Equilibrium of the human body and the gravity line: the basics

Bipedalism is a distinguishing feature of the human race and is characterised by a narrow base of support and an ergonomically optimal position thanks to the appearance of lumbar and cervical curves.

The pelvis, adapted to bipedalism, may be considered as the pelvic vertebra connecting the spine to the lower limbs. Laterally, the body's line of gravity is situated very slightly behind the femoral heads laterally, and frontally it runs through the middle of the sacrum at a point equidistant from the two femoral heads.

Any abnormal change through kyphosis regarding the spinal curves results in compensation, first in the pelvis through rotation and then in the lower limbs via knee flexion. This mechanism maintains the line of gravity within the base of support but is not ergonomic. To analyse sagittal balance, we must thus define the parameters concerned and the relationships between them.

These parameters are as follows: for the pelvis: incidence angle, pelvis tilt, sacral slope; for the spine: point of inflexion, apex of lumbar lordosis, lumbar lordosis, spinal tilt at C7; for overall analysis: spino-sacral angle, which is an intrinsic parameter.