Fatigue is a common sense caused by crushing labor, stressful social events and various illnesses. It is usually judged by their subjective symptoms, but it should be evaluated in an objective perspective. Here we show that the decrease of working efficiency and sympathetic hyperactivity are associated with mental fatigue state caused by prolonged mental workload. Recently we made a new mental fatigue model of healthy volunteers caused by long-term computerized Kraepelin test (CKT) workload. CKT is our new software for automatically checking the calculation capability, with which it is easy to determine the reaction time (RT), coefficient of variance of reaction time (CV), and accuracy of the answers (AC) during tasks. We put 24 healthy volunteers into the fatigue state by subjecting them to 120 minutes’ CKT workload, and then studied the changes in fatigue sensation, RT, CV, and AC before and after the CKT workload. The fatigue sensation, RT, and CV were clearly increased by the fatigue-inducing task and recovered during the resting period. We also studied the changes in autonomic nerve activity by using heart rate variability analysis. The low/high frequency component ratio (LF/HF) was signifi-cantly increased by the fatigue-inducing task and decreased by resting, suggesting that mental stress causes a relatively sympathetic nerve activity-dominant state. Therefore, our new fatigue model involving a long-term CKT workload is a good mental fatigue model to provide much information about the fatigue state simultane-ously, and the increase of RT, CV, and proportion of sympathetic activity (LF/HF) are associated with mental fatigue state. These might be useful objective biomarkers or evaluating a mental fatigue state.
It is well known that stressful social events frequently become the trigger for acute mental fatigue and sometimes cause problems with mental health and chronic fatigue, occasionally resulting in death in the case of overwork [1-3]. In Japan, 60% of the general adult population complaints of fatigue and one third of the population suffers from chronic fatigue [
In 2009, the new study group for fatigue supported by the Japanese Ministry of Health, Labor, and Welfare (Head: Kuratsune H) was set up, and the sensitivity and the specificity of several makers of fatigue state including autonomic nerve activity have been studied. Recently we demonstrated that autonomic nervous alterations are associated with daily level of fatigue [
Our co-workers K. Yamaguti and T. Sasabe devised the new software named the Computerized Kraepelin Test (CKT) for conveniently checking the calculation capability. Using this software, it is easy to determine the reaction time (RT), coefficient of variance of reaction time (CV), and accuracy of the answers (AC) during this test. Since the long-term CKT workload causes mental fatigue state, there is a possibility that a mental fatigue model of healthy volunteers would be produced by using this workload. Furthermore, as mentioned above, the CKT itself is the software for checking certain kinds of brain functions. Therefore, it also is available for checking changes in brain function during the fatigue state caused by the long-term CKT workload. In this present study we evaluated the fatigue sensation, and brain function in terms of RT, CV, AC, and autonomic nerve function during, before and after the long-term CKT workload in healthy volunteers.
The subjects in this study were 24 students in Kansai University of Welfare Sciences (4 males and 20 females, 19 - 22 years of age). The study protocol complied with the Helsinki Declaration for Human Experimentation. The protocol was approved by the Ethics Committee of Kansai University of Welfare Science, and all subjects gave their written informed consent for the study.
We conducted a 2-day experiment. At the first and second morning without eating breakfast, all subjects assessed their subjective fatigue levels by using the visual analog scale (VAS) from 0 (minimum) to 100 (maximum). Their baseline autonomic nerve activities were determined by using heart rate variability analysis of the data from a 5-minute electrocardiogram (ECG) before the fatigue-inducing task. After this analysis, each subject performed a 2-hour fatigue-inducing task (see details in the following paragraph) without rest. After this task, they assessed their subjective fatigue levels by using VAS at 0, 20, and 60 minutes after the start of their rest period [
In this study, computerized Kaepelin test was used as a fatigue-inducing task. Two digits from 0 to 9 and one “+” symbol were displayed at the top of the laptop computer screen (
Since the response of most of the examinees during the first 5 minutes of this fatigue-inducing task was slow because of their inexperience, we evaluated the RT, CV, and AC at 6 - 10 minutes, 61 - 65 minutes, and 116 - 120 minutes after the beginning of the task as the change in response due to the fatigue state.
Furthermore, we also evaluated the RT, CV, and AC for 4 minutes after a 1st 20- and a 2nd 35-minute rest for evaluating the change in the response due to their recovery by resting (
ECG signals were collected on-line via 3 electrodes attached to both wrists. These ECG signals were collected at 1000 Hz and converted from analog to digital with a 12-bit resolution. R-wave peaks were detected by using an LRR-03 device (GMS Co., Tokyo, Japan). R-R wave variability was measured as an indicator of autonomic nerve activity. The system calculated instantaneously the R-R intervals as the difference between successive R-wave peaks.
Frequency-domain analyses of HRV were performed on a collected R-R interval time series of 5 consecutive minutes by using the maximum entropy method (MemCalc; GMS Co., Tokyo, Japan) [
Differences in variables between groups were assessed by using a multivariate analysis of variance (type II MANOVA), which was performed on the fatigue VAS score, LF/HF ratio, RT, and CV of CKT. R version 2.8.0 was used to perform all multivariate analyses of variance [
The fatigue VAS (fVAS) scores of the 24 students before and after the long-term (2-hour) CKT workload are shown in