The natural limit is about what your cardiovascular system, muscles, tendons and bones can handle w/o damage.
On the world’s strongest man competitions seeing people have nosebleeds (I assume due to too much abdominal pressure bursing blood vessels, I don’t know), broken bones or torn muscles/tendons isn’t uncommon. Someone I knew a bit who used to compete in them (he competed at state and national levels, not international) said abuse of painkillers was rampant because of how painful it is to do that stuff.
Also natural is a weird word. Some people have a genetic mutation that limits their bodies production of myostatin, so they have huge muscles w/o much effort. Their limitations will be far higher than those w/o the mutation.