Exercise guide
Jackknife Sit-Up
The Jackknife Sit-Up raises the upper body and straight legs toward each other from a fully extended floor position. The abdominals control the torso while the hip flexors contribute substantially to lifting the legs.
Training context
Setup Cues for Jackknife Sit-Ups
Begin on a mat with enough room to extend both the arms and legs. The lower back should remain comfortable before the limbs leave the floor.
Bent-knee or shorter-range versions may be used before full straight-leg repetitions. The hands do not need to reach the feet for the repetition to be effective.
Technique overview
Lift the torso and legs together
Start with the arms overhead and legs extended. Raise the torso and legs at the same time, reaching the hands toward the shins or feet without jerking. Pause briefly, then lower both ends slowly. Reduce the range if the lower back lifts abruptly or the legs drop during the return.
Treat this as an overview and use a range, pace, and setup you can control.