According to petMD, weakness of the hind limbs in dogs is a common symptom of degenerative myelopathy, a progressive disease that affects the nervous system and spinal cord. Degenerative myelopathy is more common in older dogs, and there is no specific cause for the disease. Additional signs include loss of muscle mass and exaggerated spinal reflexes.
VCA Animal Hospitals explains that weakness of the back legs in dogs often occurs with lumbosacral syndrome, a condition that affects the lower parts of the spinal column. Lumbosacral syndrome is caused by pressure on the nerves that exit the spine. As the condition progresses, symptoms include pain and wincing when pressure is applied to the back and slow rising and lameness in the hind legs.
According to Kilgore Small Animal Hospital, intervertebral disc disease, also referred to as slipped discs, is a common condition in dogs that causes weakness of the hind legs and pain and stiffness. Slipped discs are more common in dogs with shorter legs and longer backs. If discs slip suddenly, it is referred to as acute disease. Discs can also slip in a slower fashion, leading to pinched nerves and chronic back pain. Slipped discs in dogs are often treated with steroids or surgery.