Anatomy: Isolated Muscles of the Trunk - In The Round

Work in progress...

Another post regarding anatomy here so I will be brief. You see below Posterior, anterior and lateral views of the skeleton.  The muscles will be laid starting with a few deep layer muscles outward to the top layer muscles.  In lieu of time and space, the number of muscles are limited, but for figure drawing purposes these should get you moving forward. 

Shown below; Pectoralis Minor (green): origin; rib 3-5, insertion coracoid process of scapula. Pulls scapula (shoulder) forward toward the anterior chest.
Also shown Sternocleidomastoid (teal): origin; clavicle and manubrium, insertion mastoid process. Pulls skull toward chest also turns skull left or right.

Erector Spinae (light green) reefers to a group of muscles that run along the spinal column including the Sacrospinalis and holds the body erect.  Origin; sacrum, lumbar and lower portion of thoracic spine. Insertion; ribs, upper thoracic spine, cervical spine and skull. Keeps torso erect in standing position, pulls rib cage upright from bent over position, also aids in twisting the torso.

Rhomboid Major and Minor (light orange): Origin; Thoracic spine vertebrae 2-5, insertion; vertebral border of scapula. Pulls scapula back and rotates it upward medially.

Infraspinatus (dark orange), Teres Minor (red): Origin; interior spine of scapula , insertion; greater tubercle of humerus.  Rotates humerus towards back.
Also shown, Teres Major (blue): Origin; Lowest corner of scapula (inferior angle), insertion; just below head of humerus. Allows for forward rotation of humerus, also aids bringing arm down from raised position.