Locking bow arm shoulder

[Ninnescah Hog]

Havn't been shooting well lately.  I was told I am not locking my bow arm shoulder.  Need help.  Not sure I know what that means.  And what tips do you have to help me make sure I always do this?

[moebow]

Just keep the shoulder down and back and keep your bow arm 90* to your spine.  I've never heard of "locking" the shoulder so don't know what source you've gotten that from.

Arne