Way back when iPods were a twinkle in Steve Jobs’ eye, gym rats sweated to fast-paced music, but generally couldn’t tell you how many beats per minute their favorite workout tunes contained.

In those formative years of fitness training, a song’s actual tempo was known to your instructor alone, says Alyssa Shaffer, fitness director of Fitness magazine.

“The shift has been going from instructor-based routines, to the whole iPod revolution. Now you have the ability to create your own playlist and your own intensity mixes,” she says.

Instructors have always relied on the ability of a song, depending on its “beats per minute,” to boost heart rates and return them to normal.

Even for the most enterprising of exercisers, though, it wasn’t easy to assemble a personal workout tape with the right pulse for the right moves.

“All of a sudden, the power has shifted to the everyday person,” Shaffer says.

For fitness buffs, it’s now second nature to know that music in the 115 to 120 BPM range is ideal for walking, while music for cardio workouts can range from an easy 120 to a nausea-inducing 180 in a spinning class.

Anyone with a computer and an MP3 player can tailor a workout soundtrack according to the desired BPM. With software that manipulates a song’s BPM and BPM-counting gadgets on the market, staying on top of the beat becomes even easier.

There is also Exerciseradio.com, a streaming site established by New York City-based trainer Terri Walsh. The site’s four channels offer music with BPM counts suitable for strength training, cardio workouts, kickboxing and boxing and yoga.

The free Internet radio station is designed to “cover the spectrum,” Walsh says. “Someone who is advanced should be able to use it as well as someone who is just starting out.”

Since January, Steve Boyett, known professionally as disc jockey Steveboy, has compiled music mixes with a high BPM count for a regular podcast called Podrunner. In recent weeks, Boyett’s free mixes have soared to the top of the iTunes list of its 100 most popular podcasts.