The American College of Sports Medicine (2) and the United States Department of Health and Human Services (15) recommend that healthy adults, under 65 years of age, complete a strength training routine that includes core exercises twice a week. They specifically advocate core training as a means to improve stability and maintain mobility. There are countless exercises that target the primary core trunk muscles (abdominal and lumbar) with the aim of providing these benefits. However, it is unknown as to which exercises elicit the greatest activation thereby enhancing functional gains and peak performance.
Core exercises are also commonly implemented within both sport and rehabilitation objectives to maximize strength, improve endurance, and reduce injury. Therefore, multiple past studies have focused on specific techniques to optimize trunk muscle activation (4,6,7,11,12,14). These studies are similar in that they all use surface electromyography (EMG) to evaluate muscle activity of the core. But the focus of each article differs with respect to the exercise type, position, support, plane, loads, and equipment. Monfort-Panego et al. (12) completed a literature synthesis and concluded that there were 5 critical criteria when selecting optimal core-strengthening exercises; hip flexion, upper body assistance, lower body orientation, surface angle, and abdominal bracing. The majority of the reviewed studies focus on isolation core exercises that target the primary muscle of the anterior trunk, the rectus abdominus (RA). We assume that isolation exercises would elicit the greatest activity of this primary core muscle. However, no study has provided a clear demonstration that isolation exercises are superior to integration exercises that also target the distal muscles of the core, such as the deltoid and gluteal groups.
In short, despite the extensive previous research, one question that still remains is “Do isolation or integration exercises elicit the greatest activation thereby optimizing functional and performance gains (1)?” We define integration core exercises as movements that require activation of the distal trunk muscles (deltoid and gluteal) and activation of the proximal trunk muscles (abdominal and lumbar) in comparison with isolation core exercises that only require activation of the proximal trunk muscles. Understanding the difference between these 2 core-training strategies will enable personal trainers, sport coaches, and medical providers to suggest the optimal type of exercises during a strengthening routine. Thus, our purpose was to evaluate the muscle activity of distal and proximal muscles during a series of both isolation and integration exercises. We hypothesized that isolation exercises would elicit greater activity of the primary abdominal and lumbar muscles compared with integrated exercises.
Experimental Approach to the Problem
To demonstrate which type of exercise, isolation or integration, elicits the greatest activation, we measured surface muscle activity of 6 core muscles, both proximal and distal during a randomized sequence of isolation and integration exercises.
Twenty, healthy college students, 10 men and 10 women, completed the protocol (age 22.47 ± 3.00 years, height 1.70 ± 1.00 m, mass 68.50 ± 14.83 kg, mean ± SD). The Pennsylvania State University human participants Institutional Board approved the research and appropriate consent was obtained pursuant to law.
In order to be included in the study, potential participants were required to be currently meeting the American College of Sports Medicine weekly guidelines for physical activity of 150 minutes of moderate-intensity cardiorespiratory exercise, 2–3 days of major muscle group resistance exercise, and 1–2 days of flexibility exercises (2). After screening, testing was completed within an 8-week period during the fall academic semester. All the tests started between 9 AM and 11 AM, and the participants were instructed to eat breakfast and hydrate adequately before attending the single session.
We measured EMG signals using a wired amplifier system (Bortec Octopus AMT-8, Calgary, AB, Canada) with a bandpass filter setting of 5–500 Hz and collected the data at 1,000 Hz using EVaRT software. The preamplifiers achieve a common mode rejection ratio of 115 dB at 60 Hz and the input impedance was <10 kΩ with our gel electrodes. We placed 1-cm × 1.5-cm bipolar, silver-silver chloride, surface electrodes (Vermed, A10041, Bellows Falls, VT, USA) with an interelectrode distance of 2 cm in the direction of the muscle fibers, over 6 muscles of the trunk. Before electrode placement, we prepared the skin and performed a series of measurements to locate the muscle centers of the anterior deltoid (AD, 4 cm inferior to the clavicle), RA (2 cm lateral from the umbilicus), external abdominal oblique (EO, directly superior from the anterior superior iliac spine, halfway between the iliac crest and the ribs at a medially oblique angle), lumbar erector spinae (LE, 2 cm lateral from the lumbar-3 vertebra), thoracic erector spinae (TE, 2 cm lateral from the thoracic-12 vertebra), and gluteus maximus (GM, half the distance between the greater trochanter and the sacral vertebra at a laterally oblique angle). We verified that the position of the electrodes was functionally correct and that cross talk between the muscles was negligible with a series of flexion and extension exercises suggested by Cram and Kasman (5) and Winter et al. (16).
Each participant completed a standing trial, a treadmill level walking trial at 1.25 m/s, and a series of core exercises (Figures 1–7). A complete data set was comprised of the successful completion of 16 randomly assigned isolation and integration exercises. The participants performed each exercise dynamically at a cadence of 15 flexion and extension cycles per minute set to a metronome with the exception of the hover and balance tasks, which were performed statically.
The participants were introduced to an exercise with an investigator performing the movements and then practiced each exercise with verbal cuing from the investigator before recording. The exercises were performed in random order and we will report 7 exercises that exemplify the differences between isolation and integration exercises.
For the determination of mean activity, we created linear envelopes from the EMG signals by high pass filtering the data at 20 Hz, full wave rectifying, then low pass filtering at 5 Hz and calculated the mean EMG amplitude for 15 seconds of activity data. To compare isolation and integration exercises between participants, we normalized the activity of each muscle during a core exercise to the activity during level walking. Next, we completed a repeated-measures analysis of variance (ANOVA) to determine if there was a difference in muscle activity across the multiple conditions. Finally, if there were significant differences within our ANOVA results, we performed Newman-Keuls post-hoc tests to investigate the significant differences between exercises for each muscle. Significance was defined as p ≤ 0.05.
Overall, our results demonstrated that the activation of the abdominal and lumbar muscles was greatest during the integration exercises that required activation of deltoid and gluteal muscles. One straightforward example of this is the comparison between a traditional isolation exercise of a crunch and an integration exercise of the hover with hand reach.
During the isolation and integration exercises of a crunch and hover, respectively, the rectus abdominus is the primary active muscle. However, during the hover with hand reach exercise, both the rectus abdominus and EO activity was 27% greater than during a traditional crunch (both values, p < 0.05). To add, AD and LE activities were over 2 times greater during the integration exercise (both values, p < 0.01) (Figures 8 and 9).
Moreover, we incorporated a balance component to the integration exercises with the mountain climber plank. The participants completed a plank with alternating hip and knee flexion to the contralateral elbow. This addition of a higher center of mass and the tripod stance resulted in a significantly greater activity for all the muscles. In detail, RA and AD activity were >7% greater (p < 0.05) than the forearm hover while the EO and GM activity were 11% and 70% greater respectively (both values, p < 0.05). This example illustrates the beneficial consequences of adding various levels of difficulty to continually provide unique challenges for each individual. This is also an illustration about how core exercises can be completed with numerous options to provide an optimal core-training session for all ability levels (Figure 10).
For the oblique crunch and side hover exercises, the rectus abdominus and external obliques were the primary active muscles. Although, the average normalized value of the rectus abdominus was less than the forearm hover exercises from above, the external oblique activity was 25% greater (p < 0.05). When comparing the isolation and integration exercises, the largest difference in muscle activity was for the TE and LE, which were 3 and 2 times greater (both values, p < 0.01), respectively, during the side hover. This extreme difference is an illustration of how the integration exercises may be a superior choice for a training regimen as they target a wider range of muscles for a more comprehensive strengthening effect (Figures 11–12).
For the upper body extension exercises, we compared an isolation double arm extension and an integration horse stance pointer with resistance tubes. The primary muscles for both exercises were the erector spinae and the overall average activity during the integration task was 38% greater (p < 0.01) during the integration task. Because of the contralateral limb movements of the integration exercise, the EOs were 3 times greater than for the isolation exercise (p < 0.01). This result provides additional evidence that complex movements stimulate the targeted muscles groups and other primary groups (Figures 13–14).
To sum, we reject our hypothesis that isolation exercises would elicit greater activity of the primary abdominal and lumbar muscles compared to integrated exercises. Based on muscle activity, integration exercises that require activation of the distal trunk musculature would potentially be optimal in terms of maximizing strength, improving endurance, enhancing stability, reducing injury, and maintaining mobility when completing the core-strengthening guidelines. The current results illustrate that these integration exercises elicited the overall greatest muscles activity while challenging coordination and balance.
During hover and pointer exercises, the muscles of the shoulder and hip provided body weight support and position steadiness. To add, abdominal and lumbar muscle activity was greatest when balance was challenged, by adding complex movements to these traditional core exercises. For example, RA and LE activity increased from a forearm hover position with both hips extended, to a cross over mountain climber with alternating hip and knee flexion to the opposite limb. Similarly, we reached the same conclusion for the posterior core exercises. Both thoracic and lumbar activities were greatest during the integration pointer exercise compared with a double arm extension. The integration exercises challenge postural stability and balance resulting in the activation of the distal musculature. But this additional activation at the shoulder and hip does not come at the expense of the primary abdominal and back musculature activation. In fact, the rectus abdominus and LE were generally 20% greater during integration exercises compared with the isolation exercises.
Arokoski et al. (3) also compared abdominal and erector spinae activity during exercises with and without a balance component. For example, the participants completed a bridge exercise with both feet on the ground and with one leg lifted. The average electromyographic amplitude was also at least 20% greater in the rectus abdominus, longissimus thoracis, and multifidus muscles with 1 leg lifted and was 200% greater in the external oblique muscles. These are examples of how exercises can be modified to increase the intensity for individuals with varying levels of experience and strength. Thus a personal trainer or rehabilitation specialist could begin the program with simple isolation exercises as the client or patient gains strength and progress to more integrated, complex variations.
One of the most compelling reasons to complete a core-strengthening program as an aging adult, recreational athlete, or sports professional is to reduce the chance of injury. Leeton et al. (10) reported that injured collegiate athletes had significantly less strength in the core musculature, especially the hip abductors. Similarly, Hewitt and colleagues (8,9,13) conducted multiple studies with a focus on the connection between neuromuscular trunk training and knee injury. They concluded that for collegiate athletes non-contact knee injuries were less frequent in the group of participants that completed preseason core exercises. These previous studies emphasize the importance of completing a core-strengthening program, and our current results aid in refining the physical activity recommendations by demonstrating the enhanced muscle activity during integration exercises.
In summary, a comprehensive, core-strengthening program would incorporate a unique combination of both isolation and integration exercises. Isolation exercises are simple, single joint movements, which target proximal trunk muscles and are easy to complete. Integration exercises are complex, multijoint movements that elicit greater proximal trunk muscles activity and distal trunk muscles activity. Because of the nature of the exercises, integration exercises elicit activity from a broader range of muscle groups while challenging the sensory systems for balance simultaneously. In short, we suggest that personal trainers, sport coaches, and medical providers incorporate integration exercises into their prescription of core-strengthening exercises. This is particularly critical as many clients, athletes, and patients are searching to minimize the time of training by completing a quality, efficient regimen. Thus, when completing the core strength guidelines, a routine that incorporates integration exercises with the activation of distal trunk musculature would be optimal in terms of maximizing strength, improving endurance, enhancing stability, reducing injury, and maintaining mobility.
The authors thank Zach Clark and Riley Sheehan for assistance with data collection and processing. In addition, they thank Dan Cohen and Susan Trainor for directing the CXWORX by Les Mills program and Corey Baird for providing the instructional video.
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