Cut the crap exercises for leg development. The only exercises needed are squats and either stiff-leg deadlift, romanian deadlift, goodmornings or GHD for the hamstrings. There is little more exhilarating then loading up a bar and getting underneath it to squat.
If your lower back is bad, there is a reason. It can be due to weak abdominals and obliques, it can be due to tight hamstrings, tight hip flexors or poor form. Get the issue corrected and your back problems will disapear.
I have a tendancy to have excessive lordosis. My job has me sitting all day so my hip flexors end up shortened and my hamstrings, lengthened. I have to stretch my hip flexors and ankles on a daily basis. The latter is to prevent knee issues. I suffered a calf tear years ago and the mobility of my left ankle needs constant attention or I start to have knee pain (the knee compensates for lack of ankle mobility).
I do lots of heavy situps, leg raises and sidebends to work the core and keep a healthy back.