Author Response to Invited Commentary by Heathcock

We thank Heathcock for taking the time to write her commentary1 regarding our study.2 Her comments are interesting, and our subsequent investigation into these ideas has led us to uncover some enlightening evidence in relation to the topics raised in the commentary. We will attempt to review each of the topics raised and the evidence associated with these topics. Heathcock1 suggests that “the measurements taken in this study were largely of the soleus muscle” secondary to knee contractures. There are 3 important issues to consider when contemplating this perspective. First, we measured infants born full term and infants born preterm over 3 different ages. In the infant born full term at the newborn measurement, knee extension is limited. During data collection, we were careful to ensure that we extended the knee as much as possible, but in the newborn infant born full term, the knee has a small flexion contracture that limits full extension. Although this muscle may not have been fully lengthened, it was lengthened as much as possible. This limitation to extension was not an issue at 6 or 12 weeks of age in the infants born full term or in the infants born preterm at any age. Second, we do not know which structures are limiting knee extension. If the gastrocnemius muscle was the structure limiting knee extension, then by extending the knee fully, we lengthened the muscle. Based on research by Brown and Swenson,3 the difference in ankle dorsiflexion in infants born full term at newborn age with the knee flexed to 90 degrees versus fully extended was 3 degrees in boys and 2 degrees in girls. This difference in ankle dorsiflexion with full extension and 90 degrees of knee flexion is small, and changes observed at the ankle with the small …