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Relative Activity Abdominal Muscles During Commonly Prescribed Strengthening Exercise

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    Relative Activity of Abdominal Muscles During Commonly Prescribed Strengthening Exercises

    Clinical Movement Science Laboratory, Division of Physical Therapy Education, University of Nebraska Medical Center, Omaha, Nebraska 68198.


    A common belief held by many rehabilitation professionals is that weakness or imbalance of the abdominal musculature can result in postural malalignment and contribute to the development of low back pain. Kendall et al. (12) address this concept at length in their textbook on muscle testing and function. A key component of their approach to evaluating and treating low back pain involves specific strength assessment of the upper portion of the rectus abdominis (URA) muscle vs. the lower portion (LRA). Other authors have advocated evaluating or strengthening these separate components of the abdominal musculature for treating patients with low back pain and for improving performance in athletics (4, 10, 14, 20–22, 25, 26, 29, 30).

    In addition to the influence the URA, LRA, and external oblique (EOA) muscles may have on low back problems, many individuals are even more concerned with the aesthetics of this area. Numerous abdominal exercises have been prescribed to address the need for “tightening up” specific areas of the trunk. Commonly prescribed exercises proposed for area-specific activation of abdominal muscles include (a) supine trunk flexion activities to emphasize the upper rectus, (b) supine posterior pelvic tilting and lifting the lower half of the body for the lower rectus, and (c) trunk flexion with rotation and vacuum (or “sucking in the gut”) exercises for the obliques (14).

    Although the rationale for attempting to selectively strengthen specific components of the abdominal musculature appears sound, there is a lack of tenable evidence documenting the ability of such exercises to selectively activate different components of the abdominal musculature. The majority of published studies dealing with this issue have had small sample sizes (n 10) and, in most cases, their conclusions have been based on non-normalized electromyographic (EMG) data (3, 6–9, 15, 17, 18, 19, 22–24, 31, 32). Failure to normalize EMG data before quantitative analysis introduces confounding variables not related to muscle function (e.g., skin impedance, electrode orientation, amount of subcutaneous tissue) and thus compromises the validity of the results (1, 13, 28).

    There are, however, several other studies that have used normalized EMG data to assess abdominal muscle activity during exercise. A recent study by Andersson et al. (1) investigated the activity of abdominal and hip flexor muscles during various static and dynamic strengthening exercises. They used two different methods of EMG normalization and concluded that relative EMG values may serve as useful guidelines for selecting exercises to target specific trunk muscles if the EMG data have been appropriately normalized.

    Ekholm et al. (5) evaluated several abdominal strengthening exercises, including a standard trunk curl and a trunk curl with rotation. They reported that the trunk curl with rotation provided a high activation of the oblique abdominal muscles relative to the URA, thus providing some support for the use of the trunk curl with rotation as a strengthening exercise for the obliques. They did not, however, directly compare EMG findings from the standard trunk curl with the trunk curl with rotation findings.

    Axler and McGill (2) assessed spinal compression in conjunction with abdominal EMG activity during 12 different abdominal exercises. Their primary goal was to determine which exercises would provide maximum abdominal muscle strengthening with a minimal amount of spinal compression. In contrast to the results reported by Ekholm et al. (5), Axler and McGill found that the trunk curl with rotation produced greater mean EMG activity in the URA than in the EOA muscle group. They concluded that a variety of abdominal muscle exercises are required to sufficiently challenge all of the abdominal muscles. They did not report the LRA EMG activity findings and did not directly compare EMG findings between each of the 12 exercises tested.

    Lastly, Shields and Heiss (27) evaluated rectus abdominis, EOA, and internal oblique muscle activity during an isometric trunk curl and a double straight leg-lowering exercise. They demonstrated a high degree of test–retest reliability of the normalized EMG data, lending support to the applicability of this approach for evaluating abdominal strengthening exercises. Their results indicated that the trunk curl exercise resulted in similar levels of activity in the rectus abdominis and EOA muscles, but they did not evaluate upper and lower portions of the rectus abdominis separately. A final point of interest in the data presented by Shields and Heiss was their observation that distinctly different patterns of abdominal muscle activity occurred between subjects during the double straight leg-lowering exercise.

    On the basis of our review of the literature, we concluded that there is limited support for the ability of specific exercises to differentially activate different portions of the abdominal musculature, but insufficient data to evaluate the effects of some of the most commonly prescribed abdominal exercises. Thus, the purpose of this study was to determine the relative EMG activity of the URA, LRA, and EOA muscles during performance of 5 commonly prescribed abdominal exercises. On the basis of the traditional descriptions of the effects of these exercises, we hypothesized the following: (a) the trunk curl exercise would elicit the greatest EMG activity in the URA muscle, (b) the reverse curl and v-sit exercises would elicit the greatest EMG activity in the LRA muscle, and (c) the trunk curl with a twist and vacuum exercises would elicit the greatest EMG activity in the EOA muscle. The goal of this study is to provide sound evidence for prescription of these specific exercises.



    Twenty-five subjects (10 men and 15 women) participated in the study. Their average age, height, and weight were 26.7 ± 5.8 years, 172.2 ± 10.3 cm, and 68.9 ± 17.9 kg respectively. Subjects were excluded if they were pregnant, were experiencing low back or abdominal pain, or if they had any abdominal musculature wounds (surgical or traumatic) within the past year.
    Experimental Design

    A repeated-measures design was used to analyze the effect of various abdominal-strengthening exercises on the mean EMG amplitude of different portions of the abdominal musculature. All subjects performed 5 different abdominal exercises during a single testing session. The order of the exercises was randomized. For each muscle group, the null hypothesis was that there would be no difference in mean EMG amplitude between exercises.

    EMG data were collected using pairs of bipolar, silver/silver chloride surface EMG electrodes (8 mm diameter; 12 mm interelectrode distance) with on-site preamplifiers (Therapeutics Unlimited, Iowa City, IA). EMG signals from the preamplifiers were fed to a GCS 67 multichannel EMG amplifier (Therapeutics Unlimited), where they were high-pass filtered (cutoff = 75 Hz), amplified (gain = 500–2,000, as appropriate), and converted to RMS signals (time constant = 55 ms) before being sampled at a rate of 4,000 Hz per channel by a BioPac (BioPac Systems, Goleta, CA) 16-bit analog-to-digital converter and stored for off-line analysis. In addition, a custom-designed potentiometer device was used to track the relative movement of the trunk and pelvis during the 4 variations of the sit-up exercise. A string was attached to the subject at the level of the C7 spinous process (for the exercises in which the trunk was raised) or the S2 spinous process (for exercises in which the pelvis was raised). The string was threaded through a hole in the midline of the plinth on which the subject was lying and attached to a potentiometer to provide a voltage signal proportional to the height that the trunk (or pelvis) was raised above the plinth. This signal was fed to the A/D converter and was used to identify the beginning and end of each repetition of the exercise.

    Subject Preparation. Upon arrival at the lab, subjects provided informed consent and then read a written description of each of the 5 exercises that they would perform (see below). They were then allowed to ask any questions they had concerning the exercises and were provided with a visual demonstration of the exercises if requested. The skin was prepared by shaving (when necessary) and cleansing with isopropyl alcohol. Electrode placement was standardized by placing 3 pairs of bipolar EMG electrodes on the abdomen on the right side of the midline in the following positions: (a) over the URA muscle belly halfway between the umbilicus and xiphoid process and 3 cm to the right of midline; (b) over the LRA muscle belly halfway between the umbilicus and pubic symphysis and 3 cm to the right of the midline; and (c) over the right EOA muscle halfway between the right anterior superior iliac spine and the lowest rib, with the electrodes oriented at a 45° angle superolaterally to inferomedially. These electrode placements are based on those described in previous studies (2, 17). The use of this surface electrode placement for recording from the EOA is supported by a previous study by McGill et al. (17), who compared EMG data from deep trunk muscles using surface and needle electrodes and concluded that the surface electrodes adequately represented the EMG amplitude of the EOA muscles. The common reference electrode was placed on the middle third of the right forearm along the ulnar border.

    Exercises Performed. Each subject performed the following exercises in a randomized order: (a) trunk curl. Subject lies supine with arms crossed over chest, hips flexed to approximately 45° and knees flexed approximately 90° with feet flat on plinth. Subject was instructed to curl head, shoulders, and trunk up until shoulder blades clear the plinth. (b) Reverse curl. Same position as (a) followed by subject being instructed to raise the lower half of the body (including buttocks if possible) off the table as far as possible. (c) Trunk curl with twist. Same position as (a) except the left lower limb is positioned to allow the left ankle to rest on the right knee while the left lower limb is completely relaxed. Subject is instructed to curl head, shoulders, and trunk diagonally, moving the right shoulder toward the left knee. (d) V-sit. Same position as (a) except arms are placed straight at sides. Subject is instructed to curl head, shoulders, and trunk up until shoulder blades clear the plinth while simultaneously raising the lower half of the body (including buttocks if possible) off the table as far as possible. (e) Vacuum. Subject stands with hands on hips, feet at shoulder width and knees extended. Subject is instructed to exhale completely followed by sucking their abdomen up and in as far as possible.

    Data Collection. After being instrumented, subjects performed several repetitions of the V-sit exercise to determine the appropriate gain for EMG amplification. The subject then performed the 5 exercises in the predetermined, randomized order. Two types of data were collected. Isometric (ISO) data consist of the mean amplitude of the rectified EMG during 3 repetitions of a 3-second, maximum-effort, isometric hold. Dynamic (DYN) data consisted of the mean amplitude of the rectified EMG during 3 complete repetitions (concentric + eccentric) of each exercise. The rate of performing the DYN repetitions was controlled by performing the exercises in time with a metronome set at a rate of 108 b·min−1. The subjects were instructed to complete each phase of the exercise (concentric and eccentric) in 3 beats of the metronome. For each exercise, ISO data were collected first, followed by DYN data (for all exercises except the vacuum, which is typically used only in an isometric fashion). Adequate rest was allowed between trials to avoid fatigue.

    Reliability of EMG Measures. To determine test–retest reliability, 7 subjects were asked to perform the same series of exercises on 2 consecutive days and intraclass correlation coefficients (ICC) were calculated on the basis of a one-way repeated-measures analysis of variance (ANOVA). The ICC values (n = 7) for test–retest reliability of the normalized EMG measures used in this study demonstrated generally good reliability, with all value coefficients equal to or greater than 0.75 except for the EOA values during DYN testing. The ICC values for the ISO exercises were: URA = 0.97, LRA = 0.90, and EOA = 0.78. The ICC values for the DYN exercises were: URA = 0.84, LRA = 0.75, and EOA = 0.73.

    Statistical Analyses

    Before statistical analyses, all EMG data were normalized in relation to the maximal value obtained during any of the exercises for each muscle from each subject during ISO testing (maximum voluntary isometric contraction [MVIC]). For each muscle group, a one-way repeated-measures ANOVA was performed to test for significant differences in mean EMG amplitude between exercises. Separate ANOVAs were performed for the DYN and ISO data, α = 0.05. For significant exercise-dependent effects, Tukey post-hoc multiple comparison procedures (all pair-wise comparisons) were used to identify specific differences between the exercises. All statistical analyses were carried out using SigmaStat, version 2.0 (Jandel Scientific, San Rafael, CA).


    URA EMG Activity

    The results of the ISO testing revealed that the trunk curl, reverse curl, and the trunk curl with twist all produced significantly greater URA EMG activity (p < 0.05) than the vacuum exercise, and that the trunk curl and trunk curl with twist also produced significantly greater URA EMG activity than the v-sit. The results of the DYN testing (which did not include the vacuum exercise) did not demonstrate statistically significant differences in URA EMG activity during performance of the 4 DYN exercises. Figure 1 provides a complete summary of the results for the URA under ISO and DYN conditions.
    LRA EMG Activity

    The results of the ISO testing revealed that the reverse curl generated significantly greater LRA EMG activity than did all the other exercises. Once again, the vacuum exercise elicited significantly lower LRA EMG activity than each of the other 4 exercises tested. The reverse curl also produced significantly greater LRA EMG activity than the other 3 exercises during the DYN data collection. In addition, the v-sit resulted in significantly greater LRA EMG activity than the trunk curl and the trunk curl with twist under DYN conditions. Figure 2 summarizes the activity of the LRA.
    EOA EMG Activity

    During ISO testing, there were no significant differences in EOA EMG activity between the 5 exercises studied. The DYN exercise testing EMG values for the EOA revealed that the reverse curl and the v-sit both result in significantly greater EOA EMG activity than the trunk curl or the trunk curl with twist. Overall results for the EOA are summarized in Figure 3 .


    Hypothesis Regarding URA Activity

    On the basis of the claims of Kendall et al. (12), we had hypothesized that of the 5 exercises tested, the trunk curl exercise would produce the greatest amount of URA EMG activity. Our data do not support this. The trunk curl was no more effective than the reverse curl or trunk curl with a twist during the ISO testing, and during DYN testing all 4 exercises produced similar levels of URA EMG activity. Likewise, Axler and McGill investigated abdominal exercises similar to the reverse curl and trunk curl in our study, and reported no significant differences in normalized EMG activity of the URA muscles (2). These findings contradict the claim made by Kendall et al. that the supraumbilical portion of the rectus abdominis is emphasized with trunk flexion exercises, whereas the infraumbilical portion is emphasized with posterior pelvic tilting (12). Thus, although the functional differences proposed by Kendall et al. seemed logical on the basis of anatomical and clinical observations, our EMG data do not support the first hypothesis.
    A possible explanation for these opposing viewpoints could be related to individual variability among subjects seen in the results of a study by Shields and Heiss (27). Our data also demonstrated considerable variability between subjects with respect to which exercise resulted in the greatest amount of URA EMG activity. For example, during ISO testing, the greatest URA activity occurred during the trunk curl for 7 subjects, during the reverse curl for 8 subjects, during the twist curl for 8 of the subjects, and during the v-sit for the remaining 2 subjects. Taken as a whole, our data show that the vacuum exercise produces minimal URA activation, whereas the curl, reverse curl, twist curl, and v-sit are all capable of eliciting relatively high levels of URA muscle activity. Given the minor differences in mean activity levels elicited by these 4 exercises and the fact that each of them elicited maximal URA activity in different subjects, all 4 would appear appropriate for URA strengthening.

    Hypothesis Regarding LRA

    On the basis once again of Kendall et al.'s suggestions regarding rectus abdominis function, we hypothesized that the reverse curl and the v-sit exercises would produce the greatest amount of LRA EMG activity. Our findings with respect to the reverse curl exercise clearly support this hypothesis. Mean EMG values elicited by the reverse curl were significantly greater than those elicited by any of the other exercises under both DYN and ISO testing conditions. With respect to the v-sit exercise, there was limited support for the hypothesis. During DYN testing, the v-sit produced significantly greater LRA activity than the trunk curl or twist curl exercises, but there were no significant differences between those 3 exercises during the ISO testing. On the basis of these findings, we would recommend the reverse curl as the best exercise of those tested for emphasizing LRA muscle activity.
    Hypothesis Regarding EOA Activity

    Our final hypothesis was that the trunk curl with twist would produce the greatest amount of EOA EMG activity. The traditionally cited support for this hypothesis is the anatomical orientation of the muscle fibers in this muscle group, but our EMG data do not support this hypothesis. Under ISO testing conditions, all 5 exercises resulted in mean EOA EMG values of 60 to 75% of maximum, with no significant difference between the exercises. During the DYN testing, both the v-sit and the reverse curl elicited significantly higher EOA activity than the curl or twist curl. Thus both the ISO and DYN data tend to refute the hypothesis that the twist curl exercise preferentially activates the EOA muscle group, whereas the DYN data support the use of the v-sit and reverse curl to target the EOA muscle group.
    Additional Considerations

    On the basis of the premise that the EMG activity level of a muscle during exercise needs to reach at least 60% of MVIC for strengthening to occur (16), our data suggest that all 4 of the DYN exercises (curl, reverse curl, twist curl, and v-sit) would be appropriate for strengthening all 3 muscle groups (URA, LRA, and EOA). The wide variability among the subjects regarding which exercise resulted in the greatest amount of URA EMG activity suggests that clinical EMG devices would be useful for selecting the best exercise for an individual. Although such devices are inappropriate for analysis of muscle onset timing (11) and have limitations with respect to quantitative analysis of EMG amplitude (13, 28), they can be useful for comparing values obtained from a single muscle during a single testing session as an aid in choosing an appropriate exercise to maximize the activity of a specific muscle group.
    Lastly, even though the ISO vacuum exercise resulted in the lowest EOA activity level as compared with the other 4 exercises, it did reach the strengthening threshold (see Figure 3 ). The EMG activity levels recorded for the URA and LRA muscles during performance of this exercise were well below the strengthening threshold (see Figures 1 and 2 ). This suggests that the vacuum exercise may be useful as a means of strengthening the EOA relative to the URA and LRA muscles.

    Practical Applications

    The findings reported here have direct clinical implications for the prescription of abdominal strengthening exercises. Our findings confirm that, at least for the LRA and the EOA muscle groups, there were indeed significant differences in the amount of EMG activity elicited by the abdominal exercise variations tested. The reverse curl exercise appears most likely to target the LRA, whereas the v-sit and reverse curl tend to elicit the greatest EOA activity. For the URA, all of the exercises except the vacuum produced similar levels of activity. Finally, although the vacuum exercise produced slightly less EOA activity than the other exercises tested, the finding that it elicited extremely low levels of URA and LRA activity suggests that it may be useful for strengthening to EOA without concomitant strengthening of the rectus abdominis.


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    Figure 1.Histogram depicting the percentage of MVIC of the upper rectus abdominis muscles during both the isometric and dynamic phases of the exercises. *, significantly greater than vacuum; **, significantly greater than v-sit and vacuum

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    Figure 2.Histogram depicting the percentage of MVIC of the lower rectus abdominis muscles during both the isometric and dynamic phases of the exercises. *, significantly greater than vacuum; **, significantly greater than trunk curl and trunk curl with twist; ***, significantly greater than all other exercises

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    Figure 3.Histogram depicting the percentage of MVIC of the external oblique abdominis muscles during both the isometric and dynamic phases of the exercises. *, significantly greater than trunk curl and trunk curl with a twist

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    The purpose of this study was to determine the relative electromyographic (EMG) activity of the upper and lower rectus abdominis and the external oblique muscles during 5 commonly performed abdominal strengthening exercises. Twenty-five healthy subjects participated in the study. EMG data were collected under isometric and dynamic conditions. The reverse curl resulted in the greatest amount of lower rectus activity, the v-sit and reverse curl exercises resulted in the greatest amount of external oblique activity, and the trunk curl, reverse curl, trunk curl with a twist, and v-sit all resulted in similar amounts of upper rectus EMG activity. The vacuum exercise resulted in moderate levels of external oblique EMG activity but very low levels of activity in the rectus abdominis. Our findings support the concept that abdominal strengthening exercises can differentially activate various abdominal muscle groups, but contradict some traditionally held assumptions regarding the effects of specific exercises.

    Reference Data:Willett, G.M., J.E. Hyde, M.B. Uhrlaub, C.L. Wendel, and G.M. Karst. Relative activity of abdominal muscles during commonly prescribed strengthening exercises.

    Key Words: rectus abdominis, external oblique, electromyography

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