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Waist · Body Weight
Jack Knives
Jack Knives is an intermediate isolation strength exercise that works the abs (waist), with the hip flexors assisting. It needs no equipment beyond your own body weight. A typical prescription is 3 sets of 10-15 reps, and it burns about 5.5 calories per minute.
- Target
- Abs
- Body part
- Waist
- Equipment
- Body Weight
- Category
- Strength
- Mechanic
- Isolation
- Force
- Push
- Sets × reps
- 3 × 10-15
- Calories / min
- ≈ 5.5 kcal
Primary muscles
- Core · Rectus Abdominis, Transverse Abdominis, Obliques
Secondary muscles
- Hip Flexors · Iliopsoas
How to do Jack Knives
- Lie flat on your back with your legs extended and your arms stretched on your sides.
- Simultaneously lift your upper body and legs towards each other, reaching your hands towards your feet. Engage your core to lift both your torso and legs.
- Try to touch your hands to your feet at the top of the movement, forming a "V" shape with your body.
- Slowly lower your upper body and legs back to the starting position with control. Repeat for the desired number of repetitions.
Form tips
- If the full movement is too challenging, modify by bending your knees slightly.
- Focus on steady breathing, exhaling as you lift your body and inhaling as you lower it.
- Ensure your lower back remains pressed into the floor during the movement to prevent strain.
Jack Knives alternatives
More abs exercises
Jack Knives FAQ
What muscles does Jack Knives work?
Jack Knives mainly works the abs in the waist (Rectus Abdominis, Transverse Abdominis and Obliques). The hip flexors work as secondary muscles.
What equipment do you need for Jack Knives?
None. Jack Knives is a body weight exercise, so you can do it at home or anywhere with enough space.
How many sets and reps of Jack Knives should I do?
Start with 3 sets of 10-15 reps. Pick a load or variation where the last one or two reps are hard but your form stays clean.
Is Jack Knives good for beginners?
It's rated intermediate. Beginners can work up to it once they control simpler waist exercises.
How many calories does Jack Knives burn?
About 5.5 calories per minute for a person of around 70 kg, based on a MET value of 4.5. Ten minutes of work is roughly 55 calories; heavier people burn more.
What is a good alternative to Jack Knives?
Other exercises for the abs include Resistance Band Lying Bent Knee Raise, Landmine Kneeling Twist, Band Bicycle Crunch and Dumbbell Decline Sit-Up.
For developers
Use Jack Knives in your app
Get this exercise and 1,900+ others from the WorkoutX API: 720p MP4 animations at 30 fps without the watermark, male and female models, muscles, instructions and form tips in 7 languages.
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