When was the last time you purchased underpants just for your workouts? Never? Us, too. But there may be a strong case as to why you should invest in panties for the gym.

If you're someone who works out in their lace thongs, listen up. Wearing breathable underwear is key when breaking a sweat. If you opt for silk or lace undergarments, moisture can get trapped which increases your risk of getting a yeast infection. Instead, go for pairs made of lycra or cotton.

That said, wearing the right fabric isn't the only important factor -- the style of your underpants matters, too. "Wearing a thong during exercise can bring E. coli living in the rear toward the front as you move, increasing your risk for urinary tract infection," Dr. Nicole E. Williams of the Gynecology Institute of Chicago told Elle.com .

Other things to avoid? Your old underwear. "If your underwear is old, the elastic becomes exposed and can potentially give you a rash, called irritant contact dermatitis," Sandra Lee, M.D., a board-certified dermatologist, said in an article in Women's Health.

For those who like going commando, it's not all bad news. David Bank, M.D., founder and director of the Center for Dermatology, Cosmetic & Laser Surgery, told Shape.com, "If you frequently wear bottoms with moisture-wicking capabilities, then underwear is not necessary ."

Shop some great workout undies below, along with some pants/shorts perfect for going commando: