Both in broilers and breeders, very early lameness can occur when excessive amounts of proteins are given. This lameness can express itself already within the first week of life, but can continue to give leg problems to a later age.
The leg problems can be noticed as spraddled legs and other malfunctions, but in general it can be observed that the birds are walking/moving with obvious pain.

Although there is still some discussion going on about the exact reason for this lameness, a possible explanation is that the bird uses the extra protein for energy supply. This means that the protein is burned, but that the resulting Nitrogen, Sulfur and Phosphorus cannot be sufficiently removed from the body, and forms deposits in the joints, among other tissues.
An additional factor can be that the extra protein stimulates the bone growth to a too high level, resulting in weak bones.

Varies strains need various protein levels in the feed. That is not because the birds have a difference in protein requirement for that age, but because the feed uptake is different, and as a result the uptake of protein is different.
When feed intake of a flock is reduced with 10%, for instance due to its genetic capacity, we need to increase the protein level with 10% to compensate. However, if the protein uptake is normal, the same increase of 10% will result in an oversupply of protein.