FITBODY Exercise Library · ABS
Frog Crunch
The frog crunch combines a traditional abdominal crunch with a bent-knee, turned-out leg position and lower-body movement. Learn proper frog crunch form, muscles worked, sets and reps, variations, and common mistakes.
What Is a Frog Crunch?
A frog crunch is a dynamic abdominal exercise that combines a crunch with lower-body movement. You lie on your back with the soles of your feet together, knees opened outward, and legs elevated. From there, your shoulders and pelvis move toward one another as you reach forward between your knees.
The turned-out leg position gives the exercise its name. Unlike a standard floor crunch, the legs are not planted on the floor. Holding and moving the legs adds a hip-flexion component while the abdominal muscles produce and control the curling motion of the torso and pelvis.
This makes the frog crunch useful when you want a bodyweight ab exercise that involves both ends of the body without requiring equipment. It is also easy to modify by changing how much the legs move, how far you curl, or whether you keep the feet elevated throughout the set.
How to Do Frog Crunches
The mistake I see most is women trying to make the crunch bigger by yanking the head forward. Your hands reaching between your knees should come from your ribs curling toward your pelvis, not your chin dropping to your chest. Keep some space under your chin and think about lifting your shoulder blades with your abs while your hands simply follow the movement.
Frog Crunch vs. Regular Crunch
Both exercises include trunk flexion and train the abdominal muscles, but the lower body changes the challenge. In a traditional floor crunch, the feet normally remain supported while the rib cage curls toward the pelvis. In a frog crunch, the legs are elevated and move as part of the repetition.
That does not mean the frog crunch uniquely trains separate “lower abs.” The rectus abdominis is one continuous muscle running along the front of the abdomen. Research comparing electrical activity at upper and lower recording sites has found either small differences or no meaningful difference between those regions during several abdominal exercises.
What does change is the overall muscular demand. Research on curl-up variations has shown that hip position can alter activity in the internal and external obliques and rectus femoris. The frog crunch has not been directly tested in that study, so those findings help explain the mechanics but should not be treated as a direct comparison between frog crunches and regular crunches.
If you want another crunch variation with a different resistance profile and range of motion, see my exercise ball crunch guide.
What Muscles Do Frog Crunches Work?
The rectus abdominis is the primary abdominal muscle involved in the curling portion of the frog crunch. Your internal and external obliques contribute to trunk control, while the hip flexors help hold and move the elevated legs.
Because both the upper and lower body are moving, the frog crunch requires coordination between trunk flexion, pelvic control, and hip flexion. That makes it different from describing the movement as simply an “upper ab” or “lower ab” exercise.
See the full muscle breakdown
- Rectus abdominis: Produces trunk flexion as your rib cage curls toward your pelvis and contributes to controlling the return to the floor.
- Internal and external obliques: Assist trunk flexion and help control the torso as your upper and lower body move together.
- Hip flexors: Help maintain the elevated thigh position and contribute as the legs move toward the torso.
- Deep core muscles: Contribute to trunk and pelvic control throughout the movement.
- Adductors: The inner-thigh muscles are placed in a shortened or lengthened position depending on hip angle and help control the turned-out frog position, but the frog crunch should not be considered a primary adductor-strengthening exercise.
Sets and Reps
Frog crunches work well as a controlled bodyweight abdominal exercise. Choose a repetition range that allows you to maintain the same trunk, neck, and pelvic control from the beginning of the set to the end.
| Goal | Sets | Reps |
|---|---|---|
| Learning the movement | 2 to 3 | 6 to 10 |
| Core strength | 3 to 4 | 10 to 15 |
| Core endurance | 2 to 4 | 15 to 20 |
Rest about 30 to 60 seconds between sets as needed. Stop the set when you can no longer control the movement without pulling your neck, swinging the legs, or excessively arching the lower back. To make the exercise harder over time, first improve your range and control, then add repetitions or use a more challenging variation. See my guide to progressive overload for women for more on progressing exercises without simply adding weight.
Frog Crunch Variations and Modifications
Upper-Body Frog Crunch
Keep your feet supported on the floor with the soles together and knees opened outward. Curl your shoulders upward while leaving the lower body relatively still. This reduces the lower-body demand and can make it easier to learn the abdominal curling motion.
Frog Crunch with Smaller Leg Movement
Set up with the feet elevated but move the knees only slightly toward your torso during each repetition. A smaller range can help you maintain better pelvic control if the full movement causes your lower back to arch or your legs to swing.
Frog Crunch with a Pause
Pause for 1 to 2 seconds at the top of each repetition while maintaining your curl and continuing to breathe. The pause removes momentum and makes it easier to tell whether you are controlling the position or simply swinging through it.
Slow-Eccentric Frog Crunch
Perform the upward portion normally, then take about 3 seconds to lower your shoulders and legs toward the starting position. Slowing the return increases the time your abdominal muscles spend controlling the movement without requiring external weight.
Common Frog Crunch Mistakes
- Pulling the head forward. Keep your neck comfortable and let your shoulders rise because the torso is curling, not because you are forcing your chin toward your chest.
- Swinging the legs. The knees should move because you are controlling the hips and pelvis. If momentum is doing most of the work, shorten the range or slow the repetition.
- Forcing the knees too far apart. Open your knees only as far as your hips comfortably allow. The goal is not to force a deep inner-thigh stretch.
- Letting the lower back arch excessively. Control the pelvis as the legs move away from your torso. Reduce the leg range if your back begins lifting farther from the floor than you can comfortably control.
- Turning the exercise into a race. Fast repetitions make it easier to rely on momentum. Use a deliberate curl and controlled return so you can feel when your form begins to change.
Frog Crunch FAQs
What is a frog crunch?
A frog crunch is a bodyweight abdominal exercise performed on your back with the soles of your feet together and your knees opened outward. Your legs remain elevated while your shoulders and pelvis move toward one another during the crunch. This combines trunk flexion with hip and lower-body movement. The result is a dynamic core exercise that also involves the hip flexors.
What muscles do frog crunches work?
Frog crunches primarily train the rectus abdominis while the internal and external obliques contribute to trunk control. The hip flexors also work because the legs remain elevated and move toward the torso. Other trunk muscles contribute to controlling the pelvis and spine during the repetition. The exercise should not be described as isolating separate upper and lower abdominal muscles because the rectus abdominis is one continuous muscle.
Are frog crunches better than regular crunches?
Frog crunches are not automatically better than regular crunches, but they create a different challenge. A traditional crunch emphasizes curling the torso while the lower body is supported, whereas a frog crunch also requires you to control elevated and moving legs. Research on curl-up variations shows that changing hip position can alter activity in the obliques and hip flexors, although the frog crunch itself was not tested in that research. Choose the variation that fits your training goal and that you can perform with good control.
Do frog crunches work the lower abs?
Frog crunches involve the entire rectus abdominis rather than isolating a separate lower-ab muscle. Research comparing upper and lower rectus abdominis recording sites has found no meaningful difference between those regions during several abdominal exercises. Moving the legs does increase the involvement of muscles that control the hips and pelvis, which can make the exercise feel different from a regular crunch. It is more accurate to describe frog crunches as a combined trunk and lower-body core exercise.
Do frog crunches make your waist smaller?
Frog crunches can strengthen abdominal muscles, but they cannot selectively remove fat from your waist. Changes in waist size and abdominal definition depend on factors that include overall body composition, nutrition, activity, and resistance training. Ab exercises can develop the muscles underneath body fat without determining where fat is lost. For a broader approach, see my guide to body recomposition for women.
How many frog crunches should I do?
A useful starting point is 2 to 3 sets of 6 to 10 controlled frog crunches while you learn the movement. Once your form is consistent, sets of roughly 10 to 20 repetitions can work well depending on your goal and current ability. More repetitions are not better if your neck begins pulling forward or your legs start swinging. Progress the exercise when you can maintain control through the entire set.
Are frog crunches safe or good for women over 40?
Frog crunches can be a useful abdominal exercise for women over 40 when the hip position and curling motion are comfortable. Age alone does not determine whether the exercise is appropriate. Start with a smaller range or supported feet if controlling both the upper and lower body at once is difficult, then progress as your strength improves. Persistent back, neck, hip, or pelvic symptoms during the exercise should be assessed rather than trained through.
References
- Kim, Soo-Han, and Se-Yeon Park. “Effect of Hip Position and Breathing Pattern on Abdominal Muscle Activation During Curl-Up Variations.” Journal of Exercise Rehabilitation, vol. 14, no. 3, 2018, pp. 445 to 450, doi.org/10.12965/jer.1836170.085. Twenty healthy, physically active men performed curl-ups using different hip positions and breathing patterns. Hip position affected internal oblique, external oblique, and rectus femoris activity, while rectus abdominis activity did not significantly differ between the tested conditions. The participants were men and the study did not test frog crunches specifically.
- Clark, Kathryn M., Laurence E. Holt, and Joy Sinyard. “Electromyographic Comparison of the Upper and Lower Rectus Abdominis During Abdominal Exercises.” Journal of Strength and Conditioning Research, vol. 17, no. 3, 2003, pp. 475 to 483, doi.org/10.1519/1533-4287(2003)017<0475:ECOTUA>2.0.CO;2. The researchers found no significant differences between upper and lower rectus abdominis EMG activity during six abdominal exercises. This was a small pilot study with eight healthy adults and did not test frog crunches.
- Willett, Gilbert M., Jennifer E. Hyde, Michael B. Uhrlaub, Cara L. Wendel, and Gregory M. Karst. “Relative Activity of Abdominal Muscles During Commonly Prescribed Strengthening Exercises.” Journal of Strength and Conditioning Research, vol. 15, no. 4, 2001, pp. 480 to 485, doi.org/10.1519/00124278-200111000-00013. Twenty-five healthy participants performed five abdominal exercises while researchers measured upper and lower rectus abdominis and external oblique activity. The results showed that exercise selection can alter abdominal muscle activity, but did not support several traditional assumptions about selectively targeting abdominal regions. Frog crunches were not among the exercises tested.
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I’m Julie Lohre, an IFBB Fitness Pro and certified personal trainer and nutrition specialist. For more than 20 years I have helped women over 40 get stronger, leaner, and more confident through personalized online coaching built around their real lives.