LAS VEGAS -- Wearables that measure how fast you run are common enough, but a wearable that controls your speed is much rarer. Sony's new Smart B-trainer earphones and associated app not only keep track of your vitals, but also encourage you to speed up and slow down in order to keep your heart rate exactly where it should be for your workout.

I went ears-on with the Smart B-trainer at the Consumer Electronics Show (CES) 2015, and while the convention center wasn't the ideal place for a run, I got a general idea of what set the device apart from its competitors. By using both voice and music, the Smart B-trainer can modulate your workout and tell you to speed up or slow down to get the best workout.

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A Sony representative informed me that keeping your heart rate within a target zone while running is an efficient way to burn fat. This means that neither a pleasant jog nor a protracted sprint will get the job done as well. When a user first obtains a Smart B-trainer, he or she inputs his or her weight, height and other parameters, and the app develops an appropriate workout.

As users run, an automated voice will give them encouragement, as well as tell them to run faster or slower, as their hearts rate fluctuate. The B-trainer earbuds monitor heart rate by measuring blood flow behind the ears. Over time, the B-trainer app learns how many beats per minute each song in your library has, and brings up appropriate music to help you set a slow or fast pace. (There are default tracks as well, for users who don't want to supply their own music.)

Since I didn't get to run with it, it's hard to say how well the Smart B-trainer actually works. However, the earbuds were comfortable, and the app looked comprehensive, featuring vital sign measurements and a GPS to measure runs. If consumers take a shine to the Smart B-trainer, Sony need not stop at running; rowing, cycling or any other exercise that involves a rhythm could be next.

The Smart B-trainer will launch in the first quarter of 2015, but does not have a set price yet.

Marshall Honorof is a Staff Writer for Tom's Guide. Contact him at mhonorof@tomsguide.com. Follow him @marshallhonorof. Follow us @tomsguide, on Facebook and on Google+.