Not so much for muscle building but for toughening up the shoulder area for the pressure and friction of a yoke and canoe hauling is to hike with a loaded backpack or more specifically, a rucksack. Forego any kind of waistbelt and let your shoulders do all the work. I load mine with 30lbs of sand and set off on a fairly rugged trail. There is definitely some tenderness across the shoulders the first few times until the area toughens up for the season.