His back is only flesh and blood. Why doesn't lifting huge weights injure it?
Most likely his entire skeleton (except his head and his left arm) was reinforced, otherwise his legs would also injure him when running. (Not to mention when landing a high jump.)
This troper has heard, second-hand, that one episode mentioned reinforcements that got implanted in his back for exactly this purpose.
The made for TV movie made much later than the series does mention this reinforcement.
Martin Caidin, the creator of the character and also the writer of non-fiction works on aviation and science, has been quoted as saying he knows aspects of the bionic man were physically impossible, but a level of suspension of disbelief is needed in order for the concept to work.
I recall that the bone marrow in his bones had been replaced with metal and the marrow transplanted onto the outsides of his bones to continue generating blood or something.
The novelisations stated that Steve and Jamie's skeletons were strengthened. In fact, the entire bionic conversion was explained in detail in every book (though, hilariously, the description for Steve Austin's conversion was more or less just straight copied for the Jamie Sommers novels, resulting in her being given bionic enhancements that were exclusively Steve's.
So exactly how did the Venus probe manage to not explode during the long trip from Venus to Earth through the vacuum of space, when it couldn't even handle being lifted a few hundred feet up from the Earth's surface before exploding?
Fatigue stress from all the forces the probe was subjected from launch, including the Venus flyby and re-entry into Earth's atmosphere? Eventually every material will fail given enough stress applied to it.