unstiffeningn.
Etymology: Previous versions of the OED give the stress as: unˈstiffening.
the world > life > the body > loose or stiff condition > [noun] > action of producing
1832 S. T. Coleridge (1895) 761 A sort of unstiffening of my long dormant joints and muscles.
This entry has not yet been fully updated (first published 1926; most recently modified version published online June 2019).