Study Design A prospective study of healthy volunteers Objectives The influence of the sagittal alignment of the spine and its influence on the extension reserve have been reported in the literature. However, specific analysis of the intrinsic coxofemoral and extrinsic pelvic component in subjects without any spinal or hip pathologies remains poorly reported. The aim of this study was to categorize the hip extension capacity, or extension reserve (ER), in relation to spinal sagittal alignment whilst standing, in a young healthy population. We hypothesized that the global extension reserve (GER) of the hip joint was influenced by the sagittal spinopelvic alignment. Methods We evaluated the ER of 120 healthy Caucasian volunteers (56 females (46.7%), 74 males (53.3%); mean age 25.6 years), using low dose radiographs from an EOS® X-ray imaging system in 2 functional positions; neutral standing position and in a forward lunge position. The GER is defined as the sum of the intrinsic (hip) extension reserve (IER) and the extrinsic (pelvic) extension reserve (EER). Cases were grouped into 4 sagittal alignment subtypes according to the Roussouly classification, and analyzed. Results Global extension reserve values were not significantly different between the 4 Roussouly subtypes ( P = .094), nor between patients with a sacral slope (SS) <35° (types 1/2) and ≥ 35° (type 3/4) ( P = .837). Statistically significant differences were seen between IER in each subtype ( P = .015), and EER ( P = .006). No difference in ER was seen between subtypes regarding pelvic incidence (PI). Conclusion An inverse relationship was found between IER and EER among Roussouly subtypes. The range of motion of the pelvis and the hip joint was higher in patients with a greater sacral slope.