Investigating the Applicability of User Models for Motion-Impaired Users
Simeon Keates, John Clarkson, Peter Robinson · 2000 · Proceedings of the Fourth International ACM Conference on Assistive Technologies (Assets '00) · doi:10.1145/354324.354354
Summary
This paper from the University of Cambridge investigates whether the Model Human Processor (MHP) — a foundational user model from Card, Moran, and Newell's "The Psychology of Human-Computer Interaction" — accurately describes how motion-impaired users interact with computers. The MHP decomposes human-computer interaction into three sequential stages: a perceptual cycle (perceiving a stimulus), a cognitive cycle (deciding what to do), and a motor function cycle (executing the physical action). The theory assumes certain actions, particularly key releases and repetitive button presses, are automatic and require no additional cognitive processing. The researchers tested this with two sets of user trials: first at the Papworth Trust with users who had conditions including cerebral palsy, muscular dystrophy, Friedrich's ataxia, and tetraplegia; and then a second more detailed set with four users (two mildly and two more severely impaired). Tasks included measuring perceptual response times, cognitive cycle times, and motor function times through button activation exercises using mice, keyboards, trackpads, and an EasyBall device.
Key findings
The individual MHP component values (perceptual cycle ~100ms, cognitive cycle ~90-120ms) were broadly comparable between able-bodied and motion-impaired users. However, the motor function time was approximately 50% slower for motion-impaired users (mean 300ms vs. 70ms for able-bodied). The critical finding was that when these components were combined into a complete interaction sequence, the MHP model broke down for motion-impaired users. While able-bodied users'response times matched the theoretical predictions, motion-impaired users consistently showed response times approximately 200ms longer than predicted — equivalent to one or two additional cognitive cycles. Detailed analysis of button-down and button-up times revealed that actions the MHP assumes to be automatic (like releasing a key) actually require explicit cognitive planning for motion-impaired users. For the most severely impaired user (P17, with continuous tremor and clenched hands), the interaction process included extra cognitive cycles for both pressing and releasing buttons, with the user appearing to be in a perpetual "learning mode" where each movement required conscious deliberation.
Relevance
This research exposed a fundamental flaw in applying standard HCI user models to people with motor disabilities: the assumption that physical interactions become automatic does not hold when motor control requires conscious cognitive effort. The practical implication is significant — interface designers and usability practitioners cannot simply take able-bodied interaction models and "add a bit" of extra time for disabled users. Instead, they must measure actual interaction patterns, which may involve qualitatively different processes, not just slower ones. The findings argue for supportive interface design that minimises the need for additional cognitive cycles — for example, providing visual feedback to confirm button presses, reducing the precision required for input actions, and offering positive reinforcement. This work contributed to the broader inclusive design methodology developed at Cambridge and remains relevant to anyone designing adaptive interfaces or setting timing parameters for motor-impaired users.
Tags: motor impairment · user modeling · universal access · human-computer interaction · cerebral palsy · input devices · usability testing · inclusive design