Hey!
First I want to apologize for my bad english. I hope You understand my text 
For a few years now I have been training quite heavy weight training and running, both intervals and long distance running.
In October -17 I bought F1 as well as all stretch programs. I have now trained F1 twice a week and stretch all the stretches once a week. As i started stretching, I got a big humility in being stiff as a stick and having extremely short hamstrings (and short in overall)
The last months I have started to feel pain and feel tiredness in my hamstrings. Is it because the body is not used to stretch, and stretch + hard intervals have become too much for my hamstrings !?
However, my lower back problems have improved since I started training F1 and all stretching.
Yours in stiffness
joakim