Changes in rectus abdominis and multifidus muscle activity during various leg-raising exercises in the supine position.

[Purpose] The purpose of this study is to compare the rectus abdominis and multifidus muscle activity and rectus abdominis/multifidus ratio ratios during various leg-raising exercises. [Subject and Methods] This study was conducted on 10 healthy males. Leg-raising exercises were performed in four different positions.
[Results] The activity of the rectus abdominis was greatest for raising the legs while flexing the trunk. The rectus abdominis/multifidus ratio value of co-contraction of the rectus abdominis and multifidus was lowest for raising the legs while flexing the knees.
[Conclusion] Raising the legs while flexing the knees, which provides the significant co-contraction of the rectus abdominis and multifidus, is recommended for treating vertebral instability in patients with back pain.

INTRODUCTION

A curl-up is static and cannot stabilize the vertebrae because it focuses only on strengthening the rectus abdominis (RA), not the internal or external obliques that stabilize the vertebrae1). A curl-up that involves raising the legs can be performed not just by healthy people but also by those with compromised back and hip joints to improve health and prevent musculoskeletal disease2). Raising the legs stabilizes the pelvis and prevents the body from tilting forward from the downward traction of the hip joint flexor muscles1, 2). This increases the stability of the vertebrae like a corset and prevents overload3). Researchers have studied various leg-raising exercises to increase activation of deep muscles in the trunk4). However, most studies have focused on the ratio between the RA and rectus femoris muscles to raise the legs, and few studies have focused on stabilizing the lumbar vertebrae by comparing co-contraction between the rectus abdominis (RA) and multifus (MF) muscles. The purpose of this study is to compare the rectus abdominis and multifidus muscle activity and rectus abdominis/multifidus ratio ratios during various leg-raising exercises.

SUBJECTS AND METHODS

This study was conducted on 10 healthy males ages 19–25 years (age: 23.4 ± 2.12 years, height: 175.9 ± 3.96 cm, weight: 67.9 ± 10.18 kg). The purpose and methods of the study were explained to the subjects, and written informed consent was obtained, in keeping with the ethical principles of the Declaration of Helsinki. An MP150WSW (BIOPAC System, Goleta, CA, USA) was used to measure the activity of the rectus abdominis and multifidus. An electromyogram (EMG) disposable electrode (EL 503; BIOPAC System) was used. The MP150 system was used to process the surface EMG analog signals. The electrodes were attached to the bellies of the RA along the muscle line when the trunk was flexed in a supine position. The electrodes were attached to the bellies of the MF along the muscle line while the pelvis was tilted forward in a prone position. Leg raises were completed in four different positions. All exercises were completed in a supine position, and hip joint flexion was performed at 60°. A total of four types of leg raises were performed randomly. First, subjects lifted their legs while completely unfolding the legs (exercise 1). Second, subjects lifted their legs while bending the knees at a 45° angle and completely unfolded the legs at a hip joint flexion of 60° (exercise 2). Third, subjects lifted their legs while fixing the trunk with a belt under the armpit (exercise 3). Fourth, subjects lifted their legs while completely unfolding their legs with the trunk flexed at a 30° angle (exercise 4). A 3-min break was given between exercises to minimize muscle fatigue. Data were analyzed using SPSS for Windows ver. 20.0 (SPSS, Chicago, IL, USA). A one-way repeated-measures analysis of variance was used to assess differences in RA and MF activity and the RA/MF ratio under the various exercise conditions, with the significance level set to α=0.05.

RESULTS

The activity of the RA was greatest for raising the legs while flexing the trunk (40.4%), followed by raising the legs while fixing the trunk (21.5%), raising the legs while flexing the knees (17.8%), and raising the legs in a supine position (17.8%). The activity of the MF did not differ significantly among the exercises (exercise 1, 6.0 ± 5.9%; exercise 2, 5.8 ± 5.9%; exercise 3, 6.0 ± 6.1%; exercise 4, 5.7 ± 4.3%). The RA/MF ratio value for co-contraction of the RA and MF was the lowest for raising the legs while flexing the knees (4.6 ± 2.5), followed by raising the legs in the supine position (5.7 ± 3.5), raising the legs while fixing the trunk (7.3 ± 4.0), and raising the legs while flexing the trunk (14.0 ± 9.6).

DISCUSSION

The results of this study suggest that raising the legs while flexing the trunk selectively strengthened the RA. However, the RA/MF ratio value of co-contraction of the RA and MF was the lowest for raising the legs. Richardson et al. reported that the MF and transverse abdominis must be activated to stabilize the vertebrae while minimizing contraction of global muscles such as the RA and the erector muscles of the spine5). They also reported that the MF and transversus abdominis, which are local muscles, contract separately from the global muscles and that excessive exercise of global muscles causes overload to the vertebrae, increasing pain for patients with back pain and negatively affecting the waist. They added that a clinically safe and effective exercise method should be identified to avoid excessive exercise and prevent structural damage. Raising the legs while flexing the trunk can be effective for increasing the strength of the RA, but it cannot stabilize the vertebrae because of the relatively lower co-contraction with the MF. The results showed that raising the legs in a supine position and raising the legs while flexing the knees resulted in the lowest activity ratio. Co-contraction with the MF would be more effective for stabilizing the vertebrae6). However, raising the legs in a supine position can cause unwanted movement due to external resistance applied to the trunk muscles. Therefore, maintaining a static posture of the trunk is an effective method of stabilizing the trunk4, 6). This means that the proper exercise is to strengthen the abdominal muscles and MF while stabilizing the vertebrae. Therefore, raising the legs while flexing the knees, which provides the significant co-contraction of the MF and RA (RA/MF ratio), is recommended for treating vertebral instability in patients with back pain. This study had some limitations. First, this study was conducted in a laboratory setting with a healthy population. Second, the surface electrodes can only be used to collect muscle activity on surface muscles.

Funding

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1B03035485).

Conflict of interest

None.