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

Abstract

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.

Fig. 1

Inverted pendulum model, where M is the total of mass of the body minus the mass of the stance foot, g is the gravity constant, L_e is the equivalent length of the body (see Obusek, 1995), F’ = F’ (θ, θ̇) is the escapement force provided by the muscle contraction during push off phase by the opposite leg, k is the coefficient of stiffness determined by the elastic tissues and active muscle tension, and c is the damping coefficient. θ is the angular displacement of the inverted pendulum with reference to vertical-up direction measured in clockwise direction.

Fig. 2

Time line of data collection sessions for toddlers with TD and DS, ellipses reflect points in time at which groups had equal numbers of months of walking experience.

Fig. 3

Mean overgound dimensionless stiffness plotted by group across walking experience. Preadolescents with TD and DS from Ulrich et al. (2004) are included for comparison purposes (see discussion).

Fig. 4

Dimensionless stiffness for each toddler with TD and DS at each test session, as a function of dimensionless speed, with line of best fit for each group.

Fig. 5

Mean treadmill stiffness values plotted by group, by speed, by age. Mean overground stiffness at the same ages superimposed on the graph. Note: The number of participants decreased as speed increased (mean number of toddlers at 40% [7], 75% [6.25], and 110% [4.25]).

Fig. 6

Mean overgound dimensionless impulse values plotted by group across walking experience. Preadolescents with TD and DS from Ulrich et al. (2004) are included for comparison purposes (see discussion).

Fig. 7

Dimensionless impulse for each toddler with TD and DS at each test session, as a function of dimensionless speed, with line of best fit for each group.

Fig. 8

Mean treadmill impulse values plotted by group, by speed, by age. Mean overground impulse at the same ages superimposed in the graph for comparison. Note: The number of participants decreased as speed increased (mean number of toddlers at 40% [7], 75% [6.25], and 110% [4.25]).