The rectus abdominis is the large muscle in the mid-section of the abdomen. It enables 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 helps to rotate the spine, increases pressure within the abdomen (needed for certain functions such as defecating), and assists with breathing movements. It stretches from the front of the abdomen to the back of the torso.
The external oblique muscles run diagonally down and in from the lower ribs to the pelvis. They allow you to bend sideways and rotate your torso.
Another important group of muscles related to posture is the erector spinae. These run parallel with the spine to extend the vertebral column, produce erect posture, and allow the spine to flex from side to side. They include the iliocostalis lumborum, longissimus, and spinalis. These muscles are commonly associated with lower back pain.
The tensor fasciae latae are a set of small muscles that connect the ilium to the iliotibial tract and help the gluteus maximus. They also allow the hips to extend out away from the body and help you keep one foot in front of the other.
The latissimus dorsi muscle is the widest and most powerful back muscle. It extends from the upper arm bone to the hip bone and joins the abdominal and pectoral muscles. It helps with forced breathing by raising and lowering the ribs.
The longissimus is an important muscle that helps you to keep your head up and bend it to the side.
Below the end of the spine near the pelvic bones are the so-called deep gluteal muscles. These all enable movements of 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 originate 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 important abdominal muscles include:
- Gluteus medius: One of three gluteal muscles, the gluteus medius connects the ilium to the top of the femur. It controls the level of the hips and allows the thighs to rotate.
- Gluteus maximus: The largest and outermost muscle of the buttock, the gluteus maximus attaches to several places in the pelvis and thigh. It allows you to extend your upper leg, spread it, and turn it outward.
- Serratus anterior: A group of muscles connected to the rib cage, which help stabilize the shoulder.
- Psoas major: These muscles connect the lower part of the spine to the ilium and the femur and aids in flexing the hips.