Under Armour Inc. Chief Executive Officer Kevin Plank plans to sell as many as 2.1 million shares in the sportswear company he founded, a stake valued at more than $72 million as of Friday.

Plank entered into a prearranged stock trading plan that will be active for nine months starting in October, the company said on Friday in a regulatory filing. If he sells all these nonvoting shares, he would still own almost 32 million Class C shares.

The executive, who started Under Armour in 1996, enacted a similar stock plan last year. Still, the latest move gave a jolt to investors, who see Plank as the driving force of the business. The shares fell as much as 2.7 percent to $38.66 on Friday, when stocks were generally up.

Plank has a net worth of $3.2 billion, according to the Bloomberg Billionaires Index, and much of that is tied up in Under Armour stock. Shareholders approved the creation last year of the nonvoting Class C shares, which allowed the executive to sell some of his holdings without losing influence. Through his Class B shares, which have 10 times the voting power of Class A stock, Plank controls the majority of the company’s voting rights.

Under Armour shares were already down 4.5 percent this year through Thursday, hurt by concerns about mounting competition and slowing growth.

Under the trading plan announced Friday, Plank can personally sell as much as 1.9 million Class C shares. His charitable foundation can sell an additional 200,000 shares.

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