In 'The Walking Dead' universe how fragile are the walkers physically? How tough are their bodies/bones compared to living humans?
In virtually all zombie fiction, including what I have seen of TWD television show and the comics, zombie bones are just as strong as the human's were at the time of reanimation. An elderly woman who suffered from osteoporosis would still have brittle bones, but the majority of walkers would have typical bone strength.
As far as the durability of the rest of their bodies, zombies begin just as durable as the humans they once were. As they age, they tend to deteriorate and decompose. Any wounds do not heal, but instead become starting points for further damage.
In the late stages of decomposition, corpses tend to be relatively 'mushy', which would reduce a zombie's durability. They do seem to decompose much more slowly than normal corpses, of course, so there's no telling how long it will take for them to reach that stage.
It is even conceivable that zombies in unusual environments (say, a hot & arid desert) would mummify. If they were to mummify or otherwise be preserved, their flesh could be hardened (like leather) and be slightly tougher.
If we're talking about TWD zombie bones as compared to real human bones, it seems clear that, although no one really talks about it, the zombies' bones, especially their skulls, are much less robust than our own.
As I said, no one seems to mention it on the show, but we can see it quite clearly. People routinely crush zombie skulls by stomping on them, hitting them with fairly flimsy objects, etc, and the most popular method of killing zombies is now the classic stab-in-the-head.
While I don't recommend that you try this at home, I can assure you that stabbing someone in the face won't usually result in the knife piercing their brain cavity. The skull is surprisingly thick, robust, and extremely tough. It is very difficult to penetrate it, especially with something like a knife or screwdriver.
The shape of the human skull - almost spherical in the upper portion - is such that a thrust with a knife is almost certain to be deflected and slide off to one side, which might inflict quite a bit of damage to the scalp and/or face, but the skull itself will suffer little or no damage whatsoever. Even if you managed to strike the skull perpendicular to the forehead, for instance, the knife might very well stop short, creating a possibility that you will injure your own wrist, or even worse, that your hand will slip forward and be sliced open by the blade.
As for stomping on a head, this might cause a concussion in a human victim, but it probably wouldn't crush the skull outright. And yet we see, again and again, zombie skulls popping open like grapes when the slightest amount of pressure is applied.
Moving on to blunt objects, such as baseball bats, rocks, clubs, and so on, again, the injuries inflicted upon zombies are far more serious than would be the case if the target was a living person. Yes, of course a powerful blow from a bat would fracture a human skull, but fractures aren't enough to kill a zombie. You need to destroy the brain, not merely create a few hairline fractures in the bone around it. Once again, attacks which would only hurt a person are consistently shown to kill zombies.
In season 3, we saw a zombie in the prison, whose hands were cuffed behind his back, snap his own arm in half with very little effort. This too would be almost impossible for a living human being to do. And in the season 5 finale, Rick was able to slowly shove a gun barrel through the roof of a zombie's mouth and into the cranial cavity. Also something that you couldn't do to a human without a tremendous amount of force.
We've also seen two zombies being killed by holding a relatively thin branch perpendicular to their heads, at about mouth-level, and pushing with very little force. Branches less than an inch and a half in diameter are not as strong as a human skull. The branch should break before the skull does.
And in season 4, when zombies began to fall through the ceiling of a big box store, a drop of about 15-20 feet, they literally popped when they hit the ground, almost like water balloons. A person who fell the same distance would probably break some bones, but they certainly wouldn't explode.
All in all, we are never told that zombie skeletons are far less robust than human skeletons, but all the evidence points in exactly this direction. Zombie bones are much more brittle and prone to breaking than human bones.
In a recent interview, Fear the Walking Dead showrunner Dave Erickson said that on that show, zombies will be more difficult to kill than they are on the original The Walking Dead series, because Fear the Walking Dead takes place at the beginning of the outbreak, whereas TWD is now at least a couple of years into it. As a result, zombie skulls on FTWD are stronger and harder to penetrate than has been the case on TWD.
Erickson said one of Kirkman's notes for Fear was that the zombies be different than how they were first seen on the original show. With the early onset of the outbreak, skulls will still be hard and the point, Erickson said, is to see a character trying to stab a walker in the head and not be able to penetrate the skull. "The point was it's hard physically to kill somebody," he said.
In fact, we've already seen this happen, on the second episode of the new series.
Basically if they keep getting fresh meat, they are fine, fine meaning can bite through human flesh easily and chase humans at brisk walking speed. If remain unfed, then chances are the walkers are weak enough to be harmless, meaning you can outrun them, trip them over and then go for their head. Compared to humans, they ain't that strong at all. But because they are dead, their flesh gets hardened more than usual due to lessened or no blood circulation due to heart not functioning and hardened flesh is much tougher than live flesh. So their strength is not insignificant but since their fleshes and bones are dead, they are weaker than human. A man can clench his fist so tight that he can punch so hard that it will make hole thru three walkers. Can a walker clench his muscles, tighten his biceps, make a long run with load? So compared to humans, they ain't much physically. But since their aim is not to wrestle you and win a WWE contest, all they care is to bite you. So if they are in multiple numbers, they are pretty overwhelming in physical perspective but only because they want to bite you. Even a human pitted against a pair of cats can be defenseless if all the cats want to do is bite the man.