Originally Posted by Mothra
Lots of stretching and warm-up before pushing hard will help make the most of what you have, but you might need more than that. The real bottom line is that your back-muscles are facing stronger loads than they're used to. With normal daily walking around and other upright sports, your back only has to support the weight of your upper-body. And since it's mostly vertical, the spine takes the load, not the back-muscles, which is only used to keep the upper-body upright.
With cycling, your back is bent over and more of the upper-body's weight needs to be supported by the back-muscles. Imagine laying on your belly on the bed. Then slowly scoot your upper-body off the side while trying to keep it horizontal. Imagine having your entire upper-body off the side of the bed so that your waist is over the edge (just your legs on bed) , can you imagine the amount of force your back-muscles needs to exert to keep your upper-body horizontal and not drop over the edge?
That's the same with cycling and even more. That's because not only does your back have to support your body's weight, it has to counteract the forces of your leg muscles as well. On each downstroke, your legs push your entire body upwards. You counteract that by pulling up on the bars and using your back muscles to force your body back down. Compare the size of your back muscles with the size of your leg muscles, and you'll see an imbalance.
To add to what you have stated above, the lower back is put under most stress if strain is placed on it when one is in a seated leaning forward position as in the seated racing cycling position and is aggravated further by rough road surfaces. There is a special powerful linear pedalling style in which the arms are used in addition to the hips/legs when generating the pedal power, the working arms support all upper body weight while the hips and working arms supply all pedalling resistance leaving the lower back in a relaxed state even when turning the highest gears. Anquetil used this technique, what a pity the experts ignored it. Pedalling and lifting weights have a lot in common where the lower back is concerned, there is a safe and dangerous way of doing both.