I'm not sure it's because "our muscles" aren't as strong, but it's true that overall we're not nearly as strong as other primates our size. A 200-pound chimpanzee is *far* stronger than a 200-pound human. Part of it is that the chimp's muscles attach farther from the joints than ours do, making for more muscle leverage.
I can't find a reference to it at the moment, but I once read of a zoo that had a bar in their chimp cage attached to a spring scale, to measure the chimp's strength. One day one of the chimps got especially ticked off and with one hand yanked the handle to a pull of nearly 1000 pounds.
A couple years ago at one of Rolf Anderson's Bay Area MEMS Journal Club meetings I heard with mine own ears a researcher discuss using MEMS strain gages to measure the strength of individual muscle fibers. He made the 1/10 as strong comment as if it was an "everybody knows" fact.
No one challenged it, and there were a few knowing nods among the other biotech types in attendance.