You will get your heart pumping and burn 420 calories  per hour (about 17 percent more than in a traditional mat class)  

括号中部分为420 calories per hour的非限制性定语，可以改写为非限制性定语从句。括号可以改为逗号。

You will get your heart pumping and burn 420 calories  per hour，about 17 percent more than in a traditional mat class。

You will get your heart pumping and burn 420 calories  per hour, which is about 17 percent more than in a traditional mat class.

比较级可以被副词、数量词、分数词、百分数词等修饰，表示程度。例如：This box is heavier than that one. This box is 10 percent heavier than that one.

由于百分数是修饰比较级的，应该放在比较级之前，不能放在than从句中。