We here on Earth, live under a 1 grav world.
Humans can live in lighter gravity, but the lesser gravity leads to bone lose and muscle atrophy.

What about the other direction? How how heavy can the gravity before people cannot live in it?
If a kid was born on a 2 grav world, it would be normal for his 180 lbs to be 360 on his world. I think he would live, but he probably would have bone and joint problems in later age.
Anyone know or have any what the upper limit would be?
Any links to living under various levels of gravity?

This is probably more akin to general discussion.

Since humans have never lived in heavy G involvements, we've been unable to do any extensive research about it.

We had all kinds of crazy worst case theories what would happen when we would experience zero G for the first time. It was believed that our eyes would deform, that we'd be continuously dizzy in such environment, many many bad stuff. Studying long term effects of zero/low gravity on humans is an ongoing study. We're starting to understand what it does to us and how we can combat the negative effects but we're still working it out. We still have no idea how pregnancy would be affected in zero g environment.

I suppose a likely scenario of long term effects of living in a heavy g world would contain the following: One noticeable effect would be increased muscle build up to cope with the heavy environment. It would be a strain on the heart since it would be harder to circulate the blood since gravity is pulling at it more. Issues like worn down joints (like you mentioned) is a probable long term health effect. Any other array of effects could be possible but we wouldn't know really until we try. Shame that there are no good candidates for heavy G colonies floating around, near us.

Humans can't take high G's for more than a few minutes. Look up the research into High-G manuevers done by the pilots of jet aircraft. Fighter pilots take lots of high-gs all the time.

I remember reading that women are better at taking high Gs than men.

True, it's because of the difference in body mass, the different make up of their systems. They have a lot more fat, whereas we have more muscle... That affects everything from hi-G tolerance, to cold survival, to heat tolerance, survival without foot, alcohol and poison tolerance, bouyancey and more...
There are advantages and disadvantage for both.

But the pilot thing is an extreme example really since very few people undergo those levels off acceleration, positive and negative.
The fact is though that we ALL constantly experience situations at more than 1G, it's NOT rare by any stretch of your wildest imaginings: in cars, elevators, skiing, toboggans, trains, on horse back, and a zillion other common everyday situations.

1 G is: gravitational acceleration - 9.796 m/s^2 (meters per second squared)

Yiu might only experience that acceleration on separate parts of the body for mere milliseconds at a time, or you might experience it on the whole of the body for longer, depending on the situation, but it's usually pretty short. The shorter time you measure these things happening though, usually the greater the strain you can observe in that time.
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As for life at constant 2Gs, you would adapt or die.
Succeeding generations would of course find that easier and easier.

You're thinking of fighter pilots or astronauts experiencing extreme acceleration. In those cases they're experiencing very high pull that's 10 or 12 G's. I think the record is 46.2 G's but those are deadly levels. One of the most damaging factors is when you combine these extreme pulls with vibration. Since your internal organs are subject to enormous strains while at the same time are being vibrated.

I don't think we've ever been able to simulate a constant exposure to high gravitational pull like a factor of 2, for an extended time like weeks or months.

In the 1950s and early 1960s the USAF did experiments in sustained high G exposure. It's just that the easiest way to get it is acceleration and manuver, but the results and experience are the same. It's all in those boring documentaries on the development on the first generations of Mach capable aircraft and farther, higher, faster. You get into the natural metal century series fighters and you learn this stuff. I know guys obsessed with this stuff and others who were on the ground when it happened and the F-100 Super Sabre was the state of the art.

It's one of those things that we take for granted today but was the cutting edge back in the day when a Mach 1 aircraft and the area rule effect was brand new.

But in those tests weren't they simply checking to see how their pilots would cope with extreme forces like 10 G and see how long they could stay conscious?

Correct me if I'm wrong.