The effects of curl-up exercise in terms of posture and muscle contraction direction on muscle activity and thickness of trunk muscles

BACKGROUND: The curl-up exercise is widely used in clinical practice for strengthening abdominal muscles, but has been applied without a systematic method. OBJECTIVE: The purpose of this study was to determine the most effective method considering the angle and muscle contraction direction during the curl-up exercise. METHODS: Fourteen healthy males performed the curl-up exercise according to contraction direction (concentric and eccentric) and angle (30 degrees, 60 degrees, and 90 degrees). The muscle activity of the rectus abdominis (RA), external oblique (EO), internal oblique (TO), and iliopsoas (IP) was measured using electromyography (EMG), and the muscle thickness of transversus abdominis (TrA) was measured using ultrasonography. RESULTS: The activities of the abdominal muscles (RA, EO, and IO) decreased with increasing angles (30 degrees, 60 degrees, and 90 degrees) (p < 0.05). There was no significant difference between eccentric and concentric contractions. The thickness ratio of TrA was the largest at an eccentric curl-up at 30 degrees , and the smallest at a concentric curl-up at 30 degrees (p < 0.05). CONCLUSIONS: The most effective angle for curl-up was 30 degrees. Although there is no difference in the direction of muscle contraction, eccentric curl-up at 30 degrees could be considered the most effective posture for abdominal strengthening considering the importance of TrA.