Basically I got 18 footballers from my university, and split them into 4 groups. To one of these groups I told them carry out a lower-body flexibility training routine (a set of stretches) as many times as they could for four weeks, aiming to stretch at least a couple of times a day 5 times a week. To another group, I told them that any time they were due to play football they needed a sports massage, any time they had been playing football they needed a sports massage, any time they were feeling tight - they needed a sports massage (they also did this for the same four weeks). To the next group, I told them that they would be doing a combination of BOTH - they would do their lower-body stretches every day as much as possible and also come in for sports massage before and after games and training for the same four weeks. My final group was simply a control group who would do neither. They would not receive massage and would not be given the flexibility training routine, they were instructed to not change anything about their routines - ie. if you don't normally stretch before games, don't start stretching before games!
I measured my players hamstring flexibility once before the experiment, and then once a week every week after with a simple range of motion test (example demonstrated below).
The footballer extends the leg as high as possible whilst maintaining hip flexion
Obviously, the control group barely improved because they didn't change anything (average gain of 1 degrees in 4 weeks) - all of them decreased in range of motion (flexibility) at some stage. My group receiving regular sports massage on average increased their flexibility by 12.2 degrees but showed inconsistent results that were beginning to plateau, while the flexibility training group improved slightly more with average gains of 12.75 degrees and more consistent results with potential for further improvement.
But here we go.
Receiving sports massage AND carrying out flexibility training together gave an average improvement of 20.2 degrees over 4 weeks! So when you consider a score of 90 degrees would be considered perfect flexibility, and some of these players went from scores of 55 to 75-80 in no time at all - the potential for injury-free football seems very achievable with a good balance of stretching, massage and (what I would add to my next study) strength and conditioning training!