stood up upon his feet. Note David's three attitudes: lying on the earth as a penitent (2Sa_12:16. Compare Psa 51); sitting before Jehovah as a worshipper (2Sa_7:18. 2Sa_17:16); and standing on his feet as a servant (28, 2).
the ark. See note on 1Ch_13:3 and Exo_25:22.
and = even.
the footstool of our God = the Ark. Figure of speech Anthropopatheia.App-6.