Heart rate will always be high on the motorcycle for a couple of reasons:
1. The bike weighs roughly 220 pounds.
2. You are adding velocity that your body is attempting to push and pull around; using every muscle in your body.
3. Your quads (top of your thigh muscles) are the largest consumers of oxygen in the body. If you are in the correct attack position, your quads are fully engaged - heart rate goes up.
4. We tend not to breathe very deeply, limiting our oxygen uptake - heart rate goes up.
5. When momentum is lost and we bounce off of every obstacle on the track - heart rate goes up.
One of the biggest revelations that our riders experience is that they actually go faster, longer when they "slow down". Think about Jeremy McGrath, he was so smooth on all of his landings that he could go fast all day long. Was he in shape, yes, but there was something about the way Jeremy rode (still rides) that allows him to conserve energy and go so fast for a long period of time (Pourcel would be another example when he was at PC).
Here are a few things you can do to make riding and racing a little easier:
1. Learn how to belly breathe (let me know if you need some help with this).
2. Focus on smoothing out your riding style - land on the backside of all the jumps; execute better throttle control, etc.
3. Build your aerobic engine
4. Improve your overall body strength
5. Enhance your flexibility (your heart rate is lower when your muscles move without restriction).
Please let me know if you have any questions!
-Robb Beams/MotoE (email@example.com