The main finding of the present study was that crunches performed on Swiss ball with added elastic resistance elicited higher normalized rectus abdominal activity than crunches performed on an isotonic training machine when normalized for training intensity. In contrast, the flexed hip position during the seated crunch in machine resulted in higher rectus femoris activation as compared to the Swiss ball crunch.
The current data indicate that sitting crunches in an exercise machine designed to isolate the abdominal muscles does not target this muscle group to the same extent as the supine crunch on the Swiss ball although both exercises caused high activation. Biomechanically, the seated position in the machine with near 90 degree flexion of both knee and hip promotes assisted hip flexor activity from the rectus femoris while bending the torso forward. The rectus femoris muscle functions over two joints as both a knee extensor and a hip flexor, however the authors hypothesize that the static flexed position of the knee makes it a dominant hip flexor. Further, the hold and fixation of the feet by the ankle bar can contribute to additional rectus femoris activity.28
Previous authors have reported that increased external load in the abdominal crunch exercise does not enhance rectus abdominis activity but instead increases the activation of the hip flexors.29,30
However, in the current study this was only the case for seated crunch in machine whereas the elastic assisted crunch on Swiss ball resulted in very high rectus abdominis nEMG with a concomitant low rectus femoris activity. The authors observed activation levels of more than 100% of nEMG for rectus abdominis during the Swiss ball crunch whereas values ranging from 30–60% of nEMG have been reported in the literature for abdominal exercises on the Swiss ball.10,23
Thus, it seems that the added external load provided by the elastic resistance can maximize abdominal activity and limit hip flexor activation simultaneously. Importantly, the muscle recruitment during these two exercises was equally high regardless of gender, age and pain intensity.
High activity from the hip flexors, such as rectus femoris or iliopsoas, can be unsuitable for persons with low back pain or lumbar instability in general. Increased hip flexor activity will cause an anterior tilt, increased lumbar lordosis, which potentially creating anteriorly directed shear forces on the lumbar spine. This combination may contribute to the genesis of low back pain.23,30,31
Therefore, abdominal crunches performed in an exercise machine, in a seated position may not be desirable for individuals with lumbar disk pathologies, low back pain, or weak abdominal musculature due to high rectus femoris activity. Instead, crunches with elastic resistance on Swiss ball could serve as isolated daily routine abdominal exercise for both prophylactic and rehabilitation purposes where limited hip flexor moment is desired.
In contrast to the seated crunch, crunch on a Swiss ball provides a neutral starting hip position, which seems to minimize hip flexor activity during the exercise. During a traditional crunch the resistance is provided solely by the body mass and the lever arm is constantly decreasing from start to end of the concentric (lifting) phase. However, by adding elastic resistance the loading is more uniform during the entire range of motion due to the elongation of the elastic material and the concomitant decrease in body mass lever arm as the concentric phase advances. Thus, at starting supine position with the greatest lever arm the traction from the elastic tubing is small compared with the resistance produced at higher elongation levels during the end of the concentric phase. Besides these biomechanical differences, the labile surface of the Swiss ball might also have contributed to the contrasting activation strategies of the rectus abdominis and rectus femoris muscles. It has been speculated, that the unstable surface provided by the ball alters proprioceptive demands thereby stimulating the core muscles to a greater extent than stable services, which may be important for balance and stability.10–13
However, in the present study no differences in oblique or erector spinae activation were observed, indicating that these muscles were not affected by exercise type. It is beyond the scope of the current study to determine the role of the local core muscles during these two exercise tasks as this would require intra-muscular EMG.
To obtain proper strength adaptations exercises that produce EMG activity of at least 60% of isometric MVC are recommended.21,22
Thus, both exercises were able to induce sufficient EMG activity to provide a stimulus for strengthening of the rectus abdominis and the obliques and should therefore also be considered as a high threshold rehabilitation tool by health professionals. The suggestion by Behm,14
that Swiss balls are useful for providing an exercise condition capable of increasing stability, balance and proprioception but not muscle strength may need to be re-evaluated in light of the current findings regarding the abdominal crunch performed on a Swiss ball with added elastic resistance. Future studies should address the more long term strength adaptations that may occur, along with changes in stability and proprioception in order to determine this.
Low back pain is traditionally associated with repetitive load handling and heavy manual labor. However, office work with a high degree of chair confinement (prolonged sitting) is also a frequently reported risk factor.32
Self-reported low back pain in subjects in the current study did not affect the muscle activation during the two exercises in office workers. However, caution should be applied to this interpretation, as sensitization could have depressed muscle activation during the MVICs and thereby influenced the normalization of EMG. As strong abdominal muscles provide support for the lumbar spine during everyday movements strengthening the abdominal muscles may decrease the occurrence of low back pain.7,9
However, controversy exists with this assertion, and some authors have suggested that if the global muscles are over trained before the local muscles are sufficiently developed, it could result in situations where the force produced by the global muscles can not be controlled by the local musculature.4
Hence, abdominal training on the Swiss ball with added elastic resistance should be introduced thoughtfully, with gradual intensity progression in order to ensure optimal local musculature development before focusing on strengthening the global muscles.
Evaluations of abdominal exercises using nEMG comparisons has been based on identical movement velocity or cadence normalization using a metronome13,23,33,34
rather than loading intensity. In the present study identical contraction time and intensity (3 reps of 10 RM load) were used as normalization mediators to secure valid results. Further, the participants were accustomed to the exercises and performed a 10 RM test to determine appropriate intensity a week prior to testing. This study compared relative level of muscle activity across the two exercises. Thus, the effects of crunch on Swiss ball with added elastic resistance on muscle strength cannot be directly measured and a randomized controlled trial would be necessary to draw such conclusions. Further, the study population included working aged adults only, which limits the reported lack of an age effect to this relative narrow age-range.