Flexion is an anatomical term that refers to a movement produced at a joint by a muscle or muscles that causes the angle of the joint to decrease relative to its anatomical position. For example, when a person is standing normally, the hip joint is considered to be in a neutral or 180-degree position. Flexion at the hip occurs when that person raises her knee into the air, thus bending the hip and decreasing the joint angle at the hip from 180 degrees to 90, if the knee is brought to hip height. In order to produce this movement, the muscles being flexed require the coordination of tendons, ligaments, cartilage, and bones, all structures around the joint working together.
A large number of joints in the human body are capable of flexion and its opposite movement, extension. The elbow joint flexes when the arm bends at the elbow, the shoulder joint flexes when the arm is raised straight in front of the body, and even the joints between the vertebrae can be flexed, causing the spine to curl forward as it does during an abdominal crunch. While there are exceptions, most flexion and extension occurs in the sagittal or front-to-back plane of movement. Joint movements that occur in the frontal plane, or to the side of the body, such as raising one’s leg to the side, are typically known as abduction and adduction.
At each flexible joint in the body, there is a primary and often secondary muscle group responsible for flexion at that joint. In the hip, for instance, the primary muscles are the iliopsoas, the tensor fasciae latae, and the rectus femoris, collectively known as the hip flexors. Secondary muscles involved in hip flexion include the sartorius, gracilis, and adductors longus and brevis.
When a person raises her knee from a standing position to hip height, flexing the hip to 90 degrees, the brain first sends a signal through the motor neurons of the peripheral nervous system that tells the hip flexor muscles to fire. The hip flexors then contract, or shorten in length. Because they are attached to the hip and femur bones via tendons at either end, much like a series of pulleys and cables, they move the bones around the joint, pulling the femur bone forward as the knee lifts. This complex system results in a seemingly simple action: flexion of the hip joint to move the leg forward in space, just as occurs during walking, running, climbing, and other forward movements.