When I first became a certified personal trainer about 16 years ago, there were still a lot of fitness trainers recommending crunches and sit-ups to strengthen the abs.
While there can still be some benefits of crunches and sit-ups, I almost always recommend that my clients perform planks or other core exercises to strengthen the muscles worked by sit-ups instead.
That said, if your current fitness routine does include sit-ups or crunches, it can be helpful to know what muscles crunches work and what muscles sit ups work so that you can find potentially more effective alternatives to strengthen the same sit-up muscle groups.
So, what muscles do sit ups work? Are the sit-ups muscles worked and the crunches muscles worked the same? What other exercises will strengthen the crunches muscles more effectively?
In this exercise guide, we will discuss how to perform sit-ups, how to perform crunches, the muscles worked by sit-ups and crunches, and some more effective substitutes to sit-ups for the abs.
We will look at:
- How Do You Do a Sit Up?
- What Muscles Do Sit Ups Work?
- What Muscles Do Crunches Work?
- Other Exercises to Strengthen the Muscles Used for Sit-Ups
Let’s jump in!
How Do You Do a Sit Up?
Before we look at the muscles worked by sit-ups and crunches, let’s review how to do sit-ups and how to do crunches properly.
The key to ensuring that sit-ups are both safe and effective in terms of actually engaging the muscles targeted by sit-ups is contingent upon performing them properly.
Here are the steps for how to do a sit-up:
- Lie on your back with your knees bent and your feet on the floor. Ensure your feet are shoulder-width apart and your toes are pointed straight ahead.
- Try to press your lower back into the ground.
- Cross your arms over your chest or place your hands behind your ears, gently cradling your head.
- Inhale, engaging your abs to lift your shoulder blades and torso all the way up off the ground until your torso is fully upright, perpendicular to the floor. Lift slowly using your muscles, not propelling your body up with momentum.
- Slowly lower your torso back to the starting position, not allowing gravity to take over and push you down.
Here is how to do a basic abdominal crunch:
- Lie on your back with your knees bent and your feet on the floor. Ensure your feet are shoulder-width apart and your toes are pointed straight ahead.
- Try to press your lower back into the ground.
- Cross your arms over your chest or place your hands behind your ears, gently cradling your head.
- Inhale, contracting your abs to lift your head and shoulder blades off the floor while keeping your lower back and hips down on the floor. Instead of thinking about curling up so that your face is looking at your feet, think about lifting your upper body with your abs as you keep your gaze pointed at the ceiling. (Think “up” not “forward.”).
- Pause and hold this lifted position, squeezing your abs and thinking about sucking in your stomach to press your belly button and lower back into the floor.
- Slowly lower your head and shoulders back to the starting position.
From a movement pattern perspective, the primary difference between sit-ups vs crunches is that with sit-ups, you are moving through a larger range of motion to lift your entire torso off of the ground into a fully upright position.
With a crunch, you are just lifting your head and shoulder blades off of the floor.
What Muscles Do Sit Ups Work?
A sit-up is a bodyweight exercise that primarily strengthens your abdominal muscles.
The primary muscles worked by sit-ups is the rectus abdominis group, which is the “six-pack“ ab muscles that run down the center of your torso, allowing for flexion of the stomach area.
Additionally, sit-ups work your hip flexors.
The hip flexors are a group of muscles along the front of the hip and pelvis on either side of your groin that help flex the leg or bring the leg up towards the trunk.
The primary hip flexor muscles worked by sit-ups are the psoas major and the iliacus (together called the iliopsoas), but the rectus femoris (one of the quad muscles), sartorius, and pectineus also are involved in hip flexion and can be strengthened by sit-ups.
The hip flexors play an important role in forward movement during running and walking because they flex the hip so that you can lift your leg and advance it forward for the next stride when transitioning from the stance phase of the gait cycle to the swing phase.
Other muscles worked by sit-ups include the internal and external obliques, which are the muscles on the sides of your abs, and the transversus abdominis, a deep core muscle that encircles your entire trunk.
The reason that the obliques are muscles worked by sit-ups is that all of the core muscles work together to raise the trunk and provide stability to the spine when you are performing dynamic movements such as trunk flexion with the sit-up exercise or abdominal crunch.
Additionally, the obliques contract isometrically during sit-ups and crunches to prevent lateral bending and rotation of your trunk as you lift your torso off of the ground.
Similarly, the deep transversus abdominis is one of the muscles worked by sit-ups and crunches to increase intra-abdominal pressure for spinal stability as you flex your spine.
In other words, these muscles worked by sit-ups provide stability so that the prime movers, or primary muscles worked by sit-ups have a stable base of support upon which they can contract to generate movement.
What Muscles Do Crunches Work?
The primary difference between the muscles worked by crunches and the muscles worked by sit-ups is that sit-ups target the hip flexor muscles more than crunches.
Although both exercises are performed lying on your back with your knees bent and feet flat on the floor, a full sit-up involves lifting your entire trunk off the floor into an upright sitting position.
Crunches only involve raising your shoulder blades off of the ground, which largely eliminates the engagement of the hip flexors, particularly if your feet are not held down.
For this reason, as long as you do not have contraindications to spinal flexion, crunches are generally better than sit-ups if your goal is to strengthen your abs.
This is because the muscles worked by crunches are isolated more to the abdominal muscles already discussed, mainly the rectus abdominis with secondary involvement of the internal and external obliques as well as the deep transversus abdominis.
In contrast, both the rectus abdominis abs and the hip flexors are the prime movers in the sit up exercise.
Other Exercises to Strengthen the Muscles Used for Sit-Ups
Although sit-ups and crunches are beginner-friendly calisthenics and can help strengthen your abdominal muscles, doing crunches is not the most effective way to strengthen your abdominal muscles or improve functional core strength.
Additionally, for certain populations, performing crunches is contraindicated because it causes flexion of the spine.
If you suffer from osteoporosis or low bone density in your spine, or you have degenerative joint disease in your spine, spinal stenosis, or herniated discs, doing crunches can increase your risk of further damage, such as causing vertebral compression fractures or exacerbating narrowing between your vertebrae.
Women who are pregnant or men or women who are dealing with diastasis recti should also avoid doing crunches because of the increased intra-abdominal pressure and positioning inflicted by the crunches exercise movement pattern.
Additionally, most people use momentum, gravity, and/or their hip flexors rather than really engaging the abs in a functional way.
That said, core strength is vital for helping maintain proper posture, preventing low back pain and other injuries, improving movement mechanics, and optimizing exercise performance, whether running, cycling, hiking, or lifting weights in the gym.
Therefore, you still want to strengthen the muscles worked by sit-ups.
Here are some more effective alternatives to crunches and sit ups that target the same muscles in a more functional, effective, and safe manner:
- Planks
- Pallof presses
- Bicycle crunches
- Reverse crunches
- Turkish get-ups
- Hanging leg raises
- V-ups
- Stability ball or suspension strap tucks
- Dying bugs
- Russian twist
- Cable machine crunches
Remember, using proper form and technique when performing sit-ups and crunches is crucial for ensuring that these ab exercises are not only effective at strengthening your abs, but also safe for your back, neck, and hip flexors.
For more core workout ideas, check out our guide here.
