The effects of curl-up exercise in terms of posture and muscle contraction direction on muscle activity and thickness of trunk muscles

Abstract

BACKGROUND:

The curl-up exercise is widely used in clinical practice for strengthening abdominal muscles, but has been applied without a systematic method.

OBJECTIVE:

The purpose of this study was to determine the most effective method considering the angle and muscle contraction direction during the curl-up exercise.

METHODS:

Fourteen healthy males performed the curl-up exercise according to contraction direction (concentric and eccentric) and angle (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ). The muscle activity of the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and iliopsoas (IP) was measured using electromyography (EMG), and the muscle thickness of transversus abdominis (TrA) was measured using ultrasonography.

RESULTS:

The activities of the abdominal muscles (RA, EO, and IO) decreased with increasing angles (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ) ( $p<$ 0.05). There was no significant difference between eccentric and concentric contractions. The thickness ratio of TrA was the largest at an eccentric curl-up at 30 ${}^{\circ}$ , and the smallest at a concentric curl-up at 30 ${}^{\circ}$ ( $p<$ 0.05).

CONCLUSIONS:

The most effective angle for curl-up was 30 ${}^{\circ}$ . Although there is no difference in the direction of muscle contraction, eccentric curl-up at 30 ${}^{\circ}$ could be considered the most effective posture for abdominal strengthening considering the importance of TrA.

Keywords

Abdominal muscles electromyography ultrasonography

1. Introduction

The abdominal muscles are extremely important muscles to help the upper limbs and lower extremities move and to maintain stability in the movement axis [1]. Abdominal muscles are one of the muscles emphasized in the rehabilitation of musculoskeletal and central nervous systems for injured patients [2]. They play an important role in reducing back pain, improving exercise capacity, and maintaining spinal stability [3]. Abdominal muscles can be divided into superficial muscles and deep muscles. The superficial muscles are located on the surface, such as the rectus abdominal muscle (RA) and external oblique muscle (EO), and are involved in overall trunk stability. The deep muscle is the transversus abdominis (TrA) and internal oblique muscle (IO), which contribute to lumbar stability. Among them, the TrA plays the most important role in trunk stabilization and motor control of the pelvic [4, 5].

Strengthening exercises for the abdominal muscles can improve trunk control and reduce the stress on the lumbar spine [6, 7]. Recently, strengthening exercises for the abdominal muscle have been widely used for rehabilitation of elite athletes to improve their performance and patients with back pain [8]. Various abdominal muscle strengthening methods can be used for rehabilitation by understanding how to induce muscle activity. There are several exercises that can be performed to strengthen the abdominal muscles in accordance with posture, such as the curl-up, V-sit up, reverse crunch, double leg raises, and exercises with suspension [6, 9, 10]. Abdominal strengthening exercises applied in various postures can differentially activate various abdominal muscles [10]. The muscle strengthening method according to the muscle contraction direction is divided into concentric and eccentric contraction. Concentric contraction occurs when the length of the agonist is shortened and the force generated during contraction exceeds the force exerted on the muscle. Eccentric contraction occurs when the length of the agonist is longer and the force generated inside the muscle is less than the force externally applied. Eccentric contraction has lower energy consumption and lactate production than concentric contraction [11]. In addition, eccentric contraction can produce 20–60% greater force than concentric contraction [12]. A greater level of muscle damage with eccentric contractions can be completed, which leads to more hypertrophy and hence is often applied as a muscle strengthening exercise [11]. Therefore, the eccentric contraction has often been applied to muscle strengthening exercise.

Of the various abdominal strengthening exercises, the curl-up exercise is mainly used for strengthening abdominal muscles and improving endurance [13]. Previous studies have compared muscle activity through electromyography (EMG) when curl-up, and have focused on whether curl-up affects specific muscle strengthening [10, 14, 15]. Among the various muscles, iliopsoas is a hip flexor muscle, which increases the excessive activity of the hip flexor muscle during curl-up, increasing the lumbar lordosis and increasing the load on the spine [16]. Several studies have suggested that curl-up exercise is an effective way to activate RA and EO [17, 18, 19]. Furthermore, previous studies on the curl-up exercise have been conducted mainly on the superficial abdominal muscle including the RA and EO, as well as on deep abdominal muscles such as IO and TrA [9, 20, 21]. However, TrA and IO are deeply located muscles and it is difficult to measure muscle activity. For this reason, ultrasonography is often used as a method for measuring muscle thickness of deep abdominal muscles such as the TrA [22]. Ultrasonography can be used to noninvasively and directly measure abdominal muscle thickness. McMeeken et al. concluded that the muscle thickness measured via ultrasonography and muscle activity measured via EMG of TrA were significantly correlated [23]. However, little research has been reported on the change in thickness of TrA through ultrasonography during the curl-up exercise. In addition, the curl-up exercise is often performed without any specific method in the clinical environment. Therefore, the factors (muscle contraction direction and angle) contributing to the strengthening of the abdominal muscles during curl-up exercise should be considered.

The purpose of this study was to determine the most effective angle and muscle contraction direction during the curl-up exercise according to various angles by evaluating the muscle activity and deep muscle thickness.

2. Methods

2.1 Participants

Fourteen healthy male college students (age, 20.13 $\pm$ 1.81 years; weight, 75.70 $\pm$ 10.74 kg; height, 1.73 $\pm$ 4.46 m [mean $\pm$ SD]) volunteered for this study.

Inclusion criteria: sufficient muscle strength and range of motion, in order to be able to perform the posture required for the experiment; voluntary participation in the study.

Exclusion criteria: rehabilitation treatment within the last six months due to damage of the upper and/or lower limbs; absence of back pain or musculoskeletal disorders; subjects who cause compensation pattern in curl-up performed in this study; subjects with neurological symptoms (e.g., numbness, tingling, paraesthesia).

Each participant was notified of the benefits and risks of the experiment before signing the informed consent form. This study was approved by the Kyungnam University Institutional Review Board.

Figure 1.

Flowchart of the study.

2.2 Procedure

Prior to the experiment, all participants practiced the curl-up exercise so that they could accurately perform it. First, concentric curl-up exercises were performed at different angles (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ), and eccentric curl-up exercises were performed at different angles (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ). Participants had 30 second rest between measurements for each angle. EMG measurements were performed for each angle (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ) of the concentric and eccentric curl-ups. After EMG measurements, ultrasonography was used to measure the TrA thickness, and the angles of postures were the same as those measured by EMG (Fig. 1).

The concentric curl-up exercise was performed in a crook lying position in the supine position. Position with 90 ${}^{\circ}$ knee flexion, with both legs fixed by the measurer, both participant’s hands being placed behind the head, and the fingers interlinked. At this time, the participants were instructed to lift their head and shoulders from the floor at angles (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ). The eccentric curl-up exercise was performed from a sitting position by 90 ${}^{\circ}$ knee flexion and placing their head and trunk as close as possible to the knees. Both hands were placed behind their head and with fingers interlinked, then their head and shoulders were lowered to the floor at different angles (90 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 30 ${}^{\circ}$ ). The curl-up angles were measured between the mid-axillary line and parallel with the floor. A goniometer (Baseline ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Plastic Goniometer, HiRes ${}^{\text{TM}}$ 360 Degree Head, 12-inch arm, USA) was used to measure the angle. Each position was held for 5 seconds with a 30 seconds rest between measurements (Fig. 2).

Table 1
Comparison of the muscle activity (%MVIC) of abdominal muscles by the selected angle on curl-up

	Curl up 30 ${}^{\circ}$	Curl up 60 ${}^{\circ}$	Curl up 90 ${}^{\circ}$	$p$
RA
Concentric	65.58 $\pm$ 15.81 ${}^{\text{a}}$	47.83 $\pm$ 17.08 ${}^{\text{b}}$	28.06 $\pm$ 8.25 ${}^{\text{c}}$	0.000 ${}^{*}$
Eccentric	71.17 $\pm$ 17.39 ${}^{\text{a}}$	42.75 $\pm$ 9.06 ${}^{\text{b}}$	29.47 $\pm$ 7.53 ${}^{\text{c}}$	0.000 ${}^{*}$
$p$	0.382	0.338	0.641
EO
Concentric	51.86 $\pm$ 15.69 ${}^{\text{a,b}}$	39.54 $\pm$ 9.74 ${}^{\text{a,b}}$	18.04 $\pm$ 7.02 ${}^{\text{c}}$	0.000 ${}^{*}$
Eccentric	48.46 $\pm$ 12.32 ${}^{\text{a}}$	38.03 $\pm$ 7.31 ${}^{\text{b}}$	15.10 $\pm$ 8.91 ${}^{\text{c}}$	0.000 ${}^{*}$
$p$	0.530	0.645	0.342
IO
Concentric	52.01 $\pm$ 11.51 ${}^{\text{a,b}}$	43.37 $\pm$ 12.57 ${}^{\text{a,b,c}}$	33.62 $\pm$ 14.97 ${}^{\text{b,c}}$	0.003 ${}^{*}$
Eccentric	50.98 $\pm$ 15.01 ${}^{\text{a,b}}$	40.09 $\pm$ 9.26 ${}^{\text{a,b,c}}$	31.14 $\pm$ 15.00 ${}^{\text{b,c}}$	0.001 ${}^{*}$
$p$	0.840	0.439	0.665
IP
Concentric	42.78 $\pm$ 14.76 ${}^{\text{a}}$	26.78 $\pm$ 15.24 ${}^{\text{b,c}}$	15.59 $\pm$ 8.11 ${}^{\text{b,c}}$	0.000 ${}^{*}$
Eccentric	34.16 $\pm$ 23.71 ${}^{\text{a,b}}$	40.87 $\pm$ 20.15 ${}^{\text{a,b}}$	17.03 $\pm$ 7.77 ${}^{\text{c}}$	0.005 ${}^{*}$
$p$	0.258	0.047 ${}^{*}$	0.635

$p<$ 0.05, Value are presented as mean $\pm$ standard deviation. Note: same alphabet means non-statistically difference between group based on Turkey multiple comparison test. RA, rectus abdominis; IO, internal oblique muscle; EO, external oblique muscle; IP, iliopsoas muscle.

Figure 2.

Curl-up according to angle and muscle contraction direction.

2.3 Measurements

2.3.1 Surface EMG

In this study, the surface EMG (Delsys Trigno Wireless EMG, MA, USA) was used to measure the muscle activity of the participants. The sampling rate of the conduction signal was set to 2000 Hz, and the band width was set to 20 to 450 Hz. To measure the surface EMG, the electrodes were attached to the right side of the RA, EO, IO, and IP. To precisely measure the signal, impurities (sweat and foreign matter) at the attachment site were removed, the skin was cleaned with alcohol and the electrodes were attached. Surface electrodes were attached parallel to the muscle fibers on the dominant right side as follows: on the RA at 3 cm lateral to the umbilicus, on the EO at the muscle fiber direction between the inferior border of rib and the iliac crest, on the IO at approximately 1 cm medial to the anterior superior iliac spine, and the IP at the medial site of the rectus femoris under the inguinal ligament. After the sensors were attached, a maximal voluntary isometric contraction (MVIC) was performed in the manual muscle test to standardize the muscle activity potential. The curl-up exercise was measured once for 5 seconds at each angle. The EMG signal was recorded for 5 seconds, and 1 second of signal was trimmed from the beginning and end of the EMG signal, and the remaining 3 s signal was converted to root mean square (RMS). The RMS of each muscle was converted to a percentage (% MVIC) and used for the analysis of results.

2.3.2 Ultrasonography data

In this study, ultrasonography (SONOACE X8, Samsung Medison Inc., Korea) was used to measure muscle thickness of participants. B-mode imaging was performed to view the static cross-sectional area of the muscle. The thickness of the TrA was measured horizontally with a 7.5 MHz linear probe with the center of the probe at the top of the iliac crest centered on the right axillary line in the supine position [22]. Measurements were taken at rest for each angle of the concentric and eccentric curl-up exercise. The thickness of the TrA was converted into a ratio based on muscle thickness at rest:

(thickness ratio $=$ thickness at contraction/ thickness at rest)

2.4 Statistical analysis

All statistical analyses were performed with SPSS Version 18. One-way ANOVA tests were used to compare muscle activity and thickness ratio of abdominal muscles according to angle and muscle contraction direction. The post-hoc Tukey’s test was used to detect significant differences in relative muscle activity and muscle thickness ratio between specific tasks ( $p<$ 0.05). In addition, independent $t$ -tests were used to compare the muscle activity and thickness ratio according to muscle concentric direction. The level of statistical significance was set to $p<$ 0.05 throughout all statistical analyses.

Table 2
Comparison of the muscle thickness ratio of TrA by the selected angle on curl-up

	Curl up 30 ${}^{\circ}$	Curl up 60 ${}^{\circ}$	Curl up 90 ${}^{\circ}$	$p$
TrA
Concentric	0.43 $\pm$ 0.03 ${}^{\text{a\phantom{,c}}}$	1.34 $\pm$ 0.30 ${}^{\text{b,c\phantom{a,}}}$	1.37 $\pm$ 0.42 ${}^{\text{b,c}}$	0.000 ${}^{\ast}$
Eccentric	1.50 $\pm$ 0.53 ${}^{\text{a,b}}$	1.16 $\pm$ 0.22 ${}^{\text{a,b,c}}$	1.15 $\pm$ 0.28 ${}^{\text{b,c}}$	0.006 ${}^{\ast}$
$p$	0.000 ${}^{\ast}$	0.102	0.117

$p<$ 0.05, Value are presented as mean $\pm$ standard deviation. Note: same alphabet means non statistically difference between group based on Turkey multiple comparison test. TrA, transversus abdominis.

3. Results

3.1 Comparison of abdominal muscles activity according to angle and muscle contraction direction

During concentric curl-up exercise, activity of the RA decreased significantly with increasing angle ( $p<$ 0.05). There was no statistically significant difference in muscle activity between the 30 ${}^{\circ}$ and 60 ${}^{\circ}$ in the EO, but the activity was significantly lower at 90 ${}^{\circ}$ ( $p<$ 0.05). There was no significant difference in muscle activity between 30 ${}^{\circ}$ and 60 ${}^{\circ}$ , or 60 ${}^{\circ}$ and 90 ${}^{\circ}$ in the IO, but the muscle activity at 30 ${}^{\circ}$ was significantly higher than that at 90 ${}^{\circ}$ ( $p<$ 0.05). Similarly, there was no significant difference in muscle activity between 60 ${}^{\circ}$ and 90 ${}^{\circ}$ in the IP, but muscle activity at 30 ${}^{\circ}$ was significantly higher than that at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ ( $p<$ 0.05).

During the eccentric curl-up exercise, the muscle activity of the RA and EO decreased significantly with increasing angle ( $p<$ 0.05). There was no significant difference in muscle activity between 30 ${}^{\circ}$ and 60 ${}^{\circ}$ , and 60 ${}^{\circ}$ and 90 ${}^{\circ}$ in the IO, but the muscle activity at 30 ${}^{\circ}$ was significantly higher than that 90 ${}^{\circ}$ ( $p<$ 0.05). Furthermore, there was no significant difference in muscle activity between 30 ${}^{\circ}$ and 60 ${}^{\circ}$ in the IP, but the muscle activity was significantly lower at 90 ${}^{\circ}$ ( $p<$ 0.05).

There was no significant difference between eccentric and concentric contraction in terms of the angle (Table 1).

3.2 Comparison of the TrA thickness ratio according to angle and muscle contraction direction

During the concentric curl-up exercise, the comparison of the thickness ratios of TrA between 60 ${}^{\circ}$ and 90 ${}^{\circ}$ were not significantly different, but the thickness ratios of TrA at 30 ${}^{\circ}$ was significantly thinner than at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ ( $p<$ 0.05).

During the eccentric curl-up exercise, the comparison of thickness ratios of TrA between 30 ${}^{\circ}$ and 60 ${}^{\circ}$ , and 60 ${}^{\circ}$ and 90 ${}^{\circ}$ were not significantly different, but the thickness ratios of TrA at 30 ${}^{\circ}$ was significantly thicker than at 90 ${}^{\circ}$ ( $p<$ 0.05).

There was a significant difference in the thickness ratio of TrA between concentric and eccentric curl-up at 30 ${}^{\circ}$ , but there was no significant difference at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ (Table 2).

4. Discussion

In this study we tried to determine the most effective exercise by comparing muscle activity and muscle thickness according to the angle and muscle contraction direction during curl-up exercise.

RA, EO, and IO showed the largest muscle activity at 30 ${}^{\circ}$ during curl-up exercise in terms of angle. Escamilla et al. reported that the muscle activity of RA and EO was highest when the shoulders were lifted from the floor during curl-up exercise [6]. Parfrey et al. had subjects perform the curl-up exercise according to the distance the hand moved along the floor (5, 10, and 15 cm) in the extended elbow posture, flexed or extended knee, and fixed or non-fixed feet [3]. The effects of hands moving 5 cm along the floor are similar to those of a curl-up at 30 ${}^{\circ}$ , which involves lifting the shoulders from the floor. This suggests that the activity of RA was highest during a curl-up at 30 ${}^{\circ}$ . Furthermore, as the distance of the hand moved distally, i.e. as the angle increased, the activity of RA decreased because the body resistance torque decreases as the angle increases. The curl-up mentioned above was similar to the concentric curl-up at 30 ${}^{\circ}$ in this study. A concentric curl-up at 30 ${}^{\circ}$ showed the greatest activity in abdominal muscles, consistent with previous studies. Andersson et al. reported that when subjects performed the bent knee curl-up exercise by healthy subjects, RA was reported to be activated at an average of 88–92% at 10 ${}^{\circ}$ , 80–84% at 30 ${}^{\circ}$ , and 45–46% at 60 ${}^{\circ}$ of the trunk flexion angle [24]. These results showed that as the trunk angle increases, the external moment of the upper body decreases; therefore, the muscle activity decreases as the angle increases. In this study, the maximal abdominal muscle (RA, EO, and IO) activity was achieved during the curl-up exercise at 30 ${}^{\circ}$ , and muscle activity decreased as the angle increased. Kim et al. [9] also reported muscle thicknesses during concentric curl-up exercises at different angles (30 ${}^{\circ}$ , 60 ${}^{\circ}$ , and 90 ${}^{\circ}$ ). There was a significant difference in the thickness of the RA, and the angle at exercise was an important factor affecting the strengthening of the abdominal muscles [9]. In this study, there was a significant difference in the muscle activity according to the angle, such that the angle during curl-up was an important factor affecting abdominal muscle strength.

Curl-up affects hip flexor muscles as well as abdominal muscles. Parfrey et al. and Escamilla measured the activity of rectus femoris when concentric curl-up was performed, assuming that the activity of the hip flexor muscle is almost identical to that of rectus femoris [3, 6]. Parfrey et al. reported that hip flexion was most active in partial curl-up (i.e., when the hand moved 10 cm) [3]. Escamilla [6] found that when performing seven abdominal machines and two general abdominal exercises (bent knee sit-up, crunch), the bent knee posture showed high activity in the hip flexor muscles. The results of previous studies are inconsistent with this study [6]. In this study, the activity of hip flexor muscle was greatest at 30 ${}^{\circ}$ of concentric curl-up. The opposite reason for this study was that the iliopsoas muscle was directly measured by EMG and the foot was fixed. Also, in this study, we focused on exact angle to perform curl-up. This suggests that the angles were increased more than curl-up, which is actually done in clinical practice, leading to a higher activity of iliopsoas. Therefore, it is considered that the individuals with lumbar instability or weak abdominal muscles should be careful when applying curl-up.

In a study by Andersson, eccentric muscle activity is lower than that of concentric muscle, and eccentric contraction is less active than concentric contraction muscle activation to produce a specific force [24]. Crommert [21] found that eccentric muscle activity was lower than that of concentric muscle when curl-up in various postures for healthy women. The muscle activity was higher for the concentric contraction than the eccentric contraction. In the similar moments production, the concentric contraction shows greater activity than the eccentric contraction [21]. However, Kim and Lee did not show any significant difference in upper RA and EO between concentric and eccentric sit-ups, but lower RA and IP were more active in eccentric sit-up [14]. In this study, there was no significant difference in all muscles except IP during curl-up according to concentric and eccentric curl-ups. This result is presumed to be due to the fact that the subject is limited to a strong man.

The TrA is located horizontally in the deep abdomen. The TrA does not help with trunk flexion, but the TrA increases muscle activation with other abdominal muscles due to increased loading during the curl-up exercise [21]. TrA acts like a corset during the contraction, increasing the muscle thickness and stability of the lumbar and pelvis [25]. In this study, the thickness of TrA through ultrasonography was significantly thicker at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ compared to the 30 ${}^{\circ}$ in the concentric curl-up exercise. In the eccentric curl-up exercise, there was a significant difference at 30 ${}^{\circ}$ compared to that at 90 ${}^{\circ}$ . Kim et al. reported that there was no significant difference in the thickness of TrA during the curl-up exercise at different angles [9]. It has been reported that lateral abdominal muscles including TrA mainly play a role in maintaining trunk stability rather than exercise motion itself. TrA contributes to inter segmental control of the lumbar spine rather than to assist in a specific torque direction during the curl-up exercise [22]. In the present study, during concentric and eccentric curl-up exercise, the thickness of TrA was least at the initial stage of movement (that is, when concentric 30 ${}^{\circ}$ and eccentric 90 ${}^{\circ}$ curl up), but the thickness of TrA increased after increasing the angle. This suggests that TrA acts mainly to maintain trunk stability at the beginning of the movement, but as the angle increases, the load increases and the action of TrA increases. Therefore, in order to strengthen the TrA, it can be assumed that the movement angle should be increased during concentric and eccentric curl-up exercise.

Based on this evidence, there was no difference between concentric and eccentric curl-up, but considering TrA, the most effective way to strengthen abdominal muscles is thought to be eccentric curl-up at 30 ${}^{\circ}$ . This suggests that the use of a precise angle and muscle contraction direction in clinical practice may have a greater impact on muscle strength improvement.

The limitations of this study are as follows. First, the number of subjects was small, and the subjects were composed of healthy young men, making it difficult to generalize the research results to a wider population. Secondly, we did not apply ultrasonography to all abdominal muscles. Therefore, the correlation between muscle activation and muscle thickness could not be compared. Thirdly, we did not measure the angle of motion for more subdivisions. Fourth, the contraction direction and angle were not randomized in the measurement, and fatigue was not taken into consideration. Therefore, future studies should address these limitations.

5. Conclusion

Curl-up is a method of strengthening abdominal muscles that is commonly used in clinical practice. When comparing muscle activity and muscle thickness during curl-up according to angle and muscle contraction direction, the most effective angle was at 30 ${}^{\circ}$ . Although there is no difference in the direction of muscle contraction, eccentric curl-up at 30 ${}^{\circ}$ could be suggested as the most effective posture for abdominal strengthening considering the importance of TrA. Consequently, using accurate angles and muscle contraction directions for strengthening abdominal muscle strength in clinical practice may have a greater impact on muscle strength improvement.

Footnotes

Acknowledgments

The authors would like to thank all participants for their cooperation in this study.

Conflict of interest

The authors have no competing interests to report.

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The effects of curl-up exercise in terms of posture and muscle contraction direction on muscle activity and thickness of trunk muscles

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

Table 1 Comparison of the muscle activity (%MVIC) of abdominal muscles by the selected angle on curl-up

2.3.1 Surface EMG

2.3.2 Ultrasonography data

2.4 Statistical analysis

Table 2 Comparison of the muscle thickness ratio of TrA by the selected angle on curl-up

3.1 Comparison of abdominal muscles activity according to angle and muscle contraction direction

3.2 Comparison of the TrA thickness ratio according to angle and muscle contraction direction

4. Discussion

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Comparison of the muscle activity (%MVIC) of abdominal muscles by the selected angle on curl-up

Table 2
Comparison of the muscle thickness ratio of TrA by the selected angle on curl-up