Muscled Mutants are an evolution of the Normal Mutant.
Muscled mutants behave very similarly to normal mutants, chasing after Albert West when they find him and lunging at him when they are close enough. Unlike normal mutants, these will punch with each lunge, and have a moderate chance to fracture West's bones. In addition, they can attack with slightly more frequency than normal mutants.
Being in melee range can be dangerous, so either a hit-and-run method with a melee weapon or using a gun is recommended. In particular, they have a larger hitbox than normal mutants, so an SMG will hit relatively often and can kill them quickly and from a safe distance.