The company recently reported quarterly revenues of more than $1 billion for the first time. Sales of its activewear are expected to grow 25% next year.
Under Armour, which started out selling shirts that keep you warm on cold days or cool on hot ones, arguably invented the concept of performance wear.
And its technology appears to be ahead of Nike in some ways, according to several Wall Street analysts.
"Under Armour is ahead of Nike in terms of how they are integrating technology into their proposition," Neil Saunders, CEO of consulting firm Conlumino, wrote in an email to Business Insider.
At its spring/summer 2016 showcase, Under Armour showed Business Insider a new kind of "cooling" technology it was featuring in its latest apparel lineup.
This latest venture, called Cool Switch, looks like a simple printed pattern - but it actually features a sweat-activated cooling technology. As soon as you start sweating during a workout, the apparel will start cooling you down.
Mallory Schlossberg/Business Insider
Under Armour's Cool Switch technology.
In September, Nike told Business Insider about AeroReact - a sweat-activated cooling technology that the company said had been three years in the making. That product reacts to moisture vapor and then cools the runner down.
Nike remains the number one apparel company in the United States.
Though Nike's women's business is thriving, Under Armour - complete with Misty Copeland as its spokesmodel - has a powerful women's business, too.
The brand is going far to keep women on the go with as little sweat as possible. Under Armour's head of women's design, Kate Williams, showed Business Insider a microthread fabric that eliminates sweat and microbes as an example.
The also brand showed Business Insider apparel with wrist holes, so you can check out your Apple Watch while you're out for a run.
Ultimately, Under Armour's success comes down to the fact that the brand has a great product selection.
"Looking at the product mix, it is clear that Under Armour now has a 'head to to' approach to sporting and fitness which is both attracting new customers to the brand, as well as increasing cross-selling opportunities and average ticket values from existing consumers," Saunders wrote in a note.