Nike shares rose nearly 6% in early trading on Wednesday after Apple CEO Tim Cook doubled his personal stake in the sportswear maker, raising his bets on the margin-pinching turnaround efforts led by CEO Elliott Hill.

Cook, who has been on Nike's board since 2005, bought 50,000 shares at $58.97 each, according to a regulatory filing. As of December 22, he holds about 105,000 shares, which is now worth nearly $6 million.

It was the largest open market stock purchase for a Nike director or executive and possibly the largest in more than a decade, said Jonathan Komp, analyst at Baird Equity Research.

"Cook's move as a positive signal for the progress under CEO Elliott Hill and Nike's "Win Now" actions," Komp said.

The purchase comes days after Nike reported weaker quarterly margins and sluggish sales in China even as CEO Hill tries to revive demand through fresh marketing plans and innovation focused on running and sports, while phasing out lagging lifestyle brands.

He has also attempted to mend Nike's ties with wholesalers such as Dicks Sporting Goods to increase visibility among shoppers amid stiff competition from newer brands.

However, the strategy has strained Nike's margins, which have been declining for over a year, while its efforts to grab back its premier position in discount-friendly China appears to be faltering.

Cook has been a lead independent director of Nike since 2016 when co-founder Phil Knight stepped down as its chairman.

The Apple CEO "remains extremely close" with Knight, Komp said, adding that he has advised Nike through key strategic decisions including Hill's appointment last year.

Nike's shares have slumped nearly 13% since it reported results on December 18, and are on track for the fourth straight year of declines. They were trading at $59.44 on Wednesday.