Life Time Fitness CEO Bahram Akradi has bought a 5.3 percent stake in struggling Northern Oil & Gas, and it looks like he plans to be an activist investor.

Akradi has paid $16 million for 3.4 million shares of Wayzata-based Northern Oil, according to a Tuesday filing with the U.S. Securities and Exchange Commission.

In the filing, Akradi said "he expects to engage in discussion with management, the board and other shareholders of [Northern] and other relevant parties" about many aspects of the company, including its operations, management and financial condition. Akradi might buy more shares in Northern, too, the filing said.

Akradi could not be reached for comment on Tuesday.

Northern invests in oil leases and drilling projects in North Dakota. The company has been hammered by the drop in oil prices over the past two years, and lately has experienced top management turmoil, with its longtime CEO being forced out.

Northern's stock, which briefly topped $30 at its 2011 peak, is trading near its all-time low.

It closed Tuesday at $2.25, up 2 cents.

Akradi founded Chanhassen-based Life Time, one of the most prominent fitness chains in the Twin Cities. While not known for investments in the oil sector, Akradi said in an SEC filing that he believes Northern's stock is "undervalued and is an attractive investment."

Akradi will have company in talking to management about fixing Northern Oil.

A billionaire Texas oilman and luxury hotel chain owner, Bob Rowling, has amassed a nearly 20 percent stake in Northern's stock through several firms. They control a big chunk of Northern's unsecured debt, too, and a representative of Rowling's interests, Michael Frantz, was appointed to Northern's board in August.

Michael Reger, Northern's CEO and co-founder, was terminated in August after he told the board that federal securities regulators might be pursuing an enforcement action against him. He is suing for wrongful termination. Reger is being scrutinized in an SEC investigation of possible stock manipulation in the shares of Wayzata-based Dakota Plains Holdings, a separate oil field services company.