Lumbar Strain: This is the most common cause of lower back pain radiating to the back of the legs, often described as muscle pain. It occurs due to overstretching or tearing of the muscles in the lower back.

Lumbar Disc Herniation: This condition occurs when the soft inner gel of the disc leaks out through a tear in the outer layer, putting pressure on the nerves and causing pain that radiates down the back of the legs.
Degenerative Disc Disease: Wear and tear on the spinal discs can cause pain in the lower back that radiates to the legs, especially with activities that put stress on the spine.
Sacroiliitis: Inflammation of the sacroiliac joints, which connect the sacrum to the ilium bones, can cause pain in the lower back that radiates down the back of the legs.

Cauda Equina Syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, leading to pain, numbness, and weakness in the legs, as well as potential bladder and bowel dysfunction. It requires immediate medical attention.
Spinal Stenosis: Narrowing of the spinal canal can put pressure on the nerves, causing pain, numbness, and weakness in the legs.
Osteoporotic Compression Fracture: Weakened bones can lead to fractures in the spine, causing sudden and severe back pain that can radiate to the legs.

Spinal Tumors: Both benign and malignant tumors can cause lower back pain that radiates to the legs by compressing nerves or weakening the spine.
Infectious Spondylitis: Infections of the spine, such as osteomyelitis or discitis, can cause severe back pain and require prompt antibiotic treatment.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues, which can lead to joint hypermobility and musculoskeletal pain, including lower back pain radiating to the legs.