This is a relatively simple exercise, and I thought that almost everyone knew how to do it. But my dad asked me the other night about exercises to strengthen his abs and back, and I had to demonstrate this one. So I decided to share it.
This exercise will strengthen your core and lower back, and since you also use your shoulders and legs to stabilize your back, you might get sore in those areas as well (at least at first). This is a particularly useful exercise for cyclists (we do it in my spinning class) because it teaches you to keep a flat back and stable core, which promotes a more effective power transfer from your core through your hips to your legs.
From a push-up position, bend your elbows 90 degrees and lower your weight onto your forearms. Your elbows and shoulders should both be at about a 90 degree angle, and your head, neck, back, and legs should all be in line; imagine a pole extending from the top of your head through your spine and down to your ankles. Try to keep your core tense and stable.
Start out holding yourself in plank position for 30 seconds. Build up to 2 minutes. You can also lift up one arm at a time, one leg at a time, or an opposing arm and leg. But you can work your way up to that.