[Starman]

Why are they so strong?

[Jonegy]

Because they need to be -

Take hold of the top of your door frame and tell me how many one handed "pull-ups" can you do? - but then we (humans) don't need it - we 're not swinging through the trees for the best part of the day.

[Thomas1970]

Hi Starman,
Can't answer this definitively, but I do believe Jonegy is on the right track with environmental or situational necessity.
On a biological level there are probably many reasons for this.
First off, the chimp's greatly enlongated limbs, or muscles specifically, provide a longer fulcrum when pulling against the bone, likely lending to a far more powerful contraction. Try pulling a nail out of wood for instance by gripping the hammer close to the head, as oppsed to the end of the handle.
Secondly, there are two types of muscle fiber, "fast-" and "slow twitch" of which human beings generally have a relatively equal proportion. Most top athletes on the other hand are probably either relatively gifted, or "genetic freaks" depending on how you want to look at it. Olympic sprinters for instance have a far greater ratio of fast twitch fibers which lend themselves to quick reaction and explosive power. Endurance runners on the other hand, must necessarily have far more slow twitch fibers. No conclusive studies have ever really been done into the effects of training, if any, in altering this balance.
There is one other factor though. At no given time do most of a human's muscle fibers ever really fire in unison. Typically they are activated in stages. From an evolutionary standpoint, this is necessary to survival. However short the time frame, muscles fibers need to chemically "reset" so to speak after every contraction, and it wouldn't serve to be temporarily immobile if being approached by a wild animal.
Though through specific training exercises, particularly pliometrics, muscles can be trained to fire more uniformly, producing far greater power. This is the secret to certain other athletes such as high jumpers. And certainly one factor, why even the most ordinary of people have been exhibited to accomplish feats such as lifting a car off someone in an emergency. Though not always necessarily "physically impossible", humans typically are simply not evolutionarily programmed to function in such a dramatic fashion.
Offhand, I would as well guess, that most other primates possess a slightly higher ratio of fast twitch fibers in addition to their elongated limbs. And it is probably likely their muscles function quite naturally, much more the way that the high jumper has rigorously learned.
But, then again, I'm a massage therapist with something of a background in biotechnology. I'm certainly no zoologist or wildlife biologist. So take this theory for what it's worth, my opinion. :D

[StuMegu]

I think hereditry has something to do with this. In a group the Alpha male is the strongest and usually has his way with the females! This means that generally all subsequent generations inherit the genes from the strongest male of the previous generation. In that way the entire group benefits greater overall strength which is critical if you come across another similar group and have to fight.

Humans, I believe, went down a different path in evolution. Instead of growing stronger every generation they eventually grew cleverer! ( why fight when you can throw rocks and build traps! ) and that might explain the difference between the two!

Only my limited thoughts on the matter :)

[worthbeads]

I think hereditry has something to do with this. In a group the Alpha male is the strongest and usually has his way with the females! This means that generally all subsequent generations inherit the genes from the strongest male of the previous generation. In that way the entire group benefits greater overall strength which is critical if you come across another similar group and have to fight.

There's survival of the fitest for you