Rectus abdominis activity during three sit up exercises with different range of motion

Purpose: When abdominal muscles are trained, it is well known that different exercises could elicit a greater activation of different abdominal muscles (Andersson et al. 1997); sit up is one of the most used exercises for rectus. Rectus abdominis (RA) is a muscle composed by more bellies interleaved with tendinous intersections; due to this anatomical feature different RA parts could be activated by the range of motion (Parfrey et al. 2008). The aim of this study was to analyze the emg activity of the rectus during sit up exercises performed with three different range of motion. Methods: Four subjects were tested. Emg activity was recorded from six rectus abdominis bellies (upper URA, medium MRA and lower LRA part) and both rectus femoris (RF) with a Trigno wireless system (Delsys, 1000 Hz) synchronized with an optoelectronic system that captured the positions of 6 markers bilaterally placed on the knee, hip joint center and acromion for shoulder segment (Vicon, 100 Hz). Three different exercises, crunch 20° (20), sit up 45° (45) and full (90°) sit up (full) were performed. For each exercise five repetitions were recorded with 3 s rest inbetween. The RMS of each belly/muscle was calculated in a 3° range and normalized to the MVC value. Left and right sides and repetitions were averaged and compared among exercises. Results: RA activation decreases by increasing exercise angle in full and 45, whereas RF shows a parabolic profile with a peak in the middle of the exercise range of motion. When exercises were compared, some differences were detected: MRA shows a 50% more activation in 45 and full compared with 20; URA activity is 40% higher in full than 45 in the 70-43° range; RF activity is 55% lower in 20 compared with full and 45 and 27% higher in full than 45 in the 70-43° range. Conclusion: RA activity over the whole sit up range of motion decreases, whereas RF increases highlighting that small ranges should be used for training only RA. Moreover, in crunch exercise RF activity is negligible. References: Andersson et al. (1997). Abdominal and hip flexor muscle activation during various training exercises. Eur J Appl Phisiol Occup Physiol. 75, 115-23. Parfrey et al. (2008). The effects of different sit- and curl-up positions on activation of abdominal and hip flexor musculature. Appl Physiol Nutr Metab. 33, 888-95.