It is often hypothesized that young children's difficulties with producing weak-strong (iambic) prosodic forms arise from perceptual or linguistically based production factors. A third possible contributor to errors in the iambic form may be biological constraints, or biases, of the motor system. In the present study, 7 children with specific language impairment (SLI) and speech deficits were matched to same age peers. Multiple levels of analysis, including kinematic (modulation and stability of movement), acoustic, and transcription, were applied to children's productions of iambic (weak-strong) and trochaic (strong-weak) prosodic forms. Findings suggest that a motor bias toward producing unmodulated rhythmic articulatory movements, similar to that observed in canonical babbling, contribute to children's acquisition of metrical forms. Children with SLI and speech deficits show less mature segmental and speech motor systems, as well as decreased modulation of movement in later developing iambic forms. Further, components of prosodic and segmental acquisition develop independently and at different rates.