I pretend to pull a hamstring?

Hi.
I watched a flick, 'While you were sleeping'
and this is a conversation between Lucy and Jack.
Lucy saved his life by jumping in front of a train.
He was taken to the hospital and he is recovering.
Now they are in a hospital, shooting the breeze.

This is a dialogue between them.
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- l don´t think l´ve done anything truly heroic in my whole life.
l chased a purse-snatcher once.

- Well, that´s something.

- l pretended to pull a hamstring.

- Well, most guys probably wouldn´t have chased after him in the first place.
===========================

I got one puzzling phrase - pretend to pull a hamstring.

The Korean subtitle says,
I bit off more than I can chew. That was more than I can afford to do.
or something like that.'

Not sure what that idiom exactly means, I looked up some online dictionaries,
only to find 'hamstring injuries, symptoms, treatment and others.'

Is it obviously an idiom widely used - pretend to pull a hamstring?

Any comment would be appreciated.
Thank you.