I have been in ongoing discussions with friends about this for literally decades. I maintain that a person's innate flexibility is very limited, and cannot be significantly increased by stretching. People who are flexible at 16 years old remain flexible throughout life (unless they become more or less inert in their lifestyle), and people who are inflexible when young will remain inflexible throughout their lives. I started running for health reasons in 1975. Before and after each run, for 35 years, 3-4 times a week, I stretched my various leg muscles and back. When I could no longer run, I continued my stretching regimen in The Gym, and I stretch basically every other day. That's OVER 40 YEARS of stretching faithfully on a regular basis. And I am no more flexible today than I was on that first day in 1975. While seated on the floor, with both legs stretched out in front of me, I can barely get within a foot of my toes with my fingertips. After ten minutes of grunting and groaning, I can get to within 6 inches. And I use various "tools" to help me go past the point of comfort: ropes, belts, and so on. Do you know of anyone who has significantly increased their flexibility by stretching (or Yoga)? I don't.