INTRODUCTION:
Low back
pain (LBP) is thought to affect approximately 80% of all adults
at some point in their life. Exercise programs have been
recommended as one of the more effective treatments for LBP, and
trunk strength is an important component of those programs.
Advocates of exercise programs feel that trunk strength is an
important component for both the treatment of LBP and the
prevention of injury to the back. Because the strength of the
abdominal muscles as well as the back and hip extensor muscles is
important in protecting against back injury, these muscles should
be the target of a back exercise program.
PURPOSE:
In order to
address the needs of clinicians treating low back pain, Fitness
Plus, Inc. has developed a series of three exercise machines that
were designed to strengthen the back and abdominal musculature.
According to the manufacturer, the Low Back Unit targets the
erector spinae, gluteus maximus and biceps femoris muscles. The
Abdominal Unit targets the rectus abdominus, as well as the
internal obliques and external obliques, and the Rotary Torso
Unit trains the internal and external obliques, the erector
spinae and rectus abdominus. Although these claims have been made
by the manufacturer, there are no studies available to
substantiate these claims. Therefore, the purpose of this study
was to describe the muscle activity that occurs during exercise
on these machines.
METHODS:
Subjects. Fourteen,
healthy, male subjects (20 to 40 yrs of age) volunteered to
participate in this study. Instrumentation. Surface electromyography (sEMG) was
used to assess muscle activity in the back, abdominal and hip
musculature during exercise. For the Low Back Unit, the rectus
abdominus (RA), gluteus maximus (GM), biceps femoris (BF) and
erector spinae (ES) were monitored. For the Abdominal and Rotary
Torso Units, the muscles studied were the RA, ES, right and left
external obliques (REO, LEO), and the right and left internal
obliques (RIO, LIO). A Penny & Giles
electrogoniometer was used to measure the trunk ROM. The sEMG and
goniometric data were telemetried to a Noraxon Telemyo8 receiver
and digitized by a NET 486DX computer. The data was stored and
then later analyzed using the Norquest data collection software
that is included with the system. Procedure. Exercise on the Low Back Unit
consisted of each subject completing three sets of three
repetitions of back extension while lifting 5, 25 and 50 lbs.
Exercise on the Abdominal and Rotary Torso Units consisted of
each subject performing three repetitions lifting 25 lbs and
three repetitions with maximal resistance (55 to 75 lbs). Data Analysis.
To normalize the data, the sEMG
data for each of the muscles was expressed as a percentage of
that muscle's maximal voluntary contraction (MVC). Descriptive
statistics were used to analyze both the muscle activity and
joint ROM that occurred with each of the exercises.
RESULTS:
Low Back Unit: T
he
results showed that exercise on the Low Back Unit recruited the
Erector Spinae, Gluteus Maximus and Biceps Femoris muscles
(Figure 1). The muscle activity in all three of the muscles
increased with increases in resistance. As was expected, the RA
muscle showed very little activity during the exercise. The
highest level of muscle activity in the ES, GM nd BF was elicited
during the concentric (i.e. trunk extension) phase of the
exercise.
Figure 1. EMG activity during exercise on the Low Back Unit.
Abdominal Unit:
The
results showed that exercise on the Abdominal Unit recruited the
Right and Left External Obliques, the Right and Left Internal
Obliques, and the Rectus Abdominus (Figure 2). The muscle
activity increased with increases in resistance. There was
minimal activity in the Erector Spinae. The highest levels of
muscle activity occurred during the concentric (i.e. trunk
flexion) phase of the exercise.
Figure 2. EMG activity during exercise on the Abdominal Unit.
Rotary Torso Unit:
The
results showed that exercise on the Rotary Torso Unit elicited
the highest level of activity in the Right and Left External
Obliques and the Right and Left Internal Obliques (Figure 3).
There was moderate activity in the Erector Spinae muscles and
minimal activity in the Rectus Abdominus. In all cases, the
muscle activity increased with increases in resistance.
Figure 3. EMG activity during exercise on the Rotary Torso Unit.
DISCUSSION:
Overall, the
results of this study showed that each of these machines
recruited the muscles that the manufacturer claimed were active
during the exercise. During extension on the Low Back Unit, it
appears that the ES are recruited to extend the spine, whereas
the GM and BF are working to posteriorly rotate the pelvis. Both
the GM and BF were more active with increasing angles of trunk
flexion, this was an expected finding based on other studies of
trunk and pelvic musculature.
As was expected, the Abdominal Unit recruited the RA and Oblique musculature; all of which are prime movers for trunk flexion. The Abdominal Unit is designed so that the subject only lifts the weight during trunk flexion. During trunk extension, the weights push the pad upward and the subject is not required to lift the bar, therefore the trunk extensors (i.e. ES) are not needed and were only minimally active during the exercise.
The Rotary Torso Unit appeared to selectively recruit the Oblique musculature. This again was an expected finding since they are the prime movers for trunk rotation. The REO and LIO were most active during rotation to the left and the LEO and RIO were the most active during rotation to the right. This synergistic action has been well described in other research studies. The RA was relatively inactive. This may have been due to the fact that the arms are hooked over the arm pads, and as such do not allow the trunk to go into flexion during the rotation movements. However, the ES muscles were moderately active indicating that the subjects extended their trunks against the back pad. This action may have helped stabilize the trunk during the exercise.
CLINICAL IMPLICATIONS:
The
Low Back Unit would be recommended for back patients who need to
strengthen their ES, GM and BF. However, because of the
potentially high level of ES activity combined with trunk
flexion, clinicians supervising patients with a history of a
herniated disc should caution the patient to avoid excessive
resistance combined with lumbar flexion which could put the
patient at risk for increased intervertebral disc pressure.
Overall, the combined use of these three machines provides
recruitment of all the major trunk muscles and therefore should
increase muscle strength. The increase in trunk muscle strength
should offer protection to the lumbar spine during functional
activities.