Ask HN: Electric Massage Cushions?

I thought this may be of interest to us desk workers...

I've recently been trying a cheap unbranded massage cushion on my work chair. It has two rotating heads and seems quite good for keeping my middle back more mobile.

Alas the things aren't very durable. The rotating heads are driven by a plastic worm gear, which if you lean in hard - as I often do - soon wears out and starts skipping.

Has anyone found a similar product that can deliver a hard massage without breaking? If so which? Have you looked inside to check whether the gears are plastic or metal?