There are three primary analysis and design methods that ergonomists might use to analyze manual material handling work: psychophysics, biomechanics, and physiology.
Biomechanical methods use posture, gender, anthropometry, and forces to calculate resultant muscle force requirements and bone and joint compression forces. The calculated values are then compared to accepted limits for working populations. Biomechanical analysis methods are useful when analyzing high exertion tasks, but often do not consider the effects of the dynamics, repetition, or duration of the task or job.
In a process called metabolism, the human body converts food and oxygen to chemicals necessary to power muscles. As physical activity increases, so does the demand for the chemicals that power muscles. The body responds by increasing the heart and breathing rates, which provides more oxygen and delivers the necessary chemicals to the muscles faster (the blood stream also “cleans” the muscle tissue by carrying away chemical byproducts). When the energy expenditure rates exceed the body’s energy producing rates, physical fatigue occurs, and a cardiovascular accident, such as a heart attack, can result. Muscle fatigue compromises work accuracy, productivity, and safety.
Though there is substantial theory underlying psychophysics, in simple terms, it is a research method that takes human perceptions into account. Liberty Mutual successfully applied the method to lifting/lowering tasks, carrying tasks, and pushing/pulling tasks.
For more information about manual material handling analysis techniques, see Ergoweb’s Applied Workplace Ergonomics Manual.