Background: To date, reliability of ultrasound imaging (USI) measures of the abdominal muscles in children during the active straight leg raise (ASLR) test has not been confirmed. The aim of this study is to assess the intra-rater reliability of USI in measuring of the thickness and percent thickness change of the external oblique (EO), internal oblique (IO) and transversus abdominis (TrA) on both sides of the body during the ASLR test. Material/Methods: Thirty-two healthy children between 10-12 years old participated in the study. Three repeated USI measurements were recorded, in supine and resting positions and during the ASLR test. In supine position, measurements were collected at the end of normal expiration. In the case of ASLR, measurements were collected when the person undergoing examination touched the transverse delimiter with the distal part of the lower leg. A repeat session of the measurements was carried out 6 to 8 days later. Results: The use of mean value of the three repeated measurements showed that ICC3.3 fluctuated from 0.81 to 0.97 for OE, OI and TrA in resting position and during the ASLR test. In terms of percentage change of muscle thickness, the ICC3.3 result obtained for TrA, OE and IO were 0.86, 0.83 and 0.78, respectively. Conclusions: The USI evaluation of the lateral abdominal muscles thickness at rest has high reliability in children between 10-12 years old. The USI of the actual thickness and reflex activity of OE, OI and TrA during the ASLR test, based on three repeated measurements, is also reliable. It is recommended to perform three repeated measurements for children between 10-12 years old. There is still a need to seek solutions that will decrease the SDD value while evaluating the reflex activity of the lateral abdominal muscles during the ASLR test.