Abstract: Objective To investigate the reliability of the morphological measure of medial gastrocnemius by musculoskeletal ultrasound imaging on the standing and prone positions, and compare the differences in the structural parameters of the medial gastrocnemius in different positions. Methods From December, 2017 to May, 2018, 30 healthy young subjects were measured the muscle thickness, fascicle length and pennation angle of medial gastrocnemius on the prone and standing position respectively by two testers with musculoskeletal ultrasound. One of the testers re-tested three days later. The intra-class correlation coefficient (ICC), measurement standard error (SEM) and minimum detectable change (MDC) of all muscle morphological parameters in different positions were calculated respectively, and the differences of the morphological measurement of the medial gastrocnemius between these two positions were compared. Results In the prone position, the ICC of inter-rater was 0.932 to 0.943 and the ICC of intra-rater was 0.880 to 0.915. In the standing position, the ICC of inter-rater was 0.922 to 0.938, and the ICC of intra-rater was 0.839 to 0.925. There were significant differences in muscle thickness, fascicle length and pennation angle of medial gastrocnemius in standing position and prone position (P < 0.01). Inter-rater SEM of muscle thickness and fascicle length was less in standing position than in prone position, while intra-rater MDC of pennation angle was less in prone position. Conclusion It is reliable to measure medial gastrocnemius in standing and prone positions with ultrasonography. Morphological measurement of gastrocnemius is more accurate in standing position, while the measurement in prone position is more sensitive.

Key words: musculoskeletal ultrasound, muscle thickness, fascicle length, pennation angle, reliability