Jackknife Sit Up

How to do it

1. Lie flat on a mat with the legs straight and the arms stretched overhead on the floor.
2. Brace the stomach and press the lower back toward the floor.
3. Lift the legs and upper body at the same time, folding at the hips.
4. Reach the hands toward the shins at the top, balancing on the hips.
5. Lower the arms and legs together slowly to just above the floor.

Tips

• Move arms and legs at the same speed so they meet in the middle.
• Bend the knees slightly if you cannot keep the legs straight.

Common mistakes

• Letting the legs and arms crash back to the floor.
• Arching the lower back as the legs lower.
• Only lifting the legs while the torso stays down.

Muscles worked by the Jackknife Sit Up

• Upper abs Primary35%
• Lower abs Primary30%
• Obliques Secondary18%
• Hip flexors Secondary14%
• Quads (rectus femoris) Minor3%

Share per muscle in the Zulta muscle model. The model spreads the load of one rep over 40 muscles, based on EMG and biomechanics research.

Sitting all the way up adds hip flexion (Escamilla 2006). Legs and trunk meet: lower abs and hip flexors join. Curling the ribcage toward the pelvis is spinal flexion by rectus abdominis, with the obliques assisting; a full sit up adds the hip flexors.

Jackknife Sit Up alternatives

Same equipment

• Tuck UpBodyweight
• V-UpBodyweight

Different equipment

• Decline Weighted Sit UpOther equipment
• Weighted Sit UpOther equipment

Easier

• Butterfly Sit UpBodyweight
• Sit UpBodyweight

Make it harder

• V-UpBodyweight
• Decline Weighted Sit UpOther equipment
• Weighted Decline CrunchOther equipment
• Weighted CrunchOther equipment

Make it easier

• Butterfly Sit UpBodyweight
• Sit UpBodyweight
• Weighted Sit UpOther equipment
• Long Arm CrunchBodyweight

Similar exercises

• Decline Sit UpBodyweight
• Decline Bench CrunchOther equipment

Frequently asked questions

What muscles does the Jackknife Sit Up work?

Mainly the upper abs and lower abs. The obliques and hip flexors assist.

Is the Jackknife Sit Up good for beginners?

The Jackknife Sit Up is rated intermediate. If you are new, learn the Butterfly Sit Up first and build up from there.

What is a good alternative to the Jackknife Sit Up?

For example the Butterfly Sit Up, Sit Up and Tuck Up. They train mostly the same muscles.

How do you make the Jackknife Sit Up harder?

Add weight or reps, or move on to the V-Up and Decline Weighted Sit Up.

Sources

• Escamilla et al. 2006, Phys Ther 86(5):656-671 (traditional and nontraditional abdominal exercises)

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