This proposal will be the first to experimentally and computationally assess the mechanical impact of closed reduction treatment on infants with developmental dysplasia of the hip (DDH), enabling the development of innovative pediatric rehabilitation interventions to decrease treatment duration and improve outcomes. DDH requires medical treatment in 2/1000 infants. Early diagnosis and treatment with a Pavlik harness, a 70-year-old solution that holds the infant’s hips in a flexed and abducted position for 24 hours/day for up to 20 weeks, are key to correcting DDH. However, the treatment has a 20% failure rate, requiring surgical intervention. Motion, muscle activity, and adequate joint contact are key to neonatal musculoskeletal development, yet no research has sought to understand the role of passive lying, active kicking, and body position on the development of the infant hip. Our central hypothesis is that a quantifiable combination of active muscles and hip position produced by the Pavlik harness induce a specific but currently unknown mechanical loading at the hip joint, during passive lying and active motion, which encourages correct hip development. Aim 1 will quantify passive and active hip mechanics of infants undergoing DDH treatment. Infants newly diagnosed with DDH will undergo 2 biomechanical test sessions in our laboratory before and after Pavlik harness treatment. We will monitor DDH infants in their daily life using activity sensors to quantify body orientation and time spent passive versus active. Clinical outcomes will be tracked during and after treatment during 6 visits over 2 years. This Aim will lend insight into patient selection for closed reduction treatment, help define the ideal treatment duration, and lead to directed therapies. Aim 2 will evaluate mechanical loading of the healthy hip by the Pavlik harness. The magnitude, direction, and frequency of forces at the hip due to active kicking attempts or passive lying while wearing the Pavlik harness are unknown, preventing the development of new therapies for DDH. We will instrument the Pavlik harness with sensors to evaluate the forces applied to the harness during wear to understand how the harness works to mechanically load the healthy infant hip during activity and passive lying. Aim 3 will develop computational models of the infant hip undergoing closed reduction. Experimental muscle and hip motion of DDH infants and patient-specific geometry from Aim 1 and forces induced on the Pavlik harness during active motion and passive lying from Aim 2 will serve as inputs to musculoskeletal and finite element models. These computational models of the infant hip joint undergoing closed reduction will support the development of improved DDH therapies. The expected outcome of this work is a more complete understanding of how the Pavlik harness works to translate passive and active forces to the infant hip. The results will provide valid experimental data and computational model...