In training the Upper Limb, the goal is to move the hand. The shoulder is the rest of the hand. To manage the quality of the hand movement and the quality of the shoulder, there must be enforcement with lots of specific exercises. The pecs are included in the upper limb.
You strengthen the upper limb in order to avoid the following: carpal tunnel • tennis elbow • golf elbow • frozen shoulder • rotator cuff tear (s) • biceps tendonitis.
In strengthening the Trunk, the large spinal muscles must be trained along with the movement and support of the muscles. The abdominals (obliques, rectus abdominis, transverse and pyramidalis) also need to be trained. These muscles are not only functional but also aesthetic. That is why clients learn hundreds of different exercises.
Butterfly exercises for the rhomboids are grouped with the spine, not with the upper body.