## 4 Discussion  
To our knowledge, this is the first study to identify inter-individual responses to the performance of mental tasks using a sitting and standing desk, as well as when sitting is interrupted by intermittent walking. Further, this study is the first to use machine learning to predict changes in various cognitive and mood outcomes using trait mental and physical energy and fatigue. The current findings add significantly to the literature, as they may help explain some of the inconsistent results that have previously been reported for both standing desks [14] and studies using similar mental task protocols to understand various nutritional interventions [10; 11; 24; 25; 45; 46; 69]. Our findings support the use of the low-cost and time efficient survey to measure trait mental and physical energy and fatigue in identifying inter-individual responses to various interventions. The need to measure trait mental and physical energy and fatigue is best illustrated when evaluating the interesting results for changes in POMS Fatigue. Our study finds that regardless of intervention, there is a small but significant increase in feelings of fatigue over the course of the 1.5 h of the performance of mental tasks. However, when the data are examined more closely, individuals who are low trait mental fatigue reported a significant increase in feelings of fatigue with the performance of mental tasks, and when examining the data between interventions, the biggest increase in feelings of fatigue for low trait mental fatigue individuals occur during the standing and walking interventions, with small but not significant increases in fatigue in the sitting condition. Interestingly, there were no significant differences in feelings of fatigue between interventions. This suggests that if we had not differentiated participants by trait moods, we might have incorrectly stated that performing cognitive tasks increases feelings of fatigue, but there are no significant differences between interventions. By splitting the groups based on their traits, we were able to identify that low-trait mental fatigue individuals report significant increases in feelings of fatigue in the standing and walking interventions, but individuals who are high trait mental fatigue report no changes in feelings of fatigue. The null findings by [PERSON] and colleagues [70], who report a change in moods for their participants, but report no significant differences between sitting or standing desk interventions, may warrant further consideration. Specifically, perhaps the findings of [PERSON] and colleagues [70] might have changed if their methodology had accounted for trait mental and physical fatigue. The current results are in line with previous studies reporting an increase in POMS confusion when performing cognitive tasks while seated [11] and significant improvement in POMS confusion scores when performing cognitive tasks while standing [71]. However, the findings of [PERSON] and colleagues [71] were based on the chronic use of standing desks across several weeks, while the present findings suggest that standing desk use may reduce feelings of confusion acutely. Our findings suggest that use of standing desks in a classroom setting may provide an easy to implement method of reducing feelings of confusion felt by participants performing 1.5 h of cognitive tasks. We also report that while there was a decline in feelings of vigor during all 3 interventions, with the standing intervention resulting in the largest decline in feelings of vigor compared to the sitting and walking interventions. Interestingly, when examining mental and physical energy separately, we find that performing mental tasks while standing increase mental energy; however, feelings of physical energy declined. This nuance may help explain the findings of previous work examining standing desks and moods that supports the chronic use of standing desks to increases feelings of vigor [14].