Choi, JH, Kim, DE, and Cynn, HS. Comparison of trunk muscle activity between traditional plank exercise and plank exercise with isometric contraction of ankle muscles in subjects with chronic low back pain. J Strength Cond Res 35(9): 2407–2413, 2021—This study aimed to compare the effects of 4 different ankle conditions on the activities of rectus abdominis (RA), external oblique (EO), transversus abdominis/internal oblique (TrA/IO), and erector spinae (ES) muscles during plank exercise in subjects with chronic low back pain (CLBP). Twenty-two subjects with CLBP participated in this study. The subjects performed the traditional plank and plank with 3 different ankle muscle contraction types (isometric contraction of ankle dorsiflexor, plantarflexor, and without ankle muscle contraction). Surface electromyography was used to measure the activities of RA, EO, TrA/IO, ES, tibialis anterior, and gastrocnemius muscles. A 1-way repeated-measures analysis of variance was used to assess the statistical significance of activities of the RA, EO, TrA/IO, and ES muscles. The activities of RA, EO, and TrA/IO muscles were significantly greater in the plank with isometric contraction of ankle dorsiflexor (PlankDF) than in the other 3 plank exercises. No significant difference in the activity of ES muscles was revealed during the 4 plank exercises. The activities of all abdominal muscles during PlankDF were significantly higher than those during the traditional plank, as well as during the plank with isometric contraction of ankle plantarflexor (PlankPF) and the plank without ankle muscular contraction (Plankw/o), and more than 60% of maximal voluntary isometric contraction was observed. Thus, PlankDF could be applied not only as a rehabilitation strategy for patients with decreased core stability owing to weakness of abdominal muscles but also as fitness program for improving core strength.