Effects of specific exercise instructions on abdominal muscle activity during trunk curl exercises

STUDY DESIGN: A repeated-measures, counterbalanced design. OBJECTIVES: To test whether subjects could learn and retain the ability to alter the relative activity of abdominal muscle groups when performing trunk curl exercises. BACKGROUND: Although trunk curl exercises are widely prescribed, a disadvantage of trunk curls is that they primarily activate rectus abdominis, while the internal and external oblique abdominis muscles are considered to be more important contributors to lumbar stability. METHODS AND MEASURES: A convenience sample of 25 subjects performed trunk curl exercises in accordance with 3 different sets of instructions: nonspecific instructions (NS), instructions intended to emphasize rectus abdominis activity (RE), and instructions intended to emphasize oblique abdominis activity (OE). Electromyographic (EMG) activity was recorded from the upper and lower rectus and the internal and external oblique abdominis muscles while a physical target was used to insure that the trunk was raised to the same height for all conditions. Normalized root-mean-square EMG amplitude measures were used to test for instruction-dependent changes in the relative EMG activity of the rectus and oblique muscle groups. RESULTS: Following a single, brief, instruction session, subjects performing trunk curls had significantly greater normalized oblique:rectus EMG ratios when following OE instructions (mean [+/- SD] oblique-rectus ratio, 1.45 +/- 0.34) than when following RE (mean [+/- SD] oblique-rectus ratio, 0.76 +/- 0.24) or NS (mean [ISD] oblique-rectus ratio, 0.63 +/- 0.23) instructions. Retesting 1 week later indicated that subjects retained this skill. CONCLUSIONS: With minimal instruction, subjects are able to volitionally alter the relative activity of the oblique and rectus abdominis muscles when performing trunk curls. Incorporating instructions emphasizing oblique abdominis activity into lumbar stabilization programs appears promising and has potential advantages over other approaches to altering abdominal muscle activity during trunk