Final answer:
Lumbar flexion stress primarily affects the intervertebral disc in the lower back, leading to a herniated disc and compression of spinal nerves. This can cause pain and muscle weakness in the affected areas. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs.
Step-by-step explanation:
Lumbar flexion stress can affect various structures in the lower back including ligaments, muscles, intervertebral discs, and nerves. However, the primary structure that is stressed during lumbar flexion is the intervertebral disc. When bending forward, the anterior portion of the intervertebral disc is compressed, while the posterior portion expands. If the posterior anulus fibrosus (outer part of the disc) is weakened, it can result in a herniated disc, where the nucleus pulposus (inner part of the disc) protrudes through the anulus fibrosus and compresses a spinal nerve.
The compression of the spinal nerve can lead to pain and/or muscle weakness in the body regions supplied by that nerve. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs, which can cause sciatica. Sciatica is characterized by radiating pain from the lower back down the thigh and into the leg.