What is "vital formations"?

In [this](https://buddhism.stackexchange.com/a/8282/382) answer by @yuttadhammo, he quotes a section of the Visuddhimagga: > After reciting the Pātimokkha, it seems, on the Uposatha day of the full moon, one of the two elders who were brothers went to his own dwelling place surrounded by the Community of Bhikkhus. As he stood on the walk looking at the moonlight he calculated his own vital formations, and he said to the Community of Bhikkhus, “In what way have you seen bhikkhus attaining Nibbāna up till now?” Some answered, “Till now we have seen them attain Nibbāna sitting in their seats.” Others answered, “We have seen them sitting cross-legged in the air.” The elder said, “I shall now show you one attaining Nibbāna while walking.” He then drew a line on the walk, saying, “I shall go from this end of the walk to the other end and return; when I reach this line I shall attain Nibbāna.” So saying, he stepped on to the walk and went to the far end. On his return he attained Nibbāna in the same moment in which he stepped on the line. >-- Vism. VIII.244 (Nyanamoli, trans) I'm echoing @Parag's inquiry, from the comments, here: what is meant by "vital formations" above?