How to build mass on my legs, despite a bad back?

I prolasped a disc in my back 4 years ago and injured the si joint a year and a half ago. Consequently this means i can not really do squats, cleans, deadlifts... basically any form of free weights to build my legs up Does anyone have any routines that could help me with regards to this.

leg day-
 
Lounges with dumbells 6-8 reps for 3 sets
ham plate pushes 8-10 reps for 2 sets
step ups with dumbells- 8-10 reps for 3 sets
calf raises with dumbells- 8-12 reps for 4 sets
 
calfs performed twice a week

We should live our lives as though Christ was coming this afternoon. - Jimmy Carter

SHould i stay away from leg press and extensions then? Could you also explain ham plat pushes? Thanks.

That sounds awful, Mickey.  Mind sharing how it happened?  Was it lifting related?  I am sort of worried that many years from now my back might go bad from lifting, and we might learn from any mistake you might have made.
 
Thanks.

Squat 385, Bench 235, Deadlift 450 @ 181.

Both times it was a question of too much weight, too much ego. A friend of mine had just told me he had power cleaned a hundred kgs. I went straight to the gym and done the same, but i leaned too far back and the disc just popped. A truely sickening noise!! Next i was performing some bent over rows with around 100-110 kgs. My back was strong enough but for such heavy weights my core wasn't. The lesson is to make sure your core muscle are strong enough before you tackle such weights. ALso make sure your technique is exemplary. I'm only 22 and i now walk around with a limp and have a curve in my spine.Got to start doing pilates apparently, and that is only gonna cut into my weights sessions!!!!!!!

SHould i stay away from leg press and extensions then?

 
I think those two are your best best now.  For thighs... just something like -
 
Leg Extensions
Leg Press
Lying Leg Curls
 
With the Leg Extensions first there is no strain to the lower back at all and it will allow you to Pre-Exhasut your thighs for when you move on to Leg Presses so that your thighs fail before the surrounding muscle groups, like glutes/lower back.  Also, just as a reminder... the higher your foort placing is on the leg press, the more hams and glutes it fill recruit, so... a higher foot placing may make it uncomfortable for your lower back.
 
If you are doing the extensions slow and controlled with a huge squeeze at the top of the movement, your legs should be pumped big time by the time you get to the Leg Press.  Also, playing around with the leg press backboard may give you the chance to find that sweat spot... as well as seeing how far it's possible for you to lower the weight.
 
Take it slow with the lying leg curls... soem people fling the weight up in a sea-saw manner... to ensure that you are just using ham strength you can rest your elbows on the pad.  This way you won't do any levraging by pulling on the handles.  Strict form with a nice squeeze at the top of each movement is the way to go man, sorry to hear about your injury... I hope everything works out.

Thanks so much for the onfo weapon x. gonna do a leg sdession today, will let u know how it goes!!Like the tip about the foot placing, never knew that. Just quickly, will pre exhausting my legs lead to less growth, as i,ll be less likely to push a heavy weight?!

Thanks for telling.  Im worried that because of my ****ty deadlift form I might injure something so I am going light on deadlift until I get back to college (better gym with trainers).

Squat 385, Bench 235, Deadlift 450 @ 181.

Thanks so much for the onfo weapon x. gonna do a leg sdession today, will let u know how it goes!!Like the tip about the foot placing, never knew that. Just quickly, will pre exhausting my legs lead to less growth, as i,ll be less likely to push a heavy weight?!

 
I actually think that Pre-Exhasting can lead to better growth for the intended muscle being worked, as well as prevent injuries in the long run.  It doesn't seem to work for everything, but the idea is that you begin with a isolation movement to bring that intended muscle group being worked to falure early on, that way when you decide to do a more compound movement... that intended muscle group will fail before the stabalizers do.  Like when doing Squats... the major muscles being worked are going to be your Quads/Hams/Glutes/Lower back.  It it's your Quad day, and you decide to Squats first... depending on your structure, the strong lower back/glutes & hams can almost take away a lot of the stress on the intended muscle being worked... the quads.  By doing something like Leg Extensions first, basically a isolation movemnt for the Quads... you are working that muscle early on to ensure that you Quads fail before the surrounding muscle groups on the more compound movements.
 
Yes, you will have to use less weight, but your goal should be "to put the maximum stress on the intended muscle being worked", not Powerlifting.  This also allows the lifter to get a better mind to muscle connection with the intended muscle being worked, you can really get your head in there and squeeze the hell out of the muscle.  This is just something that people do... it's the opposite end of the spectrum compared to just lifting weight to see how much you can lift.  It actually gets your mind into things a lot better... just to get an idea, here are other pre-exhaust methods for other bodyparts...
 
Shoulders (Jay Cutler Favorite)
DB Lateral Raises
Seated DB Presses
 
Back (Jay & Dorian...)
Machine Pullovers
Pulldowns/Chins
 
Back Extensions
Deadlifts
 
Legs (Lee Haney, Dorian, Ronnie, Jay, everyone I can think of...)
Leg Extensions
Squats/Leg Presses
 
Chest
Flies
Presses
 
Biceps
Incline DB Curls
BB Curls
 
Triceps
Pressdowns
Skullcrushers
 
etc...
 
The list goes on and on, I just threw those guys names in there because I've seen there videos.  Hope this helps.  Later.
 
 

Tried it out, it was a killer. DIDn't go too heavy but really controlled the weight. Had to walk a mile home which was hell!! Thanks for the good advice weaponx. One thing i am worried about is it becoming the same old same old. Will have to try and sort this back out!!!

Well try to work it in front of you, I mean with barbell on your shoulders, it presures your back much lighter then back squat...