You may want to look at some of the history on Swedish Gymnastics, which started with Pehr Henrik Ling (1776-1839). No real research studies, but certainly addressed the issue of stretching.

There are several research studies on the Extensibility of muscle and muscle stretching. The Extensibility of Muscle by Gregor Brodie M.D. Physiology Laboratory Kings College in London

Journal of Anatomy and Physiology 1895

and for human muscle

Physical Therapy in Stiff and Painful Shoulders

Dr. Frank Granger Physical Therapy 1922 Vol 2. Issue 3 pages 5-8

If you obtain this physical therapy article or others can lead you to research related to stretching.

I hope this helps

Thank you all for the helpful suggestions