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February 11, 2022 9 min read

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Knee ups have also sometimes been called reverse crunches or leg pull-ins and they work to target your abdominal muscles (also referred to as your core).

In this article, we'll take a look at how to do knee ups to build a stronger core.

Abs - Anatomy Muscles

A Brief Analysis of The Core

Your core is home to some of the hardest working muscles in your body. We’ll describe these muscles more technically later in this article, but basically these muscles are the ones around your pelvis, lower back, hips, and abdomen. They contract and assist with movements that require twisting, bending, reaching, pulling, pushing, balancing, and standing.

A strong core gives you better stability and balance for daily tasks and athletic activities. Building strong muscles in this area can also help you avoid injuries and chronic low back pain. To improve your core strength, you need to perform specific exercises that target your abdominal muscles. The knee up is an intermediate-to-advanced-level exercise that, when done correctly, will strengthen your abdominal muscles.

The relationship between your abs and your core is similar to the relationship between your biceps or triceps and your arms: one is a muscle and the other is an area made up of multiple muscles. By exercising your core, you are trying to focus on what is happening in the center of your body.Your core muscles extend from your neck down to your pelvis. (What does this paragraph mean? One is a muscle and the other is made up of multiple muscles? You can delete it completely and the article immediately makes more sense)

They incorporate a whole lot of muscle groups, such as: 

  • Traverse abdominis: wraps around the front and side of your trunk. This muscle stabilises the pelvis. Internal and external obliques: extend diagonally from ribs to pelvis and allow you to rotate your trunk. 
  • Rectus abdominis: more commonly referred to as your 'six-pack' muscle. When you bend forward you're using this. 
  • Multifidus: back muscle that supports your spine. 
  • Erector spinae: back muscle that extends your trunk and helps you stand up straight. 

Knee ups will certainly help you create abdominal hypertrophy and directly load your rectus abdominus. If you plan to do this type of workout with the proper form and on a consistent basis, you will experience a very deep abdominal burn and this means that you will be focusing mostly on all of the muscles that can help you build an impressive six-pack with high visual impact. Many people think that dropping body fat is the best way to get a noticeable six-pack, which can often be true, but adding some more depth to your abs will help as well.

As mentioned earlier, there are a few good reasons to train your core.

First, a strong core can improve your posture, as has been measured by significant improvements in postural stability tests. Core training can help you to stand a little taller. Secondly, a strong core can help you build muscle in other areas. If you have a weak core, it can cause a forward pulling and a rounding of the shoulders. Core exercises will fix that rounding and give you a much better shape in your chest muscles. 

For most people, their deep core muscles are not as strong as they should be.

In particular, runners with weak deep core muscles are at an increased risk of developing lower back pain. This pain is a consequence of poor form caused by a weak core. Once you start to slouch when you run, you close down the amount of oxygen going into your lungs. It is better to lift your head up and pull your shoulders back to get more air in your lungs because your muscles don't work without oxygen.

The whole system of getting oxygen to muscles and holding yourself upright relies greatly on your core. Generally, if you want to improve your functional performance you need to work on your core. Knee ups can be a very important way for you to start doing this. 

Engaging Your Core 

Knowing the exercises that will work your core is one thing, but if you learn how to engage it properly, you'll know how to work your core whatever muscle you are trying to train. As you start to engage your core more often, it will soon become almost a subconscious action that will really help you maintain an appropriate position and posture and also give you more support, balance, control, and strength overall. Engaging your core is the process of bracing your body as you would when you are about to be punched in the stomach.

There is quite a simple way to learn how to engage your core.

First, lay on your back with your knees bent and feet flat. Press your back into the ground and try to slide your hand under your lower back. Try to prevent your hand from passing through the gap by pressing down slightly and tensing your core muscles. Hold this position for up to 10 seconds and rest, and then repeat the exercise three to four times. As you get used to this position, try standing and contracting.

Once you know what you should be feeling, it makes it easier to contract and engage your core on demand.

Some of the key things to remember are that your glute muscles, stomach muscles, and breathing patterns should all contribute to engaging your core. Also, the contraction should not be so hard that it becomes impossible to contract and breathe at the same time. Keep reading to learn how to perform a knee up safely, the muscles used, and other exercises you can do to supplement this move and strengthen your core.

How To Perform Knee Ups With Proper Form 


Like most exercises, it is always vital that you use the proper form to get the maximum benefit and to prevent possible injuries. When it comes to knee ups, you should easily be able to do two or three sets of about 12 repetitions. Always maintain good technique throughout all of your sets and repetitions. For example, at the top of the movement, don’t roll in towards your body. Make sure your form stays tight so that you do not sway forward and back or side to side.

The knee up is quite a straightforward exercise that only requires a flat bench in terms of equipment. This means that you can do it in your home gym as part of a full home workout. Before you begin, make sure there is enough room around the bench. You will want your feet to touch the floor in the starting position and your arms to be slightly out to the sides when you are holding onto the back of the bench.

  1. Lie with your back on a flat bench with your feet on the floor. 
  2. Make sure your head is close to the end of the bench, but not hanging off the back of it.
  3. Bring your feet up onto the bench and place them flat on the surface with your knees bent and touching.
  4. Put your hands behind your head and grab the bench, one hand on each side, palms facing each other, not down. Keep your elbows bent.
  5. Engage your core by drawing in your navel and contracting your abdominal muscles.
  6. Contract your glutes and extend your legs into the air by lifting your hips off the bench. Make sure to keep your abs contracted. Think about lifting through your heels and press your feet up towards the ceiling. The bottom of your feet should be facing the ceiling at about shoulder-width.
  7. Point your toes in towards your shins. Pause as you keep your abdominal muscles tight, and reverse the movement until your hips are touching the bench. This is now the starting position.
  8. With your legs extending up, repeat the movement. 
  9. Perform up to 3 sets of 10 to 12 repetitions.

Some Knee Up Variations

To make the knee up a little easier, you can decrease the distance between the bench and your hips during the initial part of the movement. To make the knee up a little more difficult, you might want to consider using a decline bench. This puts your body at an angle and requires more balance and activation of your core muscles. Additionally, to make this move even harder, you can increase the distance you bring your hips off the bench.

Muscle Groups Used 

The knee up is a very focused exercise that works the abdominal muscles of the upper body. These muscles include the rectus abdominis, the external obliques, the internal obliques, and the transverse abdominis. Since you need to contract your glutes to lift your hips off the bench, these primary muscles also get a full-body workout, as well as your hip flexors.

As you grip the top of the bench for stability, you will also feel your arms, chest, and upper back tighten up. However, these secondary muscles mostly act as stabilizers and they are not the main muscles at work during the knee up. It is an exercise that is mostly designed to work your abs, so keep that in mind as you are pursuing your overall fitness goals and workout plans.

Safety Precautions

There are a few more important things to know about knee ups. For example, always be aware of your back position and make sure that it is straight and never arched in any way.

Also, as is the case for most exercises, you will need to think about  scapular retraction, which is a squeezing of the shoulder blades that is good for your posture. But when you are doing knee ups, you actually need to protract your shoulder blades. 

Since the knee up requires you to lie flat on your back, pregnant women should avoid doing this exercise. It can also be hard for obese people to complete the movement required by this exercise. Also, if you have been experiencing any neck issues, knee pain, or lower back pain, it might be better for you to try a different exercise, reduce the number of reps you are doing, or ask your personal trainer to help you with the movement.

If you feel any pain during this exercise, stop what you are doing, and review all of the steps we explained above. Because of the position of your body, watching yourself do a knee up is not especially easy. To make sure your form is correct, consider asking your personal trainer (or your gym buddy) for some help.

Alternative Exercises 

Like many other exercises, the knee up is also known by a variety of different names. Some of the moves that are similar to the knee up and work the same muscles include the reverse crunch on a bench and the leg pull-in. If you are not quite ready for the knee up, or if you are looking for other moves to strengthen your core and especially your lower abs, here are some other exercises that will also specifically target your abdominal muscles and give you a great abs workout:

  • Reverse crunches
  • Sit-ups
  • Bicycle crunches
  • Planks
  • Hanging knee raises
  • Hanging leg raises
  • Flutter kicks

Overall Analysis

If you follow a well-designed strength training program and if you try to eat as well as you can, you might find yourself easily gaining impressive amounts of muscle and strength. But usually, after 6 to 12 months of doing knee ups, your progress may slow down considerably. You will need to create a long-term plan to keep getting stronger. A good way to add extra bulk and power is with the Mass Stack Strength & Lean Muscle Builder.

Do a bit of research into which type of diet might be best for your specific needs. There are so many different types of wellness diets that you will need to select one that matches your goals of either  bulking your body up a lot more or shedding excess body fat and slimming down as much as you can.

As with all types of exercise, knee ups can offer you all kinds of benefits and rewards. By simply including knee ups and other bodyweight exercises in your core workout routine, you will quickly begin to realize several specific benefits. Strengthening and maintaining a healthy core is key to improving athletic performance, performing daily activities, and preventing injuries. Knee ups are excellent ab exercises that will strengthen your abdominal muscles. You can do knee ups individually, add them to a resistance training session, or include them in your regular workout.

If you really want to see continual progress and keep strengthening your abs, always include proper warm-ups, rest, and a good nutrition program. Always remember that weightlifting and cardio  can both burn calories.

Your results will be based on all of these factors, and also on how well you can recover from your workouts. To allow your body enough time to recover, we suggest resting for 24 to 48 hours before training the same muscle groups.