Latissimus Dorsi, known as Broadest muscle or simply as Lats is widest and most powerful muscle of the back. It is a large, flat, triangular muscle covering the lower back. It arises from the lower half of the vertebral column and iliac crest (hipbone) and tapers to a rounded tendon inserted at (attached to) the front of the upper part of the humerus (upper-arm bone).
Fitness tips: LATS also latissimus dorsi. Why girls need to train the Lats?
Developed latissimus dorsi visually makes your waist thinner. And if you spend much time on the development of the abdominal muscles and forget about back muscles then your figure will be look like a stump without a waist.