Introduction Compared with the sit-up, the risk for the lumbar spine compression in the crunch is reduced [1,2], but the abdominal muscles are less activated. So, it is important to find the most effective ways to maximize abdominal activation when using the crunch for abdominal strength training. The purpose of the present study was to compare the activation level of abdominal muscles in the first repetition of each set of crunches performed with different external loads. Methods 13 subjects (5 females, 8 males), performed crunch exercises (knees flexed 90º, legs fixed) with no external load and with loads representing 100, 80, 60, 40 and 20% of the 1RM, with 5` rest between sets. Surface bipolar EMG electrodes were placed on muscles of both sides, except for the RF: the upper portion (RAUR and RAUL) and the lower portion of the Rectus Abdominis (RALR and RALL), the External Oblique (OER and OEL) and the Rectus Femoris (RFR). The root mean square (RMS) of the EMG was calculated for the first repetition of each load. Mean values and SD of normalized RMS were calculated. Differences between conditions were tested using the one way ANOVA for repeated measures. Post-hoc Bonferroni test was used to detect significant differences between specific loads (p < 0.05). Results Each subject performed the maximum number of repetitions in each percentage of the maximal external load: 20% (30±2.8), 40% (26±3.8), 60% (22±5), 80% (14±2.5). When the crunch was performed without external load (0%) the subjects only performed six repetitions. We used an average of the percentage values of all studied abdominal muscles as a representative value of abdominal synergy (Ab Syn).
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