Widely used in the

human factors and ergonomics Human factors and ergonomics (commonly referred to as human factors) is the application of psychological and physiological principles to the engineering and design of products, processes, and systems. Four primary goals of human factors learnin ...

field, Rohmert's law states that the maximum

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

one's

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...

can exert decreases exponentially from the time one begins continuously exerting the force. It is commonly used to calculate "maximum holding time" for any particular task. Maximum force decays exponentially due to the amount of

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

(in the form of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

and ATP) the body is able to supply to the muscles. The

circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

s keeps muscles flooded in

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...

s at all times, so that muscles have a supply of fuel-ready to burn at any given moment. A task requiring maximum force burns a large amount of those nutrients at the onset of the task; the circulatory system is then unable to replenish the nutrients at a rate fast enough to maintain the maximum force for long. As a result, the maximum force the muscle is capable of producing is limited by the bottleneck in

availability, and decreases exponentially. Imagine a theoretical

arm wrestling Arm wrestling (also spelled armwrestling) is a sport with two opponents who face each other with their bent elbows placed on a table and hands firmly gripped, who then attempt to force the opponent's hand down to the table top ("pin" them). The s ...

match with two perfectly matched opponents, each exactly as strong as the other. They both begin the match by exerting maximum force on each other's hands, but very soon, their arms get fatigued and the actual force being exerted on each other's hands drops off quickly. They are still exerting as much force as they can, but their muscles are burning energy faster than can be replenished, and their maximum force is decreasing exponentially. Eventually, their arms are completely fatigued; they are basically just holding hands and applying what little force their muscles can muster, wondering when the other will give up. Rohmert's law suggests that, for example, static gripping loads be not more than 15% of the load required for a squeeze-operated tool.Dan MacLeod, ''The rules of work: a practical engineering guide to ergonomics'', CRC Press, 2000,
p. 106.
an
p. 152.
/ref> While Rohmert's law applies to ''maximum'' force, the inverse is true as well; the less force one is asked to exert, the longer one will be able to exert that force before their muscles become fatigued. If one is asked to exert zero force, they can theoretically hold the position indefinitely. Rohmert's law is true across all humans. While everyone has a different initial maximum force they can apply, their maximum force will decrease according to the same exponential curve as everyone else.

References

{{Muscle-stub Ergonomics Muscular system