This is a very detailed response. I've tried to get very specific and answer several of your questions.
There are several factors we need to look at when considering the strength of a gorilla compared to a human:
1) Neurological efficiency.
2) Types of Muscle Fibers available (I, IIA, IIB).
3) Joint, Tendon, bone structure (this was mentioned by one member).
4) Insertion points (this was discussed by one member). This is related to the leverages of the muscle attachments.
5) Muscle Belly Length (Longer Muscles are better).
5) Muscle mass (this was mentioned).
7) Motivation.
Gorillas are not built exactly like human beings and comparing their strength to human beings requires some guesswork. Gorillas, like most other primates, have an upper body structure built for certain types of powerful movements. They would struggle to play the piano or manipulate small objects, but they could bend a license plate a lot easier than most humans.
We are built for fine motor skills and moderate amounts of power (ie. throwing with accuracy). As mentioned by another poster, gorillas have better leverages than humans in their arms. Pound for pound, a large part of their muscle mass is in their arms, shoulder, back and chest. Their total muscle mass per pound of bodyweight isn't that much different than humans though. But then again, that's because they have such heavy bones and because their lower bodies have less proportionate muscle mass.
Guessing how much they could actually bench press is very difficult. I've never heard a report describing the distribution of fast twitch fibers in a gorilla, although I expect it to be extremely high in the arms and upper back. Moreover, we don't know how well a gorilla’s nervous system works when it recruits muscle fibers. During an adrenaline rush humans can sometimes exert forces that will break their bones. This means that we have the POTENTIAL for moments of great strength-- even if we are skinny.
You see, humans have weaker bones and so our nervous system generally prevents us from recruiting all our muscle fibers for a single movement. If it didn't we would break ourselves! A gorilla has bones that are very thick, so it wouldn't have to have nearly as strong of an inhibitory response. A gorilla can probably recruit a lot more fibers in the upper body whenever it wants to. Some humans that are gifted in strength sports have a high distribution of fast twitch fibers and great neurological efficiency. This is probably far more common amongst silverback gorillas.
So let's look at these factors. A big silverback gorilla has more mass in its arms, chest, shoulders etc. than even very large human beings. It probably has a lot of fast Twitch fibers in these areas. Its upper body has bone structure, insertion points, long muscle bellies and excellent tendon/ligament structure designed for brute strength movements. Its bones are built to protect it during power movements and hence it probably doesn't have nearly as strong of an inhibitory response when it engages in power movements.
Considering these factors, it’s obvious that a gorilla is VERY STRONG in the upper body, for certain movements.
But does this mean a gorilla can lift 4,000 lbs over its head? Hell no. To me, that sounds like fantasy. The gorilla is not designed for pressing things over its head. It's designed for pulling, bending, and grabbing things. Just look at its anatomy.
Considering the above factors you could safely say that a silverback gorilla is 4 or 5 times stronger than a healthy and well built human male (IN THOSE TYPES OF MOVEMENTS) but not when it comes to doing things that require movements that are not natural for a gorilla. Again, a gorilla could bend things very easily. It could snap things, tear things and grab things with crushing force. It could probably rip a telephone book in half without much practice. Its power in these movements would be truly scary compared to a human.
However, I wouldn't be surprised if a professional world class power lifter could bench press more than an untrained gorilla. True, the gorilla would have far more potential than an average human at the bench press, but nobody knows how well the gorilla could do in a bench press if it was trained. Sorry, but I wouldn't be surprised if a silverback struggled to get past 500 pounds. I also wouldn't be surprised if it managed to do 2,000 pounds. We just don't know.
As explained, there are too many factors involved. We don't know how all the factors listed above would work together for the gorilla, in this type of movement.
This is about as honest and accurate of an answer you will get.
One thing we do know is that chimpanzees have been tested in pulling, gripping and dead lifting movements and they exceeded untrained humans by several times (per pound of bodyweight)-- as much as 4 times or even more.
The Guinness book of World Records once estimated that a silverback gorilla could dead lift 1,800 (based on the results of the tests on Chimps). They stated that this would POSSIBLY be true if the silverback was trained. The world record for the dead lift by the strongest human on the planet is around 900 lbs. These estimates are believable and realistic, but there is still a lot of guessing involved.
Notice that these are pulling movements and gripping movements, not pressing movements. Pressing 4000 pounds above the head is unrealistic.
It's much easier to get these animals to do pulling and grabbing types of movements because they are built for them and they come naturally.
By the way, people have actually competed Grizzly bears against lions and the Grizzly Bears almost always won. Grizzly bears were also pitted against bulls and they usually won. Grizzly bears would break the backs of these other animals.
But that's rather violent and repulsive. I'd rather compare their brute strength and power, not which animal can kill the other.
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