If you're serious about strengthening your abs, you should be doing some sort of leg raises.
They're such a good exercise because they work all parts of your abs, especially the lower abs (which are sometimes neglected).
Strengthening your muscles evenly is an important part of avoiding injury. Weak and inactive lower abs will eventually lead to lower back pain. Not good.
If you haven't done any sort of core exercises for a few months or more, then start with the basic deep core exercises before you continue.
Many of us (in fact most of us) have weak deep core muscles. It's crucial to activate those muscles before moving on to more vigorous ab exercises (like this one).
If you don't engage your deep core muscles first, then your hips will be doing most the work in these exercises. That's a double whammy because you won't be strengthening your abs and you'll be putting undue stress on your lower back.
If you're aiming to get a 6-pack, then you'll also have to do
fat burning exercises,
to burn that belly fat covering your abs. Also make sure you're eating a healthy, balanced diet.
When you have good control of your deep core, you'll be ready for leg raises. You still have to maintain good technique throughout the exercises, to make sure your abs are the main muscles being worked.
Remember, it's not about how many reps you can do, or how fast you do them. Slow, controlled movements will keep you injury free and get faster results.
Here are the main variations for leg raises. They get progressively harder, so start with the first one and then try the others.
1. Lying Leg Raise
Lie on your back on an exercise mat (or soft ground). Wedge your hands under your butt.
Slightly raise your shoulders and feet off the floor (keep a slight bend in your knees) - that's the starting position.
Using your abs, raise your legs to approx. 30 degrees. Keep the rest of your body steady.
Then slowly lower your legs back to the starting position (don't let them touch the floor).
Repeat until you complete your reps.
Tips:
Exhale as you lift, inhale as you lower.
Keep your lower back touching the floor and your neck neutral (don't let your chin move up or down) throughout.
Focus on bracing your core and keeping your body balanced with a neutral back.
2. (Captain's) Chair Knee Raise
This is best done on gym equipment called the "captain's chair". But you can improvise if you have 2 heavy (stable) stools. Make sure they can take your weight. With one on each side of you, rest your forearms on the stools.
Using your abs, bend your knees up towards your chest.
Then slowly lower them back down.
Repeat until you complete your reps.
Tips:
Exhale as you lift, inhale as you recover back down.
Keep your back neutral throughout.
Don't use momentum to swing your knees up.
3. Roman/Captain's Chair Leg Raise
This is the same as No.2 but this time keep your legs straight with a slight bend in the knees throughout.
Using your abs, lift your legs up until they're parallel to the floor.
Then slowly lower them back down.
Repeat until you complete your reps.
Tips:
Exhale as you lift, inhale as you recover back down.
Keep your back neutral throughout.
Don't use momentum to swing your legs up.
This exercise may be hard using stools, if you have access to a roman/captain's chair then use that.
4. Hanging Knee Raise
Hold on to a chin-up bar, or any frame that you can grip on to that is high enough. Use overhand grip with your hands shoulder width apart. Hang with your arms straight.
Focus on your abs, bend your knees up until they're level with your waist.
Then slowly lower them back down.
Repeat until you complete your reps.
Tips:
Exhale as you lift, inhale as you recover back down.
Keep your back neutral throughout.
Don't use momentum to swing your knees up. Keep it slow and steady going up and coming back down.
5. Hanging Leg Raise
This is the same as No.4 but this time keep your legs straight with a slight bend in the knees throughout.
Focus on your abs, lift your legs up until they're parallel to the floor.
Then slowly lower them back down.
Repeat until you complete your reps.
Tips:
Exhale as you lift, inhale as you recover back down.
Keep your back neutral throughout.
Don't use momentum to swing your legs up. Keep it slow and steady going up and coming back down.
Don't relax after bringing your legs back down, keep your abs braced and slowly go straight into the next rep.
Leg Raises - Videos
Enjoy these leg raises! Check out these quality videos to see the exercises in action: