Mechanical Work in Human Movement

Mechanical work in human movement is energy transferred when a force acts through a displacement. It describes a physical interaction, such as a person lifting an object, rather than how strenuous an activity feels. For a constant force, work depends on the distance moved and the component of force along that movement. It is measured in joules .¹

Work can be positive, negative or zero. A force acting in the direction of displacement does positive work on the object; a force opposing displacement does negative work. Lifting a load involves positive work by the lifting force, while controlling its descent involves negative work by the supporting force. Holding the same load stationary does no mechanical work on that load, because it does not move, even though holding it can be tiring .¹

Muscles can both produce and absorb mechanical energy. An active muscle that shortens performs positive work, whereas one that lengthens under load absorbs energy and performs negative work. These lengthening, or eccentric, contractions help control and brake movements. Negative work therefore does not mean that a muscle is inactive .²

Mechanical work must also be distinguished from metabolic energy expenditure, the energy the body uses to sustain activity. Eccentric contractions generally require less metabolic energy than shortening contractions for a given muscle force. Consequently, the direction and amount of mechanical work alone do not fully describe an activity's energy demands .²