What do you prefer crunches or planks

How can you train abs without hurting your back?

In addition to the exercises recommended in the article you cited (planks, bridges, leg raises, bird dogs, and "stirring the pot"), full-body exercises that rely on your core muscles are also a great way to improve abdominal fitness, often strengthening your back at the same time and thus help prevent injuries instead of potentially causing them.

Some examples of this include swimming (especially dolphin, butterfly, and other strokes that rely on their stomach - kicking dolphins with flippers is one of my favorite ab workouts, possibly just because it's so fun), tai chi ( choose a style appropriate to your fitness level), surfing (or skateboarding or slacklining; anything where you need to balance your body), and rock climbing (if done right, focus on balance and technique rather than just arm strength). You will have a hard time finding someone to be a regular with those who do not have a nice, strong stomach.

If you're just looking for an exercise to add to your workout, I would go for boards. The plank and the exercises built on it are incredibly strong stomachs - we use those on my swim team as daily abdominal exercises and they are intense. As with all exercises, form is crucial if you want to avoid injury. So double-check and make sure you get it right.