Emergence of neuromuscular patterns during walking in toddlers with typical development and with Down syndrome.

During the development of walking, toddlers with Down syndrome (DS) and typical development (TD) face challenges controlling muscles, joints, and body segments. Toddlers with DS have additional challenges including increased joint laxity and decreased muscle tone and show delayed walking onset; the underlying activity of the neuromotor system remains unclear. Here we investigated the emergence of muscle activity from walking onset through 6 months of practice in eight toddlers with DS and eight with TD. We monitored the activity of core gait muscles and motion of leg segments as toddlers walked at their self-selected speeds. At walking onset muscle bursts were frequent with inconsistent burst durations. Over time, both groups of toddlers began to activate their leg muscles by using energy-efficient strategies: decreased muscle burst frequency (Wilks' Lambda=0.364, F(12, 103.476)=4.009, p< .001) and increased muscle burst duration (Wilks' Lambda=0.346, F(12, 71.727)=2.946, p= .002). Toddlers with TD increased normalized inter-burst intervals over time but toddlers with DS decreased these interval durations. By 6 months of experience toddlers with TD showed an efficient synergy among muscles, allowing increased relaxation time between bursts. Toddlers with DS improved the rhythmicity of their muscle burst, sustaining longer bursts but timing remained inconsistent. We propose increased muscle burst duration in toddlers with DS may add control by stabilizing their lax joints. Thus, their similar yet different emergent strategy may reflect their unique biomechanical and neurophysiological constraints and represent an efficient control strategy.