Baker's Cyst: A fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee, often associated with knee arthritis or injury. It is a common cause of pain in the back of the knee, especially when walking.

Hamstring Strain: Injury to the muscles at the back of the thigh, which can cause pain in the back of the knee, especially during physical activities like walking.
Calf Strain: Similar to a hamstring strain, but affecting the muscles in the lower leg, which can also radiate pain to the back of the knee.
Popliteus Tendinitis: Inflammation of the tendon that connects the popliteus muscle to the bone, leading to pain at the back of the knee.
Meniscal Tear: A tear in the cartilage of the knee joint can cause pain in various locations, including the back of the knee, especially with activities like walking.

Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg, which can cause pain, swelling, and warmth in the affected leg. Although less common, it's a critical diagnosis to consider due to its potential for severe complications like pulmonary embolism.
Neurogenic Claudication: A condition related to spinal stenosis, where narrowing of the spinal canal leads to pressure on the nerves, causing pain in the legs, including the back of the knee, when walking.

Popliteal Artery Entrapment Syndrome: A rare condition where the popliteal artery is compressed, leading to pain in the back of the knee and leg during exercise.
Gastrocnemius or Popliteus Muscle Tear: Although muscle tears can occur, they are less common causes of pain specifically in the back of the knee compared to other diagnoses listed.
Tumors: Both benign and malignant tumors can occur in the soft tissues around the knee, including the back, but are rare causes of pain in this area.