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 reserve of hip extension is necessary, because if there is a limitation during displacement, the femur would force the pelvis, by ligamentary and muscular tension, to have bending movements , with every step. Thus, it is through repeated hyperlordotic movements that the subject can maintain the fluidity of movements of walking, but when used regularly, it will lead to rapid degeneration of the spine and correlate with low back pain as has been demonstrated by several authors [1–4] (Fig. 2).

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.

If there is a sagittal imbalance , it can be represented by two components: anterior spinal imbalance and femoral flexion. To calculate the necessary correction, one must add these two components. However, the subject has a reserve extension which can be called “Actual Extension Reserve ”, which should be subtracted from the formula. This formula, in this representation, will allow only a balanced upright position and, as we have seen, a reserve extension is still necessary. This reserve extension, can be called “Theoretical Extension Reserve ”, should be added to the formula [5] (Fig. 3).

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 [9]. 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 [10] in the upright neutral position and in two positions tested.

This study shows that the active pelvic retroversion method gives a significantly inferior result compared to the lunge position method. In addition, the implementation of active movement by retroversion of the pelvis presents difficulty for many patients with paradoxical results and measurement failures (Fig. 4).

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.

This is a dynamic radiological assessment that can be criticized for its reliability, repeatability, and the influence of pain. However, this assessment can give information on the reserve of extension so far unknown in clinical applications. Therefore, we have decided to make this assessment systematic for each patient where lumbar surgery could change the sagittal balance. The exploration includes full spine and spinal dynamic radiographs, X-ray in the lunge position for right and left femur as well as in neutral position (Fig. 5).