It takes a few hundred muscles working in constant synchronization to keep a person upright in a saddle.  No one ever thinks about it in that sort of detail.  Once you sit your butt on a rock, you are using very few of them.  Riding a horse all day makes you tired.  Riding for great lengths of time is necessary if your livelihood is tied up in cattle.  When you get ready to move them out for sale or for wintering, it's a pretty good idea to know where they are and how many you still have.  You have to cover a lot of territory in the saddle and in a pickup truck, riding.  Then loading and unloading your horse,  Then riding some more.  Makes a person tired just thinking about it, don't it?