Sri Aurobindo
Letters of Sri Aurobindo
Volume II - Part 2
If you mean by failure the weakness of the body, it is due probably to your having unduly strained it in obedience to rajasic vital impulses, an effect which was increased by vital relapses 
into tamas and the struggle of the vital attacks you had. But also it often happens even in the ordinary course of Yoga that physical strength is replaced by a Yogic strength or Yogic life force which keeps up the body and makes it work, but in the absence of this force the body is denuded of power, inert and tamasic. This can only be remedied by the whole being opening to Yoga shakti in each of its planes – Yogic mind force, Yogic life force, Yogic body force.