If there is any question, see a doctor as a slipped disk would be made worse by increased exercise. If the problem is muscle imbalance, read on. First, a little background. The back, or more properly the abdomen, is definitely the weak link for many riders. Cyclists have huge leg muscles but don t have the torso strength to support and resist the tremendous forces which those big legs can generate.
Any force directed into the pedals also goes up into the torso.
And as a corollary, a rider will never get stronger by pushing pedals alone as long as their torso continues to absorb the force generated by the legs. Our evolution has given us musculature designed for quadripedal animals, and the muscles which could support a hanging, horizontal spine can't easily stabilize a vertical one subjected to all the impacts and forces our upright posture dictates. However, we have potentially quite strong abdominal musculature to provide stiffness to the entire torso to support and reflect the force of the legs, whether pushing away against the ground in quadripedal running, or pushing against the pedals in riding a bicycle. Riding with undeveloped abs is something like riding a bike with a cracked frame - all the energy gets dissipated in flexion, and doesn't get you down the road. Posture problems occur when we stand but are even more significant when we sit or ride a bike.