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.
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.
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.