You may have seen them in the gym: half of a large rubber ball that's flat on one side. They're often blue in color and look like a gigantic outtie belly button. ?What are those things?? you wonder. Well they're BOSU balls (or balance trainers). It's the latest rave to hit fitness centers across America.

BOSU is an acronym that stands for "Both Sides Up". You can use the balls, also referred to as trainers, or balance trainers, on either side. Whether the rounded bouncy dome part is up or the flat 25 inch platform side is up, you'll get different types of balance challenges.

This cross-training fitness invention has its origins in the field of medicine, balance, functional and sports specific training. It offers a different means to make exercise more appealing and effective for average people, fitness fanatics and highly trained athletes.

So what do you do with it? You can walk, run, step, hop, jump and leap on the BOSU trainer. You can work it at an easy steady rate-pace that can be maintained for long periods or push the intensity with anaerobic intervals. Or do some stretching on it. Active stretches use the muscles of the body to move a body part whereas passive stretching uses gravity or an outside force to put stretch-tension on the target muscle(s). While standing or kneeling on the dome, you can move the upper body lower, higher, to the sides or by reaching for and picking up cards from various locations on the floor, or by touching and/or relocating cones that have been placed in close proximity to the dome. It's great for working your trunk too. Maintaining spinal alignment is important to low back health and sports performance.

Many sports teams use BOSU training too. Says Bennie Wylie, Assistant Strength Coach of the Dallas Cowboys "I implement the BOSU trainer during our off-season program for core strength training and rehabilitation of ankle and knee problems. It is a great piece of equipment and gives us an important extra dimension to our overall fitness program. Almost all of the players use the BOSU trainer in some fashion on a regular basis."

So the next time you overhear a conversation in the grocery store or your friend tells you that she's going to the 5:30 p.m. BOSU class at the Y, you'll know what they're talking about. Better yet, give it a try yourself!