I'm pretty surprised they recommend crunches with you having a back issue... but your PT should be able to recommend some extremity-strengthening moves you can safely add to your core routine. Are you doing any push-ups now? That's just adding an elbow bend to a good form high plank (which requires good hand and wrist strength all by itself, besides the entire sheath of the core). Step-ups are another one ... core strength to align your pelvis; foot, ankle and hip stabilizer strength to align your knees; then the quads and hammies to lift you up onto the box.
Speed comes from what you put behind you. - Judi Ketteler