The kinematic, abdominal and hip flexor muscle electromyographic characteristics of four different trunk flexion exercises

BACKGROUND: Recent evidence suggests that a modified sit-up (MSU) that emphasizes use of the trunk flexor muscles elicits greater abdominal muscle activity and less hip flexion compared to the traditional sit-up. METHODS: This study examined the kinematic and abdominal and hip flexor muscle electromyographic characteristics of four different trunk flexion exercises, the MSU, trunk curl (TC), trunk curl on an exercise ball (TCB), and freestyle sit-up (FS), to determine the most effective exercise for trunk flexor activation. Twenty trained subjects (age: 22.4±3.6 years) performed the four exercises in a counterbalanced order for 30 seconds, with 10 minutes rest between exercises. Normalized electromyographic (EMG) and joint kinematic data were compared among exercises using one-way ANOVAs with repeated measures. Pairwise differences were identified post hoc. RESULTS: Mean external oblique EMG was greater for MSU than TC (P=0.018) and TCB (P=0.017) and greater for FS than TC (P=0.001) and TCB (P=0.001). Mean rectus abdominis EMG was greater for FS than TCB (P=0.009). Mean rectus femoris EMG was greater for FS than TC (P=0.012). Hip flexion was greater for FS than MSU (P<0.001), TC (P<0.001), and TCB (P<0.001) and greater for MSU (P=0.002) and TCB (P<0.001) than TC. Trunk flexion was greater for MSU (P=0.047), FS (P=0.002), and TC (P=0.025) than TCB. CONCLUSIONS: The MSU and FS required greater abdominal muscle activation than TC and TCB. However, hip flexion was greatest for FS, which may result in greater strain on lumbar spine. The MSU may be a better exercise choice to emphasize the trunk flexor muscles for skills that require trunk flexion.