The Scot, 25, won the 1500m and 3,000m events in Glasgow two years after claiming both in Belgrade.

Muir has also clinched two Diamond League titles and a European Championships gold in the former distance.

"She's racking up the medals and wins in unbelievable style," Young said.

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"So she's a proper world star - beginning to move into a world star of sport, never mind athletics."

On the final lap of her 3,000m final on Friday, Muir seared away from the field, while she led the pack for much of Sunday's 1500m showpiece.

"The race on Friday was one for the ages," Young told BBC Scotland. "And then to come back [on Sunday] and run a 57-second last 400m, to just run away from a super field, people running national records for their own countries - that's special.

"She's been described as looking like a proper sprinter. And you see [European and Commonwealth champion sprinter] Dina Asher-Smith joking that she wouldn't fancy racing her over 200m, and [former Olympic heptathlon champion] Denise Lewis raving about how she took off like a sprinter at the end of a fast 3,000m.

"I'm pleased to see that, we've been working on it, and it's just added one more element. She can run long, she can run fast and now she can sprint pretty much faster than anyone else as well and that makes her a very hard athlete to beat."

Young believes Muir will be "disappointed if she doesn't come away with a medal" at the World Championships in Doha during September and October.

"It's still a long way away, but if she stays healthy and stays in this kind of form she will be very hard to beat, and you'd certainly hope to put her in for a medal," he added.