I found the logical combination of words, selecting ‘add constraint’ and then ‘limit rotation’ but when I turn on the restraint buttons, it pops the knee up in a pose when I still had them all set to 0. I’m trying to limit the rotation in the sense that when in pose mode I don’t want it to be able to be rotated past a certain point, how do I do that? Why is it popping up the knee like this when it’s still on 0?
Changed question slightly.
you have it limited so it can’t move any where
it’s got nowhere it can go
check your bone axis and then change the relevant axis to limit to 0 to -90
or whatever the co-ordinance will be
Try setting your CSpace to Local Space.
Edit: The reason that happens is because the Z-axis of your bone is not facing upward initially, like it is in worldspace. So the constraint makes that so!
I see! Okay, it works a lot better on local. Thank you!
You’re welcome 
How come you want to limit the rotations though? It can easily make the rig cumbersome to work with, since the values can still go higher, AND the rotations will likely start popping…
Just stick an IK chain on the leg, and you won’t have to worry about it
I put together a leg rigging tutorial adapted from a Maya tutorial awhile ago, it may be of interest to you?
There is also this thread, which I have started as a resource for rigging in the community (a kind of one-stop shop if you will). Since I’ve found rigging information in Blender to be scattered and scarce. If you’re interested.