I'd rather not use a stepper motor for this job. When you push back on a
stepper hard enough, the shaft then steps back to the previous phase that
matches the power being put into the motor.
I'd rather get a large diameter/high torque DC motor and drive it with a
small fraction of the rated power (maybe 2V max. on a 12V motor) to provide
a resistance to the motion. An old cordless drill motor will probably be
very nice for this job.

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Why do penguins walk so far to get to their nesting grounds?