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Jackknife

4 reps · advanced

Take care: An advanced inversion: skip it with a neck or back problem, osteoporosis, high blood pressure or glaucoma, and during pregnancy; never turn the head while the hips are up.

Lying on the back with the arms pressing into the mat, the legs lift to vertical and roll over until they are parallel to the floor; then they shoot up toward the ceiling, lifting the hips until the body is one straight line from the shoulders to the toes, the weight on the shoulder blades. The spine rolls down with the legs kept vertical, and the legs lower to start again.

-30°Ankle dorsiflexion
3°Knee flexion
90°Hip flexion
90°Trunk from vertical
24°Arm to trunk
53°Elbow flexion
20Ankle spacing, cm
50Hand spacing, cm
-1Knee outside ankle, cm
–Knee cave, ° inside toe line
–Load left / right, %
0Pelvis drop, cm

Load and joint demand

Bodyweight: no weights. Joint demand below is your own body's weight in this position.

Shoulder, per side
11 / 11

N·m now / peak over the rep.

Faults

Faults grow with knee bend and apply to lifts standing on the floor. The fault check flags a knee caving more than 10° inside the line of the toes and a load split beyond 60/40. Knee cave shows best in the front camera.

Body

Femur changes keep standing height fixed by shortening or lengthening the torso. Longer femurs force more forward lean in any squat.

Muscles worked

Abs and core
35 pts
Hamstrings
16 pts
Glutes
16 pts
Lats
12 pts
Triceps
12 pts

Effort and your quota

5/10 effort5fatigue per set

4 slow, controlled reps. Pilates is practised for control, balance and mobility, never to failure, so this does not change with your limits.

Level: advanced. One round counts like light conditioning work: 32 legs points, 12 push points, 12 pull points, 12 arms points, 35 core points.

Prime movers

Rectus abdominis, transversus abdominis, obliques, gluteus maximus and hamstrings (straightening the hips), latissimus dorsi and triceps (pressing the arms down).

What to look for

  • Press the backs of the arms into the mat to lift, not the neck.
  • Reach the legs up, not over the face; the weight stays on the shoulder blades.
  • Roll down slowly, keeping the legs straight up.
How the model works, and its limits

One 3D skeleton drives both views. Segment lengths and widths are Winter's anthropometric ratios of height; segment masses and centre-of-mass positions follow Dempster's data. For free-standing lifts the whole-body angle is found numerically so the combined centre of mass of lifter and load sits over the middle of the foot. Arms and legs use two-link inverse kinematics with a pole direction. Rep timing follows a minimum-jerk profile.

Balance is quasi-static, so ballistic lifts are shown slowed down. Cable pull and pad support forces are not modelled. The spine is one rigid segment and joints have no range-of-motion limits or muscle forces.