Ask police defensive tactics (DT) instructors what's the most frequent cause of injury in their classes and they'll point the finger at an unlikely hazard: mats.

Mix a certain type of mat with a certain type of shoe, and you've got a recipe for leg, knee, ankle, and foot injuries, according to Bob Bragg, director of instructor training at the Washington State Criminal Justice Academy.

Veteran police trainer Ed Nowicki agrees: "If you've got rubber shoes and rubberized mats and you go to do a twist or a pivot, you can really hurt your ankle."

Bragg argues that bare feet on wrestling mats are just as hazardous, if not worse. "People get their toes sprained that way when their feet sink in to the mat and their toes get twisted."

For mat work, he favors wrestling shoes. As for mats themselves, tatami-style judo mats or gymnastic mats are Bragg's preference, but he says no mat is perfect.

"My advice is don't use mats when you don't need them," he says. "Quite often people will go into a room to work on stand up striking skills with no intention of taking anyone to the ground and they get injured on the mats when they twist a knee or a foot. If you don't have a perceived need for a padded surface, don't use mats."