An Electromyographic Analysis of
Commercial and Common Abdominal
Exercises: Implications for Rehabilitation
and Training
R...
There are numerous exercises used for abdominal
strengthening. Many of these exercises also activate
extraneous (nonabdomi...
The purpose of this study was to test the effective-
ness of 7 popular commercial abdominal machines
and 2 common abdomina...
bent-knee sit-up (Figure 8) and crunch (Figure 9).
No subject had prior experience in performing the 7
commercial abdomina...
Procedures
All subjects became familiar with all abdominal
exercises during a pretest session that took place
approximatel...
a slow and controlled manner using the 3-second
cadence previously described and 1-second rest be-
tween repetitions. With...
TABLE1.AverageEMG(±SD)foreachmuscleandexerciseexpressedasa%ofmaximumisometricvoluntarycontraction.
UpperRectus
Abdominis*
...
FIGURE 10. Upper rectus abdominis normalized mean (SD) EMG activity among exercises.
FIGURE 11. Lower rectus abdominis nor...
FIGURE 12. External oblique normalized mean (SD) EMG activity among exercises.
FIGURE 13. Internal oblique normalized mean...
exercises activated abdominal musculature by actively
flexing the trunk by concentric contractions during
the initial port...
Biomechanical Differences Between the Crunch and
Bent-Knee Sit-up
It should be noted that not all abdominal exercises
invo...
latissimus dorsi. Both the sternal pectoralis major
(lower fibers) and latissimus dorsi contract eccentri-
cally during th...
ately placed surface electrodes accurately reflect
(within 10%) the muscle activity within the internal
or external obliqu...
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Paper 01 emg abdominal escamilla feb 2006

  1. 1. An Electromyographic Analysis of Commercial and Common Abdominal Exercises: Implications for Rehabilitation and Training Rafael F. Escamilla, PT, PhD, CSCS1 Michael S.C. McTaggart, MS2 Ethan J. Fricklas, MSE3 Ryan DeWitt, MPT4 Peter Kelleher, MPT4 Marcus K.Taylor, PhD5 Alan Hreljac, PhD6 Claude T. Moorman, III, MD7 Study Design: A repeated-measures, counterbalanced design. Objectives: To test the effectiveness of 7 commercial abdominal machines (Ab Slide, Ab Twister, Ab Rocker, Ab Roller, Ab Doer, Torso Track, SAM) and 2 common abdominal exercises (crunch, bent-knee sit-up) on activating abdominal and extraneous (nonabdominal) musculature. Background: Numerous abdominal machine exercises are believed to be effective in activating abdominal musculature and minimizing low back stress, but there are minimal data to substantiate these claims. Many of these exercises also activate nonabdominal musculature, which may or may not be beneficial. Methods and Measures: A convenience sample of 14 subjects performed 5 repetitions for each exercise. Electromyographic (EMG) data were recorded for upper and lower rectus abdominis, external and internal oblique, pectoralis major, triceps brachii, latissimus dorsi, lumbar paraspinals, and rectus femoris, and then normalized by maximum muscle contractions. Results: Upper and lower rectus abdominis EMG activities were greatest for the Ab Slide, Torso Track, crunch, and Ab Roller, while external and internal oblique EMG activities were greatest for the Ab Slide, Torso Track, crunch, and bent-knee sit-up. Pectoralis major, triceps brachii, and latissimus dorsi EMG activities were greatest for the Ab Slide and Torso Track. Lumbar paraspinal EMG activities were greatest for the Ab Doer, while rectus femoris EMG activities were greatest for the bent-knee sit-up, SAM, Ab Twister, Ab Rocker, and Ab Doer. 1 Associate Professor of Physical Therapy, California State University Sacramento, Sacramento, CA. 2 Graduate student (at the time of study), Duke University Medical Center, Durham, NC. 3 Student (at the time of study), Duke University Medical Center, Durham, NC. 4 Student (at the time of study), California State University Sacramento, Sacramento, CA. 5 Lieutenant, Medical Service Corps, US Navy, Naval Health Research Center, San Diego, CA. 6 Associate Professor of Kinesiology and Health Science, California State University Sacromento, Sacromento, CA. 7 Associate Professor of Orthopaedic Surgery, Duke University Medical Center, Durham, NC. The protocol used in this study was approved by the Institutional Review Board at Duke University Medical Center, Durham, NC. The authors of this manuscript affirm we have no financial affiliation (including research funding) or involvement with any commercial organization that has a direct financial interest in any matter included in this manuscript. Address correspondence to Rafael Escamilla, Associate Professor of Physical Therapy, California State University Sacramento, Department of Physical Therapy, 6000 J Street, Sacramento, CA 95819-6020. E-mail: rescamil@csus.edu Conclusions: The Ab Slide and Torso Track were the most effective exercises in activating abdominal and upper extremity muscles while minimizing low back and rectus femoris (hip flexion) activity. The Ab Doer, Ab Twister, Ab Rocker, SAM, and bent-knee sit-up may be problematic for individuals with low back pa- thologies due to relatively high rectus femoris activity. J Orthop Sports Phys Ther 2006;36:45- 57. Key words: EMG, low back pain, lumbar spine, rectus abdominis, sit-up S trong abdominals are im- portant for stabilizing the trunk and helping unload stress in the lum- bar spine.3,13 Abdominal muscles (rectus abdominis, exter- nal oblique, internal oblique, and transverse abdominal) are com- monly strengthened by actively flexing the trunk with a concentric muscle action or by resisting trunk extension (due to an external force such as gravity) with an iso- metric or eccentric muscle action. Journal of Orthopaedic & Sports Physical Therapy 45 RESEARCHREPORT Journal of Orthopaedic & Sports Physical Therapy Official Publication of the Orthopaedic and Sports Physical Therapy Sections of the American Physical Therapy Association
  2. 2. There are numerous exercises used for abdominal strengthening. Many of these exercises also activate extraneous (nonabdominal) muscles, such as the hip flexors, lumbar paraspinals, or upper extremity mus- culature, which may or may not be beneficial. For example, high activation levels from the hip flexors and lumbar paraspinals tend to generate a force couple that attempts to anteriorly rotate the pelvis and increase lumbar lordosis. When coupled with weak abdominal musculature, activation of these ex- traneous muscles may increase the risk of low back pathologies. Understanding the muscle activation generated by different abdominal exercises is useful to therapists and other health care or fitness specialists who develop specific abdominal exercises for their pa- tients or clients to facilitate their rehabilitation or training needs and objectives. For example, abdomi- nal exercises that actively flex the trunk may be problematic for individuals with lumbar disk patholo- gies due to increased intradiscal pressure18 and lum- bar spine compression,3 and for individuals with osteoporosis due to the risk of vertebral compression fractures.21 However, these same individuals may be asymptomatic during abdominal exercises that main- tain a relatively neutral spine and pelvis. In contrast, individuals with facet joint syndrome, spondylolis- thesis, and vertebral or intervertebral foramen stenosis may not tolerate exercises such as the Ab Slide and Torso Track due to the extended spine position. There are numerous commercially available ab- dominal machines that are believed to be effective in activating abdominal musculature and minimizing low back stress, but there are little or no scientific research data to substantiate these beliefs. While there are numerous studies that examined muscle activity during more traditional abdominal exercises, such as the sit-up or crunch exercises,3,14,16,26,27 there is a scarcity of data related to the use of abdominal machines. A limited number of studies compared select abdominal muscle activity while performing exercises using the Torso Track, Ab Doer, Ab Shaper, Ab Flex, and Ab Roller,6,7,12,24,26 but there are no studies that we are aware of that have quantified abdominal muscle activity while using the Ab Twister, Ab Rocker, Super Abdominal Machine (SAM), and Ab Slide. Moreover, when using abdominal machines, the extent of recruitment of extraneous musculature, such as low back or upper and lower extremity musculature, is currently unknown because there have been no studies that have reported extraneous muscle activity while performing these exercises. It is also unknown how abdominal machines compare to traditional abdominal exercises, such as the sit-up and crunch, in recruiting the abdominal musculature. Abdominal machines use various techniques to target different muscles. For example, some abdomi- FIGURE 1. Ab Rocker. FIGURE 2. Ab Roller. nal machines allow only uniplanar motions, such as trunk flexion, while others use multiplanar motions, such as trunk flexion and rotation. It is commonly believed that performing simultaneous trunk flexion and rotation recruits the external and internal ob- lique musculature to a greater extent compared to trunk flexion only. However, there is currently no scientific evidence to support this assertion. 46 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  3. 3. The purpose of this study was to test the effective- ness of 7 popular commercial abdominal machines and 2 common abdominal strengthening exercises on activating abdominal and extraneous musculature. It was hypothesized that significant differences would be found in the normalized electromyographic (EMG) data of both abdominal and extraneous muscle activ- ity among exercises. METHODS Subjects To optimize the EMG signal, this study was limited to a convenience sample of 14 healthy, young subjects (7 male and 7 female) who had normal or below normal amounts of body fat for their age group. Baseline skinfold calipers (model 68900; Country Technology, Inc, Gays Mill, WI) and appropriate regression equations were used to assess percent body fat, and standards set by the American College of Sports Medicine were used to determine normal or below normal amounts of body fat.4 Mean (±SD) age, mass, height, and percent body fat were 24.1 ± 5.4 years, 58.7 ± 4.9 kg, 166.8 ± 5.9 cm, and 22.7% ± 1.9%, respectively, for females, and 24.0 ± 7.1 years, 78.6 ± 13.9 kg, 179.8 ± 4.1 cm, and 9.7% ± 4.1%, respectively, for males. All subjects provided written informed consent in accordance with the Institutional Review Board at Duke University Medical Center. FIGURE 3. Ab Doer. FIGURE 4. Ab Twister. FIGURE 5. Torso Track. FIGURE 6. Ab Slide. Individuals were excluded from the study if they had a history of abdominal or back pain, or were unable to perform all exercises pain free and with proper form and technique for 12 consecutive repetitions. Exercise Descriptions The 7 abdominal machine exercises were the Ab Rocker (Figure 1), Ab Roller (Figure 2), Ab Doer (Figure 3), Ab Twister (Figure 4), Torso Track (Figure 5), Ab Slide (Figure 6), and SAM (Figure 7). The 2 common abdominal exercises tested were the J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 47 RESEARCHREPORT
  4. 4. bent-knee sit-up (Figure 8) and crunch (Figure 9). No subject had prior experience in performing the 7 commercial abdominal exercises, but they had mod- erate experience in performing the crunch and bent-knee sit-up. The Ab Rocker, Ab Twister, Ab Doer, and SAM exercises started and ended in a seated position with a neutral spine and pelvis. The 2 common move- ments advertised for the Ab Rocker and Ab Twister were the crunch (involving sagittal plane trunk flex- ion) and the oblique crunch (moving obliquely across the body by simultaneously flexing and rotat- ing the trunk), and both were tested with rotation occurring to the left. Three common movements advertised for the Ab Doer were the body bob (frontal plane side-to-side motion), body boogie (moving in a circular motion), and good morning (involving sagittal plane trunk flexion), and all 3 of these variations were tested. The movement for the SAM involved sagittal plane trunk flexion, similar to how the Ab Rocker (crunch), Ab Twister (crunch), and Ab Doer (good morning) were performed. The Ab Roller, crunch, and bent-knee sit-up started and ended in a supine position. The crunch and Ab Roller both had 2 variations: a normal crunch involv- ing sagittal plane trunk flexion and an oblique crunch involving moving obliquely across the body by FIGURE 7. SAM. FIGURE 8. Bent-knee sit-up. FIGURE 9. Crunch. simultaneously flexing and rotating the trunk to the left. The primary differences between the 2 exercises were that during the Ab Roller the head was sup- ported by a head pad and the arms were supported by a supporting bar (Figure 2), while during the crunch the thumbs were positioned in the ears and the hands were relaxed against the head (this hand position was standardized for comfort for both the crunch and bent-knee sit-up) (Figure 9). Both varia- tions for the crunch and Ab Roller involved a curling-up motion (trunk flexion or trunk flexion with left rotation) until both scapulae were off the ground. During the bent-knee sit-up the thumbs were positioned in the ears with the hands relaxed against the head, the feet were supported and held down, the knees were flexed approximately 90°, and from this supine position the subject simultaneously flexed the trunk and hips until the elbows were even with the knees (Figure 8). The Ab Slide and Torso Track started and ended in the quadruped position (on hands and knees with hips and shoulders flexed approximately 90°), with a neutral spine and pelvis. From this position the subject straightened out the body by rolling forward in a straight line (Torso Track and Ab Slide straight) or a curved line to the left (Ab Slide curved), while maintaining a neutral spine and pelvis (Figures 5 and 6). 48 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  5. 5. Procedures All subjects became familiar with all abdominal exercises during a pretest session that took place approximately 1 week prior to the testing session. During the pretest session, each subject received instructions explaining how to correctly perform each of the abdominal exercises (each abdominal machine came with written or video instructions for its use). All exercises were performed with a 3-second cadence (1 second from start of exercise to end range, 1-second isometric hold at end range, 1-second return to starting position) and a 1-second rest between repetitions. The subjects practiced all exercises under the supervision of trained research personnel. All of the commercial exercises except the Ab Roller and Ab Slide had adjustable elastic bands to make the exercise easier or harder. To better normalize the intensity of each exercise, resistance was adjusted according to each subject’s preference and the manufacturer’s recommendation (eg, using a resistance that was not too hard but hard enough to allow the execution of at least 15 repetitions)— similar to how each subject would adjust the resis- tance and use the equipment if they purchased it for home use. The selected resistance during the pretest session was also used for that subject during the testing session. For each exercise, each subject used a resistance that enabled the subject to correctly per- form at least 15 consecutive repetitions using the 3-second cadence described above. It was not possible to normalize all exercises with exactly the same relative intensity because the Ab Roller, Ab Slide, crunch, and bent-knee sit-up used the body only as an external resistance, while the remaining exercises used resistance bands in addition to body as external resistance. In addition, even when the maximum resistance possible was used for the Ab Doer, Ab Twister, and Ab Rocker, all subjects indicated that they were capable of performing these exercises with more resistance. A metronome (set at 1 beat per second) was used to help ensure proper cadence both during the pretest and testing sessions. Once a subject was able to correctly perform each exercise with the proper cadence, a testing session was sched- uled. Neuroline (Medicotest Marketing, Inc, Ballwin, MO) disposable surface electrodes (type 720-00-S) were used to collect EMG data. These oval-shaped electrodes (22 mm wide and 30 mm long) were placed in a bipolar electrode configuration along the longitudinal axis of each muscle, with a center-to- center distance of approximately 3 cm between elec- trodes. Prior to positioning the electrodes over each muscle, the skin was prepared by shaving, abrading, and cleaning with isopropyl alcohol wipes to reduce skin impedance values, which typically were less than 10 k⍀. Electrode pairs were then placed on the subject’s right side (except for the internal oblique, which was positioned on the subject’s left side) for the following muscles in accordance with procedures previously described5,8,17,20 : (a) upper rectus abdominis, positioned vertically and centered on the muscle belly (not on tendinous intersection) near the midpoint between umbilicus and xiphoid process and 3 cm lateral from midline; (b) lower rectus abdominis, positioned 8° from vertical in inferomedial direction and centered on the muscle belly near the midpoint between umbilicus and pubic symphysis and 3 cm lateral from midline; (c) external oblique, positioned obliquely approximately 45° (par- allel to a line connecting the most inferior point of the costal margin of the ribs and the contralateral pubic tubercle) above anterior superior iliac spine (ASIS) near the level of the umbilicus; (d) internal oblique, positioned horizontally 2 cm inferomedial to the ASIS, within a triangle confined by the inguinal ligament, lateral border of the rectus sheath, and a line connecting the ASISs (it has been demonstrated that in this region only the aponeurosis of the external oblique, and not the external oblique muscle, covers the internal oblique)20 ; (e) sternal pectoralis major, positioned horizontally 2 cm medial to the axillary fold; (f) triceps brachii long head, positioned vertically over the long head muscle belly near midline of the arm approximately halfway be- tween the acromion and olecranon; (g) latissimus dorsi, positioned obliquely (approximately 25° from horizontal in an inferomedial direction) 4 cm below inferior angle of the scapula; (h) rectus femoris, positioned vertically near midline of thigh approxi- mately halfway between ASIS and proximal patella; and (i) lumbar paraspinals, positioned vertically 3 cm lateral to spine and near level of iliac crest between L3 and L4 vertebrae. A ground (reference) electrode was positioned over the skin of the right acromion process. Electrode cables were connected to the electrodes and taped to skin appropriately to mini- mize pull on the electrodes and movement of the cables. Once the electrodes were positioned, the subject warmed up and practiced the exercises as needed, then data collection commenced. EMG data were collected using a Noraxon 16-channel telemyo EMG unit (Noraxon USA, Inc, Scottsdale, AZ), and the amplifier bandwidth frequency was 10 to 500 Hz. The input impedance of the amplifier was 20 000 k⍀ and the common-mode rejection ratio was 130 dB. EMG data were sampled at 1000 Hz, recorded by a transmitter and amplifier, and broadcast to a receiver interfaced to a computer. The recorded signals were processed through an analog-to-digital (A/D) con- verter by a 16-bit A/D board. EMG data were collected during 5 repetitions for each exercise, with all exercises performed in a randomized order. Each repetition was performed in J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 49 RESEARCHREPORT
  6. 6. a slow and controlled manner using the 3-second cadence previously described and 1-second rest be- tween repetitions. With a relatively low number of repetitions performed, all subjects acknowledged that fatigue was minimized. Each testing session took approximately 45 minutes to complete. Randomly interspersed within the exercise testing session, EMG data from each muscle tested were collected during two 5-second maximum voluntary isometric contractions (MVICs). After conducting pi- lot work, we adopted the following protocols for MVIC testing, which were based on the positions that elicited the greatest MVIC for each respective muscle (all MVICs were collected on a plinth with subject in a prone, supine, or short-sitting position): (a) upper and lower rectus abdominis, body supine with hips and knees flexed 90°, feet supported, and trunk maximally flexed (ie, curl-up position), with resis- tance at the shoulders in the trunk extension direc- tion; (b) external and internal oblique, body supine with hips and knees flexed 90°, feet supported, and trunk maximally flexed and rotated to the left, with resistance at the shoulders in the trunk extension and right rotation directions; (c) sternal pectoralis major, body supine with right shoulder flexed 90° and internally rotated, the right forearm supinated, and the right elbow slightly flexed, with resistance at the right distal arm and forearm in the horizontal abduction direction; (d) triceps long head, body prone with right shoulder abducted 90° and right elbow flexed 45°, with resistance at the right distal forearm in the elbow flexion direction; (e) latissimus dorsi, body prone with right shoulder abducted 0° and extended maximally, with resistance at the right distal arm in the direction of shoulder flexion; (f) lumbar paraspinals, body prone with trunk fully extended and hands clasped behind head, with resis- tance at the shoulders in the direction of trunk flexion; and (g) rectus femoris, body in short-sitting position with hips and knees flexed 90°, with resis- tance at the distal leg in the knee flexion direction. The MVICs were collected to normalize the EMG data collected during the abdominal exercises. Each subject was given similar verbal encouragement for each MVIC to help ensure a maximum effort throughout the 5-second duration, and the subject was asked after each MVIC if he/she felt it was a maximum effort. If not, the MVIC was repeated. An approximately 1-minute rest was given between each MVIC and an approximately 2-minute rest was given between each exercise trial. Data Processing Raw EMG signals were full-wave rectified, smoothed with a 10-millisecond moving average window, and linear enveloped, then averaged over the entire duration of each exercise repetition. For each repeti- tion the EMG data were normalized for each muscle and expressed as a percentage of a subject’s highest corresponding MVIC trial, which was determined by calculating throughout the 5-second MVIC the high- est average EMG signal over a 1-second time interval. Normalized EMG data were then averaged over the 5 repetition trials performed for each exercise and used in statistical analyses. Data Analysis A 1-factor repeated-measures analysis of variance was employed to assess differences in normalized EMG muscle activity among the different exercise variations (PϽ.01). Post hoc analyses were performed using the Bonferroni test to evaluate the significance of between-exercise pairwise comparisons (PϽ.01). RESULTS Normalized EMG data for each muscle and exer- cise are shown in Table 1. Among all exercises tested, upper rectus abdominis EMG activities were greatest for the Ab Slide (straight and curved), Torso Track, crunch (normal and oblique), and Ab Roller (crunch and oblique) exercises, and lowest for the Ab Twister (crunch and oblique), Ab Rocker (crunch and ob- lique), and Ab Doer (good morning, body boogie, and body bob) exercises. Lower rectus abdominis EMG activities were greatest for the Ab Slide (straight and curved) and Torso Track exercises, and lowest for the Ab Twister (crunch and oblique), Ab Rocker (crunch and oblique), and Ab Doer (good morning, body boogie, and body bob) exercises. Graphical representations of upper and lower rectus abdominis activity ranked from highest to lowest among all exercises are shown in Figures 10 and 11. The external oblique EMG activity for the crunch (normal), Ab Roller (crunch), and Ab Doer (good morning) exercises were significantly lower compared to the Ab Slide (straight and curved) and bent-knee sit-up exercises. Internal oblique EMG activities were greatest for the Ab Slide (straight and curved), Torso Track, bent-knee sit-up, and crunch (normal and oblique) exercises, and lowest for the Ab Roller (oblique), Ab Twister (crunch and oblique), Ab Rocker (crunch and oblique), and Ab Doer (good morning) exercises. Graphical representation of ex- ternal and internal oblique activity ranked from highest to lowest among all exercises are shown in Figures 12 and 13. Sternal pectoralis major EMG activities were great- est for the Ab Slide (straight and curved), Torso Track, SAM, and Ab Twister (crunch and oblique) exercises, and lowest for the Ab Rocker (crunch and oblique), Ab Doer (good morning, body boogie, and body bob), Ab Roller (crunch and oblique), and crunch (normal and oblique) exercises. Triceps brachii long head EMG activities were significantly 50 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  7. 7. TABLE1.AverageEMG(±SD)foreachmuscleandexerciseexpressedasa%ofmaximumisometricvoluntarycontraction. UpperRectus Abdominis* LowerRectus Abdominis* External Oblique* Internal Oblique* Sternal Pectoralis Major* Triceps LongHead* Latissimus Dorsi* Lumbar Paraspinals* Rectus Femoris* AbSlide(straight)67±2672±1940±1653±1523±730±1210±43±2k 5±3fhj AbSlide(curved)61±2466±1942±1751±1520±926±1210±32±2k 9±7f TorsoTrack67±2572±1732±1858±1420±826±1110±52±2k 6±5fhj Crunch(normal)51±950±8ab 16±11af 41±94±3abgi 1±1ab 5±1g 2±1k 3±2fhj Crunch(oblique)50±1539±14ab 32±2240±116±5agi 2±2ab 8±55±3k 3±2fhj Bentkneesit-up38±12ab 44±13ab 41±1649±218±6ag 2±2ab 6±3g 4±2k 36±16 SAM42±17ab 50±20ab 31±2136±13b 26±1510±6ab 12±64±2k 20±15 AbRoller(crunch)46±1742±12ab 13±8af 38±9b 7±5agi 3±2ab 5±2g 3±2k 1±1fhj AbRoller(oblique)49±1236±16ab 20±925±11abf 5±3abgi 3±2ab 6±2g 3±2k 2±2fhj AbTwister(crunch)19±8abcde 19±10abcdefg 21±1222±9abf 13±117±3ab 5±2g 4±3k 27±19 AbTwister(oblique)20±7abcde 22±11abcfg 33±1828±11abf 22±155±4ab 6±2g 5±6k 24±14 AbRocker(crunch)15±8abcdefg 13±5abcdefg 22±1124±8abf 7±7agi 6±4ab 6±3g 4±3k 30±21 AbRocker(oblique)14±10abcdefg 14±8abcdefg 31±1823±8abf 6±6agi 7±4ab 5±2g 3±1k 21±16 AbDoer(goodmorning)14±7abcdefg 14±5abcdefg 16±11af 22±13abf 4±4abgi 2±1ab 2±2abg 15±712±11f AbDoer(bodyboogie)12±4abcdefg 11±6abcdefg 24±1031±13b 3±2abgi 1±1ab 2±1abg 13±824±19 AbDoer(bodybob)7±5abcdefg 7±4abcdefg 30±1937±18b 2±2abgi 2±1ab 1±1abg 8±316±14f *Significantdifference(PϽ.001)inEMGactivityamongabdominalexercisesbasedona1-wayrepeated-measuresanalysisofvariance. Keytopairwisecomparisons(PϽ.01): a.SignificantlylessEMGactivitycomparedtotheAbSlide(straightandcurved). b.SignificantlylessEMGactivitycomparedtotheTorsoTrack. c.SignificantlylessEMGactivitycomparedtothecrunch(normal). d.SignificantlylessEMGactivitycomparedtothecrunch(oblique). e.SignificantlylessEMGactivitycomparedtotheAbRoller(crunchandoblique). f.SignificantlylessEMGactivitycomparedtothebent-kneesit-up. g.SignificantlylessEMGactivitycomparedtotheSAM. h.SignificantlylessEMGactivitycomparedtotheAbTwister(crunch). i.SignificantlylessEMGactivitycomparedtotheAbTwister(oblique). j.SignificantlylessEMGactivitycomparedtotheAbRocker(crunch). k.SignificantlylessEMGactivitycomparedtotheAbDoer(goodmorningandbodyboogie). J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 51 RESEARCHREPORT
  8. 8. FIGURE 10. Upper rectus abdominis normalized mean (SD) EMG activity among exercises. FIGURE 11. Lower rectus abdominis normalized mean (SD) EMG activity among exercises. greater for the Ab Slide (straight and curved) and Torso Track exercises compared to all other exercises. Latissimus dorsi EMG activities were greatest for the Ab Slide (straight and curved), Torso Track, SAM, crunch (oblique), and Ab Twister (oblique) exercises, and lowest for the Ab Doer (good morning, body boogie, and body bob) and Ab Roller (crunch) exercises. Lumbar paraspinal EMG activities were significantly greater for the Ab Doer (good morning, body boogie, and body bob) exercises compared to all other exercises. Rectus femoris EMG activities were greatest for the bent-knee sit-up, SAM, Ab Twister (crunch and oblique), Ab Rocker (crunch and ob- lique), and Ab Doer (body boogie) exercises, and lowest for the Ab Roller (crunch and oblique), Ab Slide (straight), Torso Track, and crunch (normal and oblique) exercises. The relative effectiveness of exercises in muscle recruitment of the trunk, upper extremity, and hip musculature is shown in Table 2. 0 10 20 30 40 50 60 70 80 90 100Ab Slide (straight)Torso Track Ab Slide (curved) C runch (norm al) C runch (oblique) Ab R oller(oblique) Ab R oller(crunch) SAM Bent-knee sit-up Ab Tw ister(oblique) Ab Tw ister(crunch) Ab R ocker(crunch) Ab R ocker(oblique) Ab D oer(good m orning) Ab D oer(body boogie) Ab D oer(body bob) NormalizedEMG(%MVIC) 0 10 20 30 40 50 60 70 80 90 100 Ab Slide (straight)Torso Track Ab Slide (curved) C runch (norm al) SAM Bent-knee sit-up Ab R oller(crunch) C runch (oblique) Ab R oller(oblique) Ab Tw ister(oblique) Ab Tw ister(crunch) Ab R ocker(oblique) Ab D oer(good m orning) Ab R ocker(crunch) Ab D oer(body boogie) Ab D oer(body bob) NormalizedEMG(%MVIC) 52 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  9. 9. FIGURE 12. External oblique normalized mean (SD) EMG activity among exercises. FIGURE 13. Internal oblique normalized mean (SD) EMG activity among exercises. DISCUSSION Biomechanical Differences Between Flexion and Extension Exercises The Ab Slide and Torso Track were the most effective exercises in activating abdominal muscula- ture, including the upper and lower rectus abdominis and the external and internal oblique. While per- forming these exercises, the abdominal muscles con- tract in a different manner compared to performing traditional trunk flexion exercises. During the ‘‘roll- out’’ portion in performing the Ab Slide and Torso Track, the abdominal musculature contracts eccentri- cally or isometrically to resist the attempt of gravity to extend the trunk and rotate the pelvis. During the return motion, the abdominal musculature contracts concentrically or isometrically. If the pelvis and spine are stabilized and maintained in a neutral position throughout the roll-out and return movements, then the abdominal musculature primarily would contract isometrically. While performing these exercises, a relatively neutral pelvis and spine were maintained throughout the movement. In contrast, all other 0 10 20 30 40 50 60 70 Ab Slide (curved) Bent-knee sit-up Ab Slide (straight) Ab Tw ister(oblique)Torso Track C runch (oblique) SAM Ab R ocker(oblique) Ab D oer(body bob) Ab D oer(body boogie) Ab R ocker(crunch) Ab Tw ister(crunch) Ab R oller(oblique) C runch (norm al) Ab D oer(good m orning) Ab R oller(crunch) NormalizedEMG(%MVIC) 0 10 20 30 40 50 60 70 80 Ab Slide (curved) Ab Slide (straight) Torso Track Ab R oller(oblique) C runch (norm al) C runch (oblique) SAM Ab D oer(body bob) Ab R oller(crunch) Ab D oer(body boogie) Ab Tw ister(crunch) Bent-knee sit-up Ab R ocker(crunch) Ab R ocker(oblique) Ab Tw ister(oblique) Ab D oer(good m orning) NormalizedEMG(%MVIC) J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 53 RESEARCHREPORT
  10. 10. exercises activated abdominal musculature by actively flexing the trunk by concentric contractions during the initial portion of the motion, an isometric con- traction during the middle portion, and an eccentric contraction during the final portion of the motion. Understanding biomechanical differences between exercises is important because trunk flexion may be contraindicated in certain populations, such as those with lumbar disk pathologies or osteoporosis. Main- taining a neutral pelvis and spine (such as perform- ing the Ab Slide or Torso Track exercise), rather than forceful flexion of the lumbar spine (such as during the bent-knee sit-up), may be desirable for these individuals. In contrast, an individual with facet joint pain, spondylolisthesis, and vertebral or intervertebral foramen stenosis may not benefit from exercises that maintain the spine and pelvis in a neutral or extended position, such as when using the Ab Slide and Torso Track. These exercises may in fact contribute to the nerve compression. However, trunk flexion exercises, such as the crunch, bent-knee sit-up, SAM, Ab Roller, Ab Twister, Ab Rocker, and Ab Doer may be beneficial. When the lumbar spine is forcefully flexed, which may occur when performing many of the commercial abdominal machines used in the current study, the anterior fibers of the intervertebral disk are com- pressed, while the posterior fibers are in tension. In addition, in extreme lumbar flexion intradiscal pres- sure may increase several times above the normal intradiscal pressure from a resting supine posi- tion.18,19 While these stresses on the disk may not be problematic for the normal healthy disk, they may be detrimental to the degenerative disk or pathologic spine. There have only been a few studies that have compared abdominal machine exercises to the tradi- tional crunch or bent-knee sit-up exercises.6,7,12,24,26 Most of these studies compared the crunch to the Ab Roller, and like the results of the current study, there were generally no significant differences in abdomi- nal muscle activity between these 2 exercises. The biggest difference between these exercises is that the Ab Roller provides head support, which may make it more comfortable to perform compared to the crunch. The only known study to investigate abdomi- nal muscle activity between the crunch and the Torso Track and Ab Doer (good morning) was by Sternlicht and Rugg,24 and these authors found similar results as the current study: that abdominal muscle activity was significantly greater in the crunch and Torso Track compared to the Ab Doer (good morning). TABLE 2. Relative muscle recruitment of the trunk, upper extremity, and hip musculature. Note: the Ab Slide (straight and curved) and Torso Track were the exercises that produced the greatest activation of the abdominal, oblique, and upper extremity musculature, while only minimally recruiting the hip flexors. Abdominal and Oblique Muscles Upper Extremity Muscles Low Back Muscles Hip Flexor Muscles Greatest recruitment • Ab Slide (straight and curved) • Torso Track • Ab Slide (straight and curved) • Torso Track • SAM • Ab Doer (good morn- ing and body boogie) • Bent knee sit-up • Ab Rocker (crunch) • Ab Twister (crunch) • Ab Doer (body boogie) • Ab Twister (oblique) Intermediate recruitment • Crunch (normal and oblique) • Bent-knee sit-up • SAM • Ab Roller (crunch and oblique) • Ab Twister (crunch and oblique) • Ab Rocker (crunch and oblique) • Crunch (oblique) • Bent-knee sit-up • Ab Roller (crunch and oblique) • Ab Doer (body bob) • Ab Rocker (oblique) • SAM • Ab Doer (body bob) • Ab Doer (good morn- ing) Least recruitment • Ab Twister (crunch and oblique) • Ab Rocker (crunch and oblique) • Ab Doer (good morn- ing, body boogie, and body bob) • Crunch (normal) • Ab Doer (good morn- ing, body boogie, and body bob) • Ab Twister (crunch and oblique) • Ab Rocker (crunch and oblique) • Ab Roller (crunch and oblique) • Bent-knee sit-up • Crunch (normal and oblique) • SAM • Ab Slide (straight and curved) • Torso Track • Ab Slide (straight and curved) • Torso Track • Crunch (normal and oblique) • Ab roller (crunch and oblique) 54 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  11. 11. Biomechanical Differences Between the Crunch and Bent-Knee Sit-up It should be noted that not all abdominal exercises involve the same degree of flexion of the lumbar spine. Halpern and Bleck14 have demonstrated that lumbar spinal flexion was only 3° during the crunch but approximately 30° during the bent-knee sit-up. In addition, the bent-knee sit-up has been shown to generate greater intradiscal pressure18,19 and lumbar compression3 compared to exercises similar to the crunch, largely due to increased lumbar flexion and hip flexor activity. This implies the crunch may be a safer exercise to perform than the bent-knee sit-up for individuals who need to minimize lumbar spinal flexion or compressive forces due to lumbar pathol- ogy. Although the crunch and bent-knee sit-up were both effective in recruiting abdominal musculature, there were some differences. Several studies, includ- ing the current study, have shown that external oblique activity, and to a lesser extent internal ob- lique activity, are significantly greater in the bent- knee sit-up compared to the crunch.1-3,16 However, upper and lower rectus abdominis activities have been shown to be greater in the crunch compared to the bent-knee sit-up.6,14 In addition, like the current study, hip flexor activity has been shown to be greater in the bent-knee sit-up compared to the crunch.3,16 The Role of Abdominal Muscles in Trunk Stability The role of the abdominal muscles, especially the transverse abdominal and internal oblique, in en- hancing spinal and pelvic stabilization and increasing intra-abdominal pressure (IAP) has been well studied, but still remains controversial.9,10,15,22,25 IAP has been shown to unload the spine by generating a trunk extensor moment and tensile loading to the spine.11 By making the trunk a more solid cylinder by the IAP mechanism, there is a reduction in spinal axial compression and shear loads. The attachments of the transverse abdominal and internal oblique into the thoracolumbar fascia may enhance spinal and pelvic stabilization, because when these muscles contract they tense the thoracolumbar fascia. The transverse abdominal, which is the deepest of the 4 abdominal muscles, has been shown to exhibit a similar (within 15%) muscle activation pattern and amplitude as the internal oblique muscle during many of the same trunk flexion movements (eg, bent-knee sit-up, crunch) used in the current study.16,17 The highest EMG activities from the internal oblique were for the Ab Slide, Torso Track, crunch, and bent-knee sit-up exercises, which implies that these exercises may offer more effective stabilization to the spine and pelvis compared to the other exercises. Technique Variations Despite slightly greater external oblique EMG ac- tivities in oblique and curved techniques, mean abdominal and oblique EMG activities generally were not significantly different between technique varia- tions for exercises such as the Ab Slide, crunch, Ab Roller, Ab Twister, and Ab Rocker (eg, normal crunch versus oblique crunch, straight Ab Slide versus curved Ab Slide). Because simultaneous trunk flexion and rotation have been shown to increase the risk of torsional injury to the annulus fibrosis of the intervertebral disk, as well as generate relatively high lumbar compressive forces,3 and because abdominal and oblique EMG activities were generally not differ- ent between uniplanar and multiplanar trunk move- ments, the additional risks involved when performing multiplanar trunk flexion and rotation motions are not warranted for individuals who have lumbar disk pathologies. Exercise Intensity The Ab Slide and Ab Roller were the only 2 commercial exercises in which resistance could not be adjusted. This may account for more moderate amounts of muscle activity in the Ab Roller (because resistance could not be added to make it harder) and higher amounts of muscle activity in the Ab Slide (because there was no way to make it easier). However, the Torso Track, which is performed in the same manner as the Ab Slide and did have resistance bands that could be adjusted to make it easier or harder, had nearly identical muscle activity compared to the Ab Slide. The subjects used in the current study were all relatively young, active individuals who all used the Torso Track in a more difficult resistance setting. This more difficult resistance may be appro- priate for younger more active individuals, but older, less active, or weaker individuals may not be able to correctly perform the Ab Slide due to its difficulty level. In addition, all subjects set the resistance for the Ab Doer, Ab Rocker, and Ab Twister to the maximum number of resistance bands that could fit on each device. Even with maximal resistance, these 3 commercial abdominal devices recorded the lowest amount of abdominal activity. In contrast, the Ab Slide, Ab Roller, and Torso Track generated signifi- cantly greater abdominal and oblique muscle activity compared to the Ab Doer, Ab Rocker, and Ab Twister. Extraneous (Nonabdominal) Muscle Activity There are no studies that we are aware of that have reported extraneous muscle activity for abdominal machine exercises. Of the exercises tested, the Ab Slide and Torso Track produced the greatest activa- tion of the upper extremity musculature, including the sternal pectoralis major, triceps brachii, and J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 55 RESEARCHREPORT
  12. 12. latissimus dorsi. Both the sternal pectoralis major (lower fibers) and latissimus dorsi contract eccentri- cally during the initial roll-out phase to control the rate of shoulder flexion due to gravity, and concentri- cally in the return phase as the shoulders extend. Because the elbows typically remain slightly flexed and at a fixed elbow angle throughout the move- ment, the triceps brachii primarily contract isometri- cally throughout the movement. Although the hip flexors would appear to also contract eccentrically during the initial roll-out phase to control the rate of hip extension and concentrically during the return phase to aid in hip flexion, we unexpectedly found low rectus femoris activity for both the Ab Slide and Torso Track exercises. It appears that upper extremity muscles may have a greater role compared to the hip flexors in controlling and causing exercise move- ments during these 2 exercises. Although the activity of the psoas muscle was not measured in the current study due to being a deep muscle, it has been demonstrated that during exercises performed in a similar position and manner as the Ab Slide and Torso Track that psoas EMG magnitudes are low and that psoas EMG magnitudes are typically within ap- proximately 10% of rectus femoris EMG magni- tudes.16,17 From these data it can be hypothesized that psoas activity, like rectus femoris activity, is relatively low during the Ab Slide and Torso Track. However, because the Torso Track and Ab Slide are unique exercises in which psoas activity has not yet been quantified, additional studies are needed to test this hypothesis. Because the Ab Slide and Torso Track exercises produced the greatest activation of both abdominal and upper extremity musculature, these exercises may be beneficial for individuals with limited workout time and whose goal is to perform exercises that not only provide an abdominal workout but also an upper body workout. The greater relative intensity and number of muscles used during the Ab Slide and Torso Track exercises implies that these exercises may also achieve a greater energy expenditure compared to the other exercises. Moreover, tension in the latissimus dorsi in addition to the internal oblique (and presumably the transverse abdominal), which all tense the thoracolumbar fascia, may enhance trunk stabilization while performing these exercises. It should also be emphasized that cocontraction of the lumbar paraspinal muscles, along with abdominal and latissimus dorsi musculature, may enhance trunk stability and spine stiffness. Although excessive activity from the lumbar paraspinals can cause high compres- sive and shear (especially at the L5-S1) forces on the lumbar spine,3,16,23 the relatively low lumbar paraspinal activity in all the exercises tested is prob- ably not high enough to by itself cause deleterious effects to the lumbar spine. Performing exercises that generate high activity from the hip flexors and lumbar paraspinals may not be advantageous for those with weak abdominal muscles or lumbar instability, because the forces generated when these muscles act to anteriorly rotate the pelvis may increase the lordotic curve of the lumbar spine. Individuals with weak abdominal muscles or lumbar instability may want to avoid the bent-knee sit-up, SAM, Ab Twister, Ab Rocker, and Ab Doer exercises due to the relatively high rectus femoris activity. In exercises performed similarly to the exercises in the current study, psoas and iliacus activities have been shown to be similar in magnitude and recruitment pattern as rectus femoris activity.1,2,17 The psoas muscle, by its attachments into the lumbar spine, acts to hyperextend the spine as it flexes the hip during the bent-knee sit-up and similar types of hip flexion exercises, which may be detrimental to individuals with lumbar instability. It has also been demonstrated that the psoas muscle can generate compression of the lumbar spine and anterior shear force at L5-S1,16,23 which may be problematic for individuals with lumbar disk pathologies. In addition, the role of gravity in generating L5-S1 shear forces in some exercises, such as the Torso Track and Ab Slide, should also be considered when examining injury risk to the low back. Unfortunately, it is unknown how much L5-S1 shear force is generated while perform- ing the Torso Track and Ab Slide, and whether or not these forces are high enough to be problematic in some patients with low back pathologies. Effects of Electrode Placement on EMG Crosstalk The electrode positions used in the current study have been shown to minimize EMG crosstalk from other muscles.5,8,20 This is especially true for the internal oblique, which was the only muscle tested that is not a superficial muscle. The internal oblique normally lies deep to the external oblique, and therefore is susceptible to considerable EMG crosstalk from this muscle. However, it has been shown that the internal oblique is only covered by the aponeurosis of the external oblique, and not the external oblique muscle, within the triangle confined by the inguinal ligament, lateral border of the rectus sheath, and a line connecting the ASISs.20 Therefore, surface electrodes are appropriate to use for the internal oblique when electrode placement is within this area, especially when clinical questions are being discussed and if a small percentage of EMG crosstalk is acceptable. In fact, it has been shown that when performing trunk flexion exercises similar to those in the current study, mean internal and external ob- lique EMG data from surface electrodes (similarly located as in the current study) were only approxi- mately 10% different compared to mean internal and external oblique EMG data from intramuscular elec- trodes.17 These authors demonstrated that appropri- 56 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
  13. 13. ately placed surface electrodes accurately reflect (within 10%) the muscle activity within the internal or external oblique muscles. CONCLUSIONS The exercises in the current study activated ab- dominal muscles by actively flexing the trunk (crunch, bent-knee sit-up, SAM, Ab Roller, Ab Twister, Ab Rocker, Ab Doer) or by resisting trunk extension (Ab Slide and Torso Track). The Ab Slide and Torso Track exercises produced the highest activation of the abdominal and upper extremity muscles while minimizing low back and hip flexion activity. Both the bent-knee sit-up and crunch exer- cises demonstrated similar amounts of abdominal activation, while the Ab Twister, Ab Rocker, SAM, Ab Doer, and bent-knee sit-up exercises exhibited the greatest rectus femoris activity. The Ab Doer (good morning and body boogie) exhibited the greatest amount of lumbar paraspinal activity. ACKNOWLEDGEMENTS We would like to acknowledge Mike Andrawes, Tracy Lowry, and Mark Adams for all their help in data collection and analyses in this project. REFERENCES 1. Andersson EA, Ma Z, Thorstensson A. Relative EMG levels in training exercises for abdominal and hip flexor muscles. Scand J Rehabil Med. 1998;30:175-183. 2. Andersson EA, Nilsson J, Ma Z, Thorstensson A. Ab- dominal and hip flexor muscle activation during various training exercises. Eur J Appl Physiol Occup Physiol. 1997;75:115-123. 3. Axler CT, McGill SM. Low back loads over a variety of abdominal exercises: searching for the safest abdominal challenge. Med Sci Sports Exerc. 1997;29:804-811. 4. Balady GJ, Franklin BA, Whaley MH, Howley ET. ACSM’s Guidelines for Exercise Testing and Prescrip- tion. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2000. 5. Basmajian J, Blumenstein R. Electrode Placement in EMG Biofeedback. Baltimore, MD: Williams & Wilkins; 1980. 6. Beim GM, Giraldo JL, Pincivero DM, Borror MJ, Fu FH. Abdominal strengthening exercises: a comparative EMG study. J Sport Rehab. 1997;6:11-20. 7. Clark KM, Holt LE, Sinyard J. Electromyographic com- parison of the upper and lower rectus abdominis during abdominal exercises. J Strength Cond Res. 2003;17:475- 483. 8. Cram JR, Kasman GS. Introduction to Surface Electromyography. Gaithersburg, MD: Aspen Publishers, Inc; 1998. 9. Cresswell AG, Grundstrom H, Thorstensson A. Observa- tions on intra-abdominal pressure and patterns of ab- dominal intra-muscular activity in man. Acta Physiol Scand. 1992;144:409-418. 10. Cresswell AG, Blake PL, Thorstensson A. The effect of an abdominal muscle training program on intra- abdominal pressure. Scand J Rehabil Med. 1994;26:79- 86. 11. Daggfeldt K, Thorstensson A. The role of intra- abdominal pressure in spinal unloading. J Biomech. 1997;30:1149-1155. 12. Demont RG, Lephart SM, Giraldo JL, Giannantonio FP, Yuktanandana P, Fu FH. Comparison of two abdominal training devices with an abdominal crunch using strength and EMG measurements. J Sports Med Phys Fitness. 1999;39:253-258. 13. Gardner-Morse MG, Stokes IA. The effects of abdominal muscle coactivation on lumbar spine stability. Spine. 1998;23:86-91; discussion 91-82. 14. Halpern AA, Bleck EE. Sit-up exercises: an electromyographic study. Clin Orthop Relat Res. 1979;172-178. 15. Hodges PW, Richardson CA. Contraction of the ab- dominal muscles associated with movement of the lower limb. Phys Ther. 1997;77:132-142; discussion 142-134. 16. Juker D, McGill S, Kropf P, Steffen T. Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. Med Sci Sports Exerc. 1998;30:301-310. 17. McGill S, Juker D, Kropf P. Appropriately placed surface EMG electrodes reflect deep muscle activity (psoas, quadratus lumborum, abdominal wall) in the lumbar spine. J Biomech. 1996;29:1503-1507. 18. Nachemson A. Lumbar intradiscal pressure. In: Jayson MIV, ed. The lumbar Spine and Back Pain. Edinburg, Scotland: Churchill Livingstone; 1987:191-203. 19. Nachemson AL. The lumbar spine: an orthopaedic challenge. Spine. 1976;1:59-71. 20. Ng JK, Kippers V, Richardson CA. Muscle fibre orienta- tion of abdominal muscles and suggested surface EMG electrode positions. Electromyogr Clin Neurophysiol. 1998;38:51-58. 21. Ralston SH, Urquhart GD, Brzeski M, Sturrock RD. Prevalence of vertebral compression fractures due to osteoporosis in ankylosing spondylitis. BMJ. 1990;300:563-565. 22. Richardson CA, Snijders CJ, Hides JA, Damen L, Pas MS, Storm J. The relation between the transversus abdominis muscles, sacroiliac joint mechanics, and low back pain. Spine. 2002;27:399-405. 23. Santaguida PL, McGill SM. The psoas major muscle: a three-dimensional geometric study. J Biomech. 1995;28:339-345. 24. Sternlicht E, Rugg S. Electromyographic analysis of abdominal muscle activity using portable abdominal exercise devices and a traditional crunch. J Strength Cond Res. 2003;17:463-468. 25. Thomson KD. Estimation of loads and stresses in abdominal muscles during slow lifts. Proc Inst Mech Eng [H]. 1997;211:271-274. 26. Warden SJ, Wajswelner H, Bennell KL. Comparison of Abshaper and conventionally performed abdominal ex- ercises using surface electromyography. Med Sci Sports Exerc. 1999;31:1656-1664. 27. Willett GM, Hyde JE, Uhrlaub MB, Wendel CL, Karst GM. Relative activity of abdominal muscles during commonly prescribed strengthening exercises. J Strength Cond Res. 2001;15:480-485. J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 57 RESEARCHREPORT

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