The Collaborative International Dictionary of English v.0.48:

Inertion \In*er"tion\, n.
   Lack of activity or exertion; inertness; quietude. [R.]
   [1913 Webster]

         These vicissitudes of exertion and inertion of the
         arterial system constitute the paroxysms of remittent
         fever.                                   --E. Darwin.
   [1913 Webster]