David McCreight, who was most recently president of Lands' End, will report to Plank and be responsible for the core businesses, which include footwear, apparel and accessories.
Plank will remain chief executive officer and chairman.
It is the latest move by the company, which was run by Plank's childhood buddies and other close associates in its early days, to beef up its management ranks by raiding major retailers for top talent as it expands into new lines, such as off-field athletic shoes.
Plank was not available for comment yesterday. In a statement, he said, "The addition of David allows me to focus on driving growth and steering the UA brand, both in the United States and around the globe."
Analysts said the appointment was a natural progression for the company as it continues to aggressively expand. It will allow Plank to focus less on the day-to-day operations and more on new opportunities for the business.
"Clearly it sounds like Plank wants to focus on the long-term outlook," said Claire Gallacher, a retail analyst with Caris & Co. in San Francisco.
"He really wants to oversee the whole company and probably not just the domestic part of it. That would require more time and travel and he probably needed somebody that was there day-to-day."
Plank will still be involved heavily in the direction of the company, which he started in his grandmother's basement 12 years ago.
"I think it's just a function of the company really growing and expanding into new categories," said Thomas Shaw, lifestyles analyst for Stifel Nicolaus in Baltimore.
"It's building the infrastructure to support that kind of growth. These are the building blocks the company needs to grow to a $1, $2 or $3 billion-dollar company."
Shaw said Plank could also now spend time on innovation and product development, rather than the nuts and bolts of running the company.
Under Armour Inc., which started out selling its signature compression shirt and shorts made of fabrics that wick sweat from the body, has since expanded into accessories and footwear, first with football cleats and then launching a cross-trainer sneaker this year.
It plans to bring a running shoe to store shelves next year and is also working on a prototype for a basketball sneaker. It has also been pushing further into overseas markets.
In the past 12 months, it has hired several executives who have led other major retailers in expansion initiatives.
Suzanne Karkus, former president of Izod Womenswear and of the women's division of Calvin Klein Jeans, was brought in as vice president of apparel. One of the first things she did was improve the look and fit of the women's line.
The company also hired Peter Mahrer from Puma to head up the international business.
McCreight led the shift of Lands' End from a catalog company to one sold in several retail channels, including Sears stores after Sears' acquisition of Lands' End, and on the Internet. McCreight has also served as senior vice president of Disney Stores worldwide and was president at Smith & Hawken, a garden and outdoor furniture store.
McCreight, who is to join Under Armour in September, was not available for comment yesterday.
"It is a tremendous opportunity to join the Under Armour team at such an important period of growth for the company," McCreight said in a statement. "Upon meeting with the management team, it was clear to me why the brand has been so successful and why so many people want to be a part of what is happening here. I look forward to working with the entire team to help sustain the growth the brand expects to achieve well into the future."
The company said that McCreight's position will complement that of Wayne A. Marino, Under Armour chief operating officer. Marino will handle the back-end operations of the business while McCreight will handle the front end.