Introduction: Pediatric musculoskeletal (MSK) image interpretation has been identified as a knowledge gap among emergency medicine trainees. The main objective of this study was to implement a validated on-line pediatric MSK radiograph interpretation system with a performance-based competency endpoint into pediatric emergency fellowship programs and examine the number of cases needed to achieve a competency threshold of 80% accuracy, sensitivity and specificity. We further determined proportion who successfully achieved competency in a given module and the change in accuracy from baseline to competency. Methods: This was a prospective cohort multi-centre study. There were seven MSK radiograph modules, each containing 200-400 cases (demo-https://imagesim.com/course-information/demo/). Thirty-seven pediatric emergency medicine fellows participated for 12 months. Participants did cases until they reached competency, defined as at least 80% accuracy, sensitivity and specificity. We calculated the overall and per module median number of cases required to achieve competency, proportion of participants who achieved competency, median time on case, and the mean change in accuracy from baseline to competency. Results: Overall, the median number of cases required to achieve competency was 76 (min 54, max 756). Between different body parts, there was a significant difference in the median number of cases needed to achieve competency, p <0.0001, with ankle and knee being among the most challenging modules. Proportions of those who started a module and completed it to competency varied significantly, and ranged from 32.4% in the ankle module to 97.1% in the forearm/hand, p<0.0001. The overall median time on each case was 34.1 (min 7.6, max 89.5) seconds. The overall change in accuracy from baseline to 80% competency was 13.5% (95% CI 12.1, 14.8), with the respective Cohens effect size of 1.98. The change in accuracy was different between modules, p=0.001, with post-hoc analyses demonstrating that the ankle/foot radiograph module had a greater increase in accuracy relative to elbow (p=0.009) and pelvis/femur (p=0.006). Conclusion: It was feasible for pediatric emergency medicine fellows to complete each learning pediatric MSK learning module to competency within approximately one hour, with the exception of the ankle module. Learners who completed the modules to competency demonstrated very significant increases in interpretation skill.