The Journal of Strength and Conditioning Research: Vol. 13, No. 4, pp. 339–345.
Muscle Activity During Sit-Ups Using Abdominal Exercise Devices
WILLIAM C. WHITING and WILLIAM J. VINCENT
Department of Kinesiology, California State University, Northridge, California 91330-8287
STUART RUGG and ANDRE COLEMAN
Department of Kinesiology, Occidental College, Los Angeles, California 90041
ABSTRACT
The purposes of this study were to assess the activity of selected muscles used during 4 sit-up exercises with and without the assistance of abdominal exercise devices and to determine what effect, if any, the devices have on muscle activity. Nineteen young, healthy subjects completed a series of unassisted abdominal exercises (basic crunch with arms up, basic crunch with arms down, oblique crunch, and reverse crunch). The same exercises also were performed using each of 4 exercise devices. Surface electromyography was recorded from the upper and lower rectus abdominus, external oblique, rectus femoris, and sternocleidomastoid during the concentric and eccentric phases of each repetition. Repeated-measures analysis of variance analyses were used to compare mean electromyographic activity across conditions. Results showed few significant differences in abdominal muscle activation among the conditions. Some differences were noted in rectus femoris and sternocleidomastoid activity when comparing unassisted exercise and exercise using the devices. The results suggest that abdominal devices such as those tested in this study do not elicit any greater or lesser involvement of the abdominal musculature than does performing similar exercises unassisted.
Key
Words: electromyography, resistance exercise, biomechanics.
Introduction
Sit-up
exercises are used extensively to improve abdominal strength. The goals of
programs designed to strengthen the abdominal muscles include performance
enhancement, postural improvement, and lowering the risk of low back pain.
The association between poor trunk muscle strength and chronic low back
pain is well established (1,
18,
26). Weak trunk muscles and
reduced flexibility of the back and hamstrings have been identified as
risk factors in the recurrence and persistence of low back trouble (4).
Exercise programs have proven effective in remedying muscle weakness by
increasing abdominal strength (5,
12,
13,
21,
25,
27). Evidence also suggests that
exercises may prevent low back pain in asymptomatic individuals (14).
We note at
the outset the potential for ambiguity in using the term sit-up.
Sit-up can refer to the specific movement (from a supine position) of hip
flexion without lumbar flexion (21)
or can be used, as we do herein, as a general descriptor of a class of
exercises (e.g., trunk curls, crunches, reverse crunches) designed to
activate trunk and hip muscles in controlling cyclical flexion and
extension and sometimes rotation of the spine.
Electromyographic
(EMG) assessment of anterior trunk muscles, most frequently the rectus
abdominis (RA), has been widely reported across a range of activities,
including lifting (15) and
various types of sit-up exercises in adults (3,
7-10,
20,
22,
29) and children (19).
Characteristic myoelectric patterns during a sit-up include alternating
periods of concentric and eccentric abdominal activity, with lower
activity levels during the eccentric phase (7).
Regional differences in RA activation also have been reported. Sarti et
al. (24), for example, found
greater activation of the upper RA in curl-up exercises compared with the
lower RA in subjects performing the exercises correctly. Posterior pelvic
tilt exercises, in contrast, were more strenuous on the lower RA than the
curl-ups. Subjects who performed the exercises incorrectly exhibited
indistinct muscle activation patterns.
The
external oblique (EO) assists the RA in controlling spinal flexion and
extension (16) and, in concert
with the internal oblique, facilitates trunk rotation (5).
Recent work by McGill (16)
indicates that the contribution of the obliques to flexion may be
underestimated and that these muscles may play a greater role in flexion
than previously suggested.
Abdominal
strength alone is not sufficient to ensure optimal performance and
minimize the risk of injury. The antagonist muscles (e.g., erector spinae)
operate in conjunction with the abdominal muscles to control trunk
movement. In this regard, it is important to ensure proper strength
balance between the anterior and posterior muscle groups, since muscle
imbalances are implicated in spinal dysfunction (21).
As noted
earlier, most studies of sit-up exercises have used EMG to describe
movement dynamics. Other studies have reported muscle joint moments and
physiological measures (11,
23) and spinal loading (2)
during sit-ups. Axler and McGill (2)
made an especially relevant conclusion from their assessment of the
relation between EMG and loads on the lumbar spine. Using a dynamic model
(17), they assessed 12 different
abdominal exercises and concluded that evaluation of an abdominal flexion
exercise should not be based on EMG activity alone. They showed that the
activity-to-load ratio varies considerably across exercises and indicated
that EMG amplitude is not necessarily a good predictor of lumbar spine
loading.
In recent
years, various types of abdominal exercise devices have been marketed with
claims of enhancing the effectiveness of sit-up exercises and lowering the
risk of injury. Limited study has been performed on the efficacy of these
devices. The purposes of this study were to assess the activity patterns
of selected muscles used during sit-ups (both with and without the
assistance of abdominal exercise devices) and determine what effect, if
any, the devices have on muscle recruitment patterns.
Subjects
Nineteen healthy adult volunteers (10 women and 9 men) participated in this study. Subject selection was limited to individuals with sufficiently low amounts of abdominal adipose tissue to permit accurate measurement of muscle activity. For the women, mean ± SD age, height, and body mass were 21.0 ± 2.5 years, 165 ± 11 cm, and 57.5 ± 8.6 kg, respectively. For the men, age, height, and body mass were 23.4 ± 6.7 years, 178 ± 5 cm, and 80.4 ± 9.0 kg, respectively. After receiving an explanation of the experimental protocol, each subject signed a university-approved informed consent form.
Exercise Devices
Four exercise devices, selected and provided by the American Council on Exercise (San Diego, CA), were used. These devices were the Ab Roller Plus (ABR) (Quantum North America), ABSculptor (ABS) (Tristar Products), AB Trainer (ABT) (Precise), and AbWorks (ABW) (Nordic Track).
The ABR,
ABS, and ABT were similar in design. Each consisted of tubular metal
shaped to roll forward and backward as the person performing a sit-up
flexed his or her spine. The tubes extended above the exerciser and
allowed subjects to grab the tube overhead while performing “arms up”
exercises. The ABW was designed differently, with 2 padded surfaces joined
by a hinge mechanism that accommodated spine and hip flexion. Two
handlebars extended overhead to permit “arms up” exercises. The ABW also
included a roller-pad assembly designed to secure the feet with the hip
and knee each in approximately 90° of flexion. All 4 devices provided
padded support for the back of the head.
Experimental Design
After appropriate warm-up and instruction on the proper technique for executing “crunch”-style sit-ups, subjects performed 4 different abdominal exercises for each of 5 conditions: without the assistance of any device and on each of the 4 abdominal exercise devices. Condition testing order within subject was randomized. All data for each subject were collected during a single session.
The 4
abdominal exercises were a basic crunch (arms up), basic crunch (arms
down), oblique crunch (arms up), and reverse crunch (arms up). In the
“arms up” position on the ABR, ABS, and ABT, each subject held the
overhead bar. On the ABW, the subject held the handles overhead, and in
the “no device” (ND) condition, the subjects positioned their hands behind
the head (with instructions not to pull the neck forcibly into flexion).
In the “arms down” position, subjects held the supporting bar or frame in
a low position (as specified in each device's instructions), and in the ND
condition, they placed their arms across the chest.
In the
basic crunches, the feet were placed flat on the ground with knees flexed
to 90°. Subjects were instructed to elevate their trunk by lifting their
head and shoulders to a point where the scapulae were lifted above the
ground. As per manufacturers' recommendations, the oblique crunches on the
ABR, ABS, and ABT were performed by rotating the legs (with knees kept at
90° of flexion) to the left so that the lateral aspect of the subject's
left thigh was flat on the ground. Trunk movement then was performed as in
the basic crunches. Oblique crunches in the ND condition were performed
similarly.
In the ABR,
ABS, ABT, and ND conditions, reverse crunches were executed by having the
subjects hold their head and trunk in a stationary position and, beginning
with their feet off the ground and both hips and knees at 90°, curl their
legs, pelvis, and lumbar spine cephalically.
Design
differences in the ABW necessitated altered movement specifications for
the oblique crunch and the reverse curl. These changes were dictated by
the ABW's foot assembly that anchored the feet and legs and prevented the
freedom of lower extremity movement permitted in the unsecured ABR, ABS,
ABT, and ND conditions. Following the manufacturer's instructions,
subjects performing oblique crunches on the ABW rotated their head and
neck so that the subject's left cheek was flat against the supporting pad.
With the head and neck in this position, subjects performed movement
similar to the basic crunches. Reverse crunches were performed by having
the subjects pull the hinged roller assembly cephalically by flexing the
hips while keeping the trunk and head flat on the padded surface.
To ensure
temporal consistency, each sit-up was paced by metronome with each phase
(up and down) lasting 1.5 seconds. Sufficient rest (
1
minute) was allowed between trials to avoid fatigue. None of the subjects
commented that they felt fatigued at any point during their data
collection session. Each trial consisted of a series of continuous sit-ups
lasting about 25 seconds. The EMG activity was assessed for 5 consecutive
sit-ups in each series. The criterion measure was the mean EMG value for
each series.
EMG Recording
Muscle activity was measured using a standard, noninvasive EMG system (BIOPAC Systems, Inc.). Bipolar Ag-AgCl electrodes (EL208S, BIOPAC Systems) were placed on the skin overlying the right sternocleidomastoid (SCM), right upper rectus abdominus (URA), right lower rectus abdominus (LRA), right EO, and right rectus femoris (RF). An unshielded ground electrode (EL208, BIOPAC Systems) was placed on the skin overlying the acromion process. The skin over each muscle was shaved and cleansed with alcohol to reduce impedance at the skin-electrode interface. The EMG signals were sampled at 1000 Hz per channel and amplified (gain of 5,000) and bandpass filtered (10–400 Hz) using BIOPAC Systems Amplifiers. Signals were passed through a BIOPAC Systems Model MP100 connected to a Macintosh IIvx computer for analysis.
Statistical Analysis
Statistical analyses using the BMDP4V routine (BMDP Statistical Software, Inc.) were performed on the mean EMG values, using a repeated-measures analysis of variance procedure for each of the 4 exercises. Tukey's post hoc tests were performed to identify significant pairwise differences (
28). Reported differences were accepted as statistically significant at p < 0.01.Results
All
subjects exhibited higher EMG activity during the upward (concentric)
phase than during the downward (eccentric) phase. Results for each of the
4 exercises are presented herein. All references to “abdominal group”
include results for the URA, LRA, and EO.
Basic Crunch (Arms Up)
Mean EMG activity levels for the basic crunch (arms up) are shown in
Table 1
. Significant differences in
abdominal group activity were found only in the URA (ND > ABR) and in the
LRA (ABW > ABR). Mean abdominal muscle activity data for the basic crunch
(arms up) are shown in Figure 1
. Higher RF activity was found in
the ABW compared with all other conditions. The SCM was more active in the
ABR and ABS compared with the ND condition.
Basic Crunch (Arms Down)
The EMG results for the basic crunch (arms down) are shown in
Table 2. The only significant differences
in abdominal group were higher URA and LRA activity in the ABW compared
with the ABR condition. Mean abdominal muscle activity data for the basic
crunch (arms down) are presented in Figure 2
. The ABW showed higher RF
activity compared with the ND condition. Higher SCM activity was found in
the ND condition compared with the ABR, ABS, and ABT conditions.
Oblique Crunch
Mean EMG activity levels for the oblique crunch are shown in
Table 3. The URA activity was slightly
higher in the ABW compared with the other devices (ABR, ABS, ABT) but not
significantly higher than the ND condition. The LRA activity was higher in
the ABW condition than in the ABR and ABS conditions. Mean abdominal
muscle activity data for the oblique crunch are shown in
Figure 3
. The RF activity was higher in
the ABW than in the other conditions. The SCM was substantially higher in
the ABW when compared with all other conditions.
Reverse Crunch
The EMG results for the reverse crunch are shown in
Table 4. The EO activity was lower in the
ABW compared with all other conditions. Mean abdominal muscle activity
data for the reverse crunch are presented in Figure 4
. The ABW also showed lower RF
activity than ND, ABS, and ABT conditions. No differences were observed in
SCM activity. Discussion
In regard
to the abdominal muscles (URA, LRA, EO), the results of this study
generally indicate no significant difference in muscle activity for the
basic crunches and oblique crunch for any of the devices when compared
with performing the same exercises unassisted.
The higher
RF activity in the ABW for the first 3 exercises (basic crunch with arms
up, basic crunch with arms down, oblique crunch) seems consistent with the
leg position dictated by the design of the ABW. In contrast to the other 3
devices, the ABW has a roller-pad assembly that elevates the legs and
provides structural resistance to hip flexion. Subjects appeared to use
the roller-pad assembly to stabilize themselves, thus resulting in
elevated hip flexor (RF) activity.
The lower
RF (and accompanying lower URA, LRA, and EO) activity in the ABW in the
reverse crunch also seems due to the structure of the device. Reverse
crunches using the ABW appeared to involve considerable hip flexion to
pull the roller-pad assembly toward the head and trunk. It seems plausible
that this action is largely controlled by the deep hip flexors, iliacus
and psoas. If this was indeed the case (we cannot say with certainty since
EMG activity of the iliacus and psoas was not measured in this study), it
would appear to explain the observed lower activity in the RF, URA, LRA,
and EO.
Lower SCM
activity in all 4 devices compared with the ND condition in the basic
crunch (arms down) suggests a potential beneficial effect of padded head
supports. The efficacy of a head support in reducing muscle activity in
clinically relevant populations (e.g., individuals with cervical
dysfunction) needs further study.
The higher
SCM activity in the ABW for the oblique crunch is reasonable in light of
the position prescribed for performing the oblique crunch. The
instructions for the ABW device specified that the head be rotated so that
the subject's cheek is flat against the pad. This head-neck movement is
assisted by the SCM and consequently would predictably elicit substantial
muscle activity.
We note
that all subjects in this study had either no or minimal experience using
any of the devices before the study. In this sense, they must be
considered novices in terms of using such devices. However, they were all
experienced at performing crunch-type sit-ups without the assistance of
any device. We cannot predict whether further practice using the devices
would alter the muscle recruitment strategies in these individuals. In
addition, all subjects were young and well conditioned. Extrapolation of
these results to older or less fit populations, although plausible, is not
justified and requires further study.
In
practice, sit-up exercises often are performed continuously until they
elicit a burning sensation as the muscles become fatigued. Although the
subjects in this study did not exert themselves to this extent, caution is
warranted in evaluating EMG signals under fatigue conditions, since EMG
signals in fatigued muscles are known to exhibit a lower median frequency
and higher total power and peak amplitude than when unfatigued (6).
This is particularly relevant when discussing muscle activity of the
abdominal group, since these trunk flexors are more susceptible to fatigue
than the spinal extensor muscles (26).
In
conclusion, we offer a word of caution against interpreting results of
this or any other EMG study as being directly predictive of spinal
loading. As noted by Axler and McGill (2),
“although an exercise may not elicit the largest EMG activity, it is
possible that considerable activity can be effected with relatively low
loads on the lumbar spine, and vice versa.” The critical issue is how to
prescribe exercises that challenge the abdominal musculature and yet are
within the ability of a person to sustain high spinal loading. From this
perspective, no single exercise can be recommended for everyone. An
exercise that generates considerable abdominal muscle activation and high
lumbar spine forces, for example, would be contraindicated for a person
with chronic low back pain.
Practical Applications
In terms of
abdominal muscle activity, the data suggest that there is no apparent
benefit or detriment to performing these abdominal exercises with an
exercise device similar to those tested in this study. If use of such
devices encourages and motivates an individual to begin and continue an
abdominal exercise program, they would seem to be potentially beneficial.
An individual's body dimensions and exercise technique will determine
which device is most comfortable. There likely is no single “best” device
for everyone.
The purpose
of abdominal exercises is to increase the strength (“tone”) of the
abdominal musculature. Given the relatively short duration and localized
muscle involvement of abdominal training protocols, any expected caloric
expenditure would be minimal. Thus, any claims of weight loss and/or
changes in body dimensions due solely to the performance of abdominal
exercises (with or without the assistance of any device) should be viewed
with great caution. It has been well established that the most effective
means of losing weight is the combination of aerobic exercise and proper
nutrition. The performance of abdominal exercises in the absence of
regular aerobic exercise and good nutrition cannot be expected to result
in weight loss. Performance of abdominal exercises without accompanying
aerobic exercise and proper diet will result, at best, in the
strengthening of abdominal muscles underneath layers of adipose tissue (4).
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Acknowledgments
The authors express appreciation to the American Council on Exercise (5820 Oberlin Drive, Suite 102, San Diego, CA 92121–:3787), Richard T. Cotton (editor in chief, ACE Fitness Matters), and Jean Walcher for their generous support in funding and facilitating this study. Thanks are also extended to Attila Zink for his assistance in assembling the exercise devices in preparation for the study.
Click on thumbnail for full-sized image.
Figure
1. Mean muscle activity of the abdominal group muscles (upper rectus
abdominus, lower rectus abdominus, external obliques) for the basic crunch
(arms up) condition. Values are expressed as a percentage of maximum
electromyographic activity, with the maximum determined as the highest
mean activity for each subject across all conditions.
Click on thumbnail for full-sized image.
Figure
2. Mean muscle activity of the abdominal group muscles (upper rectus
abdominus, lower rectus abdominus, external obliques) for the basic crunch
(arms down) condition. Values are expressed as a percentage of maximum
electromyographic activity, with the maximum determined as the highest
mean activity for each subject across all conditions.
Click on thumbnail for full-sized image.
Figure
3. Mean muscle activity of the abdominal group muscles (upper rectus
abdominus, lower rectus abdominus, external obliques) for the oblique
crunch condition. Values are expressed as a percentage of maximum
electromyographic activity, with the maximum determined as the highest
mean activity for each subject across all conditions.
Click on thumbnail for full-sized image.
Figure
4. Mean muscle activity of the abdominal group muscles (upper rectus
abdominus, lower rectus abdominus, external obliques) for the reverse
crunch condition. Values are expressed as a percentage of maximum
electromyographic activity, with the maximum determined as the highest
mean activity for each subject across all conditions.
Table
1. Mean electromyograph values (mean ± SE) for the arms up basic
crunch (N = 19).
Table
2. Mean electromyograph values (mean ± SE) for the arms down basic
crunch (N = 19).
Table
3. Mean electromyograph values (mean ± SE) for the oblique crunch (N
= 19).
Table
4. Mean electromyograph values (mean ± SE) for the reverse crunch (N
= 19).
