This study has three phases. The first phase of the study will be the observation of early spontaneous leg movements which will be measured monthly from 1-4 months of age.

The prone locomotor intervention phase using the Self-Initiated Prone Progression Crawler (SIPPC) will occur from 5-9 months of post-term age, or end earlier if the child achieves the ability to crawl six feet. Treatment will occur at an intensity of 3 times per week for 15-30 minutes. Infants will use the SIPPC for the duration of each therapy session

The upright locomotor intervention phase using DWS will occur from 9-18 months of age, or begin earlier if the child achieves the ability to crawl six feet before 9 months of age, and end earlier if the child achieves independent walking before 18 months of age. Treatment will occur at an intensity of 3 times per week for 30 minutes. Infants will receive dynamic weight support (DWS) for the duration of the 30-minute therapy session.

During the movement observation phase, infants will wear a wireless movement sensor at each ankle for two days. Research staff will show the caregivers how to place the sensors in the morning and charge them overnight.

The training protocol for the prone training will consist of: 1) Warm-up. 2) Assisted movement of the arms and legs. 3) Calibration of the infant's arm and leg positions. 4) Self-initiated mobility for up to five minutes.

For upright training, the environment will be arranged to encourage active motor exploration and variability in walking activities.The therapist will assist the child as needed to encourage upright locomotor activities, but only the minimum amount needed to perform the task. Weight assistance will be gradually reduced as postural control and coordination improve.

The Gross Motor Function Measure-66 (GMFM-66) is a Rasch-analyzed measure of gross motor function designed for children with cerebral palsy (CP). Computation of the GMFM-66 score involves statistical weighting of the raw item scores for difficulty. Scores range from 0 to 100 with higher scores indicating more functional ability.

The Movement Observation Coding System will use video coding to assess postural control, arm and leg movements, and goal directed movement.

Wearable sensor data will be analyzed to describe leg movements produced across in the natural environment.

Error during prone locomotor training will measured using the Self-Initiated Prone Progression Crawler (SIPPC).

The cognition age equivalent obtained from the Bayley Scales of Infant and Toddler Development- Fourth Edition (BSID-IV). This value describes what age the child is currently similar to in their cognitive performance, and all values are reported in months of age. The range of values on the scale 0-42 months of age. A higher number value means a greater cognitive ability.