Halakhah su Levitico 15:9

The laws of the commandment [includes] that which they, may their memory be blessed, said that the zav imparts impurity to the [place of] sitting, laying and riding in five ways, and these are them: standing, sitting, laying, hanging from and leaning. And the understanding of riding is that it is that board which is a vessel that is made for riding (to hold on to it) that is called artson (Rashi on Leviticus 15:9). As we cannot say that we sit upon it - as if so, it would be [the same as] sitting - and this is an obvious thing. And the rest of its many details are elucidated in the tractate that is built upon it, and that is Tractate Zavim (see Mishneh Torah, Laws of Those Who Defile Bed or Seat 7).