The Engineering Projects

Does a gear exist that would allow the motor to drive two different sets of gears depending on back-driving resistance? In other words the gear would have two states:
1) There is no resistance against the gears/motor => the motor drives the high speed gear and the high torque gear remains free to rotate
2) There is resistance against the gears/motor => the motor drives the high torque gear and the high speed gear remains free to rotate
In both of these circumstances the gears should be non back-drivable so that the system can lock without the motor maintaining a torque. I'm an engineering student working on a prosthesis project and so any knowledge/experience/advice in this area would be greatly appreciated. Thanks!

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