Spinal Stenosis: This condition is characterized by the narrowing of the spinal canal, which can put pressure on the nerves. Symptoms often include pain, numbness, or weakness in the legs that worsen with standing or walking and are relieved by sitting or leaning forward. However, some patients may experience pain that worsens when seated and improves with walking, possibly due to the specific posture or movement that alleviates the pressure on the nerves.

Peripheral Artery Disease (PAD): PAD involves the narrowing or blockage of the blood vessels outside of the heart, leading to reduced blood flow to the limbs. Symptoms can include leg pain when walking (claudication) that is relieved by rest. However, some patients might experience pain at rest or in certain positions, such as sitting, which could be lessened by walking due to increased blood flow.
Musculoskeletal Issues: Conditions like muscle strain, tendonitis, or bursitis can cause leg pain that may vary with activity and position. For example, sitting might exacerbate pain in the legs due to increased pressure on certain areas, while walking could stretch or relieve pressure on the affected muscles or tendons.
Neurogenic Claudication: Similar to vascular claudication but caused by nerve compression, typically due to spinal stenosis. The pain is usually worse with standing or walking and relieved by sitting. However, the presentation can vary, and some individuals might experience worsening pain when seated due to specific nerve compression patterns.

Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the legs can cause pain, swelling, and warmth. While DVT typically presents with swelling and warmth, pain can be variable and might worsen in certain positions, such as sitting, due to increased venous pressure.
Osteonecrosis: The death of bone tissue due to a lack of blood supply can cause severe pain, especially in the hips, knees, or ankles. The pain can be constant but might worsen with weight-bearing activities or certain positions, such as sitting.
Infection or Abscess: Infections in the bone (osteomyelitis), joint, or soft tissue can cause severe pain that might worsen with movement or pressure, including sitting.

Chronic Compartment Syndrome: Increased pressure within a muscle compartment can cause pain, especially with exercise. While it typically presents with activity-related pain, certain positions, like sitting, might exacerbate the condition due to increased pressure within the compartments.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can lead to joint hypermobility and pain that might be position-dependent.
Complex Regional Pain Syndrome (CRPS): A chronic pain condition characterized by severe, burning pain at the site of an injury, which can be exacerbated by movement or position, including sitting.