Hello CityMonk,
Responding to your original (two) post(s)…
Improper forward bending heaps on to our pile as we have chosen to live in forward bending - driving, keyboarding, picking things off the floor, washing dishes, gardening, the list is extensive.
When forward bending is not done correctly (or when it is over done) then there are greater risks. As the front body is closed down the discs are “pressured” to ooze toward the back body, the spinal canal, pain, and bulging. Most disc issues occur in a forward bending action of the spine. Again, if we lived in backbend we’d likely have the opposite problem.
In asana there are a few postures where the spine is “rounded”. However, generally speaking, proper forward bending comes from a movement in the pelvis rather than a movement in the spine. And this is why there are statements about maintaining a neutral spine in forward bends. And of course there are no absolutes. But we are working toward a certain integrity in the pose even though we are not there yet AND may not be there in this lifetime.
When a person has lower back pain, forward bends are ill-advised (read: contraindicated). But forward bends certainly do play an important role in an asana practice which may ultimately balance certain students - though not all. So they are modified to fit the nature of the student.
Because the integrity (read: appropriate balance between safety and effect) of forward bends depends on mobility in the pelvis AND that mobility relies in part on hamstrings, standing forward bends are a better “start”. Why? Because seated forward bends trap the legs and limit the “out” for the student, while standing forward bends provide a latitude (read: knee flexion) for escape, release, backing off. It is also more difficult to mobilize the pelvis in seated postures since it is often stuck to a mat, the ground or what have you…unless you are practicing on ice.
Obviously a muscle has an origin and an insertion (read: two points where it attaches). Ergo moving one point away from the other is stretching the muscle. When the origin is held in place and the insertion moves OR when the insertion is held in place and the origin moves, these both lengthen the muscle. So it is anatomically “true” that when the knees are bent BUT straightening and the sitting bones are held in place OR when the knees are bent (held in place) and the sitting bones are lifting, both of these actions stretch the hamstrings. It is rare indeed to have a “never” or an “always” in yoga but it is pretty clear that when a muscle is attached and one point moves away from the other stretching occurs