Definition of PROSTRATE
1: stretched out with face on the ground in adoration or submission; also: lying flat
2: completely overcome and lacking vitality, will, or power to rise
Most massages are performed while lying flat (definition 1), but I have had a few where I was suffering from #2 and unable to rise after we were done.
Now, if you are looking for prostate massage, well softplaceto land comes to mind.