Well, this is a very broad topic so I'll try to be brief. First, I would recommend reading these two articles on heart rate training http://www.racerxvt.com/article/heart-rate-traininghttp://www.racerxvt.com/article/cardio-training
In my opinion, gyms are using heart rate training as a gimmick of sorts to get you to work "hard". As you can see from the articles, HR zones should be based on LT tests and those zones vary from sport to sport (cycling, rowing, weight lift, etc.).
If you truly want the title, I'd highly recommend joining the premium training community. That way your training program is designed specifically for motocross and not just throwing workouts at the wall and seeing what will stick. For 19 bucks it's def. worth a try and heck, the first month is free. If you don't like what you get, you can cancel.
But I digress.....let me know if you still have questions after reading the two articles. Thanks!