Sit ups. If you were to set out to design an exercise to ruin lumbar discs (the discs in your low back) you would be hard pressed to design one more effective at that task than situps. Why? The discs in your low back are damaged by repetitive flexion aka forward bending, and twisting. Situps and situps with rotation reproduce these movements exactly. Instead, try crunches (like a sit up but your low back doesn’t bend), and planks with short isometric holds.