Exertion

Exertion is a concept describing the use of physical or perceived energy. It normally connotates a strenuous or costly effort related to physical, philosophical actions and work.

Concept
In psychological terms exertion is a perceived measure of expenditure or cost. Conceptually it is also used to describe the use of power (sociological or mental) - like in (violent) force or will (philosophy), but also in economics as in cost. Philosophically it is used as a concept or percept for something different than rest - i.e activity, indicating some sort of decision making or motivation - like instinctal drive.

Physical
In physics exertion is use of energy against, or for, inertia as described by Isaac Newton's First Law of Motion. In mechanics it describes use of force against a body in direction of its motion (see vector).

Medical
In medical terms exertion is the expenditure of energy by skeletal muscles. The intensity of this action can be measured by the rate of which oxygen is expended, heat is produced and heart rate. A frequently used term is rating of perceived exertion or RPE-scale which is use of a scale to indicate a quantitive feeling of fatigue.

Cybernetics
Exertion can be proxied or extended when applied to various human-machine interfaces, like prosthesis, remote control - even for games.