Simple exercises
Some years ago a girl complained that her legs ached after a PB, so I said you need to do it more often.
The simplest way to be on a girl's back is to stand behind her and put your arms over her shoulders.
She graps your wrists and pulls you very close to her. You stand on tip toe.
She pulls your arms down in front of her as close to her body as possible.
At yhis point your weight is spread along her back, but she isn't lifting you.
She can then bend forward as far as she wishes and your weight will be along her back.
If you do this little and often her muscles will begin to get used to it and you can graduate to a PB and Donkey Ride as and when she feels ready for it.
If she is willing it will not fail.
Even a small girl can get used to a much heavier weight on her back, but it comes down to prictice and her wish to do so, and the latter is the one most likely to fail.
How long it takes will depend on how often you practice
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