(1) Active retroversion of the pelvis will help straighten the spine, but it is limited by the extension of the hip. This limit is represented by the tension of the capsule of the hip joint, the quadriceps and psoas muscles anteriorly. (2) The second step is to introduce a lordosis that will permit balance in standing position. However, this will not be enough. (3) A third step is needed with more lordosis that will create the reserve of hip extension
The reserve of hip extension can be defined as the amplitude of maximum extension of the coxo-femoral articulation relative to the vertical neutral position.
This dynamic element must be taken into account in the assessment of clinical and morphological status of patients and in the assessment of abnormalities of sagittal balance.
What is the Theoretical Extension Reserve which is necessary for smooth pelvic movement? We do not have a precise figure but based on optoelectronic studies [6–8], we can predict that slow walk requires 10° of extension reserve, for fast walking 15° and to run at least 20°.
This formula which makes it possible to calculate the correction angle necessary in the case of a sagittal imbalance, taking into account the extension reserve, does not give the information if the correction must be made at the level of the spine or pelvis, or even at the femurs. The contribution that can be obtained by stretching was not taken into account either. Thus, in practice, if a correction is necessary, one must assess whether a correction pre- or post-operative stretching could be performed, and this figure is expected to be subtracted from the formula.
How to measure the Extension Reserve ? There are two traditional ways: clinical examination and optoelectronics. Clinical examination does not make the difference in a precise way between extension movement at the level of the hip joint or in the lower lumbar spine. Optoelectronics examination is more accurate in lean subjects; however, its application is not feasible in daily clinical practice.
We propose an original method of radiological measurements using specific radiograph images . In Study 1, we compared 37 patients using two methods of measuring the extension reserve. The first method is to achieve an active retroversion movement of the pelvis in an upright position relative to the femur, while the second is to adopt a lunge position where, conversely, it is an active extension of the femur relative to the pelvis that will be applied.
A lateral radiograph with overlapped femoral heads and with a sufficient portion of the femurs as well as the lumbosacral junction allows to calculate the pelvi-femoral angle  in the upright neutral position and in two positions tested.
We concluded that only the lunge position radiograph should be used in practice. This is understandable because this is the recommended position for stretching to improve the performance of runners with improving of passive extension of the hip joint.