Rather than focusing solely on your abs and lower back for a full 20 minutes, try training your hips, torso and shoulders to be stronger and more stable as a unit. This will help decrease the wear and tear on other areas of your body and maximize performance when you're out doing the activities you love.
Try performing one movement for each of the three key areas: your hips, torso, and shoulders. And move swiftly from one move to the next. Once you've completed all three exercises, rest for 30 to 60 seconds, and then repeat the trio two or three times. Do 10 to 12 reps for each move, with the exception of the front bridge. Hold the bridge for 20-30 seconds.
Here are a couple examples:
Another option: You could save time and train your core while you work your upper body and lower body with a simple tool like a physioball. Do pushups, hamstring curls, and a lateral roll on the ball. Perform 10 to 12 repetitions of each move and repeat the circuit as time allows. This forces you to stabilize your core while getting more work done on the rest of your body at the same time.
-- Anthony Slater, head performance specialist, Core Performance Center