The influence of mental fatigue on facial EMG activity during a simulated workday

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Abstract

The present study investigated whether facial EMG measures are sensitive to the effects of fatigue. EMG activity of the corrugator and frontalis muscles was recorded during and after a simulated workday. Fatigue was evaluated in four ways: (a) the building up of fatigue effects during the workday, (b) the building up of fatigue during a test period, (c) examination of after-effects of the workday in two test sessions in the evening, and (d) comparison of subjects with a high-and low-score on an Emotional Exhaustion questionnaire. EMG activity decreased during the workday and increased again in the evening. EMG activity also increased during a test period, reflecting increased mobilization to maintain performance. High-score subjects showed a lower level of EMG activity throughout the entire workday. They reported a higher need for recovery, experienced the workday as more fatiguing, and were less well rested when getting up. EMG measures seem to reflect that high-score subjects have problems with investing sufficient energy to maintain performance during a workday.

Introduction

Fatigue is one of the most frequently reported complaints among work-related problems leading to absence and work incapacity (Foets and Sixma, 1991, Schaufeli and Houtman, 2000, Wessely, 2001). In most current theories fatigue is considered as a bio-behavioral state an individual is in, which is induced by enduring task performance; it may refer to the state during as well as to the state after that performance (Cameron, 1973, Craig and Cooper, 1992). Fatigue is a multidimensional state ‘only translated to a unitary perception by the unity of consciousness itself’ (Desmond and Hancock, 2001). Some frequent distinctions used to illustrate the multidimensionality are mental versus physical fatigue and acute versus chronic fatigue (Craig and Cooper, 1992, Gawron et al., 2001, Grandjean, 1979). Mental fatigue is a response of mind and body to the reduction in resources due to mental task execution. It warns for the increasing risk of performance failure. Under normal circumstances, people become tired by their everyday work activities, but their energetical1 resources are sufficient to meet the task demands. However, when a person is working under high levels of (mental) workload and is already fatigued (e.g., at the end of a workday), extra energy to compensate fatigue has to be mobilized through mental effort in order to maintain task performance (Gaillard, 2001, Hockey, 1997, Hockey et al., 1986). The mobilization of extra energy may result into (feelings of) acute fatigue. A subsequent return to physiological and emotional baseline levels is important. Incomplete or slow recovery from workload demands disrupts the energetic homeostasis, which in turn may lead to chronic effects on health and well being (‘slow unwinding’, Frankenhaeuser, 1979, Frankenhaeuser and Johansson, 1986). In the event of incomplete recovery, the effects of high workload demands can accumulate gradually, carry over from one day to the next (Craig and Cooper, 1992, Frankenhaeuser, 1980, Frankenhaeuser and Johansson, 1986, Gaillard, 2001, Ursin, 1980).

Fatigue resists simple measurement, which stems from the multidimensional nature of the condition (Gawron et al., 2001, Desmond and Hancock, 2001, Wessely, 2001). Therefore, fatigue is measured using several different measurement methods varying from performance based, to physiological, biochemical, or subjective measures. In most research fatigue has been measured by means of questionnaires, especially as far as work-related fatigued is concerned (e.g., Checklist Individual Strength (Vercoulen et al., 1999), General Burnout Questionnaire, Maslach Burnout Inventory (Maslach and Jackson, 1986), Shortened Fatigue Questionnaire (Alberts et al., 1997)). Although most of these questionnaires have proven to be valid and reliable, the major disadvantage remains that they are subjective assessment tools. They give limited insight into the causes and mechanisms that produce fatigue, in particular the enduring effects are unknown. Therefore, physiological measures provide important additional information about the biobehavioral state of the subject.

The present study focuses on a long tradition of using tonic EMG activity in irrelevant muscles as an index for fatigue effects. In a classic study, Wilkinson (1962) observed a strong relation between muscle tension and task performance in sleep-deprived subjects. The performance of subjects who showed a small increase in muscle tension was impaired most. Those subjects with a large increase were able to maintain their performance at an acceptable level. Thus, the larger the increase in EMG (muscular tension), the better performance was maintained after sleep loss. This finding suggests that muscle tension reflects the effort that is mobilized by the subjects to overcome their fatigue (Craig and Cooper, 1992). Under demanding conditions, like after a workday, performance level can be maintained by mobilizing extra energy through compensatory mental effort, as signified by an increase in EMG activity levels. In several experiments (Freeman, 1931, Freeman and Lindley, 1931) a high correlation has been found between EMG activity and degree of effort. Subjects with the highest average tonic EMG activity showed the smallest work decrement. Muscle tension was not taken as an indicator of fatigue per se, but rather of the effort applied in performing a task. Van Boxtel and Jessurun (1993) hypothesized that increasing EMG activity in certain facial muscles may be an expression of growing compensatory effort to keep performance at an adequate level. During a 20 m sustained attention task, performance remained stable and was associated with a gradual increase in EMG amplitude (of the frontalis, the corrugator supercilii, and the orbicularis oris inferior muscle), called an EMG gradient (Malmo, 1965). The idea of EMG amplitude as an index of compensatory effort was supported by various studies (Van Boxtel et al., 1997, Veldhuizen et al., 1998, Waterink and Van Boxtel, 1994).

The current study investigated whether tonic EMG activity of the facial muscles reflects differences in fatigue during and after a simulated workday. The aim of this study was to examine whether subjects with and without enduring fatigue complaints, as measured by a questionnaire, differed in level of EMG activity during task performance. The above-mentioned studies show that tonic activity can be considered as an index of the amount of compensatory mental effort, necessary to overcome the fatiguing effects of a workday and maintain performance at a stable level. An EMG gradient is interpreted as an indication of increasing energy mobilization in order to counteract effects of fatigue. In the present study, fatigue was measured using a multi-method approach. Three different measurement methods were used: subjective ratings (questionnaires assessing perceived mental effort, subjective fatigue, and need for recovery), performance indices (reaction times and number of correct responses on a standardized Sternberg memory-scanning task), and EMG activity measures.

EMG activity was measured during the performance of the standardized Sternberg memory-scanning task on six occasions during the simulated workday and the subsequent evening. Fatigue was manipulated in four ways: (a) the building up of fatigue effects during the simulated workday, (b) the effect of time-on-task during the 5-min task session, (c) the after-effects of the workday, i.e., performance and EMG activity in the evening, and (d) the comparison of two subject groups (high/low-score group), differing in the number of enduring fatigue complaints indicated by the score on the emotional exhaustion (EE) scale from the Dutch version of the Maslach Burnout Inventory, administered before the experiment. High-score subjects were expected to have lower levels of EMG activity during task performance than low-score subjects do. Subjects with a high score were expected to have more problems to mobilize energy, and thus would have problems with compensating their fatigue to counteract the effects of time-on-task. It was also expected that EMG activity during the evening tests would be higher for both groups than during the daytime tests. Since tonic activity can be considered as an index of the amount of compensatory mental effort, necessary to overcome the fatiguing effects of a workday, subjects would probably have to invest more effort in the evening tests in order to maintain performance at a stable level.

Section snippets

Participants

Forty-six undergraduate students participated in the experiment (24 women and 22 men, mean age=21.7 years). None of the participating students indicated to experience health problems, be under medical treatment or take medication. The two subject groups did not differ with respect to the five major personality factors (extraversion, agreeableness, conscientiousness, emotional stability, and autonomy) as measured by the five-factor personality inventory (FFPI; Hendriks et al., 1999). Subjects

EMG activity

As is illustrated in Fig. 1, the activity pattern of the two muscles was quite similar. The results of the analysis on the amplitude scores in the four short test sessions show a clear effect of test for the corrugator and frontalis muscle as shown by a negative linear trend and a significant positive quadratic trend across tests (see Table 2 for the statistical results). No significant interaction was found between group and test, and no group effect for both trends. The presence of the trend

Discussion

The present study shows that facial EMG measures differentiate between subjects with and without enduring fatigue complaints as indicated by the EE scale of the Burnout questionnaire. During all six tests, subjects with many complaints displayed less activity than subjects with few complaints. Mean muscle activity of the frontalis muscle gradually decreased during the day, whereas this activity increased in the evening. The EMG activity for both groups was higher in the evening tests than in

Acknowledgements

The present study was carried out under the Dutch Concerted Research Action ‘Fatigue at Work’ of the Netherlands Organization for Scientific Research (NWO) that funded this project.

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