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Original Research

Electromyographic Activity of the Rectus Abdominis During a Traditional Crunch and the Basic Jackknife Exercise with the Ab Lounge™

Nelson, Gail A.; Bent-Forsythe, Denise A.; Roopchand-Martin, Sharmella C.

Author Information
Journal of Strength and Conditioning Research: June 2012 - Volume 26 - Issue 6 - p 1584-1588
doi: 10.1519/JSC.0b013e318234e886
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Abstract

Introduction

Strength training for the abdominal muscles is always an adjunct to therapy when treating patients with back pain (10). In addition, strengthening the abdominals in clients suffering from neurological conditions such as cerebral palsy, cerebrovascular disease, and spinal cord injury is vital because it aids in improving posture and gait. Besides these clinical indications, strength training of the abdominals is crucial in enhancing athletic performance and body image. Consequently, strengthening exercises for the abdominal musculature have been thrust into the spotlight causing a boom in the manufacturing of abdominal exercise devices. Of these devices currently or previously sold, only a few have been found to be more effective than a traditional crunch (5,14,16), whereas others have been found to be equally or less effective at eliciting abdominal muscle activity (1,5,10,14,16).

Exercises range from what is known as the ‘traditional crunch’ to the use of specialized equipment. The traditional abdominal crunch is performed in the supine position, and it involves active flexion of the trunk through a concentric muscle contraction of the rectus abdominis muscle as the person lifts the head and scapulae off the supporting surface. Electromyographic (EMG) studies of the abdominal musculature have shown the motor units of rectus abdominis to be very active when performing this head lift from the supine position and also when hyperextending the trunk from the erect position (11).

Noble and Hildenbrand (10) compared the EMG activity elicited in the upper and lower rectus abdominis when performing a traditional abdominal crunch with use of Ab roller, Ab slide, and the fit ball. They concluded that none of the 3 pieces of equipment elicited greater activity of the upper and lower rectus abdominis when compared with the traditional crunch. Instead, both the Ab slide and the fitball resulted in a greater involvement of hip flexors, which is an undesirable feature of abdominal exercises. There was no information presented on the normalization of the EMG readings, thus making it difficult to compare the results of this study with those of other studies. Traditional and nontraditional abdominal exercises have been compared in an attempt to determine the most effective way to activate the abdominal musculature (5). The traditional exercises were the crunch and the bent knee sit up. The nontraditional exercises included devices such as the abdominal straps, the power wheel, and the ab revolutionizer. The EMG activity of the upper rectus abdominis was the highest for the power wheel (roll out), hanging knee-up with straps, and reverse crunch incline 30°.

The Ab Lounge™ is one of the more recent devices on the market, and its manufacturers claim that it is more effective in strengthening the abdominal musculature when compared with the traditional crunch. Justification for this claim is lacking. The marketing campaign for this device has resulted in clients seeking advice from their coaches and physical therapists regarding purchase of this equipment for home use. To effectively advise clients, fitness instructors and rehabilitation professionals must rely on supporting evidence. This study sought to determine whether the basic jackknife exercise on the Ab Lounge™ would result in a higher degree of upper and lower rectus abdominis activation than does the traditional crunch. The authors hypothesized that the traditional crunch would be better at recruiting both the upper and lower rectus abdominis than the basic jackknife.

Methods

Experimental Approach to the Problem

To determine if there was a difference in the degree of muscle activation of the rectus abdominis when performing a crunch or the basic jackknife with the Ab Lounge™, the subjects performed both types of exercise during a single testing session with the order of the exercise being randomized and counterbalanced. The EMG data were recorded for the upper and lower rectus abdominis while the subjects performed each type of exercise and were normalized using a maximal isometric contraction. This study was conducted in a tropical environment, and all the subjects were tested at approximately the same time of the day, in the afternoon. The tests were performed in an air-conditioned room that was maintained at a constant temperature. Before the testing sessions, the subjects had not been involved in any activity requiring physical exertion.

Subjects

Complete data were collected for a total of 22 subjects (6 men and 16 women) who were randomly selected from the Physical Therapy student population at the University of the West Indies (Mona Campus). The mean age of the participants was 20.5 ± 1.5 years, height 166.4 ± 6.2 cm, weight 64 ± 10.3 kg, and waist-hip ratio 0.7 ± 0.1. To be included in the study, the subjects had to sign an informed consent form, which was approved by the University of the West Indies Ethics Committee. The subjects were excluded if they were experiencing back or abdominal pain at the time of data collection, were involved in any regular exercise program, or had a high waist to hip ratio (men > 0.9, women > 0.8). The last criterion was included because it has been shown that significant adipose tissue around the abdomen decreases the contact between the electrode and muscle fibers (9). Ethical approval was obtained from the University of the West Indies' Ethics Committee before the commencement of this study.

Procedures

Before any testing, the subjects were familiarized with the equipment, exercise techniques, and protocols. To maintain the subjects' anonymity, a subject number was assigned, which was used on the data collection sheets, and appointments were given for evaluation. Surface EMG electrodes were placed on the upper and lower rectus abdominis, and these were kept in place throughout the experiment. Data were collected for a maximal voluntary isometric contraction after which a 5-minute rest break was given. The subjects subsequently performed 3 repetitions of the 2 types of abdominal exercises at a consistent pace. The pace was maintained by a metronome, which was set between 52 and 56 b·min−1 (3 seconds per curl-up). A 5-minute rest break was given between each type of exercise. None of the participants reported feeling fatigued at any point during the experiment.

For the maximal voluntary isometric contraction, the subjects were positioned in crook lying with the hands behind the head. They were instructed to raise the head and shoulders toward the knees, moving the trunk through the full flexion motion. With the trunk maximally flexed, the subjects were instructed not to allow movement while a posterior-directed force was applied to the anterior aspect of the acromion bilaterally, as if attempting to push the subjects back into the supine position. This position was held for 10 seconds while the EMG signals were recorded. The subjects were instructed to breathe normally during testing.

The starting position for the traditional crunch was supine on an exercise mat, with the hips maintained at a 45° angle, the knees flexed, and the feet flat on the mat. The hands were placed behind the head (Figure 1A), and the subject was instructed to lift the head and shoulders off the mat to the point where the inferior borders of the scapulae cleared the mat (Figure 1B). The subjects were instructed to exhale as they lifted the head and shoulders off the mat. The pelvis was not allowed to rotate.

Figure 1
Figure 1:
A) Start position for the traditional abdominal crunch. B) Finish position for the traditional abdominal crunch.

The basic jackknife exercise was performed on the Ab Lounge™ (Zhejiang Hotai Machinery Manufacturing Company Limited, China). The subjects were supine on the device with the knees bent and the feet placed on the foot rest. The hands were placed shoulder width apart on the overhead pull-up bar (Figure 2A). The subjects were instructed to exhale and slowly round the lower back while pulling the torso forward into a curl position (Figure 2B). At the same time, they were told to contract the abdominal muscle toward the spine. At the end of the curl, the subjects were told to inhale and slowly return to the starting position.

Figure 2
Figure 2:
A) Start position for the basic jackknife using the Ab Lounge™. B) Finish position for the basic jackknife using the Ab Lounge™.

Electromyographic activity was measured using a standard noninvasive EMG system (Grass Model System S88J, serial number 95mo722G manufactured by Grass Technologies, a subsidiary of Astro-Med Incorporated, Germany). Genuine Grass silver disc surface electrodes were used because surface electrodes have been found to be more reliable with large superficial muscles (6). Raw EMG readings were displayed on a computer. Before application of the electrodes, the area was shaved, scrubbed, and wiped clean with alcohol.

A bipolar placement was used. Two electrodes were placed 7 cm above the umbilicus on the muscle belly of the upper rectus abdominis 2 cm apart. Another pair was placed on the muscle belly of the lower rectus abdominis, 4 cm below the umbilicus, 2 cm apart. A ground electrode was placed on the skin overlying the acromion process. The electrodes were secured in place by micropore tape. Data were sampled at a rate of 1,000 Hz, amplified and bandpass filtered at 20–400 Hz, using a broad band amplifier (Grass model #15LT quad amplifier, Grass Technologies, Astro-Med Inc., Germany). The raw EMG data were full-wave rectified and normalized using a mathematical model that was set up in Microsoft Excel for Windows XP.

Statistical Analyses

To determine the degree of muscle activation of the upper and lower rectus abdominis, mean EMG values were calculated for each portion of the muscle for the traditional crunch and the basic jackknife. A one-way analysis of covariance was used to determine whether gender interacted significantly with the muscle activity. The data were checked for the following assumptions: independence of observations, homogeneity of variances, normal distributions of the dependent variables (upper and lower rectus abdominis EMG activity) and linear relationships between the covariate (gender) and the dependent variables. All assumptions were met. Once it was determined that gender had no effect on the data, a univariate analysis of variance was used to determine if there was a significant difference in the degree of muscle activity of the upper and lower rectus abdominis when the basic jackknife was compared with the traditional crunch. Differences between means were deemed statistically significant when p ≤ 0.05. All analyses were performed using SPSS version 16 for Windows (IBM® SPSS® Statistics, Armonk, NY, USA).

Results

The mean EMG activities recorded for the upper and lower rectus abdominis during the traditional crunch and the basic jackknife are given in Table 1. The mean EMG data recorded for the upper rectus abdominis were significantly higher with the traditional crunch when compared with those of the basic jackknife performed on the Ab Lounge™ (p < 0.05). The traditional crunch also produced a higher level of activity in the lower rectus abdominis when compared with the basic jackknife; however, this was not statistically significant (p > 0.05).

Table 1
Table 1:
Mean (SD) of rectus abdominis muscle activity.

Discussion

It was hypothesized that the traditional crunch would have been more effective in activating the upper and lower portions of the rectus abdominis musculature than the basic jackknife. The results of this study support the hypothesis, particularly for the upper rectus abdominis. The traditional crunch has been previously investigated in many studies with EMG values being comparable with those obtained in this study (1,2,5).

An examination of the technique used in both protocols may explain the difference in abdominal muscle activity. Performing the crunch requires a vertical lift of the trunk against gravity and moving into a flexed position without support. This requires substantial motor unit recruitment (7). Additionally, there is eccentric contraction of the rectus abdominis during controlled lowering of the trunk back to the starting position (8). During the performance of the basic jackknife, the subject holds an overhead bar. As the subject moves into flexion, there is a natural tendency to pull forward with the arms. It is possible that even though clear instructions were provided, individuals still used the upper limbs to pull themselves forward, thereby reducing the degree of activation of the upper rectus abdominis.

It was not anticipated that the jackknife procedure would produce a trend for greater activation of the lower rectus abdominis when compared with the crunch. This could be explained, however, by the hip flexion and pelvic tilting, which occurs with the jackknife exercise. The lower rectus abdominis is reportedly emphasized during the movement of the pelvis and the lower limbs toward the chest (3). A possible reason why the higher degree of activation of the lower rectus abdominis observed with the jackknife did not reach statistical significance may be because of the momentum generated during trunk flexion on the Ab Lounge™. This would passively assist hip flexion and consequently decrease the demand on the lower rectus abdominis.

The results of this study were similar to the findings of other studies comparing the use of various external devices with strengthen the abdominal musculature (5,14,15). One study reported only one of the 6 devices investigated as showing significantly higher activation levels of the upper and lower rectus abdominis than the traditional crunch (15). It must be noted that this device mimicked the traditional abdominal crunch with the only difference being the application of resistance on the anterior chest wall. Another study found that only 2 of the 8 devices investigated produced a higher degree of activation of the upper and lower rectus abdominis than was produced by the traditional crunch (5). For both devices, the technique employed for recruiting the abdominal muscles was significantly different from either the traditional crunch or the basic jackknife.

The findings of an insignificant difference in the lower rectus abdominis recruitment between the 2 protocols could be the result of crosstalk. Crosstalk occurs when the surrounding muscular contraction interferes or muddles the EMG signals of a given muscle group (4). Although considerable efforts were made to reduce this occurrence, this may not have been completely eliminated. The lower rectus abdominis lies beneath the aponeuroses of the transverse abdominis muscle, the internal and external oblique, thereby increasing the potential for crosstalk in the surface EMG signals recorded. This is unlikely to occur with the upper rectus abdominis, because this muscle lies just beneath the skin surface in the upper quadrant. Total elimination of crosstalk can only occur with the use of invasive electrodes. Another possible cause of EMG interference could have been because of electrode shifting caused by skin movement during the jackknife Ab Lounge™ exercise protocol.

An interesting observation in the study was the difference in the degree of activation of upper rectus abdominis relative to the lower rectus abdominis for each of the 2 exercises. For the traditional crunch, there was a slightly higher level of activation of the upper abdominals compared with the lower abdominals, and this pattern was reversed for the jackknife procedure. It is possible that modifications to the traditional crunch could lead to preferential activation of one segment of the muscle over another, as indicated by some researchers (13).

At the time that this study was conducted, no studies using the Ab Lounge™ were identified with which these findings may be compared. As such, further research may be necessary to confirm the results obtained in this study.

Practical Applications

The overall findings of this study have supported the hypothesis that the traditional crunch is more effective than is the jackknife exercise on the Ab Lounge™ in recruiting the rectus abdominis muscle. Clinicians and fitness professionals should consider these findings when recommending equipment and exercises designed to activate the abdominal musculature in young, healthy, sedentary individuals.

Even though the jackknife on the Ab Lounge™ was less effective than the traditional crunch, the EMG findings are useful to rehabilitation professionals when considering alternatives to the traditional crunch for clients with cervical spine injuries. Many individuals with cervical spine pathologies aggravate the problem when attempting to do a traditional crunch. This is even more so when the abdominals are weak. Kinematic data have shown that the degree of cervical spine range of motion during abdominal exercise is less when using devices that provide head support (12). The fact that the Ab Lounge™ appears to provide good support for the cervical spine and does recruit both the upper and the lower rectus abdominis to a certain degree as shown in this study may make it a viable alternative for these clients.

Acknowledgments

The authors wish to thank Ronald E. Young, Ph.D., Professor of Physiology and Ms. Dawn E. Shand of the Physiology Laboratory, the University of the West Indies, Mona campus for their assistance in providing the EMG equipment. The authors also wish to express appreciation to Mrs. Stacey Wong for supplying the exercise device for the study. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. The authors have no conflicts of interest.

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Keywords:

strength training; abdominal muscle recruitment; exercise equipment

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