Can I put the Sperax under a desk?

Partly. The rail adds front depth even folded, so most desks will not clear it the way they clear a bare pad.

Is the Strollmate stable enough without a rail?

For most walkers at 2–3 mph, yes. If balance is a concern at all, choose the rail.

The Sperax if you feel unsteady, the Strollmate if your only goal is more steps during the workday.

265 lb stated for the Strollmate against 300 lb for the Sperax. Treat stated figures as manufacturer claims and leave margin.

Head-to-head

Nothing about these two is alike except that they both move a belt. The Strollmate is a thin, cheap, desk-shaped pad you leave in place.

The Sperax is a compact treadmill with a folding rail, a 300 lb stated capacity and a 7.6 mph top speed — a machine you stand in front of, not under.

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Manufacturer-stated figures. Confirm on the Amazon listing before you buy — brands revise these between production runs.

Ultra-thin budget pick that disappears under a desk

Cheapest way to start walking at your desk, and the easiest to hide.

Riser-bar model that doubles as a light jogging treadmill

Support, capacity and speed range — the choice if walking unsupported is not comfortable for you.

Can I put the Sperax under a desk?: Partly. The rail adds front depth even folded, so most desks will not clear it the way they clear a bare pad.
Is the Strollmate stable enough without a rail?: For most walkers at 2–3 mph, yes. If balance is a concern at all, choose the rail.
Which is better for a beginner?: The Sperax if you feel unsteady, the Strollmate if your only goal is more steps during the workday.
What is the real capacity difference?: 265 lb stated for the Strollmate against 300 lb for the Sperax. Treat stated figures as manufacturer claims and leave margin.