don't make it harder

I was trying to quilt on my grandson's quilt yesterday, and it was so tiring - my shoulders were aching.

Then I realized:

Yes, I had put on the free-motion foot.
Yes, I had changed the stitch length to zero.

BUT, I hadn't put the little plastic piece over the feed dogs. Could that be why it was so hard, even if the feet were set to zero???

Will try again today...