Are these symptoms normal when exercising?

It may not be the coffee. It may be you're pushing yourself much harder than your body is conditioned for. That's when I usually get those symptoms. 2 hrs of highly strenuous activity would be enough to do it. And as far as my experience goes mountain biking, if its good terrain, there isn't enough down hill to truly recover as a rest period. I wouldn't sweat it, try to take it easier.

Sounds like you're doing too much, too soon. Back off, eat properly and increase over time. If you're just starting out, you shouldn't expect to be keeping up with big boys yet. You have to work up to that. Fitness isn't just a matter of dropping pounds, you also have strengthen your heart, lungs, muscles, etc as well. This is done through progression, not jumping in and going all out on the first day.

That's my thinking.

When I've pushed too hard I also get queasy. I know my limits now through experience and don't push so hard. I remember spending an hour sat on the kitchen floor, cold sweats, trying not to vomit. :laugh:

Seeing your doctor is not the same thing as concluding that you have a medical condition. You see your doctor to rule out anything serious. Barring anything serious, you likely need more training for those serious hills. You could choose easier routes, or take more time and more rest during the routes that you're doing.

I've recently gotten serious about mountain biking, but especially where I ride, it's pretty strenuous. Especially when climbing, I have to spend half my time sitting on logs trying to recover. After a bout of climbing, my heart is pounding, I'm breathing like a woman in labor and sweat is just pouring off of me. I'll often tremble involuntarily. Sometimes I'm also super nauseous and absurdly thirsty.

I thought it was heat exhaustion, but it happens when it's not that hot too. I drink a sugar-free electrolyte drink before and during my rides.

Is this normal? I'm hypothyroid, but my numbers are good with medication. Apart from that, no medical problems. It just seems like I can't handle intense exercise, and I'm not seeing progress in this area.

Try eating some nuts before you go...If it's low blood sugar that will help stable the shakes...You might also want to work yourself up to that intense of a ride until your body is ready for it...When my body freaks out I take it easy and push a little more each time because that feeling scares me in a way(I don't like it)I even get slight chest pain if I'm doing too much too soon..YOU"RE BODY WILL OVER COME THIS STAGE unless it's low blood sugar nuts stabilize sugar levels

I race mountain bikes and I am always very out of breath at the top of a climb. Are you riding with Flat Pedals or with Clipless Pedals? Some times when riding with Flats it can be a lot harder when climbing because you can long exert force on the downward pedal stroke, versus the Clipless Pedals you can push down and pull up to create a more effective and powerful climb.

I literally sound like I'm having an asthma attack at the top of a climb, especially when I'm putting forth a lot fo effort to get up the hill, rather than a slow pedal pace.

one word: DOCTOR

I got this way when I started ultimate frisbee.. now I eat a protein bar about half way through and drink something around 2 quarts of water throughout the game. Pretty much went away.

After a bout of climbing, my heart is pounding, I'm breathing like a woman in labor and sweat is just pouring off of me. I'll often tremble involuntarily. Sometimes I'm also super nauseous and absurdly thirsty.

Climbing hills on a bike while carrying a bunch of extra weight is extremely hard work. And it takes a long time to get good at it. And it takes practice to not constantly go into the red zone, which means it's real easy to completely blow up.

My advice is to grab a HRM and stay out of the red zone by stopping for a rest when your heart rate blows out the top of the aerobic zone.

In time, with practice, it will get better.

Thanks,everybody
It's true I am tackling some absurd climbs and I expect to have to catch my breath at the top, maybe feel weak and whatnot, but I'm getting the shaky/jittery/nausea/crazy sweating thing even early on after the second climb. I do get similar symptoms when I bike home from work after an afternoon cup of coffee. It might be that caffeine isn't the cause, but exacerbates the cause - I naturally have slightly low blood sugar. will be tracking my heart rate for a while to see how that fits in. I'll ask doctor about maybe getting a heart test too just in case, although I think that's fine.

If you have lower blood sugar in the first place then you should know that before doing any strenuous exercise you should make sure to eat/drink to boost your blood sugar first, plus bring such items with you in case of a crash. When I read your symptoms it sure sounded like a blood sugar crash to me. Being that I'm borderline Hypo, I make sure that such workouts are accompanied with things like granola/nut bars, FRUIT (pack an apple if need be), juice along with my water. I don't like that crash feeling and others, I've found, get freaked out when they see me get all shaky and my breathing gets all screwy and I nearly pass out, imagine that. :P Eat/drink before and after a workout or take with you if you are going to be gone for a long time. Sugar-free electrolyte drinks aren't doing you any favors in this.

The other suggestions of getting a better HRM certainly can't hurt either.

Smarties for reactive hypoglycemia

http://www.phlaunt.com/lowcarb/19058561.php