Nike Inc (NYSE:NKE) has long been a household name synonymous with athletic apparel, but it seems that newcomer Under Armour Inc (NYSE:UA) is becoming a viable threat to Nike’s status. Although Nike remains at the top, Under Armour recently muscled out Adidas to claim the number two slot for most popular sportswear company.

Even though Nike’s $98.41 billion market caps trumps that of Under Armour’s of $21.95 billion, Under Armour recently announced growth plans that are giving Nike investors reason to worry. On September 16, Under Armour announced an accelerated long-term net revenue target, revealing that the company anticipates revenue of $7.5 billion by 2018. This figure marks a compounded annual growth rate of 25% when compared to the company’s 2014 net revenue of $3.1 billion.

Under Armour CEO, Kevin Plank, commented, “The investments we have made and will continue to make are a testament to the extended runway of growth we see ahead and provide us with the confidence in raising our long-term net revenues growth rate target from +22% to +25%. Building off of the incredible consumer demand we are experiencing for the brand, we firmly believe we are just getting started in our pursuit to become not only the definitive performance sports brand, but a truly great global brand.”

Analysts have voiced both bullish and less-than-enthusiastic opinions on Under Armour. Last week, Morgan Stanley analyst Jay Sole maintained an equal-weight rating on the stock, noting that heavy investments limit margin growth in the near-term. However, Sole expects these investments “to deliver strong returns over time.” On the other hand, analyst Jon Kernan of Cowen reiterated his outperform rating on UA last week and raised his price target from $112 to $120, commenting that the company’s revised revenue forecast could be conservative.

Should Nike investors be worried?

Nike sponsors elite athletes such as Roger Federer, Kobe Bryant, and Maria Sharapova. Under Armour, on the other hand, prides itself on endorsing the underdog. Adrienne Lofton, Under Armour’s VP of Global Brand Marketing, explained, “We're an underdog brand. We work with athletes who most people wouldn't or didn't draft in the first round, or who they wouldn't traditionally give a prima ballerina title to. We pick that athlete with a chip on their shoulder and their desire to win because it aligns with our own attitude.”

Under Armour itself was an underdog in the field for a quite a while. CEO Kevin Plank founded the company in 1996 at age 23 and famously sold products out of the trunk of his car. The company picked up steam in 1999 after receiving product placements in two movies, and eventually went public in 2005. Is it fair to say that Under Armour has grown out of its underdog status? The company most recently reported quarterly revenue of $784 million in July; marking a 29% year-over-year increase. Although this is an impressive figure that demonstrates strong growth, it is dwarfed in comparison to Nike’s most recent quarterly revenue of $7.8 billion.

Under Armour CEO Kevin Plank does not seem worried or intimidated by Nike, commenting, “We're incredibly bullish about the momentum of the brand, the athletes we have, the stable [figures].”

Nike will be posting Q1 earnings this week for fiscal year 2016. Analysts expect continued growth, estimating that the company will post earnings per share of $1.19 on $8.2 billion in revenue. However year-over-year comparisons will be tough to achieve as Nike received a nice boost in the comparable quarter last year from the 2014 FIFA World Cup. Some analysts are optimistic heading into Nike’s earnings while others are cautious.

Analyst Christopher Svezia of Susquehanna falls in the optimistic camp, reiterating a Positive rating on Nike and with a $133 price target, up from $122. Svezia notes that Nike’s “sustainable global growth and improving operating leverage” should help the company beat earnings. On the other hand, Sterne Agee analyst Sam Poser remains cautious on stock. He reiterated a Neutral rating last week, commenting, “The current valuation, the law of large numbers’ and difficult comparisons make the growth rate very susceptible to deceleration… Nike will continue its aggressive spending for, especially due to the 2016 Olympics, and see little upside to current estimates.”

Both Nike and Under Armour investors have enjoyed increased earnings this year. In the last 12 months, Nike shares have increased over 40% from about $80 to current levels of $115. Under Armour shares have soared 50% in the last year, climbing from $66 to current levels of $101.

Will Nike remain at the top of the athletic apparel market, or will Under Armour slowly take away its market share? Find out what top Wall Street analysts think about Nike and Under Armour.