Balance balls round out your workout

For a complete fitness program, an inflatable balance ball is indispensable. It will multiply the difficulty of dozens of exercises -- push-ups, sit-ups, leg lifts -- and increase core strength by forcing you to stabilize your body through the motion. As their popularity grows, so do the variations -- as the models below illustrate.

-- Roy M. Wallack

Legs to stand on

Spri Ball With Legs: Round ball with six 3-inch-long teats.

Likes: Protuberances stop it from rolling away. They also serve as convenient carry handles for movement and storage, and make the ball a good conversation piece.

Dislikes: Doesn’t include a pump. That costs an extra $11.95.

Price: $36.95 (55-centimeter diameter); $41.95 (65 centimeters). (800) 222-7774; www.spriproducts.com.

Bottom-heavy

Reebok StayBall: Round ball with 5 pounds of sand inside.

Likes: Ball can’t roll away. The extra weight adds more challenge to leg lifts and chest presses. Foot pump and “Core Workout” DVD included.

Dislikes: None.

Price: $24.99. (800) 201-2172, www.reebokfitness.com.

Beginner ball

Valeo Stability Body Ball: Jelly-bean shape reduces instability.

Likes: Oblong shape gives a more secure feeling for new and/or older exercisers. Elongated shape adds stability because it allows only fore-and-aft movement, not side-to-side. Ideal for beginner training and rehabilitation. Includes pump and wall chart.

Dislikes: None.

Price: $24.99. (800) 634-2704; www.valeoinc.com.

Hardball

DuraBall Pro: Burst-resistant ball for heavy weight lifting.

Likes: Can be inflated more than most, which is good if you like a harder ball. It’s very safe and can handle heavy loads and bouncing. Won’t explode if pierced, just slowly deflates.

Dislikes: No pump included.

Price: $49.95 (55 centimeters); $59.95 (65 centimeters). (800) 348-8371, www.fitter1.com.