The Effects of Curl-Up and Straight Leg Raising Exercises in Different Hip Joint Positions on Abdominal Muscle Thickness

Seyyedeh Fatemeh Alavi; Nahid Tahan; Saeed Mikaili; Alireza Akbarzade Baghban

doi:10.1016/j.jcm.2023.04.005

. 2023 Jul 7;22(3):173–179. doi: 10.1016/j.jcm.2023.04.005

The Effects of Curl-Up and Straight Leg Raising Exercises in Different Hip Joint Positions on Abdominal Muscle Thickness

Seyyedeh Fatemeh Alavi ^a, Nahid Tahan ^b,^⁎, Saeed Mikaili ^b, Alireza Akbarzade Baghban ^c

PMCID: PMC10461201 PMID: 37645001

Abstract

Objective

The purpose of this study was to investigate the effect of combining 2 exercises at different angles of the hip joint on the thickness of the transverse abdominis and internal oblique muscles.

Methods

Twenty-eight healthy adults (22.7 ± 2.5 years of age; 64.93 $\pm 8.8$ kg; body mass index: 22.2 ± 2 kg/m²) performed the curl-up exercise and straight leg raise with external rotation of the hip and dorsiflexion of the ankle simultaneously. The thicknesses of transverse abdominis and internal oblique muscles were measured from right and left sides while resting, in curl up, curl up with 10º flexion of hip, curl up with 15º flexion of hip, and curl up with 10º flexion and 15º abduction of the hip. The means of muscle thicknesses were compared using repeated-measures analysis of variance and post hoc Bonferroni tests in the 5 positions.

Results

The thicknesses of transverse abdominis (P < .001) and internal oblique (P < .001) muscles were significantly greater in curl up with 10º flexion and 15º abduction of hip compared with the other positions. There was no significant difference between abdominal muscle thicknesses in measuring from either side.

Conclusion

Doing the curl-up exercise with 10º flexion and 15º abduction of the hip may be beneficial in strengthening deep abdominal muscles.

Key Indexing Terms: Resistance Training, Abdominal Muscles, Hip Joint

Introduction

Core muscles contribute to stabilization and control of the lumbar spine.¹ The transverse abdominis (TrA) and internal oblique (IO) are 2 core muscles involved in stabilizing the trunk through the connections of thoracolumbar fascia.²^,³ Core strengthening has been recommended to prevent injury³ and providing trunk stability.⁴ Weak abdominal muscles cause the pelvis to tilt forward and the lumbar arch to increase, making an individual prone to low back pain.⁵ Measuring the size (thickness) of the abdominal muscles is a relatively new method in assessing the function of abdominal muscles in healthy individuals with low back pain.6, 7, 8 Studies show decreased abdominal muscle thickness in adolescents with low back pain⁹ and less change in abdominal muscles thickness during abdominal drawing-in maneuver¹⁰ in patients with low back pain compared with healthy people.

There are several strategies for strengthening (increasing thickness) of abdominal muscles to improve functional stability and postural stabilization of the trunk.³^,11, 12, 13 Exercises that increase abdominal muscle strength and stability should include activities that challenge all these muscles in all planes of movement.¹⁴ Strategies that are used to strengthen the abdominal muscles can be in 2 forms. First, exercises that are accompanied by isometric contraction of the abdominal muscles. Second, exercises that are concentric in nature.¹⁵

In isometric exercises, the distance between xiphoid process and the anterior pelvis remains relatively constant. An example is a plank exercise. In concentric core exercises, the distance between xiphoid process and the anterior pelvis is reduced by abdominal muscles contraction. These exercises can be done in different ways such as moving the trunk toward the fixed pelvis (eg, sit-ups) or moving the legs toward the fixed trunk (eg, straight-leg raised in the supine position).¹⁶

Sit-up exercise with knees extended is used to strengthen or increase the endurance of abdominal muscles.¹⁷ Over the years many variations have been made to the sit-up exercise. Because of the high compression and shear forces on lumbar vertebrae, it was recommended to do the sit-up exercise with the knees flexed (bent-knee sit-up exercise).¹⁸ According to Vincent et al, during bent-knee sit-up exercise, the abdominal muscles were activated only in the first 20° to 30° of trunk flexion, while the remainder of the movement was accomplished by the contraction of the hip flexors. Therefore, they recommended the curl-up exercise to replace the sit-up exercise.¹⁹

In the curl-up exercise, the upper body raises above the ground to the point where the scapula is lifted off the ground.¹⁹ Numerous modifications have been made to this exercise. Their purpose is to get maximum activity of each abdominal muscles with minimum load on the spine. These modifications include changing the position of the arms, legs, or trunk. Based on Crommert et al, all abdominal muscles’ activities increase by changing the position of the arm during the curl up.²⁰ Also, Burden et al reported that fixing the legs during curl up increases activity in the abdominal muscles.²¹ Still, Andersson et al reported no difference in the abdominal muscle activity with curl up with extended or flexed hip joints.²²

The straight leg raise (SLR) exercise is used to strengthen the abdominal muscles in healthy individuals and patients with low back pain. As a stabilization and strengthening exercise, Bae et al showed that SLR that can strengthen the abdominal muscles.²³ Beales et al reported the tonic activation of IO and external oblique muscles during active SLR exercise in patients with low back pain.²⁴ Yoo et al demonstrated that modified leg raising exercises can decrease pain among patients with low back pain.²⁵

Despite many publications on abdominal muscle function during abdominal muscle training exercises, to our knowledge no study has examined the effects of combining the curl-up with SLR exercises in different lower limb positions. Therefore, the purpose of this study was to identify whether the different angles of hip flexion and abduction during curl-up exercise along with leg raising could be an important contributing factor for ultrasonic thickness of TrA and IO muscles.

Methods

Twenty-eight healthy adults (13 women and 15 men) participated in this study. The exclusion criteria were having (1) a neuromuscular or orthopedic condition and (2) low back pain or extremity pain up to 2 years before participating in the study.

Ethics

Before data collection, we provided information to all participants about the study protocols. All participants signed a written consent form. This study was approved by the research committee of Shahid Beheshti University of Medical Sciences and conducted in accordance with the Declaration of Helsinki.

Procedures

First, we determined the participant's dominant leg. This was done by asking the person to kick a ball²⁶ and fill out a questionnaire that recorded their demographic data. Before data collection, we familiarized the participants with the curl-up exercise. They were in the supine hook-lying position with pillows under the head and the knees and the hands were crossed on the chest. This was the starting position for all tests. To perform the curl-up exercise, the examiner asked the participant to perform the trunk flexion until the lower angle of the scapula was lifted off the mat. Afterward, they were instructed to perform the curl-up exercise and simultaneously the nondominant leg raise exercise with the external rotation of the hip and the dorsiflexion of the ankle in 3 certain positions of the hip (10° flexion, 15° flexion, and 10° flexion and 15° abduction; Fig 1, Fig 2). The hip angle was checked by a hand goniometer. For each participant, hip flexion and abduction angles were measured using a goniometer in the manner measured by Norkin²⁷ and marked on the side wall of the assessment bed. The participants performed a 30-second hold for each test (Figs 1 and 2).

Fig 1 — Demonstration of the curl-up with 30° hip flexion.

Fig 2 — Demonstration of the curl-up with 30° flexion and 15° abduction of hip.

Measurements

The thicknesses of TrA and IO muscles were measured from right and left sides while resting, in 10°, 15° hip flexion, 10° flexion, and 15° abduction during the exercise. Ultrasound image was taken using a 7.5 MHz transducer (HS-2100 Honda Electronic) in B-mode. The transducer was transversely placed on the antero-lateral aspect of the abdominal wall in the area between the iliac crest and the 12th rib, perpendicular to the longitudinal axis of the body. In this point the TrA and IO muscles could be readily identified. When a clear image of deep abdominal muscles was obtained, the image was frozen. Measurements were done at the end of expiration. Thicknesses of the TrA and IO muscles were measured between the inside of fascial borders using the automatic caliper in millimeters. Three images were taken in each position and their average in each image was recorded for analysis. A 10-second rest was given between each position.

Statistical Analysis

We analyzed the data using the Statistical Package for Social Sciences (SPSS), version 20, IBM Corp., Armonk, New York. The sample size was calculated by taking into account the alpha (α = .05) and beta levels (β = .2) (ie, 80% of the power of the test and extracting the values of δ = 2.2 and ∆μ = 1.2). Means and standard deviations of demographic variables of population group as well as muscle thicknesses of TrA and IO while resting and each SLR angle were used for statistical analysis.

The Kolmogorov-Smirnov test was used to assess the normality of data distribution. A repeated measure analysis of variance (ANOVA) of the 5 experimental positions (resting, curl up, curl up with 10º flexion of hip, curl up with 15º flexion of hip, curl up with 10º flexion, and 15º abduction of hip) × 2 sides (dominant vs nondominant) was used to test the main effects of type and sides of exercise and their interactions on the thickness of tested muscles. We did a post hoc test with Bonferroni when significant main effects were found. Statistical significance was set at P < .05.

Intra-examiner reliability was evaluated using type A interclass correlation coefficients with an absolute agreement definition. Two-way mixed effects model with 95% confidence interval (CI) and standard error of measurement was used to evaluate the thickness of TrA and IO muscles in curl-up exercise in 10 healthy young individuals. The measurement process was in 3 steps with an interval of 30 minutes.

Results

The participants’ mean of age was 22.7 ± 2.5 years (20-32 years). Their mean weight was 64.93 ± 8.8 kg (53-80 kg) and mean body mass index was 22.2 ± 2 kg/m². Intraclass correlation coefficients of TrA and IO muscles thickness in curl up with SLR exercises were 0.994 (95% CI, 0.983-0.998) and 0.997 (95% CI, 0.992-0.999), respectively, showing excellent reliability between the 3 scans taken for each muscle (Table 1).

Table 1.

Transverse Abdominis and Internal Oblique Muscles’ Thicknesses in Each Position in Measuring From Both Sides

	TrA (Dominant Side)			TrA (Nondominant Side)			IO (Dominant Side)			IO (Nondominant Side)
	Mean ± SD	P	Standard Error	Mean ± SD	P	Standard Error	Mean ± SD	P	Standard Error	Mean ± SD	P	Standard Error
Rest	3.56 ± 0.76		0.142	3.55 ± 0.78		0.148	7.41 ± 2.21		0.416	7.04 ± 1.69		0.319
Curl up with straight knee	3.83 ± 0.8		0.150	3.80 ± 0.75		0.141	7.97 ± 2.44		0.460	7.64 ± 1.72		0.325
Curl up with 10º flex of hip	4.37 ± 1.05	<.001^a	0.198	4.28 ± 0.93	<.001^a	0.175	9.46 ± 3.39	<.001^a	0.639	9.33 ± 2.04	<.001^a	0.385
Curl up with 30º flex of hip	4.75 ± 1.13	<.001^a	0.212	4.72 ± 0.99	<.001^a	0.186	10.10 ± 3.67	<.001^a	0.709	10.07 ± 2.20	<.001^a	0.416
Curl up with 30º flex and 15º Abd of hip	4.99 ± 1.21	<.001^a	0.229	5.15 ± 1.17	<.001^a	0.220	10.8 ± 3.75	<.001^a	0.739	10.85 ± 2.74	<.001^a	0.517

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Abd, abduction; flex, flexion; IO, internal oblique; SD, standard deviation; TrA, transverse abdominis.

P value for difference of each position compared with curl up with straight knee exercises.

TrA Thickness in Curl Up With Straight Leg Compared With Curl Up With SLR of Different Hip Angles

There was a 13.73% increase in TrA thickness (dominant side) after curl-up exercise with SLR in 10 $^{\circ}$ hip flexion compared with curl up with straight leg. There was a 24.59% increase in TrA thickness (dominant side) after curl-up exercise with SLR in 30° hip flexion compared with curl up with straight leg. There was a 30.83% increase in TrA thickness (dominant side) after curl-up exercise with SLR in 30° flexion and 15° abduction of hip compared with curl up with straight leg. The results of repeated measure ANOVA revealed a significant difference in activation of the ipsilateral TrA (P < .001) and contralateral TrA (P < .001) in all positions except resting.

There was a 12.79% increase in TrA thickness (nondominant side) after curl-up exercise with SLR in 10° hip flexion compared with curl up with straight leg. There was a 25.63% increase in TrA thickness (nondominant side) after curl-up exercise with SLR in 30° hip flexion compared with curl up with straight leg. There was a 37.63% increase in TrA thickness (nondominant side) after curl-up exercise with SLR in 30° flexion and 15° abduction of hip compared with curl up with straight leg. The results of repeated measure ANOVA revealed a significant difference in 4 positions compared with the resting position (P < .001; Table 1). There was no significant difference between TrA muscle's thicknesses in measuring from both sides (Fig 3).

Fig 3 — Comparison of TrA muscle thickness in different test positions and comparison of 2 muscle sides. 1: resting; 2: curl up with straight knee; 3: curl up with 10° flex of hip; 4: curl up with 15° flex of hip; 5: curl up with 10° flex and 15^o Abd of hip. Abd, abduction; flex, flexion; TrA, transverse abdominis.

IO Thickness in Curl Up With Straight Knee Compared With Curl Up With Different Positions of Hip

There was a 17.66% increase in IO thickness (dominant side) after curl-up exercise with SLR in 10° hip flexion compared with curl up with straight leg. There was a 25.83% increase in IO thickness (dominant side) after curl-up exercise with SLR in 30° hip flexion compared with curl up with straight leg. There was a 35.76% increase in IO thickness (dominant side) after curl up with SLR in 30° flexion and 15° abduction of hip compared with curl up with straight leg. The results of repeated measure ANOVA revealed a significant difference in 4 positions of the exercise compared with resting position (P < .001; Table 1).

There was a 22.84% increase in IO thickness (nondominant side) after curl-up exercise with SLR in 10° hip flexion compared with curl up with straight leg. There was a 33.08% increase in IO thickness (nondominant side) after curl-up exercise with SLR in 30° hip flexion compared with curl up with straight leg. There was a 24.99% increase in IO thickness (nondominant side) after curl-up exercise with SLR in 30° flexion and 15° abduction of hip compared with curl up with straight leg. The results of repeated measure ANOVA revealed a significant difference in 4 positions compared with resting (P < .001; Table 1). There was no significant difference between IO thickness in measuring from both sides, and muscles on both sides behaved similarly in different exercises (Fig 4).

Fig 4 — Comparison of IO muscle thickness in different test positions and comparison of 2 muscle sides. 1: resting; 2: curl up with straight knee; 3: curl up with 10º flex of hip; 4: curl up with 15º flex of hip; 5: curl up with 10º flex and 15º Abd of hip. Abd, abduction; flex, flexion; IO, internal oblique.

Discussion

The importance of the abdominal muscles in preventing low back pain has been suggested.¹⁴^,²⁸ Curl-up exercises have been used by therapists and trainers for therapeutic purposes in clinics and for fitness. This exercise can be used to prevent increased lordosis and pelvic tilt by increasing the tension of resting abdominal muscles. Also, different variations of this exercise have always been proposed to maximize and optimize the abdominal muscles. To our knowledge, no study has examined the effect simultaneous position of flexion and abduction hip joint on the thickness of TrA and IO muscles during curl up with SLR exercise. This study aimed to find out what changes in the angles of the lower limbs during curl up with SLR exercises can have the greatest effect on the thickness of TrA and IO muscles, as measured with diagnostic ultrasound. Our first finding suggested that doing curl-up and SLR exercises together leads to a greater increase in the thickness of the deep abdominal muscles than curl up without SLR exercise.

Studies show that muscle thickness on ultrasound is correlated to the muscle activity.⁸^,²⁹ Therefore, the increase in TrA and IO muscles’ thickness in curl up with SLR exercises compared with the curl up with straight knee suggests that there is more abdominal muscle activity in combining curl up with SLR. Recently, some studies have been done on the different lower limb positions’ effects on abdominal muscle activation during abdominal muscle training. Similar to our study, Yoon et al studied the effect of different positions of the hip joint in the frontal plan on the activity of the abdominal muscles. They reported that performing SLR exercise at 10º abduction angle compared with SLR at 30° abduction angle and SLR in neutral position leads to less activation of lateral abdominal muscles.³⁰

Our main finding shows that TrA and IO muscles are more active during curl up with SLR exercise (30° flexion and 15° abduction in hip joint) compared with other positions mentioned in this study. These results can suggest that doing hip abduction during SLR causes more activity in the lateral abdominal muscles. From a biomechanical point of view, abduction in the hip creates an additional rotational moment arm in the hip joint, which requires the abdominal muscles to act more to counteract this moment arm. In this regard, Kim et al show that different angles of the leg in bilateral leg raising exercise have different effects on the ultrasonic activity of the abdominal muscles, so that the maximum thickness of the rectus abdominal muscle is observed at 90º bilateral leg raising, whereas changing the angle of legs in this exercise has no effect on the activity of lateral abdominal muscles.¹⁰

An important point that should be considered when prescribing abdominal strengthening exercises is that minimum pressure should be applied to the spine during the exercise. For example, in a bilateral leg raising exercise, the pelvis tilts forward and the lumbar spine hyperextends, which can cause damage to the lumbar facet joints.⁵ We selected an exercise that puts minimal pressure on the spine to enable the participant to exercise without pain.

Limitations

We analyzed the ultrasonic activity of the abdominal muscles during a static muscle contraction. Although isometric contraction of muscles could be considered as a way to strengthen muscles, the results may not be fully indicative of the concentric contraction of these muscles. Second, this study was performed on healthy individuals. So, our results cannot be generalized or applied to people with back pain.

Conclusion

According to our study, adding unilateral SLR to the curl-up exercise may effectively increase the bilateral activity of deep abdominal muscles in healthy participants. When SLR was performed with abduction, this muscle's activity was increased.`

Acknowledgments

The authors thank Muhammed Hussein Mousavinasab for editing the submitted manuscript.

Funding Sources and Conflicts of Interest

This study was funded by Shahid Beheshti University of Medical Sciences. No conflicts of interest were reported for this study.

Contributorship Information

Concept development (provided idea for the research): N.T, S.M.

Design (planned the methods to generate the results): N.T.

Supervision (oversight, organization and implementation): N.T, S.F.A.

Data collection/processing (experiments, organization, or reporting data): S.F.A, S.M.

Analysis/interpretation (analysis, evaluation, presentation of results): A.A.B.

Literature search (performed the literature search): S.F.A.

Writing (responsible for writing a substantive part of the manuscript): N.T, S.F.A.

Critical review (revised manuscript for intellectual content): N.T.

Practical Applications.

•
Curl-up exercises and straight leg raise (SLR) are 2 strategies for strengthening abdominal muscles.
•
Adding unilateral SLR to the curl up increased the activity in the lateral abdominal muscles.
•
Curl up with 15° hip abduction during SLR caused more activity in these muscles.

Alt-text: Unlabelled box

References

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[bib0001] 1.Escamilla RF, McTaggart MS, Fricklas EJ, et al. An electromyographic analysis of commercial and common abdominal exercises: implications for rehabilitation and training. J Orthop Sports Phys Ther. 2006;36(2):45–57. doi: 10.2519/jospt.2006.36.2.45. [DOI] [PubMed] [Google Scholar]

[bib0002] 2.Akuthota V, Ferreiro A, Moore T, Fredericson M. Core stability exercise principles. Curr Sports Med Rep. 2008;7(1):39–44. doi: 10.1097/01.CSMR.0000308663.13278.69. [DOI] [PubMed] [Google Scholar]

[bib0003] 3.Akuthota V, Nadler SF. Core strengthening. Arch Phys Med Rehabil. 2004;85(3 suppl 1):S86–S92. doi: 10.1053/j.apmr.2003.12.005. [DOI] [PubMed] [Google Scholar]

[bib0004] 4.Malanga GA, Aydin SM, Holder EK, Petrin Z. Springer; New York, NY: 2017. Functional Therapeutic and Core Strengthening: The Hip and Pelvis in Sports Medicine and Primary Care; pp. 185–214. [Google Scholar]

[bib0005] 5.Norris CM. Abdominal muscle training in sport. Br J Sports Med. 1993;27(1):19–27. doi: 10.1136/bjsm.27.1.19. [DOI] [PMC free article] [PubMed] [Google Scholar]

[bib0006] 6.Critchley DJ, Coutts FJ. Abdominal muscle function in chronic low back pain patients: measurement with real-time ultrasound scanning. Physiotherapy. 2002;88(6):322–332. [Google Scholar]

[bib0007] 7.Tahan N, Khademi-Kalantari K, Mohseni-Bandpei MA, Mikaili S, Baghban AA, Jaberzadeh S. Measurement of superficial and deep abdominal muscle thickness: an ultrasonography study. J Physiol Anthropol. 2016;35(1):1–5. doi: 10.1186/s40101-016-0106-6. [DOI] [PMC free article] [PubMed] [Google Scholar]

[bib0008] 8.Tahan N, Rasouli O, Arab AM, Khademi K, Samani EN. Reliability of the ultrasound measurements of abdominal muscles activity when activated with and without pelvic floor muscles contraction. J Back Musculoskelet Rehabil. 2014;27(3):339–347. doi: 10.3233/BMR-130453. [DOI] [PubMed] [Google Scholar]

[bib0009] 9.Rahmani N, Mohseni-Bandpei MA, Salavati M, Vameghi R, Abdollahi I. Comparative study of abdominal muscle thickness on ultrasonography in healthy adolescents and patients with low back pain. J Ultrasound Med. 2018;37(4):905–912. doi: 10.1002/jum.14427. [DOI] [PubMed] [Google Scholar]

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PERMALINK

The Effects of Curl-Up and Straight Leg Raising Exercises in Different Hip Joint Positions on Abdominal Muscle Thickness

Seyyedeh Fatemeh Alavi, MS(PT)

Nahid Tahan, PhD

Saeed Mikaili, PhD

Alireza Akbarzade Baghban, PhD