Sk8pole in action. (www.sk8pole.com)

The invention of the wheel sometime around 4,000 BC suddenly made all kinds of things easier, from carrying to killing to athletics. Fitness is great on wheels; putting a circular ring with an axel on any athletic contrivance instantly gives it an exhilarating sense of speed that can turn any workout into fun. Here are some of the newest wheeled workout wonders sure to inspire feats of fitness at any age.

A pole vault

Sk8pole: Adjustable-length, three-section aluminum pole with a flexible rubber foot that you use to propel yourself while on a skateboard.

Likes: Great upper-body and core workout — sort of like stand-up paddle-boarding on land. Skateboarding kids may like the way it allows them to alternate leg and arm power and try new tricks. I liked the speed and the smooth, flowing functional fitness, including the surprisingly difficult challenge of balancing while switching to my weak-side hand and feet position. In that sense, repeated use will certainly help you develop better symmetry. I saw two skateboarders using a Sk8pole for several miles in the CicLAvia event in Los Angeles a few weeks ago. Length adjusts from 5.75 feet to 2.5 feet, making it easy to carry.

Dislikes: My teenage son and nephew, both into skateboarding but not fitness, liked the pole at first but soon thought it was just too much work.

Price: $49.95. http://www.sk8pole.com


Shimmy and go

ATM Sports Fliker Free Style Rider: Basic, effective three-wheel scooter for kids that is propelled by twisting your hips, swaying your body and pressing out in a skate-style carving motion. Feet don't touch the ground.

Likes: It works. A very simple, inexpensive version of a Trikke, the popular self-propelled carving vehicle that pioneered the three-wheel category (and has been reviewed favorably in this column several times), the Fliker's legs are configured in a fixed V-shape with caster wheels. As you shimmy from side to side, it moves forward instantly and fairly fast. There is no learning curve. You go! With a hand brake, it's a safe and fun transportation and fitness option for kids. Three progressively larger models fit 5- to 10-year-olds.

Dislikes: Too bad it doesn't come in an adult size.

Price: $119 to $139. http://www.fliker-scooter.com


It's a kick

Kimatek SkiMotion: Self-propelling three-wheel scooter with sophisticated energy transfer mechanism that lets you move with various creative foot motions.

Likes: A smooth, fun and fast all-body workout for kids and adults. You could easily ride an hour on this. Like a Trikke, the SkiMotion moves forward when you lean your weight and kick side to side, in a skating motion. But its four-pivot hub and swiveling caster wheels also let you propel yourself forward by moving both legs in and out in a scissor kick, as if you're standing on a ThighMaster. It has dual brake levers and folds up for easy carrying. There are two models: The larger A model with a taller handlebar stem and 100-millimeter rear wheels, for people up to 200 pounds, and the smaller kids' C model with 90-mm rear wheels, rated to 155 pounds.

Dislikes: The forward lean required by the relatively low-height, non-adjustable handlebar is not as comfortable as the upright position of a Trikke. People taller than 6 feet may have issues.

Price: $299 (A model), $279 (C model). http://www.rideskimotion.com


'Unline' skates

Inventist Orbitwheel: Minimalistic, futuristic skates, each made of a single 9.5-inch tall wheel, with platforms inside them for your feet.

Likes: Cool and different. Helps build skill, balance and coordination. Easy to carry and pack. Can be used with regular shoes. No fuss with clips and straps. You cross your legs side-to-side to move forward. Seems like something you'd see in the new James Bond movie. Looks great on the instructional video.

Dislikes: This takes time — time you may not have. I was told that inline skaters pick it up quickly, but this is not at all like inline skating, as your feet are positioned sideways. You need to start against a wall and slowly push yourself until you get some balance. Only one kid in my neighborhood worked up the courage to try the Orbitwheels; after about 30 minutes, he was fairly competent, but unenthusiastic. As for me, an old inline skater, I almost broke my hip and wrist in the first 5 seconds — and decided I'd better stick to cycling.

Price: $84.95. http://www.inventist.com

Wallack is coauthor of "Bike for Life: How to Ride to 100" and "Barefoot Running Step by Step." roywallack@aol.com