(Bloomberg) -- You’ve heard it a million times: Sit less. Yourdesk is slowly killing you.

But standing desks—and their juiced-updescendants, treadmill desks—are expensive, often require an officemanager’s approval, and simply stand out too much for the moremodest-minded among us.

Read: 8 ways to help employees find time forfitness

A slew of new exercise machines are designed toslide under your desk, giving your legs access toa mini elliptical machine or stationary bike withless expense and space—and fewer stares—than a standing desk ortreadmill rig.

In other words, they let you discreetly work out while youwork.

Read: Smartphones get peoplemoving

I put three such systems to the test.

1. Cubii ($347)

The Cubii, a sleek under-desk elliptical machine thatlooks like a tiny light bike from the movie Tron and purports toburn 120 calories an hour,recently made some waves on theKickstarter circuit.

What sets the Cubii apart is a companion app that promises totrack your workday workouts and a design that had an under-desksetting specifically in mind. According to its founders, the key isa low profile and a narrow range of motion that minimizes how highyou pump your knees.

“We researched solutions to stay more active and came acrosstreadmill desks and standing desks,” says Cubii co- founder ShivaniJain. “However, there weren't accessible to us as they wereexpensive, needed a lot of space, and required a complete change inour office furniture authorized by the HR department. We wanted tobuild something that is accessible to everyone.”

Assembling the Cubii was quick. I downloaded the app, setthe resistance dial (to max, naturally), and pumped my legs forabout three seconds before my knee first banged up against mydesk.

Rather than give up, I lowered my chair and kicked theCubii a bit further away, extending my legs and therefore reducingthe height of my knee’s zenith.

Success. Although this position—with my legs outstretched fairlyfar—made it difficult to sit up straight and maintain proper typingposture, it proved comfortable for leaning back while reading ortaking on other nontyping tasks.

The machine was also quiet. And while it would be hardnot to notice that I was doing something under my desk, unless youfind yourself panting for breath, the near-silent machine isunlikely to disturb office mates.

Still, users should come into this with realisticexpectations.

The narrow range of motion feels more like repeatedly steppingon sewing-machine pedals than using a gym elliptical.

And while the companion app allowed me to keep track of my burn,I found it a bit clunky to navigate, and it currentlydoesn’t sync up with any other fitness trackers (thecompany says that feature is coming soon).

2. Deskcycle ($159)

Deskcycle, as the name suggests, is basically atiny under-desk exercise bike: With a pedal height of just 10inches, it claims to be the lowest-profile pedal exerciser outthere.

Like the Cubii, the Deskcycle features a dial that makes it easyto crank the resistance up and down instantly.

At the lowest of its eight settings, it felt as if I wereeffortlessly pedaling a downhill bike.

At its highest, I was winded in minutes.

And while this device doesn’t feature a companion app, a digitaldisplay allows you to keep track of your workday workout, and anincluded mount even lets you plop the display on your desk,negating the need to peek under your desk to check on yourmileage.

Overall, I found the movement smooth and the workout real. Forsuch a small device, the design also felt surprisingly solid.

As with the Cubii, I achieved the best results whenlowering my chair and extending the reach of my legs—though eventhat may not be enough to keep the tallest among us from bangingour knees.

Thankfully, the Velcro-strapped bike pedals actually offer afair amount of leeway with your leg angle, making it ideal forstretching out.

And while the company recommends users have desks that are atleast 27 inches high, I found that, unless you happen to beparticularly short, you’ll almost certainly need at least a fewinches beyond this.

3. Stamina InMotion Compact Strider ($200)

The Stamina InMotion Compact Strider looks a lotlike somebody shrank—and then ripped the top off—a gym ellipticalmachine.

Unfortunately, it was nearly impossible to use under a deskwithout banging my knees.

When I stepped away from my desk, however, I found it to be apretty decent compact exercise machine.

Unlike the Cubii, which can be used only while sitting down,this “compact strider” can also accommodate standing exercisers,making it well suited for apartment dwellers looking for a smallelliptical, if not office workers looking for an at-desk fitnesssolution—though without any handles to using it while standingrequires—and will likely build up—a bit of balance.

Part of this design difference comes from the pedalposition.

While the Cubii puts the user’s foot in a toe-up position thatallows it to be used with extended legs, this one's relativelyflat-footed pedals allowed it work well when standing up straight,but it was very awkward once I stretched out my legs as far asrequired to keep my legs from banging.

It likely takes a serious multitasking mind to use any ofthese machines while concentrating on other work, but most peoplecould surely benefit from having one handy for quick middayactivity bursts.

Though if anybody has ever called you tall, try before you buy.Your knees will thank you.

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