1. Illiopsoas
2. Rectus Femoris
3. Tensor Fascia Lata
and to a lesser extent, the Sartorius, and select muscles of the both the hip abductor and adductor groups.
As stated in previous blogs, all muscles contract either concentrically (shortening under load) or eccentrically (lengthening under load) and provide varied functions as they pass through differing angles of rotation. Although almost everyone realizes the concentric contraction of a muscle group, I feel they don't realize and understand the value of the eccentric contraction in both efficiency and safety of human motion. Let's now go back to the function of the hip flexor muscles to explain this.
As previously stated, the concentric contraction of this muscle group results in movement of the thigh upward toward the abdominal region. The attachment of the head of the thigh bone (femur) in the hip joint (acetabulum) provides a ball and socket connection for a very stable axis of rotation. Increased resistance is provided by either the angle of the body in relation to gravity (lifting your knee to your chest while on your back vs. while in a standing position), leg in a straight position vs. bent, addition of increased resistance, placement of resistance along the length of the leg, or a combination of all the above. The result being increased force to the muscle thus providing the necessary increases in both strength and power to combat the opposing eccentric force. However, the efficiency and safety of this concentric movement is dependent upon the range of motion of this same muscle group while in an eccentric or lengthened position while subjected to this increased resistance.
One of the main structures providing this increased lengthening of the muscle in it's eccentric phase of motion is the connective tissue that covers the muscle fibers. It not only holds them together in unique bundles, but then exits to form the tendons that eventually connect to the surface of the bone providing a strong attachment point for joint motion. In the case of the hip flexors, increased elasticity or stretch of this tissue allows the joint to extend further into hip extension and provides increased tension at end range.
This tension causes a stretching of the elastic collagen fibers that make up the connective tissue. They give it the unique ability to store energy, and then release it, such as a spring when it has been put under tension while in a lengthened position. It is this storing and release of energy that allows for efficient motion and reduced fatigue during muscle exertion. When there isn't sufficient eccentric length and increased muscle strength to oppose this eccentric load: then injury to the tissue or the joint can result. I will expand on this further in my next blog.
