The abdominal muscles provide postural support, protect internal organs, and perform other functions. In addition, the lower abdominal muscles help protect the pelvic cavity.

The rectus abdominis is the large muscle in the center of the abdomen. It controls 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. They run vertically with the spine 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 all work on 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.”

Other muscles that affect hip movement, close to the adductors, are the psoas major and iliacus. They flex and rotate the hip and thigh.

The quadriceps are regarded as 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. 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, semimembranosus, and semitendinosus.