By RYAN DIONNE

Winter athletes know it can be difficult to find a hat that fits under a bike helmet and stays put for running. But Pearl Izumi has done just that with its Microsensor Skull Cap, an upgrade to a design that’s been around for years.

Unlike many winter caps on the market, the tight-fitting Pearl Izumi is built for performance during aerobic activity. Sweat wicks through the cap and while active you often forget it’s even on your head.

Snug like a swim cap, the Microsensor fits under a bike helmet and doesn’t move around when you’re out for a run. In my tests, the fabric wicked sweat well and blocked wind, keeping me warm without overheating for a long bike ride in 40-degree weather and on numerous short training runs.

Made of nylon, polyester and Lycra, the Skull Cap refuses to stretch out, and it fits tight against your head. Its shape lets you pull the cap down over your ears while also staying put in the front and back for all-around protection.

That said, the cap isn’t designed for women with long hair.

Many women pull their hair back or put it up while active, including my wife. Because the Microsensor Skull Cap comes in just one size, it may not fully cover a woman’s ears.

That was the case with my wife, who found it tended to ride up on the ears, exposing her lobes. This ill fit was seen with a helmet on and without.

But for men — and women with shorter hair — Pearl Izumi’s $20 cap is a must-have accessory for multi-sport athletes who aren’t afraid of the cold.

*Pros: Stays in place; Fits comfortably under a bike helmet; Wicks sweat well

*Cons: Looks like a swim cap; Not designed for women with long hair; Center seam may cause discomfort in tight-fitting helmet

*MSRP: $20

*Contact: www.pearlizumi.com

—Contributor Ryan Dionne is based in Boulder, Colo. He writes a blog on the outdoors and gear at http://explore-it.blog.com