SETUP: Lie flat on the floor underneath a Smith machine bar, knees bent so you can keep both feet flat on the ground. Position the bar at around chest level and press it up until your arms are filly extended. This is the start position.
REP: Crunch up by driving through your abs. The bar will move up, and your arms should remain straight. Come back down to the floor, and reset for the next rep.
TIP: I like to lift my butt slightly as I crunch up, tilting my pelvis up towards my abs to create a harder contraction. I also like to begin breathing out before the rep starts, as this also leads to a much better abs contraction.