Any physical object with three different moments of inertia will rotate stably about the largest and smallest moments of inertia, but is unstable when rotating about the middle moment of inertia. You can test this yourself with a videotape. Toss it into the air, spinning about its thinnest dimension, and it will continue to spin until you catch it. The same will happen if you spin the videotape about its thickest dimension (neglecting diagonals). However, if you hold the tape with the spine facing your left hand and toss it into the air, spinning the tape about its middle dimension, it will usually come down with the spine facing your right hand.
The human body has three different moments of inertia, just like that videotape. When executing a dismount somersault from a high bar the human body rotates about its unstable moment of inertia, yet people accomplish this dismount regularly. We have discovered a way to stabilize this otherwise unstable rotation, and wrote the high bar simulation to test it. The stabilization method is beautifully simple: just put the arms in the right initial positions and hold them in place with the right springs.
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