Together with Muda and Mura, Muri is a "non value added" activity which must be eliminated.
The meaning of Muri is reserved to the persons or to the machines that accomplish some type of working activity.
When it is referred to a persons it point at an activity that is difficult to accomplish because of the position assumed by the body or dangerous for the body health or that can generate sickness in the person if accomplished in a repetitive way.
When it is reffered to a machine or a tool it point at an activity carried out (by the mean) with difficulty because of overloading, strenous and intensive utilization, no maintenance or overiding utilization specs and guidelines.