Which trunk exercise most effectively activates abdominal muscles? A comparative study of plank and isometric bilateral leg raise exercises

Abstract

BACKGROUND:

Various forms of trunk exercise have been used for increasing abdominal strength. The plank exercise and bilateral leg raise exercise are one of these trunk exercises. However, there are few studies that compared the effects of these exercise variations.

OBJECTIVE:

The present study aimed to investigate how effectively the trunk muscles during plank and bilateral leg raise exercises are activated.

METHOD:

Surface electromyography responses of the rectus abdominis, internal oblique, and erector spinae muscles were investigated during the plank and bilateral leg raise exercise with different hip position. A total of 18 healthy, physically active female volunteers completed the normal plank exercise, plank exercise with placing the lower leg in a horizontal condition, bilateral leg raise exercise, and bilateral leg raise with lower leg as horizontal condition.

RESULTS:

The horizontal condition caused significant increases in activity of the internal oblique muscles compared with the general condition ( $p<$ 0.05). The bilateral leg raise exercise showed significantly greater activation in the rectus abdominis compared to the plank exercise ( $p<$ 0.05).

CONCLUSION:

The present study showed that the horizontal condition had the advantage of activating the internal oblique muscles, and the leg raise exercise is effective in strengthening global muscle such as the rectus abdominis.

Keywords

Abdominal strengthening electromyography prone bridge training

1. Introduction

It was suggested that chronic low back pain (CLBP) is one of the major musculoskeletal problems, and about 70–80% of the population experienced back pain at least once in their lifetime [1, 2]. Patient with low back pain show decreased muscle strength of the abdominals and trunk extensors, especially the strength of the abdominal muscle is weaker in individuals with low back pain than in those without [3]. Although recent systemic review demonstrated that muscle strength and endurance could not be predicting factor for low back pain [4, 5], a patient with generated low back pain tends to recover with enhancing trunk strength and endurance exercise program [6].

Strengthening exercises for the para-spinal muscles are necessary to decrease the incidences of pain [5]. Various trunk exercise programs have been used for increasing trunk muscle capacity, including bridge, plank, abdominal hollowing, abdominal bracing, reciprocal leg and arm raising in a quadruped posture, and isometric leg raise exercises [3, 7]. Among them, isometric forms exercise is preferred to adjust the patient with CLBP, which could be useful for improving strength as well as endurance.

Plank and bilateral leg raise are popular isometric exercises for patients with CLBP and healthy population 8,9. Both exercises have similar subjective difficulties and effectively enhance the trunk muscle strength and endurance, without adding an external load such as cuff or sandbag. Both exercises are known to enhance the trunk stability by activating rectus abdominis, external oblique muscles [3, 8]. Despite this similarity, there was no study which compared plank and isometric leg raise exercises. The plank is one of the closed chain exercises, which has fixed position with elbow and toe while elevating pelvic segment to upward direction and maintaining position. On the other hand, bilateral leg raise is open kinetic chain exercise, in which one has to elevate both legs slightly off the floor and maintain position. Clinical literature demonstrated that different kinetic chain exercises affect different muscle stimulations and kinesiology [10]. The present study expected that plank exercise could be more advantageous for stimulating trunk musculature by directly exposing pelvis against gravity, and bilateral leg raise could be advantageous for activating lower extremity musculatures. For investigating those exercises we have differentiated the effect for strengthening musculature and compared the plank and bilateral leg raise exercises with an electromyography study.

Using one’s body weight is known as an effective and safe exercise form. Previous researchers commonly suggested the appropriate kinematic feature should be under premised to assure strengthening effects, especially for isometric exercises [11]. During the standard plank and bilateral leg raise exercises, both hips are slightly flexed for exposing weight of pelvic and both legs against gravity. Because stretched muscle length increases the muscle tension [10], this standard position could be advantageous to abdominal muscle recruitment by reducing tension of the iliopsoas muscle. When exposure of pelvic and both legs against gravity, on the other hand, generated internal force was as increased as positioning segment vertical to gravity line (parallel to the ground). This parallel body position also could be helpful to induce co-contraction trunk musculature, maximize the exercise effects. Therefore, this study addressed the issue of comparing muscle activity between the plank and isometric leg raise exercises, and investigated effects of positioning body segment as horizontal to ground on muscle activities.

2. Methods

2.1 Participants

Participants were recruited from a local university using convenience sampling. As an inclusion criterion, it was determined that female participants must have had no musculoskeletal disorders the previous 6 months. For preventing influences of the gender differences on results of the present study, as well as concerns of the body exposure due to electrode site, the present study has been performed with female participants only.

Figure 1.

Demonstration of the bilateral leg raise exercise (A, B), and plank exercise variations (C, D). A: Bilateral leg raise exercise, B: Bilateral leg raise exercise with horizontal level, C: Plank exercise, D: Plank exercise with a horizontal trunk level.

As an exclusion criterion, participants who have had neurological disorders or medical status such as pregnancy were excluded in the present study. In addition, participants with body mass index (BMI) $\geqslant$ s25 were excluded, avoiding the potential influence of fatty tissue on measuring surface electromyographic (sEMG) activity. A total of 18 female individuals participated in this study. The subjects were 20–25 years old (21.6 $\pm$ 2.4, mean $\pm$ SD) with height and weight of 158.6 $\pm$ 4.2 cm and 55.6 $\pm$ 5.3 kg, respectively. All participants provided written informed consent according to the protocol approved by Kaya University Faculty of Health Science Human Ethics Committee. Fourteen participants were required for the present study design, which set as 80% of power, 0.6 of effect size ( $\eta^{2}$ ), and 0.05 of alpha level by using power analysis provided by the G-power 3.1.9 program (G*Power, Düsseldorf, Germany).

2.2 Instrumentation

The trunk muscle activities were measured using a Trigno wireless system (Delsys, Boston, MA, USA). The sampling rate was at 2000 Hz, and band pass filter was set between 20–450 Hz. The data were recorded with EMGWorks ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Acquisition (Delsys). The sensor (27 $\times$ 37 $\times$ 15 mm) included four skin contacts (5 $\times$ 1 mm) made of pure silver (99.9%). Five surface electrodes were attached parallel to the muscle fibers on the dominant right side as follows: on the rectus abdominis (RA) muscle at approximately 3 cm lateral to the umbilicus; on the belly of the erector spinae at the 3rd lumbar spine (ES-L3) and 2 cm lateral to the spine at the level of the iliac crest; on the internal oblique (IO) muscle at approximately 1 cm medial to the anterior superior iliac spine; on the belly of the erector spinae muscle at the level of the 10th thoracic vertebra (ES-T10) and 2 cm lateral to the spine; and on the rectus femoris (RF) muscle at approximately the midpoint at the line between anterior superior iliac spine and patella [12, 13]. After selection of the sensor, the skin of the patient needs to be prepared in order to get a good electrode-skin contact. The skin surface was shaved, in case the skin surface was covered with hair, and then cleaned with alcohol. After the alcohol was vaporized, the skin would be dry before the electrodes were placed.

Table 1
Descriptive statistics of normalized EMG data (%MVIC ${}_{\textit{RMS}}$ ) of the five muscles during plank and bilateral leg raise exercises

Muscles	%MVIC
	Plank exercise		Bilateral leg raise
	Normal	Horizontal	Normal	Horizontal
RA ${}^{\rm sa}$	51.83 $\pm$ 17.44	58.99 $\pm$ 15.19	63.79 $\pm$ 16.95	65.82 $\pm$ 18.90
IO ${}^{\rm b}$	45.78 $\pm$ 16.16	52.89 $\pm$ 20.42	47.21 $\pm$ 12.71	52.97 $\pm$ 14.33
ES-T10 ${}^{\rm a,b}$	23.42 $\pm$ 7.22	26.38 $\pm$ 9.21	17.77 $\pm$ 8.14	21.25 $\pm$ 7.95
ES-L3	13.47 $\pm$ 7.28	15.60 $\pm$ 9.16	13.72 $\pm$ 7.69	17.65 $\pm$ 13.09
RF	42.75 $\pm$ 20.38	42.00 $\pm$ 17.13	43.98 $\pm$ 30.71	47.39 $\pm$ 29.51

${}^{\rm a}$ : significant difference in exercise factor ( $p<$ 0.05). ${}^{\rm b}$ : significant difference in horizontal condition factor ( $p<$ 0.05). sMVIC: Maximum voluntary isometric contraction, RA: Rectus abdominis, IO: Interal oblique, ES-T10: Erector spine – T10 level, ES-L4: Erector spine – L3 level, RF: Rectus femoris.

Figure 2.

The normalized EMG data (%MVC) of the RA, IO, and ES-T10 during four conditions of exercises. * Significant difference between conditions ( $p<$ 0.05).

2.3 Procedures

After 10 minutes of practice and rest time for acclimation to the exercise, the subject performed four types of trunk exercises as follows in randomized order. (1) Normal plank exercise (The subjects were in the prone position and raised the pelvis from the table, with their body weight supported on the forearms and feet. Subjects were instructed not to elevate their hips, and to try to maintain the pelvic tilt angle). (2) Plank exercise with a horizontal trunk level (This was the same as the normal plank exercise condition but with addition of a step box set as horizontal trunk level under the feet). (3) Bilateral leg raise exercise (The subjects were placed in the supine position and raised both legs 10 cm from the table. Subjects were instructed not to flex their knees, and to try to maintain the pelvic tilt angle). (4) Bilateral leg raise exercise with horizontal level (This was the same as BLR condition but the posterior superior iliac spine was positioned at the edge of the table). For all four exercises, the researcher supported the feet of participants before beginning the sEMG measurements. The exercises were randomly assigned by drawing lots. Subjects were instructed not to flex their knees, and to try to maintain the pelvic tilt angle (Fig. 1).

Subjects conducted two trials under each exercise condition, and sEMG data were collected over a period of 16 seconds controlled by metronome. The participants were allowed to rest for 5 min between exercise conditions and for 3 min between trials. After providing sEMG data during the leg raise and plank exercise variations, each subject performed two trials at maximum voluntary muscle contraction (MVIC) for the included muscles against manual resistance. The maximum manual resistance for each muscle was given to participants from a female researcher, which the methods are according to clinical muscle testing literature [14]. The laboratory conditions during measurements were constantly maintained throughout the study. The highest value between the two MVIC trials was used for the normalization procedure. The normalized values of RA, IO, ES-L4, ES-T10, and RF are presented as %MVIC, and these values were used for statistical analysis.

2.4 Data analysis

The SPSS statistical package (version 12.0; SPSS, Chicago, IL, USA) was used to examine the differences in the activation of RA, IO, ES-L4, ES-T10, and RF between the exercise variations. Two-way repeated-measure ANOVA was performed to test for differences in %MVIC for each muscle activation. For the main effects derived via pairwise multiple comparisons, the Bonferroni correction was performed to identify specific differences between exercise variations. In all analyses, $p<$ 0.05 was taken to indicate statistical significance.

3. Results

The RA activity showed significant differences in factor of exercise condition, which was higher with bilateral leg raise condition compared with the plank exercise (F ${}_{1.17}=$ 8.05, observed power $=$ 0.762) ( $p<$ 0.05). The normalized sEMG data for the IO differed significantly between horizontal conditions (F ${}_{1.17}=$ 12.08, observed power $=$ 0.905) (Table 1, Fig. 2). The horizontal condition caused significant increases in IO activity compared with standard condition regardless of exercise variations ( $p<$ 0.05).

Both the factor of the exercise and horizontal conditions, significant differences in normalized sEMG values were found for the ES-T10 (Table 1, Fig. 2). The plank exercise and horizontal condition showed significantly greater ES-T10 activation, compared with the bilateral leg raise and normal conditions ( $p<$ 0.05).

4. Discussion

In contrast to the initial hypothesis, the results of the present study demonstrated that the plank and bilateral leg raise exercises similarly activated the trunk musculatures. The bilateral leg raise exercise was initially expected to activate the RF rather than trunk muscles because the exercise indirectly activated the trunk musculature by elevating both legs. However, bilateral leg raise exercise elicited greater than 45% of MVC in the RA and the IO muscles. It was suggested that exercise progression included modified bilateral leg raise as level 5 exercise, which the %MVC value of the IO and RA were around 40% [15]. It was reported that EMG signal amplitude greater than 45% MVIC on average may provide sufficient stimulus for strength gains in trunk musculature [16]. In accordance with previous study, both the plank and bilateral leg raise exercises were adequate for strengthening the abdominal muscles especially with internal oblique and rectus abdominis muscles [15, 17]. Regardless of the body horizontal condition, bilateral leg raise exercise showed greater activity of the rectus abdominis than the plank exercise. Although some literature warned the excessive lumbar lordotic curve during bilateral leg raise exercise, the exercise could benefit for gaining greater strength of rectus abdominis.

Different to rectus abdominis, result of internal oblique muscle is only affected by a factor of the horizontal condition, not from a factor of the exercise condition. The horizontal condition during exercise is related with challenging pelvic control during plank exercise and bilateral leg raise exercises. A recent study suggests that greater internal oblique muscle activation could occur when the higher adjustment of the feet height, which was associated preventing pelvic drop during plank exercise [18]. In addition, a previous study also reported that voluntary pelvic control during a leg raise exercise generated decreased pelvic rotation with greater abdominal muscle activity [19]. Both the previous two studies and present study results suggest abdominal muscle such as internal oblique could be more activated with exercise condition which required pelvic control against the increased external load. A previous study suggests that measuring the IO using sEMG indirectly measures the transverse abdominis activity [12]. Therefore, some previous studies demonstrated measures of the internal oblique abdomen as “internal oblique abdominis with transverse abdominis”. Because the transverse abdominis is regarded as a deep abdominal muscle addressing major role for preventing low back pain [20], in this perspective of muscle, the horizontal condition challenging pelvic control is effective for selectively activating deep abdominal muscles.

The 10th level of erector spine muscle was influenced by both the factor of the exercise conditions and the horizontal conditions. Different to the result of the rectus abdomen, the plank exercise more activates the erector spine rather than the bilateral leg raise exercise. This result is general and natural because the thoracic level of paraspinal musculatures are to be activated for resisting lowers of the upper body during plank exercise. On the other hand, the thoracic level of paraspinal musculatures is not necessary for directly resisting something as external load during bilateral leg raise exercises. Transfers of the internal force against the external load might be increased in horizontal conditions that make leg and pelvic positions as parallel to the ground. Therefore, horizontal condition positioned hip joint from the slightly flexed to the extended hips, and weight of the lower limb is increased. In an assumption of the muscle chain theory, 10th level of erector spine muscle might be influenced from the increased weight of the lower limb through thoracolumbar fascia. However, there was no major difference in 4th level of lumbar erector spine. We thought lumbar erector spine’s relatively short distance to the axis might affect this result. Recent finding also support this assumption. A previous study demonstrated that increased external load of the lower limb could increase the scapulothoracic muscular activity rather than para-lumbopelvic musculatures [21, 22].

This study had some limitations. The first is the lack of EMG information for the other trunk muscles, such as the external oblique and gluteus maximus, which may still be influenced by the addition of a horizontal condition during the exercises. Second, our results cannot be generalized because of the limited number of subjects included in the study. Finally, excessive lordosis caused by the anterior tilt of the pelvis cannot be measured with limited video analysis. Further investigations are needed taking these limitations into consideration.

5. Conclusion

In summary, both the plank and bilateral leg raise exercises activate the abdominal muscles, especially with abdominal flexors. Among the investigated muscles, rectus abdominis is highly activated with bilateral leg raise, in comparison to the plank exercise. The horizontal condition was shown to be effective for activating the internal oblique regardless of the exercise conditions.

Footnotes

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (no. 2017R1C1B50747 95).

Conflict of interest

The authors have no conflict of interest to report.

References

Wong

Transfeldt

Macnab

McCulloch

. Macnab’s backache. Philadephia, Williams & Wilkins, 2007.

Wang

Zheng

Lou

Liu

Cai

Hua

Wei

Shen

Chen

Pan

Chen

. A meta-analysis of core stability exercise versus general exercise for chronic low back pain. PLoS One. 2012; 17(12): 520-582.

May

Johnson

. Stabilisation exercises for low back pain: a systematic review. Physiotherapy. 2008; 94(3): 179-189.

Hartvigsen

Kongsted

Hestbaek

. Clinical examination findings as prognostic factors in low back pain: a systematic review of the literature. Chiropr Man Therap. 2015; 23: 13.

Cho

Beom

Lee

Lim

Lee

Yuk

. Trunk muscles strength as a risk factor for nonspecific low back pain: a pilot study. Ann Rehabil Med. 2014; 38(2): 234-240.

Ishak

Zahari

Justine

. Effectiveness of Strengthening Exercises for the Elderly with Low Back Pain to Improve Symptoms and Functions: A Systematic Review. Scientifica (Cairo). 2016; 16: 323-427.

Shamsi

Sarrafzadeh

Jamshidi

Zarabi

Pourahmadi

. The effect of core stability and general exercise on abdominal muscle thickness in non-specific chronic low back pain using ultrasound imaging. Physiother Theory Pract. 2016; 32(4): 277-283.

Schoenfeld

Contreras

Tiryaki-Sonmez

Willardson

Fontana

. An electromyographic comparison of a modified version of the plank with a long lever and posterior tilt versus the traditional plank exercise. Sports Biomech. 2014; 13(3): 296-306.

Andersson

Nilsson

Thorstensson

. Abdominal and hip flexor muscle activation during various training exercises. Euro J App Physiol & Occup Physiol. 1997; 75(2): 115-123.

10.

Muscolino

. Kinesiology: the skeletal system and muscle function, (2nd ed.), St Louis, Mosby; 2011, pp. 479-499.

11.

Lee

McGill

. Effect of long-term isometric training on core/torso stiffness. J Strength Cond Res. 2015; 29(6): 1515-1526.

12.

Czaprowski

Afeltowicz

Gȩbicka

Pawłowska

Kȩdra

Barrios

Hadała

. Abdominal muscle EMG-activity during bridge exercises on stable and unstable surfaces. Phys Ther Sport. 2014; 15(3): 162-168.

13.

Cram

Kasman

Holtz

. Introduction to surface electromyography, 1st ed, Aspen, 1998.

14.

Kendall

McCreary

Provance

Rodgers

Romani

. Muscles: testing and function, with posture and pain, 5th ed. Baltimore, MD, Lippincott Williams & Wilkins; 2005.

15.

Davidson

Hubley-Kozey

. Trunk muscle responses to demands of an exercise progression to improve dynamic spinal stability. Arch Phys Med Rehabil. 2005; 86(2): 216-223.

16.

Ekstrom

Donatelli

Carp

. Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises. J Orthop Sports Phys Ther. 2007; 37(12): 754-762.

17.

Kibler

Press

Sciascia

. The role of core stability in athletic function. Sports Medicine. 2006; 36(3): 189-198.

18.

Park

Yoo

. Effect of height of feet on trunk muscle activity and pelvic tilt angle during prone bridge exercises. Isokinet. Exerc. 2016; 24(30): 189-194.

19.

Park

Kim

Park

Kwon

. Effects of the pelvic rotatory control method on abdominal muscle activity and the pelvic rotation during active straight leg raising. Man Ther. 2013; 18(3): 220-224.

20.

Preece

Willan

Nester

Graham-Smith

Herrington

Bowker

. Variation in pelvic morphology may prevent the identification of anterior pelvic tilt. J Man Manip Ther. 2008; 16(2): 113-117.

21.

Comfort

Pearson

Mather

. An electromyographical comparison of trunk muscle activity during isometric trunk and dynamic strengthening exercises. J Strength Cond Res. 2011; 25(1): 149-154.

22.

Rutkowska-Kucharska

Szpala

. Electromyographic muscle activity in curl-up exercises with different positions of upper and lower extremities. J Strength Cond Res. 2010; 24(11): 3133-3139.

Which trunk exercise most effectively activates abdominal muscles? A comparative study of plank and isometric bilateral leg raise exercises

Abstract

BACKGROUND:

OBJECTIVE:

METHOD:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Participants

Table 1 Descriptive statistics of normalized EMG data (%MVIC 𝑅𝑀𝑆 ) of the five muscles during plank and bilateral leg raise exercises

2.4 Data analysis

3. Results

4. Discussion

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Descriptive statistics of normalized EMG data (%MVIC ${}_{\textit{RMS}}$ ) of the five muscles during plank and bilateral leg raise exercises