My thighs and my shoulders. I put drop bars on my mountain bike (35lbs easily) to start my training with a buddy, who is also on a heavy freakin cross-country bike. Today is our climbing day. We did 20 miles of INTENSE hills, some super steep, some stupidly long, all of them hurt. God give me my road bike. Tomorrow is the day off, Sunday is the long ride day, about 40mi+ we will be heading AWAY from the mountains though!