When you bend forward, a combined movement of the lumbar spine and the pelvis occurs.
As you begin to bend forward, this movement starts from the head and upper trunk. The pelvis shifts backwards to keep the centre of gravity over the base of support, thus balancing the body.
For approximately the first 45 degrees of forward flexion, the Extensor muscles of the spine maintain the balance of the body. The Posterior Ligaments become taut and the facet of the Zygapophyseal Joints come together, which provides stability for the Intervertebral Joints, and the muscles relax.
When the movement has reached the point where all the vertebral segments are at full range, supported by the Posterior Ligaments and Facets, the pelvis begins to rotate forward: an anterior pelvis tilt. The glutes and hamstrings control this part of the movement. The pelvis will continue to rotate forward, until the muscles are at full length.
The final Range Of Motion depends on the flexibility of the Back Extensors, the Fasciae and the Hip Extensors.
To return to an upright position, the Hip Extensor muscles rotate the pelvis posteriorly, after which the Back Extensors extend the spine, beginning at the lumbar region and working its way upward.