Originally Posted by CrankyOne

I've never seen an upright MTB? If you are leaning forward, even just a little, it makes a huge difference and will cause you to sweat a lot more. You are carrying some of your body weight on your hands, wrists, arms, and shoulders which requires considerably more effort than carrying it on your keister. You are also creating folds in your skin that increases sweat and reduces cooling ability. Were you wearing a helmet and gloves? These both reduce your body's ability to cool. Breathable white long sleeve oxford shirt that helps keep your body cooler? How hard were you pushing? These all make significant differences.