Blur the background bahind a (fast) moving object

I read a lot about masks and blurring faces but I need something else.
I have videos with moving persons in it (figure skaters) and I just like to only have the skaters in focus and the background blurred.
So far I haven’t found a way on how to accomplish this in ShotCut.

First question before getting into anything else; did you take the video yourself and if so, on what?

The way to do it is to have the same video on 2 track and add blur to the version below and add a mask to the first one that mask out every thing except the moving target.
There is no easy way to auto generate the mask and track the movement. There is some motion tracking in the next version of shotcut to be released very soon, but it only work with simple sharped mask.
To make it perfect you need to use a "Draw: Mask (glaximate) and move change the mask for each frame and that is a lot of work.

Thanks for the update.