Ankle motion and muscle extensibility during supine kicking in infants born fullterm and infants born preterm

Infants born preterm are subjected to the effects of gravity which results in prolonged lower extremity extension in the neonatal intensive care unit versus the tight flexion imposed by the uterus on infants born fullterm. This difference in prolonged positioning may alter the muscle tendon unit in the gastrocnemius/soleus and explain changes in spontaneous movement at the ankle. The purpose of this investigation is to examine measures of muscle extensibility in the gastrocnemius/soleus and ankle kinematics during supine kicking in infants born preterm and infants born fullterm. Method. Measurements of gastrocnemius/soleus muscle extensibility and spontaneous, supine kicking were assessed in 20 infants born fullterm and 22 infants born preterm at low-risk for a disorder in motor development. Muscle extensibility and ankle kinematics were assessed at newborn age (term age for the infants born preterm), 6 weeks of age (adjusted for the infant born preterm), and 12 weeks of age (adjusted for the infant born preterm). Results. Two-way, mixed model ANOVA's revealed the infants born preterm had shorter measures for the taut muscle tendon unit (AO), shorter tendon and muscle belly length (AMax), and more muscle belly extensibility (AO to AMax) on passive muscle extensibility measures. The infants born preterm also demonstrated more maximum plantarflexion and less maximum dorsiflexion during spontaneous, supine kicking. Significant differences on the main effect of age were found for taut tendon muscle unit (AO) and maximum plantarflexion. Measures of passive muscle extensibility were related to measures of ankle kinematics at all three ages. The highest correlations were found between AO and maximum plantarflexion (r = .518). Discussion. Differences were found in both measures of muscle extensibility and ankle kinematics in infants born preterm when compared to infants born fullterm. Measures of gastrocnemius/soleus muscle extensibility are related to active movement found at the ankle during spontaneous supine kicking and may partially explain active movement at the ankle. Understanding the relationship between passive measures of muscle extensibility in the gastrocnemius/soleus and active movement throughout development may suggest that muscle extensibility in infants born preterm requires intervention at an early age to decrease dysfunction at the ankle.