Can you have wastage or weakness for months, on and off, in one leg due to using a sit on lawn mower. What's the best treatment?

It depends how much vibration the machine is sending through you and how long you are using it for each time.

If it does rattle somewhat, the vibrations will be likely to be sent through the seat and up your spine which could well explain the effect upon your lower limbs. Can you pad the seat to avoid this?

To help any tissue damage caused, I'd consider some Arnica Gel and perhaps even some Ginkgo Biloba to aid circulation to the area.

Just be aware that Ginkgo won't help straight away but since the weather has turned colder and you won't be using the mower as often, this could be a good time to start.

Best wishes

Earle