The dorsal muscle group (back) is composed of a large number of muscle groups.

The main muscle groups include the large dorsal muscles (which are the largest), the trapezius muscles located at the top of the back (composed of three bundles: upper, middle and lower) and the lumbar muscles at the bottom of the back.

For others, this muscle group can be very difficult to build. If you have difficulty taking backstrokes with basic exercises such as bar pulls, insulation work on machines or pulleys should be considered.

You will also have to put your ego aside, because often practitioners try to lift far too heavy loads with an awful technique. This is to the detriment of the development of this muscle group.

Concerning stretching, their interests are often discussed, debated. From my own experience and observation, I think it is in your interest to stretch for a good reason…

Avoid increasing your muscle stiffness, which results in undeniable losses of mobility over time. This can become a real handicap.

Stretching exercises:

Stretching from the back to one arm 
Back stretching with arms stretched vertically
Stretching the back with a fixed bar
Spinal Stretch