Twelve South's new Apple Watch strap is going to make a lot of weightlifters and roller derby players very happy.

The Apple Watch's heart rate monitor is one of the most accurate options on the oximeter market — as long as you're not doing anything that involves wrist bending or heavy protective gear. Now, Twelve South may have just the armband strap to fix that problem.

To understand why the Apple Watch doesn't like it when you bend your wrist, you have to get how the device's heart rate sensor (oximeter) works. Essentially, the Apple Watch measures your pulse by shining light on the blood vessels in your wrist and measuring how fast blood pumps through said vessels. When you bend your wrist or otherwise contract muscles in your arm, however, the watch may not get an accurate reading thanks to those vessels being constricted. Things like pushups, other weightlifting, and even roller derby can interfere with how the monitor reads your pulse — leading to inaccurate workouts.

But rather than pick up a third party armband heart rate monitor to augment your Apple Watch, Twelve South has a different option for exercise aficionados: Move your watch to your bicep with its new ActionSleeve Armband strap.

The strap comes with a protective silicone outer frame to protect your watch's casing and an adjustable neoprene strap to provide a snug fit on your arm — no matter your flexing prowess.

It's also a great option for amputees and other athletes who don't always have access to their wrists — Twelve South worked with Cameron Massengale, an amputee athlete at The Citadel Military College of South Carolina, when developing the product.

I've consistently worn my Apple Watch under my wrist guard during roller derby in the years I've owned it, but Twelve South's new ActionSleeve has me very excited nonetheless — wearing the watch under protective gear is often uncomfortable, and I don't have easy access to the controls. (I even took to wearing it in Reverse Crown orientation so the crown wouldn't push against the wrist guard.) I already swap my straps when I go to exercise; swapping for an armband seems like an even better idea.

What about you, iMore? Sound like something you want to try for your Apple Watch? Let me know in the comments.

See at Twelve South