Temporal associations between physical activity, mental activity and fatigue dimensions in knee osteoarthritis : an exploratory intensive longitudinal study

Background: Fatigue may include both physical and mental dimensions. Evidence suggests that physical and mental activity may influence fatigue in knee osteoarthritis (OA). However, how physical and mental activities relate to fatigue dimensions in knee OA is unclear. Purpose: This study estimated intra-day contributions of physical and mental fatigue to general fatigue and evaluated temporal associations between physical activity, mental activity and fatigue dimensions in knee OA. Methods: An intensive longitudinal study combined with ecological momentary assessment of mental activity intensity and fatigue dimensions was conducted on 23 participants with knee OA. Physical activity was monitored continuously with an accelerometer over 7 days. Results: Physical fatigue contributed 33% more to general fatigue earlier in the day than mental fatigue, and 11% more near the end of the day. Within-day, previous general fatigue significantly and negatively predicted: future step counts, light intensity physical activity time, and light intensity physical activity + standing time. We found a significant bidirectional association between mental activity and general fatigue, a positive association between mental activity and mental fatigue and a significant negative association between mental fatigue and mental activity. Conclusion: Within-day general fatigue may be a significant fatigue dimension that reduces physical activity. Conversely there was no evidence that physical activity might contribute to lower scores on any fatigue dimensions in this population. To manage general fatigue, physical and mental fatigue might have to be targeted more precisely at different time of the day