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. Shielding while within their range is strongly recommended in order to avoid fractures and bleeding. 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. As always, a pistol is the safest option thanks to its long range.
Like all melee attacks, each of their punches has a noticeable cooldown, so an experienced player may opt to use a shotgun between their punches. Two point-blank shots from a basic shotgun will kill it on any difficulty.