The lower back muscles support upper body weight, protect body tissues and stabilize the spine. Since this area bears much of your weight and stress, it is the most likely part of the back to become injured. By understanding what these muscles do, you can help keep your lower back pain-free.
The psoas major inserts at the five lumbar vertebrae, passes in front of the pelvis and attaches to the lesser trochanter of the femur (the upper inside part of the thigh bone). The psoas major stabilizes the base of the spine, which can lengthen and move forward, and it enables external rotation in the hip.
The quadratus lumborum runs from the iliac crest and attaches to the twelfth rib and the top four lumbar vertabrae. It stabilizes the bottom rib when you are breathing and helps with side bending, lifting the hip and extending the lumbar spine. The colon, kidney, psoas major and diaphragm are in front of the quadratus lumborum.
The largest muscle of the lower back, lattissimus dorsi originates from the mid-thoracic spine to the sacrum and inserts to the top of the humerus (upper arm bone). This large muscle stabilizes the back and helps the arm to move both inward and back.
Deep Back Muscles
The deep back muscles are erector spinae and transversospinalis. Erector spinae, also known as the sacrospinalis, support the entire spinal column and attach from the sacrum to the all five lumbar vertebrae and the bottom two thoracic vertabrae. At the thoracic spine, the muscular fibers split into the iliocostalis, longissiumus and spinalis.
The deepest back muscles, transversospinalis, include three layers, starting with the deepest: rotatores, multifidus and semispinalis. These layers not only stabilize the spine, they help each vertebra move in a precise manner.
Strong abdominal muscles help keep the back in shape. The abdominal wall contains four paired muscles that fill the space between the ribcage and pelvis. Rectus abdominis, the most active muscle during sit-ups, is the outer part of the abdomen, along either side of the navel. The external oblique attaches to the outside of the lower seven ribs on each side, and the internal oblique lies just behind it. The obliques help with side bending and rotation of the spine and ribcage.
The deepest muscle, the transversus abdominis (TA), attaches from the iliac crest to the inner surfaces of the bottom six ribs. Contraction of this muscle pulls in the belly and increases curvature of the lumbar spine. Along with the multifidus, it stabilizes the spine, so a well-developed TA helps strengthen the lower back.