The shape, mobility and health of your spine is an important consideration when determining proper bike fit.
If your upper back/lower neck is too curved outward (kyphotic), there is a tendency towards more upper back pain/neck pain from the increased angle at the neck required to see the road. Decreasing the handlebar-saddle differential can help, with more or less reach determined by the impact to total curvature and shoulder-back integrity. (Wearing glasses that can slide down on the nose create similar problems).
If the low back/booty is too J-lo-like (lordodic), the differential and reach need be adjusted to keep from excessive pelvic tilt. These clients are usually more limber, sometimes to excess. If the spine is overly mobile, power transfer from sacrum to pelvis to femur is altered under load, with symptoms ranging from head to toes. And lumbar-laxity recruits erector spinae/ quadrates lumborum (low back) muscles that can actually inhibit gluteal (butt) recruitment. My job here is to find a pelvic angle conducive to stability, power and vertebral integrity, using, among other things, conformity at the thoracolumbar junction to indicate a positive outcome.
It is difficult to discern if decreased pelvic flexion is due to hamstrings or truly tight low back, because the two are connected via the ischial tuberosities (sit-bones). Tight hamstrings pull on sit-bones and increase posterior pelvic tilt; tight low back muscles (QL) create the same scenario. While the symptoms and possible solutions for these conditions are many, reducing the amount or unilateral and combined strain with shims, decreased saddle height, reduced reach and differential are typical.
I do not believe that adults can physically increase the length of developed muscle tissue, but that it learns to relax in passive stretching exercise or functional range-of-motion movements. Hard stretching alters connective tissue, and connective tissue is connective tissue… remember? It holds the body in place for muscles to move it, and if altered, alters aspects of proprioception.
A great approach to lengthen hamstrings is to lie on the floor and put the legs up the wall. The back should be flat and the hamstrings SHOULD NOT HURT. Position the butt far enough away from the wall that you can relax. 5-10 minutes in the evening will not only get the pooled blood and lymph out of your legs, it will teach your hamstrings to relax in a lengthened state.
Most vertebral wedging, disc herniation (tear), rigidity and scoliosis can be addressed on the bike, but only by someone qualified to do the job (see my section on “myths”). If you have a chiropractor, physical therapist, body worker, etc., be sure to schedule an exam with bike and trainer. I usually schedule time to attend during these sessions, to discuss what I see and collaborate with the provider to ensure a positive outcome.