The wearables market continues to be very promising, with fitness trackers and smart watches being the main drivers of volume and growth, according to the latest Tractica report. Last year, device shipments achieved higher than expected growth especially in the fitness tracker segment, in which unit sales have doubled since 2014.

The research firm predicts that annual wearable device shipments will increase from 85.0 million units in 2015 to 559.6 million units by 2021, representing a compound annual growth rate (CAGR) of 36.9%.

Tractica has upgraded its wearable device shipment and revenue forecasts significantly since the last edition of its forecast in early 2015, driven by stronger than anticipated growth of fitness trackers, a proliferation of smart watch offerings, and rapid development of the market for emerging wearable categories like body sensors, smart clothing, wearable cameras, smart glasses, and an increasing diversity of niche devices.

“2015 was the biggest year for wearables by far, with fitness trackers outperforming industry expectations and smart watches coming out strong,” research director Aditya Kaul said in a statement. “While fitness is the overwhelming driver for wearables today, there are a number of micro-segments emerging, ranging from kids’ watches to smart footwear to new healthcare devices like wearable patches, that will drive this market forward.”

Another forecast, from ABI Research, expects activity tracker shipments to hit 87 million in 2021.