I've had back of knee issues several times. Front of knee, too. You should google Baker's cyst and a few other things I've forgotten about. The fact that you've sprinted before and not had problems doesn't mean too much.
Anyone ever get this?
This happened twice in a row now.
I do 4-8 sprints.
The next day, the BACK of my knee is tight/sore/tender.
Not quite locked up, but something definitely going on.
Am I tearing something back there?
Also, I do not stretch, ever.
I have done sprints for 5 years with this not being a problem.
I've had back of knee issues several times. Front of knee, too. You should google Baker's cyst and a few other things I've forgotten about. The fact that you've sprinted before and not had problems doesn't mean too much.
If you are new to the PB - please ignore ALL of this stuff, until you've read the book, or at least http://www.marksdailyapple.com/primal-blueprint-101/ and this (personal fave): http://www.archevore.com/get-started/
Stretch warm up, also drive off your leg, your legs should finish straight, butt to foot, I had problems in my upper calf/back of knee because I was pulling my feet back instead of driving my self forward, whether it's squating or sprinting form is important
Are you male/female? Many females have underdeveloped hamstrings and this can cause knee problems.
My first thought is minor hamstring pull. This is the usual injury associated with sprints.
Try doing leg swings before your sprints and jog for a minute or two.
Leg swings - swing your leg front to back, like a pendulum. Don't force it, you will feel it loosen up and range of motion will increase. Do 20-30 on each leg. Then do a short jog.
After your sprints you should do regular stretches (but don't bounce) this will increase flexibility and help to reduce injury.