This document describes muscles of the posterior trunk region in three layers - superficial, intermediate, and deep. It then discusses the erector spinae group of deep muscles in detail, including their origins, insertions and functions in different movements. Characteristics of local and global muscles are also outlined. The document provides information on measuring thoracic kyphosis angle using a flexicurve ruler and details two corrective exercise programs - a local program and comprehensive program - that were used in a study to reduce thoracic hyperkyphosis.