Endurance Times for Low Back Stabilization Exercises

941
Endurance Times for Low Back Stabilization Exercises: Clinical
Targets for Testing and Training From a Normal Database
Stuart M. McGill, PhD, Aaron Childs, BSc, Craig Liebenson, DC
ABSTRACT. McGill SM, Childs A, Liebenson C. Endurance times for low back stabilization exercises: clinical targets
for testing and training from a normal database. Arch Phys Med
Rehabil 1999;80:941-4.
Objective: To establish isometric endurance holding times,
as well as ratios between torso extensors, flexors, and lateral
flexors (stabilizers), for clinical assessment and rehabilitation
targets.
Design: Simple measurement of endurance times in four
tests performed in random order by a healthy cohort. To
measure reliability, a subsample also performed the tests again
8 weeks later.
Setting: University laboratory.
Participants: Seventy-five young healthy subjects (31 men,
44 women).
Results: Women had longer endurance times than men for
torso extension, but not for torso flexion or for the "side
bridge" exercise, which challenges the lateral flexors (stabilizers). Men could sustain the "side bridge" for 65% of their
extensor time and 99% of their flexion time, whereas women
could sustain the "side bridge" for only 39% of their extensor
time and 79% of their flexion time. The tests proved reliable,
with reliability coefficients of >.97 for the repeated tests on 5
consecutive days and again 8 weeks later.
Conclusion: Healthy young men and women possess different endurance profiles for the spine stabilizing musculature.
Given the growing support for quantification of endurance,
these data of endurance times and their ratios between extensor,
flexor, and lateral flexor groups in healthy normal subjects are
useful for patient evaluation and for providing clinical training
targets.
© 1999 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and
Rehabilitation
r ~ H E R E HAVE BEEN MANY recent research developments
.1[ related to torso stabilization,-1 4 endurance,,56 and motor
control training 7,8 for rehabilitation of the low back. For several
years, our group has been working to quantify the stability of
the low back in a variety of occupational and rehabilitation
tasks, identify the structures that are best able to stabilize the
back, and identify exercises to enhance these stabilizers.
From the Spine Biomechanics Laboratory, Faculty of Applied Health Sciences,
Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada (Dr.
McGill, Mr. Childs); and the Los Angeles College of Chiropractic, Whittier, CA (Dr.
Liebenson).
Submitted for publication July 1, 1998. Accepted in revised form December 30,
1998.
Supported by the Natural Science and Engineering Research Council, Ottawa,
Canada.
No commercial party having a direct financial interest in the results of the research
supporting this article has or will confer a benefit upon the authors or upon any
organization with which the authors are associated.
Reprint requests to Smart M. McGill, PhD, Department of Kinesiology, University
of Waterloo, Waterloo, Ontario, Canada N2L 3GI.
© 1999 by the American Congress of Rehabilitation Medicine and the American
Academy of Physical Medicine and Rehabilitation
0003-9993/99/8008-510453.00/0
Several research questions have been systematically addressed to meet these objectives. For example, Cholewicki and
McGill 9 (and others such as Stokes and Gardner-Morse 1°)
identified several muscles that have the potential to assist in
stability. Juker and colleagues ll implanted intramuscular electrodes in the deep musculature of the torso to identify which
muscular components were recruited when stability was needed.
In their study, subjects stood upright with their arms at their
sides and loads were progressively applied to their hands,
producing spine compression in the absence of major torque
demands. This paradigm was motivated by the pioneering work
of Lucas and Bresler, 12 who found that the lumbar spine, under
pure compression, will buckle or become unstable at very low
levels of approximately 90 Newtons (or 20 pounds). Monitoring the deep muscles together with the surface musculature
demonstrated that the quadratus lumborum was the muscle
most favorably activated by the motor control system to provide
stability in this task, 13 as well as in other tasks that required
substantial flexor and extensor moment development. Although
the abdominals are important stabilizers, the quadratus was
more active in this upright posture, in the absence of bending
moments but when the risk of buckling from compression was
high. This observation, coupled with the engineering stability
analysis of Cholewicki and McGill, 9 demonstrated that the
quadratus lumborum was architecturally best suited to be the
major stabilizer of the lumbar spine. In our opinion, this
combination of data proved that the quadratus lumborum is
preferentially recruited to maintain sufficient stability in the
spine.
The next clinical question was to find the preferred method to
challenge the quadratus and enhance its stabilizing role, but in a
way that minimized loading on the spine. After assessing
muscle recruitment patterns and spine loading that resulted
from a variety of exercises, 11A3 the "side support" or "side
bridge" exercise was identified as optimizing the challenge to
the quadratus lumborum while minimizing load on the lumbar
spine. This exercise has now been incorporated into several
exercise programs throughout the world, including the American College of Sports Medicine textbook. 14 As it has gained
exposure, clinicians have been requesting guidelines for duration times and endurance relationships between different sets of
torso muscles. Thus, normal values for flexor, extensor, and
lateral flexion (and stabilizing) endurance times, and their ratios
to each another, are needed to guide clinicians in determining
training targets to use when assessing endurance in low back
rehabilitation patients.
The purpose of this study was to collect isometric endurance
times from a healthy normal population performing the "side
bridge" or lateral flexion exercise, as well as isometric flexion
and isometric extension exercises. These endurance times can
be used to form "normal" relative ratios to guide clinicians and
identify endurance deficits within specific patients.
METHOD
Seventy-five subjects were selected from a university community, 31 men and 44 women with a mean age of 23 years
Arch Phys Med Rehabil Vol 80, August 1999
942
ENDURANCE TIMES FOR LOW BACK EXERCISES, McGill
(standard deviation [SD], 2.9yrs). Given the objective of
obtaining a healthy population, a health status questionnaire
was used for screening to exclude individuals who were not in
good health, particularly those who had experienced disabling
low back pain. Additional data collected included measures of
height, weight, activity level, and dominant hand (post hoc
analysis revealed that not enough subjects were left-handed to
allow further analysis). Five subjects were selected to perform
reliability tests; they performed the tests at the initial session,
then, after 8 weeks, repeated the tests daily for 5 consecutive
days. Each new subject was familiarized with the tests. Subjects
performed the following four endurance tests, with the order
randomized among the subjects: the isometric extensor exercise, the isometric flexor exercise, and the "side bridge"
exercise on both right and left sides. A minimum of 5 minutes
was provided between efforts to facilitate recovery.
The extensor endurance test (fig 1) was modified from the
Biering-Sorensen test, 15 which has been shown to be consistently reliable as a measure of back extensor endurance) 6
Subjects laid prone with the lower body fixed to the test bed at
the ankles, knees, and hips and the upper body extended in a
cantelevered fashion over the edge of the test bench. The test
bench surface was approximately 25cm above the surface of the
floor. Subjects rested their upper bodies on the floor before the
exertion. At the beginning of the exertion the upper limbs were
held across the chest with the hands resting on the opposite
shoulders, and the upper body was lifted off the floor until the
upper torso was horizontal to the floor. Subjects were instructed
to maintain the horizontal position as long as possible. The
endurance time was manually recorded in seconds with a
stopwatch from the point at which the subject assumed the
horizontal position until the upper body came in contact with
the floor.
The flexor endurance test (fig 2) required subjects to sit on
the test bench and place the upper body against a support with
an angle of 60 ° from the test bed. Both the knees and hips were
flexed to 90 ° . The arms were folded across the chest with the
hands placed on the opposite shoulder and toes were placed
under toe straps. Subjects were instructed to maintain the body
position while the supporting wedge was pulled back 10cm to
begin the test. The test ended when the upper body fell below
the 60 ° angle.
The side bridge test (fig 3) consisted of subjects laying on an
exercise mat (thickness, 2.5cm) on their sides with legs
extended. The top foot was placed in front of the lower foot on
the mat for support. Subjects were instructed to support
themselves lifting their hips off the mat to maintain a straight
line over their full body length, and support themselves on one
elbow and their feet. The uninvolved arm was held across the
Fig 1. Subject secured into the test bench performing an isometric
extensor exercise.
Arch Phys Med Rehabil Vol 80, August 1999
Fig 2. Subject performing an isometric flexion exercise in which a
block is pulled back from the subject to assist with controlling
posture. {A) Subject seated against block at start of test. (B) The
block has been pulled back.
chest with hand placed on the opposite shoulder. The test ended
when the hips returned to the exercise mat.
During all tests, subjects were reminded to maintain the
position as long as possible. Only the subject and evaluator
were present in the testing room. Subjects were not provided
with any clues to their scores until the conclusion of the test.
Statistical analysis consisted of the calculation of reliability
coefficients on a subset of five subjects for repeated tests both
daily over 5 consecutive days and again over an 8-week period.
While t tests were conducted to assess effects of hand dominance, it happened by chance that only four subjects were
left-handed, invalidating the implications. Student t tests
(p < .05) assessed differences between pairs of scores for
similar exercises between the genders and between exercises
within each gender. (An analysis of variance was not used
Fig 3. Subject performing a side bridge exercise.
ENDURANCE TIMES FOR LOW BACK EXERCISES, McGill
943
Table 1: Mean Endurance Times (sec) With Standard Deviations and Ratios Normalized to the Extensor Exercise
Men
All
Women
Task
~
SD
Ratio
Y
SD
Ratio
~
SD
Ratio
Extensor
Flexor
Side bridge, right
Side bridge, le~
146
144
94
97
51
76
34
35
1.0
.99
.64
.66
189
149
72
77
60
99
31
35
1.0
.79
.38
.40
171
147
81
85
60
90
34
36
1.0
.86
.47
0.5
because the issues addressed in this work required comparison
of only specific cells.)
RESULTS
Mean endurance times for the exercises and ratios of
endurance times between exercises are listed in table 1. The
ratios of endurance times were normalized to the extensor-hold
exercise because subjects were able to hold their position the
longest during this exercise. There was no significant difference
in endurance times between the side bridge performed on the
left side and the side bridge performed on the right side. Women
had longer endurance times than men for the extensor exercise
(t test, p = .0018), but not the flexor exercise; men had more
endurance in the side bridge exercise (p < .0001). The ratios
for clinical use showed that men could maintain the side bridge
exercise for a period that was 65% of their extensor time
(p < .0001), whereas women could maintain the side bridge for
approximately 39 % of their extensor time ( p < .0001).
The reliability study with the five subjects found that the
repeated tests, on 5 consecutive days, produced excellent
reliability coefficients of .98 for the extensor exercise, .97 for
the flexor exercise, and .99 for the side bridge on the left and
fight sides. The reliability remained excellent over a period of 8
weeks. When all test scores, including the 8-week 5-day
repeated sessions, were added to assess reliability, the reliability
coefficients were .99 for the extensor exercise, .93 for the flexor
exercise, .96 for the right side bridge, and .99 for the left side
bridge.
DISCUSSION
Given the growing popularity of stabilization exercises, this
study was motivated by requests from clinicians to establish a
normal database for endurance times of the isometric exercises
identified in this study. Clinicians also requested the establishment of ratios of the endurance times of tasks relative to each
other. These ratios can be used to identify endurance deficits in
patients, thereby guiding clinical decisions to restore more
normal relative endurance between muscle groups and normal
function. The side bridge has been justified as an ideal training
exercise to challenge the quadratus lumbomm and the abdominal wall with minimal spinal loading. 13 This exercise, together
with the flexor and extensor exercises, may also be used to
evaluate endurance capabilities and ratios of endurance balance
around the torso. It appears that the side bridge qualifies as a
good test on the criteria of cost (it requires no special
equipment), safety (it has been documented to provide substantial challenge to the quadratus lumborum, approximately 50%
of maximum activation13), and reliability (excellent reliability,
as documented by the reliability coefficients calculated here).
There are few data sets to compare with the data from this
study. Alaranta and as s ociates 16 documented much lower endurance times in the isometric back endurance test for normal men
and women (97 and 87 seconds, respectively) 35 to 54 years of
age. Our subjects, however, were younger. We were unable to
find comparable data sets for the endurance ratios in our study.
There are several limitations to be considered for the
interpretation of these data. Our subjects were relatively young
and healthy. It was the objective of this study to establish what
is normal and healthy; however, the performance of older
individuals may be different. Our hope is that others will
establish data on older subjects and different classifications of
back pain.
Recent evidence is compelling to justify rehabilitation approaches that emphasize spine endurance and stabilization
exercises for the injured back. 1,3-5,17 The side bridge exercise
has been documented to challenge the quadratus lumborum and
the muscles of the abdominal wall to enhance spine stability. I~
The ratios we report provide clinicians with guidance for
normal endurance ratios between torso flexion, extension, and
lateral bending exercises. The isometric endurance exercises
performed in this study are intended for quantification of patient
status only. During training and rehabilitation, these exercises
would be held for shorter periods of time, with repeated sets.
Further work is required to identify optimal holding times and
numbers of repetitions.
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