Illustration of muscles of the lower back, with accompanying bones. The latissimus dorsi (above left) is a large triangular flat muscle which ex tends over the lower half of the thorax. It arises from spines of the lower six thoracic vertebrae. It wraps around the ribcage (upper right) and att aches to the humerus of the arm. It rotates the arm and lowers it. The gluteus maximus (at centre, left) is the largest muscle of the buttock. It arises from the ilium hipbone (middle right) and fused sacral vertebrae, and attaches to the femur of the leg. It supports the body trunk, extends the flexed thigh, and rotates the hips. Muscles at lower left are important for posture and walking.