If your stuff is worn out from working out, be careful: Tired sneakers and frayed parts can leave you vulnerable to pain or injury, says certified personal trainer Jessica Matthews, continuing education coordinator for the American Council on Exercise. Here's when to go shopping.

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Sports Bra: After you've used it for six months to a year.

Stability Ball: Three or more years, with regular home use.

Bicycle Helmet: Any time you've crashed or dropped the helmet hard enough to dent or crack the protective foam liner.

Shoes: Replace them after three to six months of regular workouts, or 350 to 500 miles of running or walking.

Originally published in Ladies' Home Journal, May 2009.