A recent study in The Spine Journal showed that the type of forces that your spine experiences determine what the intervertebral disc cells will do. Scientists found that heavy loads activated genes that told the disc cells to slow down their breakdown cycle. Moderate loads over a prolonged time however signaled a quicker turnover of the cells.
What this means to you and I is that moderate but repetitive compressive loads to the spine (such as a repetitive lifting job) tend to lead to quicker and more advanced disc disease than individuals who experience heavy but infrequent compressive loads (such as a weight lifter). Your gym workout is not contributing to a bad back, it is in fact preventing you from developing one!
Gwendolyn A. Sowa, J. Paulo Coelho, Kevin M. Bell et al (2010). "Alterations in gene expression in response to compression of nucleus pulposus cells." The Spine Journal 11(1) pp. 36-43; doi:10.1016/j.spinee.2010.09.01