Muscular leverage and Arthrokinematics
In the kinetic chain (nervous, muscular and skeletal systems) the bones act as levers which are moved by the force of the muscles. The levers (bones) are moved around different axes (joints) creating what is referred to as torque. Joints are either reduced eccentrically, stabilized isometrically, or overcome concentrically. The compressive load on the joint(s) is increased as the distal end of the body moves further from center of gravity and reduced as they move closer. For example when abducting the arms, as when extending them overhead in extended mountain pose, the compressive load on the joint beings to increase as you raise the arms and is greatest when the arm is at a 90º angle from the torso. Compressive load then begins to decrease as the arms reach their final position with the hands directly over the shoulders.
High hinge Vs. Low hinge movement
Beginner students may have a tendency to round their spine as they fold forward. This rounding usually begins around he thoracolumbar junction and continues through the lumbar spine to the sacrum. When students fold this way it is referred to as moving from the high hinge. If the movement occurs at the hips with the spine lengthened then it is referred to as the low hinge. In yogasana it is preferable to move from the low hinge.
Why you should bend from the low hinge
forward bending at the upper hinge is a result of contraction of the rectus abdominis (the outermost layer of the core musculature) pulling the lowest ribs toward the iliac crests shortening the front body. This results in a change to the lumbar curvature from concave to convex. This can over stretch the erector spinae making lifting from this position more susceptible to injury. Secondly folding in this way can result in herniation of the lumbar disc in the direction of the spinal chord.
If the fold is performed from the low hinge then the ribs remain lifted and the hinge happens through activation of the psoas muscles. This helps maintain the natural concave curvature of the lumbar spine and maintains a balance between the lengthening of the rectus abdominis and the erector spinae. Low hinge movement also transfers more of the load to the hamstrings which are more equipped to handle the stretch.
Putting it all together
When performing a forward fold it is best to hinge from the hips (low hinge) to maintain the natural curvature of the lumbar spine, reducing possibilities of injuring the back or spine. However, knowing what we do about compressive load it is important to maintain an engagement of the transverse abdominals to help support the spine as it moves through 90º of flexion from the hips. To do this be sure to pull the navel up and in towards the spine as you fold. Maintain lift through the chest and length through the spine. Once traveling past 90º the compressive load begins to decrease as the chest moves closer to the thighs. To return to standing soften the knees and lengthen the spine first. Engage the core by pulling the navel up and in then move from the low hinge to rise.