Measuring Unintentional Force Drift: Reliability, Number of Trials, and Age-Related Differences

Unintentional force drift (UFD) describes the decline in force that occurs when visual feedback is removed despite the intention to maintain a target force. This study determined the reliability of UFD, the number of trials required for reliable assessment, and whether reliability differed across age groups. Twenty-four healthy adults completed six trials of simultaneous bilateral upper extremity force production task (isometric elbows flexion at 20% of Maximal Voluntary Contraction under visual and no visual feedback). Relative and absolute reliability of force and force drift were assessed using intraclass correlation coefficient (ICC), standard error of measurement and minimal detectable changes. Overall, measurement reliability improved as the number of trials increased. Force showed good reliability with visual feedback and excellent reliability without visual feedback. Force drift showed moderate reliability with visual feedback and good reliability without visual feedback (ICC = 0.84). Across all participants, five trials provided a UFD measurement with good reliability. UFD measurement properties differed across age groups, with older adults showing lower ICC values, greater measurement error, and requiring more trials than younger adults. In conclusion, five trials are recommended for UFD assessment, although more trials may be required in older adults.