Jackknife Sit-Up

A beginner strength exercise for the abs, done with just your bodyweight.

Main muscles: Abs
Equipment: Bodyweight
Level: Beginner
Type: Compound
Movement: Pull

How to do the Jackknife Sit-Up

Lie flat on the floor (or exercise mat) on your back with your arms extended straight back behind your head and your legs extended also. This will be your starting position.
As you exhale, bend at the waist while simultaneously raising your legs and arms to meet in a jackknife position. Tip: The legs should be extended and lifted at approximately a 35-45 degree angle from the floor and the arms should be extended and parallel to your legs. The upper torso should be off the floor.
While inhaling, lower your arms and legs back to the starting position.
Repeat for the recommended amount of repetitions.

Sets and reps

Strength	3–5 sets of 3–5 reps, rest 2–3 minutes
Muscle	3–4 sets of 6–10 reps, rest about 2 minutes
Endurance or fat loss	2–3 sets of 12–15 reps, rest 60–90 seconds

Log every set in MyoStats and it works out your estimated one-rep max and suggests the next weight or rep target, so you keep progressing without guessing.

Alternatives to the Jackknife Sit-Up

Questions

What muscles does the Jackknife Sit-Up work?

Mainly the abs.

Is the Jackknife Sit-Up good for beginners?

Yes. It's rated a beginner exercise: start light and learn the movement before adding weight.

What can I do instead of the Jackknife Sit-Up?

Good alternatives that train the abs: 3/4 Sit-Up, Air Bike, Bent-Knee Hip Raise or Bottoms Up.

How many sets and reps of the Jackknife Sit-Up should I do?

Strength: 3–5 sets of 3–5 reps, rest 2–3 minutes. Muscle: 3–4 sets of 6–10 reps, rest about 2 minutes. Endurance or fat loss: 2–3 sets of 12–15 reps, rest 60–90 seconds.

Exercise steps and photos: free-exercise-db (public domain).