The purpose of this study was to examine the effectiveness of a commercial abdominal
machine (Ab-Slide) and three common abdominal strengthening exercises (abdominal crunch,
supine double leg thrust, and side bridge) on activating abdominal and minimizing
extraneous (nonabdominal) musculature-namely, the rectus femoris muscle. We recruited
10 males and 12 females whose mean (+/- SD) percent body fat was 10.7 +/- 4 and 20.7%
+/- 3.2%, respectively. Electromyographic (EMG) data were recorded using surface electrodes
for the rectus abdominis, external oblique, internal oblique, and rectus femoris.
We recorded peak EMG activity for each muscle during each of the four exercises and
normalized the EMG values by maximum muscle contractions (% MVIC). A two-factor repeated-measures
analysis of variance assessed differences in normalized EMG activity among the different
exercise variations (p < 0.05). Post hoc analyses were performed using the Bonferroni-adjusted
alpha to assess between-exercise pair comparisons (p < 0.002). Gender did not affect
performance; hence, data were collapsed across gender. We found a muscle x exercise
interaction (F9,189 = 5.2, p < 0.001). Post hoc analyses revealed six pairwise differences.
The Ab-Slide elicited the greatest EMG activity for the abdominal muscles and the
least for the rectus femoris. The supine double leg thrust could be a problem for
patients with low-back pathology due to high rectus femoris muscle activity.