Widely used in the

human factors and ergonomics Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are great apes characterized by their hairlessness, bipedalism, and high intellig ...

field, Rohmert's law states that the maximum

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

one's

muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

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 Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

(in the form of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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

circulatory system In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart ...

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 excret ...

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 in which two participants, facing each other with their bent elbows placed on a flat surface (usually a table) and hands firmly gripped, each attempt to "pin" their opponent's hand by forcing ...

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