When you're performing the stiff-legged deadlift, don't lock your knees out entirely. Keep a soft bend in them, and lower the weight slowly. Use your hamstrings and glutes, not your lower back, to bring the weight back up.
#FreakMode: Alex Savva's 12-Week Fitness Plan - Week 2, Day 12
Leg day is the best day. Go move some heavy weight!
Day 12: Legs