Developmental trajectory of dynamic resource utilization during walking: toddlers with and without Down syndrome.

After years of walking practice 8-10-year-old children with typical development (TD) and those with Down syndrome (DS) show uniquely different but efficient use of dynamic resources to walk overground and on a treadmill [Ulrich, B.D., Haehl, V., Buzzi, U., Kubo, M., & Holt, K.G. (2004). Modeling dynamic resource utilization in populations with unique constraints: Preadolescents with and without Down syndrome. Human Movement Science, 23, 133-156]. Here we examined the use of global stiffness and angular impulse when walking emerged and across the ensuing months of practice in eight toddlers with TD and eight with DS. Participants visited our lab when first able to walk four to six steps, and at one, three, four, and six months of walking experience. For all visits, toddlers walked overground at their preferred speeds and for the last two visits on a treadmill. Toddlers with TD and DS demonstrated clear and similar developmental trajectories over this period with more similarities than differences between groups. At six months stiffness and impulse values were higher than previously observed for 8-10-year-old children. Stiffness values increased significantly throughout this period, though rate of change slowed for the TD group by three months of experience. Impulse values rose sharply initially and slowed to plateau during the latter months. Treadmill data illustrated toddlers' capacity to adapt dynamic resource use to imposed changes in speed, particularly well after six months of practice. Consistent with our studies of preadolescents and older adults, toddlers with DS produced significantly wider normalized step width than their TD peers. We propose that the challenge of upright bipedal locomotion constrains toddlers with TD and DS to generate similar, necessary and sufficient stiffness and impulse values to walk as they gain control and adapt to playful and self-imposed perturbations of gait over the first six months. The plateau in impulse and slow-down of stiffness increases over the latter months may be the first signs of a downward trend to the lower values produced by older children with several years of walking experience.