I think flexibility and stretching is a minefield: there is as much quackery and over-simplification as there is with diet and bodybuilding.
Let me try and boil my 'flexibility rant' down to a few bullets:
- Read proper scientific material e.g. Thomas Kurz's book, Pavel's stuff
- Understand that a lot of yoga is not efficient for what you want and seeped in mumbo-jumbo
- Pilates is absolutely superb for teaching a person correct spine function and mobility, especially those with back injuries. I recommend you do a course for a few months then integrate it into your workouts from then on.
- Static/traditional stretching before exercise diminishes strength and slightly increases chance of injury
- If you need to then do dynamic stretches before exercise, static/isometric afterwards.
- Do you need to? Are you doing something atheletic, like kickboxing?
- I strongly recommend you investigate the concept of MOBILITY as well as flexibility. Imho don't train for flexility, train for mobility. I'm going to get flamed so hard here but to me the difference is like plenty of people doing yoga can touch their toes but cannot perform fluid and graceful lunges and squats.
- Hit Youtube and look at some of Kelly Starret's stuff.
- Be very fucking careful what you do having had a back injury, but similarly, over-compensating for this is the single greatest way to fuck yourself up again. Sorry for the conundrum there.
Let me finish with a tiny example to explain the last point, which is super important.
When I first started my first office job my body found itself sitting immobile for 40 hours a week. My spinal mobility dropped and I became stiff in my thoracic spine (between shoulder blades). My small, supportive muscles became weak. One morning I leaned over the sink to brush my teeth, at an angle of 3 degrees or something. Bang! Injured my back. From brushing my teeth.
In my genius I then decided that the problem was I had 'put my back in a weak position' so from then on concentrated on keeping a nice straight back in my life. That's good posture right? All that happened was I further weakened and stiffened my back. A few months later, my concentration lapsed one morning and BANG! Yanked it again, even worse this time. I must try harder to keep a straight back I thought.
Fast forward to taking Pilates. My teacher was horrified how my back was so stiff and weak. He explained to me that every vertebra should be mobile, like a link in a bicycle chain. I started exercising my back, learning how to curl each link down and up. I started deliberately doing rounded-back bodyweight deadlifts, usually on one leg. Something I'd have thought sure to cause back injury years before. The result: my back became stronger than ever, supple and mobile and I learned how to use my spine properly.
I hope this has been of some help and sorry for going off on one.