Grip force and operating forces describe the force a user can exert and the force a product demands of that user. If the required force is above what the weakest intended users can exert, a use-related risk arises, regardless of how comprehensible the operation is otherwise.
Force is a requirement, not a property
Every actuation places a force requirement on the user: drawing up a syringe, opening a closure, flipping a lever, carrying a device. This requirement is a property of the product. Whether it can be met is decided by the person in front of it.
For design, this means: The benchmark is not the average user but the weakest intended user. If the design is based on the mean, the required force lies above what a considerable share of the intended users can comfortably exert.
How widely hand dimensions and forces vary
The range between users is larger than is usually assumed in the design. The anthropometric dataset ANSUR II (survey from 2012, 4,082 men and 1,986 women between 17 and 58 years of age) shows the following for the hand, calculated from the publicly available raw data:
- Hand length: 5th percentile female 165 mm, 95th percentile male 210 mm, range 45 mm
- Hand breadth: 72 mm to 96 mm, range 24 mm
- Hand circumference: 173 mm to 230 mm, range 57 mm
A grip designed for the middle of this distribution does not fit at either end. For an instrument held for hours, this decides fatigue and precision.
Limitation: ANSUR II is a sample of US military personnel, that is, young, selected, and physically fit. The figures show the order of magnitude of the variation, not the distribution among nurses or older users. For a specific design, the target group has to be surveyed separately.
Force varies widely as well. Representative reference values for Germany come from an analysis of the German Socio-Economic Panel (11,790 people between 17 and 90 years of age, measured with a hand dynamometer): The mean grip strength reaches about 54 kg in men between 30 and 49 years of age and about 34.5 kg in women between 35 and 44 years of age, and declines with age thereafter. The study classifies values below 33 kg for men and below 21 kg for women as weak. Anyone who also intends older or frail users therefore cannot orient themselves on the mean of healthy adults.
Relevance for medical devices
With medical devices, it is an aggravating factor that operation often takes place with gloves, under time pressure, or in an unfavorable posture. All three reduce the available force compared with the laboratory value.
For the use-related risk analysis, the force requirement is therefore worth its own consideration: It can create a hazardous situation without any comprehension problem being present.
Typical weaknesses in practice
- Design for the average. The actuation force is tried out in the development team, whose members do not represent the intended users.
- Force without a numerical value. The specification calls for easy operation but states no maximum actuation force against which it could be tested.
- Lab value without conditions of use. Measurement is done with a bare, dry hand in a comfortable posture. Gloves, time pressure, and an unfavorable posture are left out.
- Once instead of repeatedly. A force that is applied effortlessly once can lead to fatigue when repeated many times over a shift.
Regulatory reference
Body forces and body dimensions are standardized in their own right. In Germany, the series DIN 33411 "Physical strength of man" (German: Körperkräfte des Menschen) contains concepts and defining parameters (Part 1, edition 2023-12) as well as values of maximal static action forces (Part 5, edition 2023-03). DIN EN 1005-3 (edition 2009-01; European standard EN 1005-3) gives recommended force limits for the operation of machinery. Body dimensions as percentile values are provided by DIN 33402-2 (edition 2020-12).
For medical devices, the MDR (EU Medical Device Regulation) requires in Annex I that risks resulting from the ergonomic features of the device be reduced as far as possible. In the usability engineering process according to IEC 62366-1, the force requirement therefore belongs to the consideration of the user interface.
The benchmark for operating forces is the weakest intended user, not the average. Hand dimensions and forces vary more widely than is usually assumed in the design, and gloves, time pressure, or an unfavorable posture reduce the available force further. An excessive force requirement can create a hazardous situation without any comprehension problem being present.
Frequently asked questions (FAQ)
Which user should an operating force be designed for?
For the weakest intended user, not the average. A common approach is a design that also reaches the 5th percentile of the weakest user group. If the design is based on the mean, the required force lies above what a considerable share of users can comfortably exert.
How large is the difference between small and large hands?
According to ANSUR II, between the 5th percentile of women and the 95th percentile of men there are 45 millimeters of hand length, 24 millimeters of hand breadth, and 57 millimeters of hand circumference. A grip for the middle of this distribution does not fit at either end.
Do gloves reduce grip force?
Yes. Gloves reduce both the force that can be applied and the tactile feedback. Since medical devices are often operated with gloves, this condition belongs in the survey of the force requirement and in the usability evaluation.
What data on grip strength exist for Germany?
An analysis of the German Socio-Economic Panel with 11,790 people between 17 and 90 years of age provides reference values by sex, age, and body height. The mean reaches about 54 kg in men between 30 and 49 years of age and about 34.5 kg in women between 35 and 44 years of age, and declines with age thereafter.
Do you want to make the ergonomic quality of your product not just plausible but demonstrable? We combine measurement data, expert assessment, and medical interpretation into an objective ergonomics analysis.
More about our ergonomic analysisSources
- IEC 62366-1:2015+AMD1:2020, Medical devices, Part 1: Application of usability engineering to medical devices
- Regulation (EU) 2017/745 on medical devices (MDR)