Performance apparel specialist Under Armour (NYSE:UA) reported strong first-quarter earnings Friday morning, but investors can't seem to decide whether they like what they see.

After quickly rising 5% at the open, shares of UA gradually fell back to even, then followed with another 2% rise later in the morning, then a dip into negative territory, and back to positive in the afternoon ... and on, and on ...

You know what? While I admittedly find these short-term fluctuations mildly amusing, I'd like to reach out and encourage you to tear your eyes away from your charts and focus on what really matters: What will Under Armour do over the long term?

The numbers
Before anything else, however, let's look at the figures the Baltimore-based company gave us Friday morning.

On one hand, first-quarter revenue increased by an impressive 23% year over year to $472 million. On the other, and in spite of that higher revenue, Under Armour actually earned 47% less than it did during the same period last year.

Yikes! Should investors be worried?

In a word: Nope.

So what happened? In the press release, the company reminded us that the drop in net income largely reflected "the planned timing of marketing expenditures." For those of you keeping track, this shouldn't have come a surprise, considering Under Armour CFO Brad Dickerson gave investors a heads-up way back in January by saying:

"We are planning to be more targeted in some of our marketing expenses this year, which we anticipate will create some significant year-over-year timing shift. The first quarter in particular is expected to see nearly 350 basis points of deleverage."

What's more, Dickerson also went on to explain that the second and third quarters would probably also reflect inconsistent year-over-year earnings as a result of this shift, and the fourth quarter would be more consistent with 2012. In the end, he concluded that Under Armour still plans to hold total 2013 marketing expenses "relatively flat as a percentage of revenues compared to 2012."

On another encouraging note, Under Armour's year-over-year inventory levels remained steady at $324 million -- a welcome relief from inventory concerns in prior years -- and the company managed to increase its cash and equivalents by 139% to $256 million, while at the same time lowering its long-term debt by 21% to $60 million.

The train's a-comin'!
As a longtime Under Armour shareholder, I'm happy the company has once again fulfilled its promise of profitably maintaining at least 20% top-line growth for the 12th consecutive quarter. In addition, this marks the 14th consecutive quarter of apparel revenues growth of more than 20% -- an impressive feat, considering Under Armour was already dominating the category that made it a household name.

Perhaps even more important, however, is the mammoth opportunity Under Armour is seizing in the athletic-footwear market, which itself, as a I pointed out earlier this week, is expected to grow by nearly 15% to $85 billion by 2018.

That's why I'm also impressed that Under Armour's first-quarter footwear revenue increased 27% to $81 million. While that might seem minuscule in the broad scheme of things, remember CEO Kevin Plank's words in 2011, when he placed Nike's (NYSE:NKE) 90% basketball shoe market share squarely in Under Armour's sights:

And what I can commit to you is that I'm not going to make predictions on exactly how much market share, but I would much rather be sitting where we are because it's coming. We will take market share. It's a freight train. And I believe that the opportunity we have is great.

Great indeed. While Under Armour may seem more like a pesky mosquito to Nike in the footwear market right now, I'm convinced it won't take long to morph into the freight train Plank described. In fact, given Under Armour's recent lawsuit against Nike over trademark infringement in its advertising, it's already becoming more evident Nike is taking Under Armour's threat seriously. 

Foolish final thoughts
In the end, however, despite the fierce competition, I'm still convinced that Under Armour's freight train has what it takes to continue full steam ahead.

Fool contributor Steve Symington owns shares of Under Armour. The Motley Fool recommends and owns shares of Nike and Under Armour. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.