While I’ve spent decades helping to test and design great workout routines utilizing the most productive exercises for each body part, it wasn’t until just a couple of years ago that I discovered one of my favorite ab exercises isn’t very efficient…

I’m speaking, of course, about hanging knee ups. This was the key exercise for lower abs which I used to always start my ab routine with. I’ve since realized that it’s far from perfect.

The problem with hanging knee ups is that there’s almost zero resistance at that bottom, which is the key turnaround point for maximum fiber recruitment. That’s not great news when you’re looking for fast results. Luckily, I had always combined those with two other exercises which were much more efficient, so I still got some decent results.

The hanging knee up is similar to a barbell curl if you think about it. There’s no resistance at the start, which is the arm’s extended position at the start of a curl. When most people curl, they tend to lean back, so there is still some resistance as soon as the arms start to bend. That’s the cheat-curl style popularized by Arnold, and it puts stress right at the max force point. It’s not always great for your back, however, and on hanging knee ups, there’s no way to overload that key point at the bottom. Cheating on this will only reduce the resistance at the bottom.

Instead of hanging, try the exercise on an incline with your head at the top of a slant board. This allows you to maintain resistance at the bottom when your feet are close to the floor and your lower back starts to arch slightly.

If that’s too difficult, start by doing knee ups flat on the floor instead. That will put the most resistance at the key spot and less at the weaker top contracted point, so they should be easier. It should look like a leg lift at first, with your knees slightly bent, and then turn into a hip roll so that your knees end up over your chest (still bent) with your feet high and over your face and your butt off the floor. Be sure to keep your hands flat on the floor at your sides for leverage.

You should be able to progress from on-the-floor knee ups to incline knee ups quickly and once you can get 15 to 20 reps, add resistance with ankle weights or with a light dumbbell secured to your feet. You could also get some ankle straps with hooks to connect your feet to a low cable, like on a crossover machine.

Once you get proficient and want to make things a bit tougher, you can try supersets with sitting V-ups. Immediately after a set of the incline knee ups go over and sit on the end of a flat bench with your hands gripping the sides and your torso angled back at about 45 degrees with your feet on the floor to start. Then, with a slight bend in your knees raise your legs as high as you can—your knees should be at about chest level. You’ll be in a V position, hence the clever name. Lower and repeat continuously with no pauses at the top or bottom until you can’t get your feet off the floor. Those will really set your abs on fire, especially if you do them immediately after a set of incline knee ups, but they work well on their own, too.