In Depth: Deep muscles
The abdominal muscles provide postural support, protect internal organs, and perform other important functions. The lower abdominal muscles help protect the pelvic cavity.
The rectus abdominis is the large muscle in the middle portion of the abdomen. It facilitates the tilt of the pelvis and the curvature of the lower spine. Next to it on both sides of the body is the internal oblique. This wide muscle rotates the spine, increases pressure on the abdomen, and aids in breathing. It stretches from the front of the abdomen to the back of the torso.
Another important group of muscles related to posture is the erector spinae. These elongated muscles run vertically with the spine, and serve to extend the vertebral column, produce erect posture, and allow the spine to flex from side to side. The muscles of this group include the iliocostalis lumborum, longissimus, and spinalis. These muscles are commonly associated with lower back pain.
Below the end of the spine near the pelvic bones are the deep gluteal muscles. These are all associated with the thigh, whether rotating it, pulling it away from the body, or stabilizing the hip joint during walking. These muscles include the piriformis, obturator internus, and gemellus inferior.
The muscles that pull the legs together, such as those needed when riding a horse, are the adductor muscles of the hip. They begin at the pelvis and attach to the femur. They are the adductor longus, adductor magnus, adductor minimus, and adductor brevis. When these muscles are strained during physical activity, the injury is commonly referred to as a “groin pull,” which is a frequent football injury.
Other muscles that affect hip movement are the psoas major and iliacus. They are located near the adductors. They flex and rotate the hip and thigh.
The quadriceps (or quadriceps femoris) are the strongest and leanest muscles in the body. These four muscles are attached to the head of the femur at the hip and are the major extensors of the knee. This means they are the muscles primarily responsible for extending the knee to a straight position. They are the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris.
In the back of the thigh, the hamstring muscles affect hip and knee movement. They begin under the gluteus maximus behind the hip bone and attach to the tibia at the knee. They are the biceps femoris (long head and short head), semimembranosus, and semitendinosus.