Down syndrome (DS) is the most common genetic condition and causes significant development delays and weaknesses in the motor, cognitive, and language domains. It is important to study these co-occurring developmental challenges and develop effective intervention strategies for positive learning outcomes in multiple domains simultaneously. It was found that motor development is closely associated with cognitive and language development in typically developing infants and advances in motor development have developmental cascade effects on other domains. However, this interrelation has not been studied in infants with DS. Our prior work has shown that gross motor experience through body-weight-supported treadmill stepping leads to an earlier onset of walking and improves subsequent locomotor ability in infants with DS. However, we do not yet know the extent to which this motor experience advances cognitive and language development in infants with DS. Research also suggests that grasping, a major fine motor skill emerging in infancy, provides the needed scaffolding for subsequent gesture and speech production. However, research on grasping and its potential effects on other domains in infants with DS remains scarce. The objectives of this proposal are to understand: the role of (a) specific gross motor experience and (b) specific fine motor experience on the motor, cognitive and language development of infants with DS. Our central hypothesis is that motor (both gross and fine) experience will help advance cognitive and language development in infants with DS. We will recruit 45 infants with DS from the greater Atlanta area and beyond. There will be three groups: (a) gross motor (GM) group (n=15), entering the study at about 10 months of age; (b) gross motor plus fine motor (GM+FM) group (n=15), entering the study at about 10 months; and (c) control group (n=15), entering the study at about 20 months. Aim 1: Determine the effects of gross motor experience on cognitive and language development in infants with DS, comparing the GM and control groups. Only the GM group will receive a home-based, parent-guided practice of treadmill stepping from 10 months of age (T1) until onset of walking. We hypothesize that the GM group will show higher Bayley scores and better gesture and word production at termination of the GM intervention (T3) and five months thereafter (T4) than the control group. Aim 2: Determine the effects of fine motor experience on cognitive and language development in infants with DS who receive the GM intervention, particularly in gesture and word production, comparing the GM and GM+FM groups. Only the GM+FM group will receive fine motor experience in practice of grasping using "sticky mittens" from 10 months of age for about five months. We hypothesize that the GM+FM group will show further improvements in motor, cognitive and language domains, particularly in gesture and word production, than the GM group at termination of the FM intervention (T2) and again at T3 and T4. Our study will gain an in-depth understanding of the cascading effects of motor experience on cognitive and language development and lay the groundwork for future clinical trials in infants with DS.