Atrophy is a pathological term which means becoming smaller than normal.
Forelimb : front leg
Hindlimb: Back Leg
There are five major reasons for lameness in animals:
1- Nerve involvement: Fore and hindlimbs have special nerves which innervate ( conduct ) with muscles. There are two kinds of nerves: sensory and motor nerves
Sensory nerves cause sense of legs or hands and motor nerves promote movements of them.If one of motor nerves is damaged in hands or legs, animals shows limping. Also disc disease causes limping of front or back legs in dogs or cats.
In long term because of lacking of using affected leg or hand , muscle atrophy is occurred. Therefore that forelimb or hindlimb seems slimmer than normal
2-Muscle damage: Myositis ( inflammation of muscle ) because of trauma, ischemia ( insufficient blood supply ) and infection can lead to limping. In this situation animals suffer from sever pain.
3-Bone fractures & joint diseases: Any trauma to hands or legs can cause fracture of a bone of them. Inflammation of a joint leads to arthritis and limping.
4-Luxation and incomplete luxation: in this situation because of trauma or congenital defect the head of a bone which is normally in a socket of the join comes out of its socket like patellar luxation.
5- Ligament damage: because of trauma rupture or incomplete rupture of a ligament can lead to limping such as cruciate ligament rupture in dogs.
It is necessary to say in a patient one or some or all of them can be seen.