For example, screw-off lids for jars should not require so much turning force that elderly people cannot remove them. Strength Strength is defined as the greatest force that can be exerted by a muscle instantaneously. There are large differences in strength both between and within individuals. For instance, more force can often be exerted by the dominant hand. Strength is measured isometrically, i.e., by the greatest among of force a muscle can exert without shortening its length Unfortunately, Americans, including Chapanis, are the last people to still use non-metric units.
Units used are ounces, pounds, horsepower, etc.
Human factors engineers in other countries should use the metric equivalents of these quantities. This is the amount of external physical work an individual can perform. It is measured in units of energy such as ft.-lb. To get an idea of how much skill is actually required in activities such as walking, you should watch a baby learning how to walk. This is measured when force is applied to a control, but the control does not move, or moves negligibly, in relation to the operator's body.