Jack Knife Crunch - Watch all active muscles

If you asked yourself how to do the JackKnife Crunch/Sit-Up, this video will help you by giving you extra anatomical data.

The Jack knife Crunch/sit-ups are a great low-impact exercise for strengthening abdominal muscles.

The jackknife provides some additional challenge and variation to the sit-up and trains both the abs and hip flexors.
The target muscles are the abdominal muscles and hip flexors.
The hip flexors connect the legs to the upper body and pull the legs up, these muscles also pull the pelvis anteriorly.

The abdominal muscles work hard statically in order to stabilize the pelvis and the spine, and also dynamically to flex the spine.

With straight legs, the load on the abdominals and the hip flexors is higher, due to higher torque.

Do you have Tight/Short Hamstrings?

In the case of untrained trainees with shortened hamstrings who can't keep their legs straight at 90 degrees and do not have strong abdominal muscles, the command to straighten legs to 90 degrees will create a very large load on the lower back!!

Therefore, if shortened hamstrings are identified, an adaptation is required, such as bending the knee.

We must maintain a neutral rigid spine without allowing movement in the lumbar spine.
The leg weight draws the pelvis forward through the hip flexors.
The abdominals are supposed to stabilize the pelvis and prevent anterior tilt (APT)

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The Muscle and Motion team