Biomechanics is the study of the mechanics of human activity and how muscular activity leads to internal loading of bones, ligaments, joints and other soft tissues in the body. Internal loading can be thought of as cumulative trauma that results from repetitive stress. Along with internal loading, muscles, bones and ligaments are subject to the concept of force where the amount of physical effort required in performing a task such as heavy lifting or maintaining control of equipment, tools or patience.
Body mechanics is a subset of biomechanics. It is based on a belief that reliance on correct body position or movements will provide protection from the force associated with lifting and moving patients. Body mechanics alone do not provide sufficient protection from the heavy weights, awkward positions and repetitive motions involved in manually handling patients.
Ergonomics is the science of fitting the job to the worker with an emphasis on reducing the problems associated with muscle and joint disorder.