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Plank jack

Take care: Jumping in a plank loads the abdominals hard: leave it out in pregnancy and postpartum.

From the top of a push-up, the feet jump out wide and back together like a jumping jack, the hands planted under the shoulders and the hips level.

33°Ankle dorsiflexion
16°Knee flexion
8°Hip flexion
72°Trunk from vertical
67°Arm to trunk
23°Elbow flexion
20Ankle spacing, cm
56Hand spacing, cm
-1Knee outside ankle, cm
–Knee cave, ° inside toe line
–Load left / right, %
0Pelvis drop, cm

Load and joint demand

Bodyweight exercise: the load slider has no effect here; body mass comes from your limits.

No gravity-based joint demand to show for this exercise.

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
60 pts
Glutes
38 pts
Calves and shins
21 pts
Shoulders
14 pts

Effort and your quota

4/10 effort4fatigue per set

Your quota per set: 30 s hard, stopping 2 reps short of failure.

One hard set earns 53 legs points, 14 push points, 60 core points.

Prime movers

Transversus abdominis, rectus abdominis, hip abductors, deltoids (holding), calves.

What to look for

  • Hips level with the shoulders; don't pike up.
  • Light, quick jumps on the toes.
  • Hands under the shoulders, arms straight.
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.