Purpose: The purpose of this study was to define how different force environments by neuromuscular diagnosis (hypertonic versus hypotonic) impact the growth and morphology of the proximal femoral and acetabular regions relative to typically developing children. Methods: Children with cerebral palsy and spinal muscular atrophy were compared with typically developing children aged 6 months to 11 years. Routine pelvic radiographs were evaluated using measures of hip geometry for the proximal femur and acetabulum. The data were analyzed using general linear models to estimate the developmental patterns according to age and diagnosis. Results: One hundred eighty-four children met the inclusion criteria: 58 spastic cerebral palsy Gross Motor Function Classification System I–V (263 hips), 32 spinal muscular atrophy (79 hips)), and 94 typically developing (187 hips) were included with a mean age of 4.9 ± 3.1 years. Using spinal muscular atrophy as a reference, significant differences in proximal femoral development included long thin versus short neck (p < 0.01) and round versus flat epiphysis (p = 0.001). A thin neck-wide epiphysis was found in spinal muscular atrophy versus thick neck-small epiphysis for typically developing (p < 0.05). The ratio of acetabular width to proximal femoral epiphysis width differed significantly for typically developing (p = 0.001) compared with cerebral palsy and spinal muscular atrophy. There was a negative correlation between migration percentage and acetabular width to epiphysis width in children with cerebral palsy, but no correlation in children with spinal muscular atrophy. Conclusion: Hip geometry was impacted by the force environment experienced during growth. These findings emphasize the crucial roles of gross motor function, muscle tone, and strength differences in determining hip morphology. Level of Evidence: III, retrospective case control.