Hi
Here it goes 2 interesting photos of Laird´s back group of muscles used during paddling, together with an article written by a famous fitness tri athletes coach here in Brazil, where SUP is getting pretty popular.
I was not able to translate the name of the muscles, but hope you guys at least can see it. Sorry for the poor english

Good paddling for all
Ivan
“People are saying that paddling a stand up board is a very good work out, and every Fitness Coach fully agrees with that. It is great for injury recovery, work the knees, legs, and gluteus.
It is a very pleasant way to stay in shape, paddling and riding waves.
These photos of Laird show the work of the superior muscle chains, remembering that the legs and abdomen work as much or more than the back and arms.”