The Nobel prize Camilo José Cela once said: "No es lo mismo estar dormido que estar durmiendo, como no es lo mismo estar jodido que estar jodiendo.".
The anecdote surrounding this funny quote illustrates well how the usage of gerund ("dormido", "jodido") and past participle ("durmiendo", "dormido") don't always carry the same meaning.
Apparently Cela, as senator at the Spanish upper house, was reprimanded for falling asleep:
Senate President: "Sr Cela, you're asleep!" (asleep = dormido)
Cela: "Not Sir, I'm not asleep; I'm only sleeping" (sleeping = durmiendo)
Senate President: "Isn't it the same?
Cela: "Of course, it isn't the same! Because it isn't the same to fall asleep than to be sleeping; as it isn't the same to be fucked than to be fucking" (fucked = jodido, fucking = jodiendo)
The ~ing form and the past participle of the verb "to lie" are "lying" and "lain". Note that "to lie" doesn't translate into Spanish as the reflexive verb "echarse", but as "estar echado". This is the reason why:
I see a man lying on the ground.
is translated as:
Veo un hombre echado en el suelo.
To understand why "to lie on the ground" translates as "estar echado en el suelo" and not "echarse en el suelo", one needs to understand the difference between a punctual and a durative verb.
"Echarse en el suelo" is a punctual verb, close in meaning to "get on the floor", because it describes an action that takes place at a particular point in time.
"Estar echado en el suelo" and "lie on the ground" are a durative verbs, because they describe an action that lasts a period of time.
To be clear, despite Camilo José Cela's claim and as stated by Emilio Gort in the comments, most Spanish speakers consider that "estar dormido" and "estar durmiendo" are virtually identical in meaning.