Background/Objectives: Duchenne muscular dystrophy (DMD) leads to postural abnormalities and increased lumbar lordosis, which may affect gait and spinal load. This study aimed to assess the reliability of sagittal spinal curvature measurements using the Rippstein plurimeter and to analyze spinal curvature in ambulant boys with DMD compared to healthy peers. Additionally, the study examined the effect of lower limb positioning in standing on sagittal spinal alignment in boys with DMD and investigated the relationship between hip adduction and extension range and spinal alignment. Methods: The study included 42 boys with DMD and 36 healthy peers aged 5–14 years. In boys with DMD, spinal curvature was measured using the Rippstein plurimeter in two positions: feet in alignment with hip joints axis and with feet together. In healthy participants, measurements were taken in the first position only. Hip adduction and extension ranges were also assessed in both groups. Results: Plurimeter measurements demonstrated high reliability. Boys with DMD showed significantly increased cervical retraction, greater sternal deviation from the vertical, and increased lumbar lordosis compared to healthy peers. Lower limb positioning (adduction) altered sagittal spinal alignment. Hip adduction and extension ranges were decreased in the DMD group and showed a correlation with spinal alignment. Conclusions: The Rippstein plurimeter provides reliable measurements and is useful for monitoring posture in boys with DMD. Reduced hip mobility and lower limb positioning influence lumbar lordosis and should be considered in physiotherapy planning for DMD.