[Grammar] they get regular exercise walking their pets

[ambitious-girl]

Hello all,
I don't know why the author used the -ing form of the verb in the bold part?

Dog owners have fewer health problems. Their hearts are stronger, because they get regular exercise walking their pets. In addition, their mental health improves. Having a dog improves their state of mind and alleviates stress, which in turn means lower blood pressure.

 

[teechar]

Their hearts are stronger, because they get regular exercise while (they are) walking their pets.

That is implied.
Have a read of the section titled: "the meaning and use of participle clauses" in the link below.

https://learnenglish.britishcouncil.org/en/quick-grammar/participle-clauses