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

Read: Smartphones get people moving

I put three such systems to the test.

1. Cubii ($347)

The Cubii, a sleek under-desk elliptical machine that looks like a tiny light bike from the movie Tron and purports to burn 120 calories an hour,recently made some waves on the Kickstarter circuit.

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

“We researched solutions to stay more active and came across treadmill desks and standing desks,” says Cubii co- founder Shivani Jain. “However, there weren't accessible to us as they were expensive, needed a lot of space, and required a complete change in our office furniture authorized by the HR department. We wanted to build something that is accessible to everyone.”

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

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

Success. Although this position—with my legs outstretched fairly far—made it difficult to sit up straight and maintain proper typing posture, it proved comfortable for leaning back while reading or taking on other nontyping tasks.

The machine was also quiet. And while it would be hard not to notice that I was doing something under my desk, unless you find yourself panting for breath, the near-silent machine is unlikely to disturb office mates.

Still, users should come into this with realistic expectations.

The narrow range of motion feels more like repeatedly stepping on 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 currently doesn’t sync up with any other fitness trackers (the company says that feature is coming soon).

2. Deskcycle ($159)

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

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

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

At its highest, I was winded in minutes.

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

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

As with the Cubii, I achieved the best results when lowering my chair and extending the reach of my legs—though even that may not be enough to keep the tallest among us from banging our knees.

Thankfully, the Velcro-strapped bike pedals actually offer a fair amount of leeway with your leg angle, making it ideal for stretching out.

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

3. Stamina InMotion Compact Strider ($200)

The Stamina InMotion Compact Strider looks a lot like somebody shrank—and then ripped the top off—a gym elliptical machine.

Unfortunately, it was nearly impossible to use under a desk without banging my knees.

When I stepped away from my desk, however, I found it to be a pretty 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 small elliptical, if not office workers looking for an at-desk fitness solution—though without any handles to using it while standing requires—and will likely build up—a bit of balance.

Part of this design difference comes from the pedal position.

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

It likely takes a serious multitasking mind to use any of these machines while concentrating on other work, but most people could surely benefit from having one handy for quick midday activity bursts.

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

Copyright 2018 Bloomberg. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.