Many people bend incorrectly at their waistline / top of the hips (figure 3) stressing the L4-L5 and L5-S1 segments rather than pivoting the pelvic bowl over the femur heads (figure 1). The degree to which the pelvis can pivot over the femur heads is dependent upon the length of the hamstrings which originate on the hips. As the pelvis pivots, the hamstrings stretch. As the hamstrings cross the knee joint, bending the knees provides slack in the hamstrings allowing the pelvis to continue its pivot. Thereafter, further bending requires spinal flexion. A moderate degree of spinal flexion is healthy, but excessive spinal flexion can cause or aggravate back pain.
Figure 1 correctly pivots the pelvis over the femur heads. Like a seesaw, as her head goes down, her tailbone goes up. Her tailbone rises to a 90° angle which demonstrates normal hamstring length. Notice how her spine remains relatively straight with its natural curves.
Figure 2 has shorter than normal hamstring length preventing her pelvis to pivot to 90°. To bend further, it would be more efficient to bend her knees to slacken the hamstrings and continue pivoting the pelvis, but since she doesn’t, spinal flexion takes over. This makes sense if her intention is to stretch the hamstrings and back muscles, but for someone with disc bulging and herniated discs, there is risk if spinal flexion is excessive. It’s a matter of degree.
Figure 3 demonstrates what happens when someone folds at the waistline / top of the hips rather than pivoting the pelvis over the femur heads. She either (1) has very tight hamstrings and overcompensates by excessively flexing her lumbar and thoracic vertebrae or (2) simply doesn’t realise where the most efficient point to bend is and folds at her waistline out of habit. Notice the limited degree of forward tilt of her pelvis and how her tailbone remains pointed downward as she rounds her back.
Hers is the case with a great many people who don’t realise that they are asking two or three lumbar vertebrae to cover a wide degree of bend even though the lumbar joint space is not designed for that range.
Transitioning from theory to action
Breaking habits is difficult. To feel how to pivot the pelvis: as your upper body lowers forward, move your pelvis back and away as if sticking your butt out. Like a seesaw, the tailbone rises as you descend. The spine remains long from head to tailbone. You’ll start feeling the hamstrings lengthening. At their maximum length, bend the knees for slack, but continue with the intent to pivot the pelvis forward, lifting the tailbone higher. Some people bend the knees, but it doesn’t help if they lose the intention to pivot the pelvis and leave the tailbone pointing down.
For more stability and length during descent, apply a yogic technique called uddiyana bandha, pulling in the lowest abdominal muscles to the pelvic bowl and lumbar vertebrae. This creates an intra-abdominal pressure to support and lengthen the front of the lumbar vertebrae. This is even more important while rising from a bent position as there is more risk of compression while rising.
Leaning over a sink
Leaning over a sink to brush one’s teeth or to shave does not require as much of a pivot of the pelvis as shown in figure 1, so short hamstring length should not pose a problem and bending the knees not imperative. Still, I suggest bending the knees, and while the upper body lowers forward, move the pelvis back and away from the sink with the tailbone pointing away. If one feels the hamstrings lengthening, that’s a good sign. Additionally, as you initiate the movement, pull in the lowest abdominals as explained above and while returning to an upright position.
Read and practice "Tight hamstring stretches."