Effects of mental fatigue on static upright stance and functional balance in older adults

Background: Cognitive influences on balance control may contribute to greater instability and falls in older adults. In support of this, old age exacerbates the effects of a concurrent cognitive task on balance. Mental fatigue is a psychobiological state experienced following prolonged demanding cognitive activity. However, its effects on static upright stance and functional balance in older adults is unclear. It is also unclear how the effects of mental fatigue and a concurrent task interact. Methods: Balance was assessed in ten younger and ten older participants before and immediately after 25 min of the incongruent Stroop colour-word test (mental fatigue) and leisurely reading (control), presented in a randomized counterbalanced order on separate days. Static and functional balance was assessed using the centre of pressure path length when standing still and the 'Timed Up & Go' test, respectively. These balance assessments were completed with and without a concurrent backward counting task (i.e. dual-task and single-task, respectively). Results: Under subjectively-confirmed conditions of mental fatigue, sway path length when standing still was 32% greater than the control condition in older adults but unchanged in younger adults. This age-dependant effect of mental fatigue on static balance was similar in single-task and dual-task trials. Mental fatigue did not significantly affect functional balance performance in either age group. Conclusions: These findings are the first to show mental fatigue to impair static balance control in older adults. Therefore, whether due to everyday activities or a symptom of disease, mental fatigue may contribute to poor balance in older adults.