The most important question for exercise-focused earphones is whether they stay in place. Sennheiser's MX 686G Sport definitely get this one right, with very secure-fitting eartips. At $69.95, the earbud-style earphones deliver surprisingly powerful bass response despite the actual earpieces being flat like the old white earbuds that used to come with iPods, thanks in part to the eartips entering the ear canal like in-canal earphone pairs do. The MX 686G would benefit from some more high-mid and high-frequency presence overall, however, and can sometimes sound overly weighted towards the bass end of the scale. They're a counterpoint to the JVC HA-ETR40, similarly well-designed earphones that sound a bit too bright and skimp on the bass.

Design
Sennheiser's Sport line has always had a distinctive look that will be a delight for some and a deal breaker for others. The earphones in the line often flaunt a neon yellowish-green as their main color, and they certainly stand out. The sweat- and water-resistant MX 686G is no exception; there are gray and white accents on the earpieces and inline remote, but this is otherwise a very neon design.

The eartips have a sort of teardrop shape that fits the ears quite securely, but the fit is also aided by an extending, adjustable stabilizing fin on the top portion of each earpiece that can exert varying degrees of pressure against your ear. It's a unique spin on the now-familiar stabilizing fin we've seen in earphones from Sennheiser, Bose, and others, and it works quite well.

Technically, these are earbuds and not earphones, meaning the actual earpieces themselves lack a nozzle that enters the ear canal. Some will view this as a technicality, since the removable, antimicrobial eartips do enter the canal, creating a secure-enough fit to ensure a consistent stereo image and solid bass response, avoiding two common issues with flat earbuds.

The flat audio cable features an inline remote and microphone with Volume Up/Down buttons that double as track navigation buttons depending on how long you press them, and a central multi-purpose button for Play/Pause/Call Answer/End. There's also a shirt clip on the cable, which helps cut down on cable thumping. The MX 686G ships with a well-padded carrying pouch for the earphones, but oddly only one pair of eartips. 

Performance
On tracks with intense sub-bass content, like The Knife's "Silent Shout," the MX 686G delivers a surprising amount of bass for earbud-style earphones. However, this comes with a caveat: The eartips, while quite comfortable and secure, are very easy to put in place so that the perforations in the nozzle through which sound travels aren't ideally aligned with your ear canal, even though the nozzle is sitting in your canal. The result of this misalignment is very limited bass response. So, if you're playing a song that should have deep lows and you don't hear them, you're going to have to twist and fiddle with the eartips until it emerges. The reward is some serious thumping low-end that, for some, will be a strong motivator while exercising.

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On tracks with less intense bass content, like Bill Callahan's "Drover," eartip placement is still very important: If you don't get things right, the track can sound thin and the stereo image will be off, resulting in one ear sounding louder than the other. When the eartips are properly aligned, this track sounds like it has big bass despite not much being present in the mix itself—Callahan's baritone vocals get an added dose of low-mid richness (which they don't really need), and the drums receive added bass presence that is pleasant. The overall sound signature sounds a bit too weighted toward the lows, and not quite crisp enough. 

The same is true on Jay-Z and Kanye West's "No Church in the Wild." There's a surprising amount of deep bass for earbud-style earphones, from the sub-bass synth hits to the kick drum loop, but the vocals are less crisp and defined in the high-mid range than they could be. It sounds powerful, but a tad muffled.

If bright, crisp treble combined with less-overwhelming bass is your thing, consider the MX 686G Sport's polar opposite in the JVC HA-ETR40, which is possibly too bright and crisp and lacking in the lows. If you're after something between those extremes, there are plenty of exercise-focused in-ear options that fit the bill: The JBL Synchros Reflect, the Audio-Technica ATH-CKX7iS, and the JLab Fit are all solid options. At $70, the Sennheiser MX686G gets a lot right in terms of fit, but the audio is not quite on par with a typical Sennheiser offering.