Simple SummaryThe purpose of this research was to understand how upper-limb kinetics behaves alongside activity time during a position-sustained isometric task and how upper-limb kinetics relates to performance fatigability. As hypothesized, there were changes in acceleration behavior indicative of the displacement of the upper limb in the direction of shoulder extension, especially in the second half of the task, and also an increasing variation of acceleration, and, thus, in movement variability, alongside activity time. However, these changes showed different behaviors between men and women, suggesting greater performance fatigability in women. Results also showed that performance fatigability was positively related to average acceleration in an early phase of activity only in men, meaning that early movement adjustments were apparently sufficient to increase activity time and, consequently, performance. According to the results of our study, upper-limb acceleration measured through a single IMU can be a useful and easy strategy to identify fatigue early.AbstractUpper-limb position-sustained tasks (ULPSIT) are involved in several activities of daily living and are associated with high metabolic and ventilatory demand and fatigue. In older people, this can be critical to the performance of daily living activities, even in the absence of a disability. Objectives: To understand the ULPSIT effects on upper-limb (UL) kinetics and performance fatigability in the elderly. Methods: Thirty-one (31) elderly participants (72.61 ± 5.23 years) performed an ULPSIT. The UL average acceleration (AA) and performance fatigability were measured using an inertial measurement unit (IMU) and time-to-task failure (TTF). Results: The findings showed significant changes in AA in the X- and Z-axes (p< 0.05). AA differences in women started earlier in the baseline cutoff in the X-axis, and in men, started earlier between cutoffs in the Z-axis. TTF was positively related to AA in men until 60% TTF. Conclusions: ULPSIT produced changes in AA behavior, indicative of movement of the UL in the sagittal plane. AA behavior is sex related and suggests higher performance fatigability in women. Performance fatigability was positively related to AA only in men, where movement adjustments occurred in an early phase, though with increased activity time.