My under-desk pedal machine has a motor that moves my legs for me — is that exercise at all?

It is not exercise, and it may still be doing something worthwhile. Those are two separate questions and it is worth keeping them apart.

Not exercise, because exercise requires your muscles to generate force. If the motor is doing the work and you feel no effort, your muscles are being moved rather than working. Nothing adapts — no strength, no cardiovascular effect, and essentially no energy expenditure beyond sitting still.

Possibly useful anyway, because the well-documented problems with prolonged sitting are not only about a lack of exercise. Sitting motionless for hours reduces circulation in the legs, and passive movement does help with that. Devices of this type are commonly aimed at exactly that, and passive motion has a real role in some clinical settings.

So: a circulation aid rather than a training device. Which is a reasonable thing to own — it is just not what the marketing implied you were buying.

One small correction to "no energy expenditure": there is a little, because holding your legs in contact with moving pedals involves some low-level muscular activity, and you are not perfectly passive.

It is a rounding error compared with actual activity, though. If the question is whether it contributes to a calorie balance, treat the answer as no.

If what you wanted was actual activity during the working day, the thing to look for is a device with resistance rather than a motor.

An under-desk pedal unit with adjustable resistance and no motor is a different product and is what most people picture when they hear the description. You do the work, you can dial the difficulty, and at a low setting you can pedal for hours while typing without breaking concentration.

The honest caveat is that even those produce fairly modest effects at desk-compatible intensities. You cannot work hard enough to train while also doing your job, and anyone who tells you otherwise is selling something. What they do provide is a meaningful increase in daily energy expenditure and a lot less time spent completely still, both of which are worth having.

A standing desk with a walking pad is the other version of the same idea and gets a lot more movement in, at the cost of being harder to type on.

The intervention with the best evidence behind it, and the one nobody wants to hear because it is not a purchase: get up regularly.

A couple of minutes of standing and walking every half hour addresses most of what continuous sitting does, and it costs nothing. The various "movement" gadgets are largely attempts to avoid needing to do that, and they mostly do not replace it.

Setting a recurring reminder and actually standing up is unglamorous and it beats every device I have bought, and I have bought several.