Today we’re going to take a closer look at the hanging knee and leg raise to ensure you’re engaging your ABS for maximum growth because, if done correctly, this can be one of the best exercises to engage the entire rectus abdominis and place a bit more emphasis on the lower abs as well!

However, if we’re being honest, this is one of those exercises where most people perform the movement incorrectly and end up training an entirely different set of muscles called the hip flexors. These muscles include the rectus femoris, the iliacus, the psoas, the iliocapsularis and the sartorius and even though all these muscles originate and insert in slightly different areas, they all do the same thing, which is flex the hip.

But why are we discussing hip flexion in an ABS video? Well, that’s because hip flexion occurs when you bring your knee up toward your chest, hip extension happens when your knee travels behind your torso and this movement pattern is key to understanding how to maximize the exercise because when most people perform the hanging knee or leg raise, they end up recruiting more hip flexors than ABS. So to understand why, let’s take a closer look at the anatomy of these two muscle groups:

The Anatomy Of The Rectus Abdominis

Although your core is comprised of many muscles, the most prominent one you’re trying to activate when training your abs is the rectus abdominis, or six-pack. Now, the rectus abdominis originates in the pubic area, more specifically, the pubic symphysis and inserts on the xiphoid process and costal cartilages of your ribs. So with that in mind, imagine the rectus abdominis as a long sheet of muscles whose main function is to FLEX THE SPINE, which means to literally make your spine round.

The Anatomy Of The Hip Flexors

The main hip flexor that ends up activated instead of the ABS when performing a hanging leg or knee raise is the iliopsoas which originates along your lower back vertebrae (T12, L1-L3), then crosses the hip joint and inserts on the femur. Its function is to flex the hip, or to bring the knee up and it’s involved in many other activities such as walking, running and even standing. But when it comes to weight lifting, I just want you to imagine the hip flexors as long, powerful muscles that cross the hip joint and elevate your upper leg towards your chest and just like most muscles in your body, the rectus abdominis and hip flexors prefer to work together.

But what ends up happening, most likely due to the name of the exercise, is that we isolate the hip flexors with very little engagement in the ABS because instead of spinal flexion occurring, we simply raise our knees to about 90 degrees and stop. If anything, the name of the exercise should be changed to the pelvis tuck because that’s essentially what it is due to the fact that the main purpose of your rectus abdominis is to flex the spine. So, if you’ve been keeping your spine straight throughout the movement as you lift your knees towards your chest, all you have to do to properly activate your rectus abdominis is bring your knees up a bit higher and you’re good to go!

But make sure you’re really flexing your abs throughout the movement and try to think of your legs as the RESISTANCE, which actually brings me to my next tip.

Intensifying The Exercise

As you become stronger you’ll need to find ways to make this exercise more challenging and that can be achieved by holding a dumbbell with your feet. Even if it’s just 5lbs to start with, once you’re able to train with sets of 15 – 20 reps in a row using only your bodyweight, you’re going to want to increase the resistance to train your ABS HARDER and continue to build blocky abs.

But Does Focusing On Spinal Flexion Actually ISOLATE The Rectus Abdominis From The Hip Flexors?

Well the answer is no. The hip flexors will always WANT to help out and that’s a good thing. However, what you DON’T want to do is focus solely on the hip flexors and ignore the engagement of the abs and for two very good reasons.

1. Because you want to train your abs so they become more defined.
2. Because the hip flexors are already quite tight on most people from sitting all day. Contracting, strengthening and shortening them further is just an injury waiting to happen. Also, since the hip flexors originate in the lumbar spine, as they become tighter they can end up pulling on the vertebrae, causing lower back pain and in some cases hyperlordotic posture.

Executing Proper Form

Now that you understand the exercise and how the muscles involved function when performing it, how do you actually use the knee raise or leg lift to train your abs? First of all, it doesn’t matter if you perform this movement hanging or in a captain’s chair and whether you perform the knee raise version or leg lift all comes down to how strong your abs are. In most cases, you’ll start off with the knee raise, then utilize the leg lift version as you get stronger.  Remember that your legs are providing the resistance your abs need to contract harder. Therefore by straightening your legs you’re simply increasing the resistance your abs need to overcome in order to flex the spine. Then as that becomes easier, you’ll graduate to the weighted knee raise.

But if I were you, I’d still start the exercise on the captain’s chair to get used to the spinal flexion portion of the movement. First, take a moment to stabilize your body in the air because momentum is NOT your friend in this situation. Next, begin tucking in your pelvis as a POSTERIOR pelvic tilt is what you’re trying to achieve as you crunch your abs from bottom to top, vertebrae by vertebrae. 

Then as your spine continues to flex, your knees should be traveling higher and higher until they reach your chest. Next, slowly control the descent as you reach a full back extension at the bottom and then re-initiate the movement for the second rep. THAT’S how you perform a leg lift!

Conclusion

As for sets and reps, when it comes to abs it varies. Although I always usually perform about 4 – 6 working sets, on some workouts I like to go as high as 30 – 40 reps per set using just body weight whereas on other days I like to train with a heavy dumbbell and keep my sets closer to 10 – 12 reps. I’ve always found that the combination of high and low volume training has worked best for me, so don’t be afraid to switch things up throughout the week and if you’re the kind of person who ALWAYS saves abs for last, try training abs first or on a day that’s normally a rest day. If you want to see changes in your mid-section, you need to get into the habit of training your abs HARD just like any other muscle group!

Written By Scott Herman