In 2013, Fitbits, Jawbone UPs, and Nike FuelBands accounted for 97 percent of all smartphone-enabled activity trackers sold at brick-and-mortar stores or through big ecommerce sites, according to NPD Group, which tracks the digital fitness device market at the point of sale. For the 52-week period beginning in early January 2013 and ending in early January 2014, Fitbit's devices accounted for 68 percent of devices sold; Jawbone UP sales accounted for 19 percent of all devices sold; and Nike FuelBand sales made up 10 percent of sales. All other activity tracking devices together accounted for the remaining 3 percent of sales.

As Fitbit announced last week -- and we reported -- NPD Group found the overall digital fitness device market to be worth $330 million in 2013. NPD Group Executive Director and Industry Analyst for Consumer Technology Ben Arnold told MobiHealthNews that the firm is tracking sales for "full body activity trackers" like those mentioned above along with sales of pedometers, sports watches, heart rate monitors, and other accessories. Arnold admitted that precisely defining the category and subcategory (activity tracker vs. pedometer, e.g.) might be a work in progress as the market is still so new.

Since NPD is tracking point of sale data for retailers the data does not include direct-from-manufacturer sales, which means buying a device straight from Fitbit's website would not be counted in NPD's analysis.

"It is remarkable that three brands have over 90 percent of the share in this tracker market," Arnold said. "These three products have been around for a while and when you think about putting something on your wrist and calculating your steps throughout the day, you probably think about one of these three products. Can some of these [newer entrants] make an impact? I absolutely think so, but what new are they bringing to the table? I don't know if it is a commodity at this point, but what fitness device doesn't keep track of your steps and help you keep track of calories and all that? A lot of these products do similar things."

Arnold said that given how new the digital fitness tracker market is, he fully expects it to double in size this year, which means there is still room for companies to carve out a niche.

"If we were talking about TVs or headphones, I'd say there probably is not room for anyone besides these big three," Arnold said. "If the market doubles, then, yes, there is room for some of these smaller players. Yes, you still need to innovate and offer a different way to do things or different designs [from the current market leaders] ... but the other big consideration is distribution. Can you align yourself with the right retailers?"

Some digital fitness device companies struggle with finding the right mix of stores to sell their products, Arnold said. Devices often don't only belong in sporting goods stores or only in big box technology retail stores.

Arnold said that the $330 million sales generated by digital fitness devices includes the entire category --meaning pedometer, sports watches, and heart rate monitors, too -- but with 72 percent, digital fitness trackers made up the largest percentage of those sales. That's about $238 million.

According to NPD's data, among retail stores Best Buy is the top seller of digital fitness trackers. Target is important too, Arnold said, but Best Buy has dedicated more floorspace to the category and features more devices.