Working in is a concept developed by Paul Chek, an internationally-renowned expert in the fields of high-performance exercise kinesiology and stress management. The idea behind working in, in contrast to working out, is that energy is being returned to the body instead of being expended. When many people begin an exercise program, they are already operating from a place of energy deficit. This means that people are already tired or not appropriately nourished and so a program that is only focused on working out will drain the body of its energy causing burnout and unnecessary pain.

A fitness program which incorporates working in emphasizes exercising in a way that is therapeutic to reinvigorate the trainee, and exercises that are typically done in workouts can be done in different ways to work in, thus improving the outcome of them. This blog entry from the CHEK institute provides more information of the concept of working in.