Jackknife Sit-Up
The Jackknife Sit-Up targets the Abdominals and suits beginner-level lifters. With 4 distinct steps, proper form is straightforward to learn and execute.
Starting position
Ending position
Muscles Worked
The abdominals handles the primary load during this movement. This compound movement keeps tension concentrated on a single muscle group.
How to Jackknife Sit-Up
- 1
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.
- 2
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.
- 3
While inhaling, lower your arms and legs back to the starting position.
- 4
Repeat for the recommended amount of repetitions.
Common Mistakes
- • Brace your core before initiating the movement — maintain tension throughout the entire range of motion.
- • Control the eccentric (lowering) phase for at least 2 seconds to maximize muscle fiber recruitment.
- • If reps become too easy, slow the tempo or add a pause at the hardest point rather than rushing through more reps.
- • Squeeze at peak contraction for a full second — this is where most of the growth stimulus occurs.
Similar Exercises
Frequently Asked Questions
What muscles does the Jackknife Sit-Up work?
The Jackknife Sit-Up primarily targets the Abdominals. It's an effective compound exercise for building abdominals strength.
Is the Jackknife Sit-Up good for beginners?
Yes, the Jackknife Sit-Up is suitable for beginners. Start with light weight to master the 4-step form before adding load.
What equipment do I need for the Jackknife Sit-Up?
You need no equipment — just your bodyweight to perform the Jackknife Sit-Up. This makes it ideal for home workouts.
Track Your Progress
RepStack logs every set, calculates your e1RM, and coaches progressive overload — automatically.