Endurance Times of the Thoracolumbar Musculature: Reference Values for Female Recreational Resistance Training Participants

Abstract

The assessment of thoracolumbar muscle endurance (TLME) is common among strength and conditioning professionals as well as clinicians desiring to quantify baseline muscle performance and determine injury risk. Reference values for such assessments are documented in the literature; however, their utility may be of limited value due to heterogeneous participant selection and limited demographic reporting. Moreover, active cohorts who engage in resistance training (RT) may reach a ceiling effect on existing reference values when testing routinely trained muscles. Thus, the purpose of this study was to establish reference values for standardized TLME tests among women who participate in recreational RT and to determine if imbalances or asymmetries exist. Participants included 61 women aged 18-59 who engaged in RT for at least one year. Flexor, extensor, and lateral flexor TLME was isometrically assessed using standardized procedures with documented reproducibility (r ≥ 0.93). Results identified significant differences (p < .001) between mean TLME times of flexors (163 +/-106 seconds) and extensors (105 +/-57 seconds). Left (66 +/-38 seconds) and right side bridge (61 +/-33 seconds) were comparable (p=0.06). Flexor to extensor imbalances were more pronounced among RT participants when compared to previously reported general population reference values, suggesting a training effect or bias. Moreover, similar imbalances favoring the flexors are a documented risk factor for low back pain. Thus, training considerations inclusive of the extensors may benefit women who engage in RT as a means of mitigating risk. Individuals evaluating muscle performance should consider reference values that represent the population of interest.

Full text

RESEARCH NOTE
ENDURANCE TIMES OF THE THORACOLUMBAR
MUSCULATURE:REFERENCE VALUES FOR FEMALE
RECREATIONAL RESISTANCE TRAINING PARTICIPANTS
WILLIAM J. HANNEY,
1
MOREY J. KOLBER,
2
PATRICK S. PABIAN,
1
SCOTT W. CHEATHAM,
3
BRAD J. SCHOENFELD,
4
AND PAUL A. SALAMH
5
1
Department of Health Professions, University of Central Florida, Orlando, Florida;
2
Department of Physical Therapy, Nova
Southeastern University, Fort Lauderdale, Florida;
3
California State University Dominguez Hills, Carson, California;
4
Department of Health Sciences, CUNY Lehman College, Bronx, New York; and
5
Department of Orthopedic Surgery, Duke
University, Durham, North Carolina
ABSTRACT
Hanney, WJ, Kolber, MJ, Pabian, PS, Cheatham, SW, Schoen-
feld, BJ, and Salamh, PA. Endurance times of the thoracolum-
bar musculature: Reference values for female recreational
resistance training participants. J Strength Cond Res 30(2):
588–594, 2016—The assessment of thoracolumbar muscle
endurance (TLME) is common among strength and condition-
ing professionals and clinicians desiring to quantify baseline
muscle performance and determine injury risk. Reference val-
ues for such assessments are documented in the literature;
however, their utility may be of limited value due to heteroge-
neous participant selection and limited demographic reporting.
Moreover, active cohorts who engage in resistance training
(RT) may reach a ceiling effect on existing reference values
when testing routinely trained muscles. Thus, the purpose of
this study was to establish reference values for standardized
TLME tests among women who participate in recreational RT
and to determine whether imbalances or asymmetries exist.
Participants included 61 women aged 18–59 years who
engaged in RT for at least 1 year. Flexor, extensor, and lateral
flexor TLME was isometrically assessed using standardized
procedures with documented reproducibility (r$0.93). Re-
sults identified significant differences (p,0.001) between
mean TLME times of flexors (163 6106 seconds) and exten-
sors (105 657 seconds). Left (66 638 seconds) and right
side bridges (61 633 seconds) were comparable (p= 0.06).
Flexor to extensor imbalances were more pronounced among
RT participants when compared with previously reported gen-
eral population reference values, suggesting a training effect or
bias. Moreover, similar imbalances favoring the flexors are
a documented risk factor for low back pain. Thus, training
considerations inclusive of the extensors may benefit women
who engage in RT as a means of mitigating risk. Individuals
evaluating muscle performance should consider reference val-
ues that represent the population of interest.
KEY WORDS lumbar spine, ratio, trunk endurance, weight
training
INTRODUCTION
An estimated 50 million people in the United States
participate in resistance training (RT) (1,15) as
a means of achieving improved health, perfor-
mance, and fitness attributes (3). Measures of fit-
ness and performance are often documented as a means of
establishing baseline attributes, as well as documenting
change, and in some cases for the recognition of injury risk.
Test selection should be comparable with an individual’s
level of function to avoid a ceiling effect. A ceiling effect
could be observed in cases where a particular test does not
possess a higher level of difficulty, and therefore cannot dis-
criminate baseline performance levels between individuals or
groups with varying performance levels (13,14). Moreover,
a ceiling effect may be present when an individual’s baseline
performance score is ranked (based on ordinal criteria) at the
highest performance level (using predetermined normative
values) of a test, despite room for improvement. Thus, ceil-
ing effects may occur as a result of a test that fails to ade-
quately challenge a specific skill set or a test that has
a ranking system inadequate for capturing higher perfor-
mance levels.
Individuals who participate in routine exercise or RT may
reach a ceiling effect on standardized tests (18) when using
reference values established by participants representing the
general population. Moreover, reference values of a particular
cohort should be based on a similar representation of the
population if an individual’s baseline muscle performance or
injury risk is being determined.
Address correspondence to Morey J. Kolber, kolber@nova.edu.
30(2)/588–594
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Trunk endurance tests are commonly performed as
a means of assessing baseline fitness attributes in the healthy
population, as well as change in response to a training
routine. In addition, testing may be performed to identify risk
for conditions such as low back pain (LBP). Although
normative values for trunk endurance times have been
reported, they are often based on a heterogeneous general
population, which may not represent those who are actively
participating in an RTor exercise regimen (9,12). Moreover,
most studies that have established reference values do not
provide necessary demographic detail beyond age, gender,
and in some cases body morphology. Procedures used to
evaluate trunk muscle endurance vary based on start position-
ing and difficulty levels. Ito et al. (9) proposed 3 requirements
for evaluating trunk muscle endurance: (1) the procedure
should be easy to perform without the need for special equip-
ment; (2) the procedure should be safe for the individual to
perform and should not exacerbate existing LBP; and (3) the
method used must have high reproducibility.
McGill et al. (12) have described 3 specific clinical tests for
thoracolumbar muscle endurance (TLME) that can be used
among strength and conditioning professionals and clini-
cians. The tests, which require no special equipment, were
found to be reproducible with reliability coefficients of
greater than or equal to 0.93. Using these tests among
a healthy cohort recruited from the general population,
(12) the authors established reference values and ratios.
Although these reference values may be used by strength
and conditioning professionals and clinicians to evaluate
baseline muscle performance and in some cases injury risk,
their utility may be of limited value in an active cohort who
routinely engages in RT. Specifically, individuals who engage
in RTmay have a training bias that lends to reference values
or muscle imbalances that are not comparable with a hetero-
geneous cohort.
To date, normative TLME times have been established for
the young healthy female population (6,12) and the young
athletic female population (7), but not for women who reg-
ularly participate in RT. Although the aforementioned
TLME study provided reference values (12) and represented
75 men and women with a mean age of 23 years, no descrip-
tion was given of the subjects’ current fitness level or partic-
ipation in exercise activities. Chen et al. (6) reported
normative trunk muscle endurance times among 28 healthy
women from the general population as well with a compara-
ble mean age of 23.8 62.4 years. While participants were
reportedly sedentary, a sample size of 28 is insufficient to use
for a reference standard.
The United States Center for Disease Control and Pre-
vention reported that the number of women participating in
RT increased significantly from the years 1998 to 2004 (17.7–
19.6%) (4). From a muscle performance perspective, one
cannot be certain that existing TLME reference values
would represent this population. Moreover, evidence sug-
gests that muscle imbalances are a risk factor for LBP; there-
fore it is important to have reference values that represent
the population of interest (2,9,11).
The purpose of this study was to identify TLME reference
values of women who routinely engage in recreational RT
using previously reported testing procedures (12) with docu-
mented reliability. We chose these particular tests, as they
possess portability and can be used across the disciplines
without special equipment. In addition, we sought to evalu-
ate TLME ratios and symmetry to determine the presence
of imbalances. Specifically, we evaluated imbalances that
have previously been associated with and predictive of
LBP. Finally, we sought to compare the reference values in
our investigation with previously reported reference values
from a study using a cohort from the general population.
Individuals who routinely assess TLME or who prescribe
exercises should be cognizant of population-specific refer-
ence values when identifying baseline fitness levels or
determining risk for injury. The value of the results from
this investigation resides in exercise prescription, as goals are
often derived from baseline measurements. Moreover, im-
pairments of TLME are often determined by existing
reference values. Identification of reference values specific to
RT participants would provide a more accurate representation
of the population of interest to determine baseline perfor-
mance. Finally, impairments or imbalances once identified are
often amenable to changes in exercise programming. Thus,
the interpretation of a test must possess a level of scrutiny,
which is dependent on accurate reference values.
METHODS
Experimental Approach to the Problem
Trunk endurance tests are routinely used in the fitness and
rehabilitation settings as a means of quantifying baseline data
and in some cases profiling ones risk for injury. The utility of
baseline data is typically recognized based on a comparison
with population-specific normative data. In the case of trunk
endurance tests, normative values do not exist in women
who participate in recreational RT.
This study was a descriptive investigation of TLME times
among women who participate in recreational RT. The
specific variables analyzed included the flexor, side bridge,
and extensor endurance tests. These tests have been the
subject of previous investigations using female athletes and
representatives from the general population. Moreover, the
tests we are investigating have established reliability, porta-
bility, and provide information specific to trunk muscle
endurance. The current research design will provide infor-
mation specific to the study purpose of collecting reference
values for women who participate in recreational RT.
Subjects
Sixty-one adult women who were currently participating in
recreational RT were recruited from local university and
fitness facilities (age, 27.6 69.9 years; body mass, 61.5 6
8.6 kg; height, 163.9 66.7 cm). Demographics are presented
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in Table 1. Inclusion criteria included women aged 18–59
years who participated in recreational RT for 1 or more years
at least 2 times per week. Before the study, participants were
screened by using the Physical Activity Readiness Question-
naire (PAR-Q) & YOU questionnaire. Any positive responses
on the PAR-Q & YOU questionnaire would preclude partic-
ipation in the study due to medical concerns. Additional
exclusion criteria included prior surgery to the lumbar spine
and insufficient English language skill to complete all ques-
tionnaires. In addition, individuals who had not consistently
trained during the past 4 weeks were excluded. Finally, we
sought to assess individuals who participated in recreational
RT; thus, individuals who participated at a competitive level
or who were training for bodybuilding, weightlifting, or
powerlifting competition were excluded as a means of mak-
ing our population more homogeneous. After explaining the
purpose of the study to participants, a University of Central
Florida Institutional Review Board approved consent form
was provided and signed before participation.
Procedures
Each participant completed 3 previously established TLME
tests following the same protocol used by McGill et al. (12).
Before the start of each test, participants were explained the
specific testing procedures as outlined below and advised
they would be requested to hold the position for as long
as possible. In addition, participants were required to
demonstrate the testing position for a brief moment of ,5
seconds to ensure they could achieve the necessary testing
requirements before the actual test. The tests were per-
formed in the following order: flexors, side bridge (per-
formed on both the left and right lateral flexors), and the
extensors. Participants were not provided with any verbal
inducements and results were not shared with participants
until test completion. Testing was performed in an air-
conditioned laboratory setting and all testing was conducted
between the hours of 8 AM and 4 PM. Times were recorded
with a stopwatch (Test Mark Industries, East Palestine, OH,
USA) and rounded to the nearest second.
The Flexor Endurance Test. The participant sat on a padded
test bench, with upper-body resting against a support set at
a608angle from the bench (Figure 1). The hips and knees
were flexed to a 908angle with the feet resting flat on the
bench. A belt was strapped across the participant’s feet for
support during the test, and the participant’s arms were
folded across her chest with hands resting on opposite
Figure 3. The extensor endurance test.
Figure 2. The side bridge test.
Figure 1. The flexor endurance test.
TABLE 1. Demographic characteristics (n= 61).
Minimum Maximum Mean 6SD
Age (y) 18 54 27.6 69.9
Height (cm) 152.4 182.9 163.9 66.7
Body mass
(kg)
45.4 86.2 61.5 68.6
Endurance Times of the Thoracolumbar Musculature
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shoulders. The participant was instructed to maintain this
position throughout the test. The test began when the
upper-body support was pulled 10 cm back from the partic-
ipant. The test ended when the participant’s upper body fell
below the 608angle, allowing the upper body to come into
contact with the support.
The Side Bridge Test. This test required the participant to be in
a side-lying position with her body in a straight line. The
lower foot was positioned behind the top foot for support.
The participant was directed to elevate the hips above the
mat to uphold a straight line along the entire body length
and to hold the position on 1 elbow and both feet (Figure 2).
The top arm was placed along the chest with hand located
on the opposing shoulder. The test was terminated when the
hips were no longer able to be held in line with the trunk.
The Extensor Endurance Test.
The participant was then asked
to lie prone with her lower
body secured at the ankles,
knees, and hips, while the
upper body was extended in
a cantilevered position over the
border of a test bench that
permitted height adjustment.
The approximate height of
the test bench surface was
25 cm above the brace support.
The participant supported
their upper body on the brace
support before exertion. At the
onset of exertion, the partici-
pant’s arms were placed across
her chest with her hands rest-
ing on the opposing shoulders
and the torso was elevated
until it was parallel with the
floor (Figure 3). The partici-
pant was directed to continue
this position for as long as pos-
sible. A stopwatch was used to
record the endurance time to
the nearest second from the point the torso was parallel until
they came into contact with the brace support.
Statistical Analyses
Data were analyzed with Statistical Package for the Social
Sciences (SPSS; IBM Software, Armonk, NY, USA) version
22 for Windows. Mean values and the associated SDs were
calculated for demographic and training characteristics of
participants. The mean and SD of each timed endurance
test was also calculated. In addition, ratios of each mean
endurance time were normalized to the mean extensor
endurance time and computed to allow comparison across
studies. A 1-way analysis of variance (ANOVA) (a=0.05)
was performed to assess differences of the timed scores
between the flexor and extensor tests and between the left
and right side bridge tests. An ANOVA was used as the test
TABLE 2. Comparison of results (mean values and ratios of endurance times) of this study with those of McGill et al.
Test
Current study,
mean 6SD (s) Ratio*
McGill et al.,
mean 6SD (s) Ratio*
Extensor 105 657 1.00 189 660 1.00
Flexor 163 6106 1.56 149 699 0.79
Left side bridge 66 638 0.63 72 631 0.38
Right side bridge 61 633 0.58 77 635 0.40
*Ratios normalized to the extensor test.
Figure 4. Mean endurance times for each of the tests (n= 61).
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is more robust than a t-test and has the ability to absorb
variance.
RESULTS
Demographic characteristics of the participants are pre-
sented in Table 1. All subjects reported participating in RT at
least 2 times per week for recreational purposes. The mean
endurance times with their corresponding SDs are presented
in Figure 4. Although the mean flexor endurance time was
the highest of all the mean endurance times, it also had the
largest variance (163 6106 seconds). The mean extensor
endurance time was the next highest (105 657 seconds),
followed by the side bridge tests (L = 66 638 seconds and
right = 61 633 seconds). Table 2 presents a comparison of
flexor endurance times between this study and previously
reported values using the general population (12). The mean
extensor and both side bridge times were shorter than those
reported in the aforementioned study of participants from
the general population (extensor: 105 657 seconds vs.
189 660 seconds (12); left side bridge: 66 638 seconds
vs. 72 631 seconds (12); right side bridge: 61 633 seconds
vs. 77 635 seconds (12)). The flexor to extensor ratio in this
study was (1.56), whereas the flexor to extensor ratio in the
study by McGill et al. (12) using a general population cohort
was (0.79). Table 3 illustrates that although no significant
between-side difference was observed in the side bridge tests
(p= 0.06), a significant difference was apparent between the
flexor and extensor tests (p,0.001).
DISCUSSION
The results of this study show that women who engage in
RT have greater endurance times in their trunk flexor
muscles compared with the trunk extensors and lateral
flexors when using the TLME tests in this investigation.
The time differences may suggest a training effect or bias.
This result is in contrast to the reference values reported in
the literature (12) among healthy nonathletic individuals
(Table 2), which showed that extensor endurance time was
the highest among the 4 muscle groups tested. A possible
explanation for this finding is the difference between partic-
ipant populations of the 2 studies. McGill et al. (12) recruited
participants with an undefined activity level from a general
population, whereas this study recruited recreational RT par-
ticipants to provide a more homogeneous sample. Partici-
pants in the study referenced in Table 2 were not defined as
being athletes or individuals who routinely exercise.
Similar findings to this study favoring longer flexor vs.
extensor endurance times have been reported for elite
athletic females (7), and intercollegiate male rowers (5).
One study, using a fairly small sample size of sedentary
women (6), identified a similar imbalance to the current
investigation, favoring longer flexor endurance hold times,
contrasting the findings of McGill et al. (12). An explanation
for this contrast, and a premise of this study, resides in the
failure of the previous studies to make an effort to define
activity levels. Specifically, having a mixed group of seden-
tary vs. RT individuals would potentially produce a consider-
able variance. Furthermore, this study found a statistically
significant difference (p,0.001) between the flexor and
extensor endurance times using a defined population, as
did investigators of a study using intercollegiate male rowers
(p= 0.001) (5). When comparing the results of our study of
recreational RT women to elite athletes (women) from 6
different sports (7) the flexor endurance times were 163
and 222 seconds, respectively. Regarding the extensor
endurance time, this study identified a mean hold time of
105 vs. 167 seconds among women defined as elite athletes
(7). Based on reviewing our results and previous data, it
seems that women who are elite athletes have a greater
imbalance of flexors vs. extensors when compared with
women engaged in recreational RT women. Furthermore,
it seems that this imbalance is not necessarily seen among
healthy women who do not have a defined participation
level in RTor athletics, which may be suggestive of a possible
training bias. This assumption of a training bias alone has
shortcomings as sedentary individuals with LBP have been
identified as having imbalances comparable with those re-
ported among athletes and individuals participating in rec-
reational RT. Moreover, the 2 studies referenced using
nonathletic individuals from the general population have
fairly small sample sizes with wide variances.
Previous researchers have identified an association
between flexor to extensor trunk endurance and LBP, with
results identifying weakness of the extensors compared with
flexors, as a factor associated with LBP (2,9,11). In a 5-year
prospective study investigating trunk endurance ratios, re-
searchers reported that lower extensor to flexor strength
was a risk factor for LBP (11). Other authors as well have
reported an association between the aforementioned imbal-
ance in studies comparing individuals with and without LBP
using isometric TLME tests (2,9).
Age does not seem to be a factor in differences of flexor
and extensor endurance times as compared with activity
level and training characteristics. The mean age for the
TABLE 3. pvalues for differences between
muscle groups.*
Left side bridge
(mean 6SD)
Right side bridge
(mean 6SD)
One-way
ANOVA
66 638 61 633 p= 0.06
Extensor
(mean 6SD)
Flexor
(mean 6SD)
One-way
ANOVA
105 657 163 6106 p,0.001
*ANOVA = analysis of variance.
Endurance Times of the Thoracolumbar Musculature
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populations in the studies by Chen et al. (6), Evans et al. (7),
and Chan (5) are similar to that of the population in McGill’s
study 23.8 62.4, 21.2 62.3, and 20.4 61.16, respectively.
Thus, age may not be as important of a factor that accounts
for differences in flexor and extensor endurance times as
activity level and training characteristics are. One consider-
ation may be that abdominal training is routinely a compo-
nent of fitness training. As such, 94% of the women recruited
in this study participated in RTactivities that directly empha-
sized abdominal musculature engagement, which would
promote trunk flexor rather than trunk extensor muscle
endurance. Unfortunately, a weakness of our study regarding
its prescriptive utility resides in our failure to capture details
on training of extensors and lateral flexors. Although it
would not change the results or prescriptive applications, it
would help to further understand potential training bias. A
second explanation for the higher flexor endurance time is
the possibility for compensatory mechanisms to be used on
this test compared with the extensor test. Based on the es-
tablished flexor test procedure, the test ends when the trunk
of the subject falls below a 608angle. During testing, it was
commonly observed by the research associates that the par-
ticipants would further flex their lower lumbar spine in effort
to maintain the testing position. This could result in 2 pos-
sible compensatory mechanisms: greater activation of the
psoas muscle with or without the external and internal ob-
lique muscles. By flexing the trunk an additional amount,
the abdominal muscles were further shortened, potentially
decreasing the mechanical advantage of this muscle group.
Thus, to maintain the flexed position, greater activation of
the primary hip flexor (psoas) was necessary to provide
stabilization. Reliance on the psoas muscle in addition to
the abdominal muscles could have resulted in an increased
flexor endurance time for the flexor test. Chan (5) observed
similar occurrences when conducting the flexor test and
proposed that in addition to further flexing the trunk, par-
ticipants may also have rotated the trunk in the transverse
plane. This would allow for greater activation of the oblique
muscles, with time-sharing between left and right sides,
resulting in a prolonged flexor endurance time. However,
these assumptions were not borne out in the aforemen-
tioned study (5) or this study and further research is nec-
essary. The authors reasoned that the amount of variance
with the flexor and extensor tests is related to the amount
of compensatory movements possible. This would explain
why a greater variance is seen with the flexor test compared
with the extensor test, which has been reported in similar
studies (6,7,12).
One additional factor to consider, based on the current
and previous investigations, is the omission of practice trials
for the specified tests (5–7,9,12). In the current investigation,
participants were asked to assume the testing position briefly
to determine their ability to perform the test, ensure appro-
priate form, and provide a degree of familiarization. Never-
theless, the lack of a practice session or trials may explain the
variance seen in this and previous studies. Incorporating
a training session before actual data collection may have
produced higher scores secondary to a learning effect. How-
ever, we did not include a learning trial, as we sought to
determine reference values using a procedure that would
be consistent with both rehabilitation and many standard
fitness assessments.
The design of the test itself could also be considered as
a possible reason why the flexor endurance time is the
greatest, with the greatest amount of variance. The trunk
flexor endurance test used in this study required the subjects
to maintain 608of trunk flexion throughout the test. Chen
et al. (6) conducted a study comparing 3 different flexor tests;
1 test was performed at 608of trunk flexion, another at 458,
and the last at a curl-up position. Both the 458and curl-up
tests were found to have shorter mean endurance times and
less variance than the 608test. The authors of the aforemen-
tioned study reasoned that the length-tension relationship
between the abdominal muscles is different for each test.
The rectus abdominis and abdominal oblique muscles expe-
rience greater lengthening in the 458position than the 608
position. Because of the increased length of the abdominal
muscles during the 458position, more force is required to
maintain the testing position. Thus, the abdominal muscles
fatigue more rapidly during the 458test, resulting in a shorter
endurance time. The curl-up position is unique in that its
fulcrum point is at T11-12, and therefore, mainly recruits
the rectus abdominis muscle alone during the endurance
test. Although these features of the curl-up test are very
different from those of the 458test, there was no significant
difference found between the endurance times of these 2
tests. Some researchers have recommended using either
the 458test or the curl-up test to measure trunk flexor endur-
ance because they are more time efficient with less variable
results than the 608test (6).
The curl-up test referenced and used in the study by Chen
et al. (6) was first proposed by Ito et al. (9) as an inexpensive
endurance test for the trunk musculature that could easily be
performed in a clinic setting. Ito et al. (9) conducted the test
on 90 healthy men and women with a mean age of 46.2
years, and 100 men and women with chronic LBP with
a mean age of 45.3 years. The purpose of the study was to
determine whether a curl-up endurance test would minimize
the amount of lumbar lordosis experienced by the partici-
pants compared with a bent-knee endurance test similar in
design to the 45 and 608tests previously mentioned by other
investigators (6) and this study. To complete the curl-up test,
participants were asked to lie supine and raise their lower
extremities with 908of hip and knee flexion. The test began
when the participants lifted the scapulae off the ground and
ended when they touched the ground. Lateral radiographs
were taken to assess the degree of lumbar lordosis present
during the curl-up test and bent-knee test. The results
showed that lumbar lordosis was significantly less during
the curl-up test compared with the bent-knee test. Other
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studies have also verified that a flexor endurance test with
a curl-up design not only provides a more neutral position
for the lumbar spine than a bent-knee design but is also
a better option for training the rectus abdominis (8,10). It
was found that there is a greater duration of rectus abdom-
inis activity, as well as greater rectus abdominis activity than
psoas activity during a curl-up test compared with a bent-
knee test (8). When considering these findings, a future study
of recreationally active females should include a curl-up test,
458bent-knee test, and 608bent-knee test to determine the
most reliable trunk flexor endurance value. In addition, since
recreationally RT participation among women is increasing
(4) and these individuals are at a higher risk for low back
injuries (3,16,17), the curl-up test is a potentially safer alter-
native for clinically testing trunk flexor endurance.
No significant differences were noted between the left and
right side bridge endurance times of this study, which was
consistent with previous investigations (5–7,12). A trend
worth noting among these tests was that the left side bridge
mean time was greater in the studies conducted by McGill
et al. (12), Chen et al. (6), Evans et al. (7), and this study.
Although these studies contain populations of various age
and activity level, this trend emerges and could provide for
future research to explain this phenomenon.
PRACTICAL APPLICATIONS
In this study, the trunk flexor endurance time was signifi-
cantly greater than trunk extensor endurance times implying
an imbalance in muscle performance among RT participants.
The ratio of flexor to extensor endurance in our study
identified an imbalance, suggesting a training effect that is
biased toward trunk flexors (abdominals). The importance of
the imbalance between the flexors and extensors lies in the
documented risk for LBP. Although we cannot state with
certainly that risk factors for LBP identified in the general
population would carry over into RT participants; it does not
seem unreasonable to recognize the potential for risk as
established in the literature. Given the risk for LBP among
individuals with flexor to extensor imbalances and the
findings of this study, individuals who participate in RT
should consider efforts to train their thoracolumbar exten-
sors to a level comparable with the flexors. Efforts to train
the extensor groups may potentially serve to mitigate the
risk profile for LBP among women who participate in RT;
however, one must recognize the inherent limitations in
generalizing risk profile from the general population to
active RT participants. Future prospective research to
determine if indeed such imbalances are predictive of LBP
among women who participate in RT as they do among the
general population would be of value.
ACKNOWLEDGMENTS
No funding was received for this investigation.
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