Frustrated with the fit of your MP3 player's stereo earbuds? I don't blame you. Some are notorious for popping out at the slightest provocation—particularly those made of hard plastic that don't come with foam covers. Others, including Apple's ubiquitous iPod earbuds, do come with the covers, but the foam tears easily and the pads are often lost. Still others—namely Creative's Zen Aurvana earbuds—fit securely but transmit tremendous, booming sounds into your ears with each slight pull of the wire (read: with every step you take while walking or jogging.) Enter the Sennheiser MX75 Sport earbuds, which are designed specifically for the fitness enthusiast.

The trick to the MX75 Sport is what Sennheiser calls its Twist-to-Fit design. That's not what I would call it; the name conjures up images of in-ear buds like the Etymotic ER6i and the Shure SE210, but these earbuds really just sit in your ear, anchored by the rubber driver surround and an additional stalk above, which has its own rubber pad that serves as an anchor. Sennheiser packs in plenty of different sizes for both the drivers and the extra stalk pad, and also throws in a set of soft foam surrounds in case you want the traditional earbud fit. (I didn't need them). There's also a cable clip and a flexible vinyl case with a button clasp in the package.

The 3.5mm headphone connector fits any standard portable MP3 or DVD player. The earbuds are resistant to both water and perspiration, and Sennheiser claims that the earbuds and cable are designed to withstand extra abuse. The product certainly gives that impression; the rubber also extends around the stalk, leaving hard plastic only for the parts that don't touch either your ear or your hand. You can choose from exactly one color, blinding green—a pretty big drawback if you're not a fan of DayGlo.

I'd characterize the MX75 sound as moderately bright (certainly not as bright as the green). Depending on the recording, music with lots of treble can sometimes sound a bit harsh, but overall, the high end is smoother than any number of the low-cost buds bundled with portable MP3 players. There's also an impressive amount of transparency and detail, particularly for the price. Muse's Stockholm Syndrome sounded as energetic and reverberant as usual, with its characteristic soft treble, whereas Depeche Mode's Precious sounded bright, airy, and punchy.

The MX75 Sport drivers feature neodymium magnets. Like most over-the-ear buds, they won't blow you away with their bass extension—there's barely any perceptible signal below, say, 60 Hz. But there is enough accurate mid-bass to add sufficient weight, especially when they're compared with Sennheiser's own MX400s and the standard Apple stereo earbuds that come with every iPod.

And unlike both of those models, the MX75 Sport earbuds don't fall out. I'm not kidding; even a moderate tug on the cable didn't loosen them much. At the same time, they were so comfortable that, while I never quite forgot I was wearing them, I never felt the need to adjust their fit or take a break, even after long listening sessions.

Don't dismiss pricier earphones from Etymotic or Shure just yet. The Sennheiser MX75 Sport headphones don't approach the level of depth and detail of higher-end models. But they acquit themselves just fine. As long as you're not looking to duplicate a high-end audio experience during your workouts, these 'phones strike an ideal balance between cost, durability, and comfort—as long as you like green.

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