Were Cro-Magnons a lot stronger than modern humans

And what about Homo Erectus, would they have had chimp like strength?

That depends on what you consider mdoern humans. Cro Magnon is Homo sapiens, just like us, but they lived over 30,000 years ago. How strong we are depends on how hard we train. It is doubtful that at an absolute scale, they were stronger than modern Olympic weight lifters or faster than modern Olympic sprinters. On average, however, they should be stronger and faster than the aveage person in the street because they had to hunt for a living whereas most city slickers rarely do any exercises at all, and most probably have jobs that requires little physical exertion.

Chimps exercise daily, by climbing trees to make sleeping nests and by walking long distances in search of food everyday. In contrast, many of us walk from the living room to the garage, drive to work and sit at a desk all day long. Therefore they would seem stronger. But if we were to compare well trained athletes versus chimps, we find that they are only about as strong as humans on an absolute scale. They are only stronger than we are (about twice as strong) if we take into account their smaller body size. The difference is genetics. We humans have deactivated some of the genes that give the chimps their strength and our muscle fibers are different. Our jaw muscles are much weaker than those of a chimp's because it allows us to have a much bigger brain. Our leg muscles are weaker because we are built for running longer distances on the wide open savanna, whereas chimps are designed for sprinting short distances through the woods.

As for Homo erectus, they have stronger jaw muscles and smaller brains than we do. They should also be similar to us in terms of arm and leg strength because they are built more like us than chimps.

They certainly would have been a lot tougher and resilient but strength-wise they wouldn't have differed much.

cro-magnons ARE modern humans.