A lying straight leg raise is a resistance-training exercise performed while lying on a flat bench. Resistance is generally applied using ankle weights, but you can also attach a cable that is connected to a weight stack around your ankle. This exercise is a single-joint movement that isolates various muscles of your hip.
Pelvic Girdle
Your pelvic girdle connects your trunk and legs. A left and right pelvic bone compose your pelvic girdle, and each is formed by three bones that fuse during puberty. Your pelvic girdle attaches to the base, or sacrum, of your spine at a joint called the sacroiliac. There is little or no movement at this joint. Your leg articulates with your pelvic girdle at a joint called the acetabulofemoral, or hip, joint. This is a ball-and-socket articulation formed by the spherical head of your femur and a deep cup on your pelvic girdle called the acetabulum.
Thigh Movements
There are seven ways you can move your thigh at the hip. You can raise it forward, to the rear, to the side and over across your other leg. You can perform all these movements sequentially and you can also rotate it inward and outward. When you raise it to the front during lying straight leg raises, the motion is called flexion. A number of muscles are involved in this movement.
Hip Muscles
Seventeen muscles cross and act at your hip joint. Some of these are two-joint muscles that also cross the knee. Hip flexion is produced predominantly by the tensor fascia latae, pectineus, iliopsoas, rectus femoris and sartorius. The tensor fascia latae and pectineus are particularly active through the first half of the joint's range. Your gracilis, adductor longus and adductor brevis also contribute to hip flexion in select conditions. Some of these hip flexors are particularly important for muscle function and form.
Rectus Femoris
Your rectus femoris is a large muscle located superficially on the front of your thigh. It is one of the four muscles that compose your quadriceps, which function collectively to extend your knee. Your rectus femoris is the only quadriceps muscle that also crosses the hip. Straight leg raises are often prescribed for patients with knee injuries because they provide stimulation to this important knee extensor without having to bend the knee.
Iliopsoas
Your iliopsoas comprises three muscles that share a common attachment on your femur. These muscles --- the psoas major, psoas minor and iliacus --- act as one muscle at the hip. The iliopsoas is the strongest hip flexor and is very important during walking, running and kicking.
Tensor Fascia Latae
Your tensor fascia latae is a small muscle located in front of and slightly lateral to your hip joint. This muscle transmits its pull down the side of your leg via the iliotibial tract, which attaches to your tibia, or lower-leg bone. This makes the tensor fascia latae an important muscle for stabilizing the knee joint in the weight-bearing position.
Sartorius
Your sartorius is the longest muscle in your body. It also crosses the knee. Your sartorius connects the outside portion of your hip to the front, middle portion of your tibia. It is a slender, ribbon-like muscle that is directed down on an angle inward on the front of your thigh. It is the most superficial of the front thigh muscles and is prominently displayed on the defined legs of competitive bodybuilders.
Abdominal Muscles
Lying straight leg raises do not specifically train your abdominal muscles because those muscles, the rectus abdominis, external obliques and internal obliques, do not cross and create movement at your hip. You might feel like your abs are working when you do lying straight leg raises because they do contract without changing length to stabilize your hips. Use exercises like crunches and reverse crunches to work your abdominals.
References
- "Principles of Anatomy and Physiology"; G. Tortora and S. Grabowski; 2000
- "Kinesiology: Scientific Basis of Human Motion"; N. Hamilton, et al.; 2008
- "ACE Personal Trainer Manual"; American Council on Exercise; 2010
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