Seating and wheeled mobility (SWM) equipment is tested according to specific standards governed at an international level by the International Standards Organization (ISO). At a national level in the United States, standards are governed by a collaboration between the American National Standards Institute (ANSI) and the Rehabilitation Engineering and Assistive Technology Society of North America (RESNA) and are referred to as the ANSI/RESNA standards. Each ISO member/country has a similar national body which may adopt their own national standard, based on the ISO standard, with or without changes (Koch, et al., 2025). 

Standards are used to disclose product information in a uniform way to help compare specific product features. Standards often describe testing methods for performance relative to a specific product feature. These standards only become mandatory when regulatory agencies write them into legislation. Many manufacturers voluntarily use some of the product testing (Koch, et al., 2025). 

What does this have to do with Dynamic Seating?

Some standards define testing methods for specific areas of the wheelchair frame and postural support devices (PSDs) which tend to break when a client exhibits a high strength extensor pattern. These tests often include the maximum force a component must tolerate before product failure. 

Let’s start by reviewing a few relevant standards

ISO 16840-3 Wheelchair seating – Part 3: Determination of static, impact and repetitive load strengths for postural support devices

This standard includes testing methods to determine the strength and durability of the seating device. Three types of tests are used to evaluate different types of performance. The purpose of static tests is to evaluate the ability of the specific seating system component to withstand stresses caused by the user exerting a force on the component. Impact tests evaluate the ability to withstand rapid dynamic forces that exceed the static force magnitude. Repetitive load test evaluates component’s ability to withstand repeated fatigue stresses and wear. 

ISO 16840-3 tests the back cushion supporting structure (the system upon which the back support is mounted, including the back canes), lateral supports, and the head support – areas often damaged by significant client forces. 

ISO back support test methods consist of static, impact, and repetitive load testing. These tests do not compare directly to specific maximum force against the back support and so are difficult to compare to research results. A 100N/22.5 lbf force is applied at the top midline of the back support during static strength testing. Impact testing uses a 25 kg / 55 lb pendulum released at specific angles to the back support. Repetitive load testing uses a force of 100 kg / 220 lbf for 1000 cycles. 

ISO 16840-3 requires a head support and mounting hardware to sustain up to 91 N / lbf in the fatigue loading testing method. Head support tests include repetitive and static.

ISO 7176-8 Wheelchairs — Part 8: Requirements and test methods for static, impact and fatigue strengths

This standard includes testing methods to determine the strength and durability of a wheelchair, including footrests and back supports. The testing method for back supports has been replaced with testing methods under ISO 16840-3 above.

Per ISO 7176-8, the maximum downward force a wheelchair footrest must withstand is 1000 N / 225 lbf (maximum client weight 100 kg / 220 lbs).

How does this translate into the real world?

Researchers have attempted to estimate the forces generated during high tone events by measuring forces against specific components. Note that peak forces on the back support and footrests can exceed body weight. This is because the person can push against multiple surfaces. Consider performing a leg press. You can probably press more than your body weight because as you press against the footplate, you are also pressing against the back support. Without a back support, you could not press nearly as much before moving posteriorly on the seat. 

Back Support Force

Hong, et al. (2006) found participants could exert up to 180 lbf against the back support. Samaneein, et al. (2013) found participants with extension exerted, on average, 60-70% of their body weight against the back support. Peak extension forces of up to 200% of client body weight against the back support were noted. Assuming a client weights 100 lbs, that client may exert up to 200 lbs of force against the back support. A client weighing 200 lbs may exert up to 400 lbs of force. While difficult to correlate with ISO standards, this degree of force appears to often exceed the forces used in the testing methods.

Footrest Force

Samaneein, et al. also found clients with extension exerted, on average, 20% of their body weight against each footplate. Peak extension forces were up to 600% of client body weight against each footplate. Assuming a client weighs 100 lbs, the client may exert up to 600 lbs. of force which is 167% higher than the 225 lbf maximum downward force of this testing method. 

Head Support Force

Aveyard, et al. (2025) found that the study participant (an individual with high-strength extensor patterns) exerted 220 N (49 lbf) against the head support and mounting hardware, over twice as much (145% higher) as the 91 N / 20 lbf testing method maximum. The researchers also found that the participant exceeded the sensor threshold three times, exerting an even higher level of force. 

Aveyard, et al. also found cracking in the back support shell (where the head support mounts to the back shell) at 8,000 repetitions of force against the head support. The testing method requires 17,500 repetitions without equipment failure. 

Summary of ISO Standard Maximums and Research Findings

Wheelchair Component	ISO Standard	Research Findings
Wheelchair Back Support	Static strength 100 N / 22.5 lbf for no less than 5 seconds, top midline of back Impact test uses 25 kg / 55 lb pendulum released at different angles to back Repetitive load 100 kg / 220 lbf for 1000 cycles	60-70% body weight, up to 200% body weight (Samaneein, et al., 2013) 100 lb client = 200 lbf  150 lb client = 300 lbf  200 lb client = 400 lbf  Up to 800 N / 180 lbf force against back support (Hong, et al., 2006)
Wheelchair Footrest	1000 N / 225 lbf maximum downward force	20% body weight, up to 600% body weight (Samaneein, et al., 2013) 100 lb client = 600 lbf / 167% higher than standard 150 lb client = 900 lbf / 300% higher than standard 200 l. client = 1200 lbf / 433% higher than standard
Wheelchair Head Support Maximum force	91 N / 20 lbf force against head support pad	220 N / 49 lbf (Aveyard, et al., 2025)  145% higher than standard
Wheelchair Head Support Maximum repetitions	17,500 repetitions	8,000 repetitions before failure (back support shell cracking), 46% of threshold

Conclusion

Test methods attempt to balance typical and extreme loading situations. Evaluating seating components under typical loading conditions seeks to ensure durability and an adequate service life. In distinction, testing to extreme loads focuses on safety during occasional and/or unexpected forces. Products should not fail catastrophically under either of these conditions. People who exhibit strong extensor tone, whole body extensor spasms, and/or large and forceful movements have been shown through research, as well as clinical experience, to often exceed typical loading thresholds. Dynamic seating can be used for clients who exceed the design specifications of their wheelchair seating and mobility equipment. A clear indicator that a client exceeds these specifications is equipment breakage. 

References:

1. Aveyard, B., Dauncey, W., Redwood-Thomas, J., Eggbeer, D., Penman, R., & Tasker, L. (2025). Bridging the gap: from standards to reality in wheelchair headrest-backrest design: a case study. Disability and rehabilitation. Assistive technology, 1–10. Advance online publication. https://doi.org/10.1080/17483107.2025.2532704
2. Hong, S. W., Patrangenaru, V., Singhose, W., & Sprigle, S. (2006). Identification of human-generated forces on wheelchairs during total-body extensor thrusts. Clinical Biomechanics, 21(8), 790-798. https://www.sciencedirect.com/science/article/pii/S0268003306000751 
3. Koch, K., Kopplin, K., & Perr, A. (2025). What are Standards for Wheelchairs and Wheelchair Seating? In M. Lange & J. Minkel (Eds.), Seating and Wheeled Mobility: a Clinical Resource Guide (2nd ed., pp. 615-633). Routledge. https://doi.org/10.4324/9781003526377.
4. Samaneein, K., Greene, P., Lees, K., and Riches, P. (2013). Comparison of Imparted Forces between Rigid and Dynamic Seating Systems during Activities of Daily Living by Children with Cerebral Palsy. Congress of the International Society of Biomechanics, Brazil.