biering- Sørensen testing. The multifidus fatigues
at a faster rate than the iliocostalis lumborum during this test
demonstrating a higher initial median frequency and normalized median
frequency slope (Ng et al., 1997).
Ng and Richardson, 1996
suggests that the modified Biering-Sørensen test with the use of
EMG power spectral analysis may be a reliable method to measure
the fatigue rate of the back muscles if cross-talk is minimized
and adds that measuring the fatigue rate of the multifidus may be
a useful clinical measure. Van Diėėn et al. (1993)
observed that the multifidus muscle at the L5 level appeared to
show the most consistent changes of the EMG power spectrum as a
consequence of fatigue.
Maintaining a horizontal position during Biering-Sørensen test (referred
to in this study as 100% of the head, arms and trunk {HAT} segments)
results in higher fatigue times than at higher levels of resistance
(Moffroid et al., 1993).
With increased fatigue times, motivation, pain levels, and alternative
muscle control strategies may play a larger role. According to the
Canadian Society for Exercise Physiology, Canadian Physical Fitness
and Lifestyle Approach manual (2004),
the back extensor endurance test with the HAT as the resistance
(modified Biering-Sørensen test) has been reported as a valid and
reliable assessment of back extensor endurance, and found to be
positively related to back health. The Canadian Society for Exercise
Physiology, Canadian Physical Fitness and Lifestyle Approach manual
(2004)
indicates that this finding supports the use of back extensor endurance
with HAT to differentiate levels of back health.
The purpose of this paper was to compare trunk and hamstrings muscle
activity in subjects with different degrees of back health (low
back pain and no low back pain) and to investigate the effects of
different percentages of HAT resistance added to the Canadian Society
for Exercise Physiology modified Biering-Sørensen test for time
to fatigue, median frequency and EMG.
| METHODS |
|
Subjects
Twenty male volunteer subjects were recruited from the university
population. These subjects were grouped into low back group
(n = 10) and control groups (n = 10). Subjects were included
in the low back pain group based on a self report of currently
having low back pain or having a history of chronic or recurrent
low back pain that limited activity. One of the researchers
was a certified and practicing chiropractor who examined the
subjects to ensure there was some degree of disability or
pain and that conversely the controls did not have significant
disability or pain. All subjects completed an Oswestry Low
Back Pain Disability Questionnaire (Fairbank et al., 1980;
Thomas et al., 1989)
as well as a numeric pain scale. Subjects in the low back
pain group had a mean age of 29.1 years (± 8.2) and mean mass
of 79.7 kg (± 11.2) as compared to 24.7 years (± 2.9) and
81.9 kg (± 7.8) for controls. Table
1 reports subject characteristics and mean scores of the
Oswestry Disability Index and 0-10 Pain scale. Oswestry Low
Back Disability Index scores were 72% lower and pain scores
96% lower in the Control group than low back pain group. The
low back pain group had an Oswestry mean score of 18.3% (±
11.8), which is clinically categorized as "mild disability"
as compared to control group that had an Oswestry of 5.1%
(± 5.5), which is also considered "mild disability".
The mean pain score from the low back pain group was 3.43
(± 2.0) as compared to that of 0.1 (± 0.4) for controls. Using
the Mann-Whittney Test, significant differences (p = 0.007)
were found between Oswestry Low Back Disability Index scores
between low back pain and Control groups and significant differences
(p < 0.001) in pain levels between low back pain
and Control groups.
The experiment was explained to the subject and any questions
or concerns were addressed and the subjects were informed
that they could withdraw from the experiment at any time.
A consent form was read and signed prior to experimentation.
The Memorial University of Newfoundland Human Investigations
Committee approved the study.
Prone back extension
The posture adopted for the test was a variation of the Bering-
Sørensen test (Biering-Sørensen, 1984)
as described and implemented by the Canadian Society for Exercise
Physiology, Canadian Physical Fitness and Lifestyle Approach
test (CSEP, 2004).
The Beiring Sorensen test was originally described by the
authors as having subjects lay prone on an examination table
and maintain an unsupported trunk (from the upper border of
the iliac crest) horizontally until they could no longer hold
a horizontal position or for a maximum of 240 seconds. The
buttocks and legs are fixed to the table with three, three
inch canvas straps. Any variations from the described methods
are known as modified Sorensen tests. Our tests differ from
the original in numerous ways, as described in our methods,
but most notably by having subjects exert force against a
strain gauge, but also in that we did not define a default
test duration of 240 seconds. All protocols were held to exhaustion
(failure to maintain prescribed force). Subjects lay prone
on a padded examination table, with the trunk of the body
extended off the edge of the table at the level of the anterior
superior iliac spine of the pelvis. The lower legs, thighs
and mid-buttocks region were restrained from motion using
wide straps attached to the examination table. A pad placed
under the ankles prevented subjects from bracing against the
table with their feet. A harness was attached around the trunk
at the T4-5 level. The strain gauge was attached to this harness
at a midline location of the trunk while the other end was
attached to an anchor plate at floor level. The harness/strain
gauge assembly was adjusted so the subject maintained a trunk
orientation parallel with the floor. The trunk was supported
against gravity during rest periods (Figure
1).
Definition of hat (head-arms-trunk segment)
Using the subject's body mass and normative data derived through
regression equations, (Zatsiorsky, 2002)
the subject's HAT mass was calculated. Using Zatsiorsky's
calculations, it was found that subjects' HAT mass was 49.11%
of their total body mass. HAT values were calculated based
upon relative mass values from in vivo investigations by Zatsiorsky,
2002
of the inertial properties of 100 physically fit young males.
These values are consistent with his regression equations,
which are:
| Head and Neck: |
y = 3.243 + 0.24x |
| Upper Arm (2): |
y = -0.142 + 0.029x |
| Forearm (2): |
y = 0.0165 + 0.0139x |
| Hand (2): |
y = 0.109 + 0. 046x |
| Upper Trunk: |
y = -0.078 + 0.0161x |
| Middle Trunk: |
y = -2.222 + 0.194x |
| Lower Trunk: |
y = -0.348 + 0.117x |
n
these equations, x = the total body mass. The sum of the y
values represents the mass of the HAT segment. The use of
HAT-related values allowed for a normalized load condition
across all subjects. These HAT-related loads, measured in
Newtons, were equal to the HAT plus additional percentages
of the HAT value of 10%, 20%, 30%, 40%, 50%, 60% and 70%.
Since the segment was held in a horizontal orientation and
the exertion was isometric, it was assumed that the resistance
force vector was vertically oriented and acting through the
centre of mass of the HAT segment.
Experimental design
The force displayed on the computer screen was calibrated
so that 10% increments of HAT were visible to the subject
for feedback. Repeated measures were taken over four sessions.
Individual fatigue tests (test sessions) were separated by
a minimum of 48 hrs and no longer than 96 hours. In each testing
session, subjects were initially asked to perform a series
of 3-5 repetitions of 2-5 s MVIA and then 7 randomly applied
2-5 s submaximal exertions of 100% -170% HAT in increments
of 10%. The subjects viewed the computer screen and attempted
to maintain the prescribed force (% of HAT).
There was a rest period of at least 2 minutes between exertions
and a longer rest period of 5-10 minutes after all submaximal
and maximal contractions were completed to minimize effects
of muscle fatigue for the subsequent fatigue protocol (Behm
et al., 2004).
Subjects had to maintain the prescribed force for the submaximal
exertions whereas they provided their greatest effort for
the maximal exertions.
Subjects were then cued for the fatigue protocol and given
standardized verbal encouragement during the effort. On each
testing session, subjects would exert one randomly chosen
force equivalent to their HAT mass plus a given percentage
(0, 20, 40 or 60%) of that HAT mass until volitional failure.
The test was terminated if the subject could not maintain
the given force as displayed on the screen, or if their torso
fell below parallel to the floor (a conclusion criterion only
necessary when assessing the 100% HAT condition). The researchers
monitored the subject's position and would give an initial
warning that the back position was not parallel. A second
warning would result in termination of the test. Subjects
used the visual feedback of a video monitor that demonstrated
the target and actual forces. Electromyographic (EMG) signals,
force and time to failure were all recorded.
Instrumentation
Surface EMG was collected using a bipolar differential collection
system (ME3000P; Mega Electronics Ltd, Kuopio, Finland) utilizing
1cm diameter silver/silver electrodes spaced 1 cm apart. This
was used to collect the electrical activities of 6 muscles
in the trunk and thigh. Channels were sampled at 1000 Hz,
band-pass filtered between 20 Hz and 500 Hz and amplified
(differential amplifier: differential gain of 1000, common
mode rejection ratio 130 dB, noise 1 µV). They were converted
from analogue-to-digital (12-bit), and stored on computer
for analysis. Signal amplification was done at the reference
electrode site to minimize signal artifacts caused by movements
and external noise.
Electrodes were placed bilaterally over the lumbosacral erector
spinae (LSES) 2 cm lateral to the L5-S1 spinous processes
and over the upper lumbar erector spinae (ULES) 6 cm lateral
to the L1-L2, spinous processes. While a number of studies
have used the L5/S1 configuration of surface EMG electrodes
for examination of multifidus, (Vezina and Hubley-Kozey, 2000;
Hermann and Barnes, 2001;
Danneels et al., 2002),
others suggest the intramuscular needle electrodes are necessary
for accurate assessment (Stokes et al., 2003).
For the present study, the EMG activity collected by the electrode
arrangement is referred to as LSES as we expect we may have
activity from more than just the multifidus. In the same way
it is expected to emphasize the measurement of the multifidus
at the lumbosacral junction with our narrow electrode placement,
we expect to emphasize the longissimus thoracis with our placement
of electrodes more lateral to the L1-L2 spinous processes.
We are aware that we may also be interpreting signals from
iliocostalis lumborum and multifidus and in this paper refer
to the observed EMG activity as ULES. Electrodes were also
placed bilaterally in the mid-belly of the biceps femoris.
Reference electrodes were placed 5-10 cm away from the collecting
electrodes for all collection arrays.
Bony landmarks and careful palpation was used to place electrodes
in the same location. Both skin marking and measurement techniques
enhanced the repeatability of electrode placement. The subjects'
skin was prepared prior to electrode placement by initially
shaving local body hair, removing dead epithelial cells with
very fine grade sandpaper and then cleansing the areas with
an isopropyl alcohol swab.
Force exerted against the harness assembly placed at the T5/T6
level was collected through a Wheatstone bridge configuration
strain gauge (Omega Engineering Inc. 55LCCA 250). The signal
was converted from analogue-to-digital (MP100 analogue-to-digital:
12-bit; Biopac Systems Inc. Holliston, MA) and stored and
analyzed through computer software. (Acqknowlege III, Biopac
Systems Inc. Holliston, MA).
Data analysis and statistics
All signals were visually inspected during real time collection
of EMG to ensure optimal signal quality. The median frequency
was calculated using a Fast Fourier Transformation (FFT) algorithm
and a Hamming window function. This was a data reduction option
available from the MegaWin software (Mega Electronics Ltd,
Kuopio, Finland) employed in the EMG data collection and analysis.
A spectral estimate was calculated using a 1024 point moving
window over the time from the initial marker flag representing
the onset of activity to the final marker flag denoting the
subject could no longer maintain the horizontal trunk position.
The change in median frequency was calculated for the time
period (Hz/sec) and employed as an estimate for muscular fatigue.
Using the same time markers, the average amplitude of the
EMG signal (aEMG) were also calculated. Descriptive statistics
were reported for fatigue time, change in median frequency,
and aEMG. These measures were compared across the conditions
of 100%, 120%, 140% and 160% HAT using an ANOVA of a 2x4 (group
x resistance) configuration (SPSS 12.0 for windows, SPSS Inc.,
US). Significance was set at p < 0.05 for all tests. Levene's
Test of Homogeneity was performed on force and EMG, to ensure
reliability in EMG electrode placement. There were no significant
differences between groups. A Bonferroni (Dunn) procedure
was used to identify the differences among the percentage
of HAT. Effect sizes (ES = mean change / standard deviation
of the sample scores) were also calculated and reported (Cohen
1988).
Cohen applied qualitative descriptors for the effect sizes
(ES) with ratios of less than 0.41, 0.41-0.70 and greater
than 0.7 indicating small, moderate and large changes respectively.
Differences between groups for the Oswestry Low Back Disability
Index and Pain Scales were analyzed with a 1 way ANOVA.
Intraclass correlation coefficients were calculated for extensor
force, EMG of each muscle during each MVIA and each percentage
of HAT. Reliability was assessed using an alpha (Cronbach)
model intraclass correlation coefficient (Cohen 1988).
The average force (N) output of the MVIA condition was compared
between groups over the four sessions using an independent
t-test.
Intraclass correlation coefficients were calculated for the
EMG of MVIA's and HAT for control and low back pain groups.
The MVIA EMG and force ICCs were separately compared with
a repeated measure 1 way ANOVA. The HAT EMG intraclass correlation
coefficients were compared with a 2x8 (Group x HAT%) configuration
ANOVA (SPSS 12.0 for windows, SPSS Inc., US) for each of the
muscle groups. Differences were considered significant if
they achieved an alpha level of p < 0.05. Bonferroni post-hoc
tests were used to discriminate between individual and significant
differences. Data in the text and figures include means and
standard deviation (SD).
|
| RESULTS |
|
Fatigue time
Figure 2 depicts the difference in fatigue time as resistance
increases from 100% to 160% HAT. Expectedly, fatigue times
decreased as resistance increased. The low back pain group
had 4.5%, 34.2%, 40.6% shorter times at 120%, 140% and 160%
of HAT respectively however no significant differences were
detected between groups.
Median frequency
Figure 3 illustrates differences in median frequency
between Control and low back pain groups for each extensor
muscle group. Median frequency decreased more as resistance
increased from 100-160% HAT. Differences were observed only
in the biceps femoris and only at higher percentages of HAT.
Table 1 reports significant
between group differences in the right biceps femoris. There
were significant pairwise differences in the left biceps femoris
at 140% HAT with 89% lower median frequency in controls. A
significant pairwise difference was also evident in the right
biceps femoris at 160% HAT with 77% lower median frequency
in controls and significance was approached (p = 0.057) at
120% HAT with 107% lower median frequency in the control group.
Average EMG (aEMG)
For the control group, the aEMG consistently increased from
100% to 160% HAT. Table 2
reports aEMG means for each group across percentages of HAT.
The aEMG was markedly increased in the control group between
the 140% to 160% of HAT condition in all extensor muscle groups.
In the low back pain group the 160% HAT condition only elicited
marked changes in the left and right ULES, but failed to show
marked differences in other muscles. Table
3 reports the interaction between groups and resistance
for each muscle group. There was 54% less ULES aEMG in control
group than in the LBP group. There was a significant difference
at 140% HAT in the left biceps femoris with 86% lower aEMG
in controls. The right biceps femoris demonstrated significant
differences; with 65% lower aEMG in controls at 120% HAT and
an 81% lower aEMG in controls at 140%.
Reliability
Table 4 reports the intraclass
correlation coefficients for all extensor muscles and compares
the mean intraclass correlation coefficients of the six-extensor
muscles for each %HAT and MVIA of Controls with that of the
low back pain group. There was excellent correlation in all
muscle groups in all % HAT in the control group, but much
less homogeneity in the low back pain group compared to the
control group.
|
| DISCUSSION |
|
Back
endurance as it relates to low back pain has received much
attention. Currently, a modified Biering- Sørensen test is
used as part of the Canadian Society for Exercise Physiology
Canadian Physical Activity Fitness and Lifestyle Approach
(CPAFLA) test. Many studies have demonstrated that differences
in fatigue times are lower in those with low back pain than
those without (Alaranta et al., 1995;
Biering-Sørensen, 1984;
Hultman et al., 1993;
Mayer et al., 1995;
Nelson et al., 1995;
Smidt et al., 1983).
This study however did not find such a clear distinction in
those subjects identified with mild low back pain disability
scores. The rigorous testing procedures outlined in our protocol
may account for differences in overall fatigue times, but
not in differences between groups. Differences in fatigue
responses were observed through EMG evidence in select muscle
groups at higher resistance of fatigue, but there were no
differences at lower percentages of HAT. Further, fatigue
time did not appear to be a sensitive measure to discern between
mild low back pain and control groups.
There was no significant difference in the fatigue times between
low back pain subjects and controls. These findings are similar
to that of Biering-Sørensen, 1984(low
back pain: 164s, controls: 195s), Sparto et al., 1997(low
back pain with a mean of 109s), McKeon (2006)
(low back pain: 15.3s, healthy males: 124.4s) and Hultman
et al., 1993(low
back pain: 134s, controls 150s). Kankaanpaa et al., 2005
also reported a lack of difference in paraspinal activation
(EMG amplitude and mean power frequency) and relative fatiguability
between low back pain participants and healthy males. In the
current study the initial series of MVIA and submaximal exertions
were performed by all subjects and therefore, should not have
been a factor in the differences found between the groups.
However even with adequate muscle recovery periods (Behm et
al., 2004),
the initial testing may account for lower fatigue times than
found in most studies. The norms for the Canadian Physical
Fitness and Lifestyle
Approach back extension fatigue test indicate that both the
low back pain (102s) and control (101s) subjects in the present
study were situated in the 50th percentile (Payne
et al. 2000).
No subject in the low back pain group in this study reported
recent severe bouts of low back pain within the past month,
but all reported recurrent or chronic low back pain that was
reported to affect their activity. Validated outcome measures
and visual analogue pain scales, while significantly different
between groups, did not convey a sense of severe pain or marked
physical disability. However subjects with similar pain history
and ranges of discomfort are likely characteristic of people
that are candidates for back assessments.
Median
frequency
Pairwise differences were only present at higher levels of
resistance. Right biceps femoris demonstrated no difference
in median frequency at 100%, but significant differences were
evident at 120% and 160%. Significant differences were also
found at the 140% HAT condition for left biceps femoris and
right ULES. These findings may suggest that the lower resistance
levels are not sufficient to delineate between groups, but
as resistance increases, more extensor effort is required
and the differences between groups occur primarily in the
biceps femoris. Significant differences at the right ULES
may also play a role. Whereas some studies have been able
to delineate between healthy and low back pain subjects with
a modified
Biering-Sørensen test (Biering-Sørensen, 1984,
Ng et al., 2002),
questions arise regarding the reliability of the test. Van
Diėėn and Heijblom (1996)
reported that test retest errors between sessions could reach
20% but that similar to the increased discrimination in the
present study at higher resistance levels, reliability increased
with increased relative force. Luoto et al., 1995
indicated that the high incidence of low back pain in the
12-month follow-up in their study was implausible suggesting
the reliability of the low back pain questionnaire was far
from complete. Similarly, the limitation of the self-reported
low back pain questionnaire (Oswestry) in the present study
is discussed in further detail in the limitation section to
follow. There is a vast spectrum of disability and pain associated
with chronic low back pain individuals. The low back pain
group heterogeneity in the present study might be considered
a reflection of that population. The wide range of disabilities
and pain levels would make it exceedingly difficult to accurately
identify or predict low back pain with a single test utilizing
a narrow range of resistance.
Average
EMG
Differences in aEMG between groups were evident in the right
ULES at 140% HAT. The only other significant differences occurred
in the left biceps femoris at 140% HAT and in the right biceps
femoris at 140 and 150% HAT. While the final product of force
output through back extension is a composite of many synergistic
muscles and recruitment strategies, it appears that the most
marked differences in muscle recruitment between groups occurred
in the biceps femoris at higher percentages of HAT. Numerically,
the low back pain group had higher mean aEMG values for the
right and left biceps femoris in 6 of the 8 measures. Conversely,
the low back pain group had numerically lower aEMG values
for the right and left LLES and ULES for 12 of the 16 measures.
Hence there was statistically significantly greater biceps
femoris activity in the low back pain group (left biceps femoris
at 140% HAT, right biceps femoris at 120 and 140% hat) with
a trend toward greater biceps femoris activity, which contrasts
with lower low back pain LLES and ULES activity. These findings
would suggest that the subjects with low back pain maintained
similar back fatigue as controls due to a greater reliance
on their hip extensor (biceps femoris) activity. It could
be suggested that the test is not simply a test of back fatigue
but also dependent upon either purposeful or automatic alterations
in motor control strategies.
The multifidus (a component of LLES activity in this study)
has been reported to fatigue at a faster rate than the iliocostalis
lumborum (Ng et al., 1997)(a
component of ULES activity in this study) leading to the suggestion
that the fatigue rate of the multifidus may be a useful clinical
measure (Ng and Richardson, 1996).
The iliocostalis and longissimus and multifidus muscles are
arranged from lateral to medial and are contained within their
own fascial compartment (Bogduk, 1980).
The lumbar portions of the iliocostalis and longissimus attach
to the mamillary, accessory and transverse processes of the
lumbar vertebrae and apart from a small number of medial slips
of the longissimus, the iliocostalis and longissimus do not
have superior attachments in the lumbar spine (Macintosh et
al., 1986).
These muscles act at a distance having fibers that do not
act in a plane parallel to compressive force, but are of a
more posterior and caudal orientation and are well suited
to resist anterior shearing forces. (McGill, 2002)
The slips of the multifidus which attach distally at the sacral
crest, interosseous sacroiliac ligament, thoracolumbar fascia
and medial edge of the iliac crest span only two or three
segments and attach to the posterior aspect of the spinous
of each vertebrae. The extension torque creates more local
compression and than does the iliocostalis and longissimus.
The disparity in configuration of these muscles highlights
why iliocostalis and longissimus are though to act as global
stabilizers where as the multifidus is seen to impart stability
on a more local level.
The lack of consistent differences in ULES and LLES activity
in the present study with significantly greater biceps femoris
EMG activity in the low back pain group would further suggest
that not just back musculature are involved in maintaining
the posture associated with the modified Biering-Sørensen
test. Based on the results of this study, using aEMG of erector
spinae muscles in low resistance modified Biering-Sørensen
tests may not be ideal when attempting to evaluate healthy
subjects from those with mild chronic or recurrent low back
pain.
Muscle
synergysm
Due to the synergism of muscles used in back extension; there
are various motor control strategies that may be employed
during a low intensity fatigue test to maintain a desired
static posture. Motor unit substitution during fatigue protocols
has been reported for a number of limb (Bawa et al., 2006,
Kouzaki et al., 2004,
Kouzaki and Shinohara, 2006)
and trunk muscles (Westgaard and DeLuca, 1999).
Kouzaki and Shinohara, 2006
reported that subjects with more frequent alternate muscle
activity experience less muscle fatigue. Muscle substitution
protects postural muscles from excessive fatigue when there
is a demand for sustained low-level muscle activity (Westgaard
and DeLuca, 1999).
It is suspected that at higher intensities (larger percentages
of HAT) there is less time for implementing a motor control
strategy that coordinates load sharing across synergistic
muscles. This may be the reason why fatigue time differences
are more pronounced at 140% and 160% HAT. For an 80kg subject,
140% HAT is 540N or 87% of maximum for controls and 132% of
maximum for the low back pain group. It is probable that at
higher percentages of HAT that approach or exceed maximal
values, there is less opportunity to employ alternative recruitment
strategies.
In an isolated case, one of the control subjects had a higher
fatigue time at 160% than at the 100% condition. When EMG
data streams were reviewed, it was evident that he had developed
a load sharing strategy between his lumbar extensors and biceps
femoris, alternating bursts of activity in each muscle group
thus creating "micro- rest periods". This case highlights
the idea that although the neuromuscular fatigue of the trunk
and hip extensors contribute to fatigue time, motor control
strategies may play an equal or superior role in the application
of fatigue protocols.
Limitations
One of the most significant limitations of this study is having
the subjects use self-report of low back pain to delineate
control and low back pain groups. Although the differences
in the pain and Oswestry scores were significant between groups,
there was considerable variability in the scores within the
low back pain group. Such variability may have reduced the
discrimination between groups. Additionally, it should be
noted that an Oswestry score of 18% classifies a subject as
having only mild lower back disability. Although the relatively
low levels of disability and pain are a likely cause for decreased
differences between groups, it can be argued that clients
with similar pain and disability characteristics are likely
candidates for conservative care treatment and likely to present
to kinesiologists or trainers for fitness appraisals.
Based on this limitation, it might be suggested that the present
HAT-based protocol would specifically aid practitioners in
classifying patients with varying degrees of mild back disability
based on the Oswestry classification. For future studies,
it is suggested that scores or other form of external assessment
be used as grouping criteria groups independent of self classification
as back pain sufferers or not. There were some limitations
in the research design. Firstly we used a relatively small
number of subjects with each group containing 10 subjects.
Secondly, a series of maximal and submaximal tests were performed
prior to the fatigue protocol. Although adequate recovery
times were used, this could have potentially led to shorter
fatigue times. Because this was done consistently on each
session and for all subjects, it is not a factor influencing
differences between groups.
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