I used to practice my Karate to 'The Eye of the Tiger' (As well as Michael Jackson's entire "Thriller" Album!)
In the days when I tried the 'gym workout' (it was included in my school tuition, so I went and found out having a workout partner helps!), they played nothing BUT heavy metal...why not, you can't workout to Lionel Ritchi, can you? (at least not with a significant other!)
I found some heavy music from Heart, Van Halen, Journey and such might help, as long as the ballads are left out. Depending on the workout, some dance-type music, might even work out...how about Chili Peppas (I think is the name...they had the tune "I got the POWER"!)
I'm personally not familiar with heavy metal, though I've rented and listened to a few songs (in my 'angry' years).