Quantifying Demand, Capacity, and Risk in Physical Ergonomics

December 7, 2020

Demand, capacity, and risk play a huge role in our understanding of what is acceptable and not acceptable to prevent illness and injury in the field of physical ergonomics.

Demand can be understood in terms of demand put on a muscle, and capacity is the ability of the muscle to meet that demand. These two elements must be in balance and exist on a continuum. When capacity is greater than demand, there is very little risk, such as holding a feather. On the other end of the spectrum, when demand is greater than capacity, there is a much higher risk, such as lifting a box that is too heavy.

It is tempting to turn this continuum into something that is clearly defined - acceptable or unacceptable. But what is an acceptable risk?

In ergonomics, Threshold Limit Value, or TLV, helps to define acceptable loads and risk. TLV can be understood as the level to which workers can regularly be exposed over a lifetime without experiencing adverse effects. Ergonomic TLVs are typically based on the capacity of female workers because on average, female muscles have lower strengths and endurances for the same absolute loads. Acceptable loads also depend on whether a task is repetitive, or whether it is done once.

For example, a motor vehicle accident may happen once in your life. With a repetition of once in a lifetime, you can have a very high force that is considered acceptable. When the force exceeds what is considered acceptable, you would expect an acute injury.

In the health and safety realm, acute injuries are often seen in accidents, like people falling off of ladders. However, repetitive stress injuries (RSIs), also known as repetitive motion injuries, are one of the fastest growing occupational injuries.

Repetitive tasks decrease the acceptable threshold of demand. For example, if someone were to carry a slightly heavy box in an awkward position 20 times a day, the repetition of this task would greatly increase their risk of injury, becoming an unacceptable load. Jobs that require sitting or standing in the same position for long periods of time, or repeatedly performing the same manual task, put workers at risk of developing repetitive stress injuries.

Carpal tunnel syndrome is a well-known RSI, but these types of injuries impact more than office workers. Other factors that contribute to injury include equipment, posture, force, adequate break time, and weight distribution.

Want to learn more? Our course, Physical Human Factors and Ergonomics, with Jim Potvin, PhD, CCPE will teach you how to identify occupational tasks that can contribute to musculoskeletal injury and fatigue, and how to quantify the acceptability of risk.