Advances in Physical Education
2013. Vol.3, No.2, 84-88
Published Online May 2013 in SciRes (http://www.scirp.org/journal/ape) http://dx.doi.org/10.4236/ape.2013.32014
84
Relationships between Sit-Ups and Abdominal Flexion Strength
Tests and the Thickness of Each Abdominal Muscle
Takanori Noguchi1, Shinichi Demura2, Kenji Takahashi3
1University of Technology, Fukui, Japan
2Graduate School of Natural Science & Technology, Kanazawa University, Kanazawa, Japan
3Teikyo Heishei University, Ichihara, Japan
Received April 1st, 2013; revised May 4th, 2013; accepted May 17th, 2013
Copyright © 2013 Takanori Noguchi et al. This is an open access article distributed under the Creative Com-
mons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, pro-
vided the original work is properly cited.
In exercise prescription and training, a simple test that accurately evaluates abdominal strength without
the use of a special device is required. This study aimed to examine the relationships between sit-ups and
abdominal flexion strength tests and between both the above tests and the thickness of each abdominal
muscle. Subjects comprised 13 healthy young males (age: 18.9 ± 0.64 yr, height: 170.2 ± 0.70 cm, and
weight: 64.9 ± 8.80 kg). Sit-ups and abdominal flexion strength tests as well as muscle thickness meas-
urements of rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles were
performed. The relationships between the measured values were examined using Pearson’s correlation
coefficient. A significant correlation (r = 0.75) was found between sit-ups and abdominal flexion strength
tests. In addition, both tests showed significant correlations (r > 0.65) with thicknesses of the rectus ab-
dominis and internal oblique muscles. From the present results, the relationship between sit-ups and ab-
dominal flexion strength tests is very strong, and both tests are related to muscle thicknesses of rectus ab-
dominis and internal oblique muscles. The sit-ups test can evaluate not only muscle endurance of abdo-
men but also static strength.
Keywords: Ultrasound Imaging; Abdominal Muscle; Muscle Thickness; Field Test
Introduction
It is important to accurately understand the effect of strength
training over specific periods of time to efficiently reinforce
strength (Pardis, Ramin, Saeed, & Mohsen, 2012; Roger &
Thomas, 2010). Further, it is important to be able to easily de-
termine muscle strength using methods such as grip strength.
However, there are some regions whose muscle strength can be
easily measured and those for which it is more difficult to be
measured. Muscle strength is broadly divided into static and
dynamic strengths, with the former being generally measured
by maximal strength, such as grip strength. Determining
maximum ability is essential for deciding relative loads in
training or exercise prescription and should be adequately
measured. The abdominal muscle groups contribute in enhanc-
ing intra-abdominal pressure, stabilizing the vertebral column,
and maintaining posture (Michael, Erik, & Udo, 2010). Further,
they are related to flexion, torsion, and lateroflexion of the
trunk and are important not only for athletes but also for com-
mon people. Hence, many training methods aiming towards
reinforcing abdominal muscle groups have been proposed
(Sands & McNeal, 2002). However, devices that can easily
measure abdominal muscle strength are less developed.
During the sit-ups test, subjects repeatedly bend and extend
the torso at the hip for a specific period of time; hence, endur-
ance of abdominal and hip muscles is largely related (Demura,
2011). The relative simplicity of the sit-ups test and not requir-
ing special devices makes this test extremely practical. There-
fore, the sit-ups test is also one of the physical fitness tests used
by the Ministry of Education, Culture, Sports, Science and
Technology of Japan. Given that maximal strength is also an
important factor that determines muscle endurance, if the
maximal abdominal strength related to sit-ups is superior, it is
assumed that abdominal muscle endurance is also superior.
Hence, we hypothesized that the relationship between sit-ups
and abdominal flexion strength tests is strong.
Miyamoto et al. (2006) reported that knee extension strength
is related to muscle thicknesses of rectus femoris and vastus
intermedius (Miyamoto et al., 2007). As muscle thickness is
also related to muscle volume (Abe, Kearns, & Fukunaga, 2003;
Akagi et al., 2009), it is inferred that if muscle thickness is
large, static strength and muscle endurance involved in the
related parts develop accordingly. Hence, it is assumed that
strength measured by sit-ups and abdominal flexion strength
tests is related to thicknesses of abdominal muscles.
This study aimed to examine relationships between sit-ups
and abdominal flexion strength tests compared to each other
and thicknesses of four major abdominal muscle groups.
Methods
Subjects
Subjects comprised 13 young males (age: 18.9 ± 0.64 yr,

T. NOGUCHI ET AL.
height: 170.2 ± 0.70 cm, and weight: 64.9 ± 8.80 kg).
All subjects were healthy and physically competent. Before
the experiment, the purpose and procedures were explained in
detail, and written informed consent was obtained from all par-
ticipants. The experimental protocol was approved by the Eth-
ics Committee on Human Experimentation of the Faculty of
Human Science, Kanazawa University (No. 2012-14).
Procedures
In this study, we measured sit-ups, abdominal flexion
strength, and muscle thickness. Each measurement device and
procedure was as follows.
Sit-Ups Test
The test was performed according to the recommended
method for physical fitness tests by the Ministry of Education,
Culture, Sports, Science and Technology. Subjects lay on their
back with their knees bent at approximately right angles while
both feet were positioned flat on the ground. They held their
hands against their chest where they must remain throughout
the test (Figure 1). During the test, an assistant held the sub-
jects’ feet placed on the ground. Subjects sat up until they
touched their knees to both elbows; then, they returned to the
floor. The movement was repeated as many times as possible
for 30 s. The assistant counted and records the number of cor-
rect completed sit-ups. The test was measured only once owing
to the influence of fatigue.
Abdominal Flexion Strength Test
A trunk strength measurement device (original model: Takei
Scientific Instruments Co. Ltd. Tokyo, Japan) was used to
measure strength of abdominal flexion. This device was newly
developed to measure isometric strength during forward bend-
ing of the trunk. While pushing the rotation lever at chest height,
the exerted strength was measured by a strain-gauge meter
connected through a pulley (Figure 2).
After adjusting the chair height to match the seated height of
each subject, subjects sat so as to match their abdominal flexion
Figure 1.
Sit-ups test.
Strain gauge meter
Figure 2.
Trunk strength measurement device.
point on the rotation axis of the lever. After placing both el-
bows on the base of the rotation lever with arms folded, sub-
jects exerted abdominal flexion strength maximally with their
timing. At that time, the tester checked that the subject did not
push with their hands or back. The measured value through a
pulley was revised by the following formula:
Abdominal flexion strength = measurement value (kg) × 0.6
After a single practice, the test was performed twice. A one
minute rest was taken between each trial to eliminate the influ-
ence of fatigue.
Thickness of Abdominal Muscles
An ultrasound imaging device (GT-101, TANITA, Tokyo,
Japan) was used to measure thicknesses of abdominal muscles.
Figure 3 shows computer displayed ultrasound images using
the B-mode method. Thicknesses of four muscles (rectus ab-
dominis, external oblique muscle, internal oblique muscle, and
transversus abdominis) were measured using a probe frequency
of 6 MHz. The thickness of rectus abdominis was measured as
the maximal width of a point 4-cm transverse from the navel,
excluding tendinous intersections. Because the other three parts
have overlapping organizations, they were measured at the
same position (Ferreira, Ferreira, & Hodges, 2004; Leonardo,
Chris, Jane, Paul, & Debra, 2009). We measured two thirds of
line drawn horizontally from the height of the navel perpen-
dicular to a vertical axillary line (Figure 4).
All measurements were completed by a single experienced
tester. The thickness of each abdominal muscle was measured
twice in an upright standing position with muscles tensed to
increase abdominal pressure.
Statistical Analyses
Intra-class correlation coefficients (ICC) were calculated to
evaluate trial-to-trial reliability. The relationships between sit-
ups and abdominal flexion strength tests that and between both
tests and the thickness of abdominal muscle was examined by
Pearson’s correlation coefficient. Statistical significance (α)
was set at p < 0.05.
Results
Table 1 shows the results of ICCs of abdominal flexion
strength test, and the thickness of each abdominal muscle. All

T. NOGUCHI ET AL.
86
variables showed a very high ICC over 0.94. Table 2 shows the
correlation between sit-ups and abdominal flexion strength tests
and that between both the above tests and the thickness of each
abdominal muscle. A significant and high correlation was
found between sit-ups and abdominal flexion strength tests (r =
0.75). Significant and moderate or higher correlations were
found between both the above tests and thickness of rectus
abdominis and internal oblique muscles (r = 0.65 and 0.79,
respectively).
Discussion
In both strength testing and ultrasound imaging analyses, the
reliability of measured values is often questioned (Hides, Mi-
okovic, Belavy, Stanton, & Richardson, 2007; Mannion et al.,
2008). ICC of abdominal flexion strength in this study was very
high (0.99). This indicates that the values measured by the
trunk strength measurement device, developed in this study,
have high reliability. In addition, ICCs of the thickness of each
abdominal muscle was also very high, with all values over 0.94.
Gill, Mason, & Gerber (2012) reported that ICCs of the thick-
ness of transverse abdominal muscle, external oblique muscle,
and internal oblique muscle measured by the same method were
approximately 0.97 - 0.98. On the basis of results, the reliability
of muscle thicknesses of individual abdominal muscles as
measured by ultrasound imaging is considered to be high.
Figure 3.
Ultrasound imaging.
Measurement of abdominal flexion strength is impractical as
a field test in actual training because it requires a specialized
device (Jungmin, Hyoseob, Se, & Yu, 2012). In contrast, the
Figure 4.
Measurment sites of ultrasound imaging.
Table 1.
Intra-class correlation coefficients (ICCs) of abdominal flexion strength, and each part of abdominal muscle thickness.
1st 2nd
Mean SD Mean SD F-value p ICC
Strength (kg) 46.7 17.84 46.7 17.76 0.02 0.90 0.99
Rectus abdominis (mm) 17.7 3.85 17.8 3.98 0.06 0.81 0.98
External oblique muscle (mm) 8.1 2.23 8.2 1.93 1.00 0.34 0.96
Internal oblique muscle (mm) 15.6 3.69 15.6 3.56 0.04 0.84 0.97
Transversus abdominis (mm) 5.2 1.19 5.2 1.53 0.00 1.00 0.94
Note: *p < 0.05.
Table 2.
Correlations between the sit-ups, abdominal flexion strength, and muscle thickness.
Coefficient of correlation (r)
n = 13 Mean SD The sit-ups p Strength p
Sit-ups (Times) 30.6 7.42
Strength (kg) 46.7 17.75 0.75* 0.00
Rectus abdominis (mm) 17.8 3.88 0.66* 0.01 0.65* 0.02
External oblique muscle (mm) 8.1 2.04 −0.15 0.63 −0.02 0.94
Internal oblique muscle (mm) 15.6 3.56 0.79* 0.00 0.69* 0.01
Transversus abdominis (mm) 5.2 1.33 −0.02 0.95 0.08 0.81
Note: *p < 0.05. strength: abdominal flexion strength.

T. NOGUCHI ET AL.
sit-ups test, consisting simply of repetitive abdominal flexion
and extension can be easily performed anywhere and requires
no equipment. The sit-ups test has been used for evaluating
muscle endurance from completed sit-ups within a predeter-
mined time or the time until completion of a predetermined
number of sit-ups (American college of sports medicine, 2000).
However, it is considered that static strength in addition to
muscle endurance of the abdomen are related to successful
completion of the sit-ups test because a high correlation was
found between sit-ups and abdominal flexion strength tests in
this study. In brief, it is inferred that individuals with superior
abdominal flexion strength can perform and maintain rapid
abdominal flexion throughout the sit-ups test.
It was reported that the magnitude of strength of a muscle
group is strongly related to its thickness (Miyamoto et al.,
2007). Hence, it is inferred that larger muscle thickness is also
related to well-developed static strength and muscle endurance
involved in strength exertion of the related parts. The abdomi-
nal muscle groups consist of the following four parts: rectus
abdominis, external oblique muscles, internal oblique muscles,
and transversus abdominis (Michael, Erik, & Udo, 2010). By
examining thicknesses of abdominal muscle groups related to
sit-ups and abdominal flexion strength, the muscle parts that
contribute to strength exertion of the abdomen can be identified.
Significant correlations were found between the sit-ups test and
thicknesses of rectus abdominis and internal oblique muscles (r
= 0.66 and 0.79, respectively). Rectus abdominis is a paired
muscle that runs vertically on the anterior abdominal wall and
is related to abdominal flexion. In additional, internal oblique
muscles are located in lateral abdominal regions and are also
related to abdominal flexion of rectus abdominis. It is consid-
ered that people who perform the sit-up exercise develop the
above muscle groups related to abdominal flexion. Miyamoto et
al. (2007) reported that knee extension strength showed sig-
nificant relationship with thicknesses of rectus femoris and
vastus intermedius muscles (rectus femoris muscle: r = 0.56,
vastus intermedius muscle: r = 0.43, and their total: r = 0.62)
(Miyamoto et al., 2007). In brief, it is suggested that people
with greater muscle thicknesses are superior in strength in the
corresponding parts. In the present results, significant correla-
tions were found between the abdominal flexion strength and
thicknesses of rectus abdominis and internal oblique muscles (r
= 0.65 and 0.69, respectively). Hence, abdominal flexion
strength of people with well-developed rectus abdominis and
internal oblique muscles may also be superior.
In contrast, the present results show that thickness of external
oblique and transversus abdominis muscles showed non-sig-
nificant relationships with sit-ups and abdominal flexion
strength tests. While these muscle groups are anatomically
related to body trunk flexion, they are perhaps more important
for lateral bending and rotation of the trunk or increasing ab-
dominal pressure. Sit-ups and abdominal flexion strength tests
used in this study may not adequately reflect characteristics of
these muscle groups. For these muscles, movements more spe-
cific to the muscle action will be examined.
Conclusion
The relationship between sit-ups and abdominal flexion
strength tests is strong. Further, both tests are related to thick-
nesses of rectus abdominis and internal oblique muscles. Static
strength in addition to abdominal muscle endurance contributes
considerably to completion of the sit-ups test. The sit-ups test is
easily executed for the evaluation of static strength as well as
muscle endurance of the abdomen. It is recommended that ef-
fectiveness of abdominal strength be evaluated on the basis of
the results of the sit-ups test.
Acknowledgements
This study was supported by the Japanese Society for the
Promotion of Science KAKENHI Grant-in-Aid for Young Sci-
entists (B) Number 24700673. The authors would like to thank
Enago (www.enago.jp) for the English language review. The
results of the present study do not constitute endorsement by
Journal of Strength & Conditioning Research.
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