Background and Aim: It was unknown whether ultrasound-measured forearm muscle thickness was impacted by pronation of the forearm. The aim of this study was to investigate the influence of forearm pronation on two forearm muscle thicknesses (MT-ulna and MT-radius). Participants and Methods: Fourteen healthy children and adolescents sat on a chair with their right arm comfortably on a table, and their hands were fixed to the board with elastic bands. The probe was placed perpendicularly over the forearm, and the angle of the board was then pronated in 58 increments from
108 to 308. The average value of the two measures at each angle was used. Results: There was evidence that MT-ulna differed across measurement sites (F 5 51.086, P < 0.001). For example, the values of the MT-ulna were 2.58 (SD 0.40) cm in standard position (08), 2.56 (SD 0.41) in
108, 2.62 (SD 0.41) in 108, 2.65 (SD 0.42) in 208, and 2.71 (SD 0.43) in 308. Followup tests found that all sites differed from each other except for
108 and
58 (P 5 0.155) and 108 and 158 (P 5 0.075). There was also evidence that the MT-radius differed across measurement sites (F 5 22.07, P < 0.001). Follow-up tests found that many but not all sites differed from each other. Conclusion: Our results suggest that MT-ulna increases and MT-radius decreases due to forearm pronation from the standard position (08). When determining the forearm position using the 95% limits of agreement, we recommend the forearm position within +-58 of the standard forearm position when measuring forearm MT.  Kulcsszavak: B-mode ultrasound, forearm muscle thickness, forearm pronation