Check out this "quick tips" article I wrote for MTBR and Girl Bike Love: Build strength now!
I wanted to give people a quick and easy workout plan when they have little time, and don't have a gym membership. When I refer to 'core' I am referring to shoulders, back, abs, etc. Don't hold yourself up by your arms on the handlebars, use that whole core so your arms can be free to flow with the bike. Enjoy!