help with basic concept explaination please!

Hey guys, I am first year physio student so please forgive my naïvety!

but just a quick question with regards physiological movements with regards to a contractile muscle lesion :

" eg. a muscle lesion in anterior fibres of deltoid will be painful on active flexion of shoulder joint and passive extension of the shoulder" - Petty.

I understand why its painful on active flexion but why passive extension?

any help is much appreciated.

With passive extension you would be stretching the anterior fibres of the deltoid which would lead to pain...