Originally (in experimental studies of muscle contraction): a load added to a strip of muscle after contraction has begun. In later use also: the force that a ventricle of the heart must generate in order to eject blood (determined largely by vascular resistance).
Mid 19th century (in an earlier sense). From after- + load.
To subject (a muscle) to an afterload.
In radiotherapy: to load (a previously implanted receptacle) with a radioactive substance; to insert (a radioactive substance) into such a receptacle.
Late 19th century; earliest use found in Brain: a journal of neurology. Probably partly from afterload, and partly from after- + load.