The purpose of this study was to measure and validate trunk muscle thickness while performing the Heel Off (HO-ex) and Bird Dog exercises (BD-ex), which are hip extension exercises in the supine position. Thirty-one healthy young males who provided informed consent were included in the study. The thicknesses of the right trunk muscles (lumbar multifidus (LM), erector spinae (ES), external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) were measured using an ultrasound machine. Measurements were taken under four random conditions: supine, HO-ex, crawling on all fours, and BD-ex. One-way analysis of variance and Friedman tests were performed to determine the differences between the conditions for each muscle thickness. LM was significantly thicker in the HO-ex. ES was significantly thicker in HO-ex than in supine, and in BD-ex than in supine, HO-ex, or crawling on all fours. EO was significantly thicker in the supine position than in HO-ex, crawling on all fours. IO was significantly thicker in the HO-ex than in the supine position. TrA was significantly thicker in HO-ex, crawling on all fours, and BD-ex than in the supine position, with no significant difference between HO-ex, crawling on all fours, and BD-ex. The results of this study suggest that HO-ex is more advantageous than BD-ex in stimulating contraction of the multifidus and IO muscles, and that HO-ex can stimulate contraction of the TrA to the same degree as BD-ex.