The initial force is the force needed to start the load moving, and the sustained force is the force required to keep the load moving.

The value of both forces is determined by using a dynamometer. For example, if the task involves pulling a cart, the dynamometer is attached to the handle of the cart and the force exerted to start moving the cart (initial force) is measured; once in motion, the force exerted to keep on pulling it (sustained force) is measured.