The activity tracker was 2014's top-selling category within wearables and will go on dominating the market this year, although new uses and applications are emerging, according to researcher Futuresource.
Some 16.8m of the devices were sold around the globe last year, with pedometers and footpods in second place, seeing 11.4m buyers.
Oliver Rowntree, market analyst at Futuresource, said 2014 was the breakout year for wearables, with the primary category driver being sports and fitness applications – with fitness devices the largest product category and activity tracking a prominent smartwatch feature.
"We expect to see large category growth with the release of the Apple Watch later this year," he said.
Futuresource full-year 2014 figures for worldwide wearables sales saw 10.2m GPS trackers sold; 6.1m smartwatches; 6.0m heart rate monitors; 0.6m wireless network-enabled watches; and 0.5m head-mounted displays.
"Consumer research we carried out in May and October with more than 8,000 respondents across the US, UK, France and Germany saw the proportion of respondents intending to purchase wearable devices in the next 12 months rise from 10 per cent in May to 16 per cent in October 2014," Rowntree said.
Since May, more current iPhone owners have said they intend to purchase wearables; this was the stand-out trend that Futuresource research found between May and October.
"IPhone owners now lead the way in all categories, particularly in smartwatches, which 17 per cent of iPhone owners expressed intent to purchase in the next 12 months, up from only six per cent in May 2014," he said.
Rowntree believed that the products will converge, with heart rate and fitness features becoming embedded in other devices. This means the leading fitness segment will begin to deflate, largely in favour of smartwatches.
Shipments of dedicated fitness devices will grow less than 25 per cent from 2014 to 2018, and connected watches by more than 1,100 per cent, Futuresource figures suggested. That's despite the fact that smartwatch battery life currently leaves something to be desired, with many brands struggling to run for even one day.
"It's worth noting that the market isn't just about the devices; it's the services and apps people are using in conjunction with them," Rowntree said.
"This will determine how they are used and the level of uptake. The future growth of the category will be determined by whether killer applications develop beyond fitness, such as augmented reality, 'life logging' and communications."
Futuresource analysts have been walking the halls at the international Consumer Electronics Show (CES) in Las Vegas, checking out technology advances and reporting their findings.
"The US currently leads the way in the two big emerging wearables categories: activity trackers and smartwatches. The US is typically an early-adopter market, and also a very Apple-centric market, as well as being the biggest per capita spender on health and fitness," noted Rowntree.
CES 2015 saw the showcasing of applications for smartwatches that go beyond the FitBit paradigm. Car maker Hyundai showed a driver alertness app and rival Audi a customised LG watch that can actually control a car. As a smartwatch on the wrist will always know where you are, car and smart home control are natural progressions for the smartwatch category.
Smart headphones were also on show. Seven per cent of all headphones now have wireless connectivity and the prices in this category are rising as well, according to Futuresource.
However, fitness bands were largely absent from CES announcements this year.
Simon Bryant, associate director of consumer electronics at Futuresource, said that post-CES the largest wearables opportunities will come from industries other than traditional CE.
"The experts in wearables are not necessarily CE vendors; they are fashion houses and lifestyle brands that can align more readily with consumer aspirations," he said.
"Smart garments will significantly disrupt dedicated wearable devices as we progress towards mainstream consumer acceptance."
The industry needs to stop thinking of wearable devices simply as CE devices that are worn on the body, Bryant said.
IT firms with CES announcements around wearables included Lenovo, which is entering the market; Qualcomm, which has about 15 wearables products so far; and Intel, with a smart-clothing computer module due out at the end of 2015.
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