Keywords

1 Introduction

Fatigue has been and continues to be a ubiquitous issue concerning human performance and safety whose influence is felt across operators, tasks, domains, and time. One study found that 20% of National Transportation Safety Board investigations identified fatigue as a contributing cause to safety incidents across domains (surface transportation, flight and marine operations, etc.) [3]. Research conducted on medical professionals attests to their fatigue-inducing working conditions (regular 24–36 on-duty shifts), their feelings of fatigue affecting their work, and the fact that it puts them and their patients at greater risk of harm [4]. Fatigue has been shown to slow response times, impair judgment, and compromise decision-making skills [5]. These tendencies would typically hamper performance across any task but would be particularly detrimental to those reliant on effective vigilance and efficient attentional allocation such as driving a semi-autonomous vehicle, baggage screening, medical diagnostic screening, anesthesia monitoring, and air traffic control. Understanding, measuring, and designing to effectively alleviate fatigue is critical to the successful operation and safety of human-machine systems across the world.

Yet despite its pervasive nature and influence, a single, agreed-upon operational definition and scheme of quantification and measurement remain elusive. This work examines these issues and seeks to create a unified, coherent, reliable, valid, and easily applicable trans-domain subjective fatigue assessment instrument.

1.1 Definitions

Defining fatigue becomes problematic as theorists disagree even upon its fundamental nature. Some consider fatigue an emotion, a subjective feeling; others a behavioral pattern, a performance outcome, or a moderator/mediator of stress. Still others would characterize it as its own unique stressor unto itself. Though the debate rages on in some academic circles, the authors would like to put forth their position on these contentions. Firstly, fatigue is a construct, a feature of the operator, not merely their behavior or a performance outcome. Secondly, performance decrements and fatigue are not wholly commensurate with one another. Fatigue may cause performance decrements, but performance decrements alone are not sufficient for prescribing a fatigue state. Thirdly, there is a pervasive tendency to equate fatigue and sleepiness. Though these states are related to each other, they are not the same. Sleepiness is the urge or tendency to fall asleep, while fatigue is more multidimensional and see Sect. 1.2 [6].

Though there are many competing operational definitions of fatigue, Soames-Job and Dalziel’s is arguably the most accurate and comprehensive. They define fatigue as “the state of an organism’s muscles, viscera, or central nervous system, in which prior physical activity and/or mental processing, in the absence of sufficient rest, results in insufficient cellular capacity or system-wide energy to maintain the original level of activity and/or processing by using normal resources” [7]. This definition clearly specifies the cause of fatigue in prior physical or mental activity, reflecting one of the key dimensions of fatigue’s nature which will be discussed shortly. The caveat specifying that fatigue manifests ‘in the absence of sufficient rest’ accurately conveys how operators can be in a fatigued state at the onset of performance as well as accounting for the demonstrable benefit of rest breaks [8]. Moreover, by specifying ‘insufficient cellular capacity or system-wide energy’, not only does this characterization encapsulate physiological processing, but it also thereby overcomes the circular reasoning inherent to most competing definitions. Finally, the last clause concerning the inability to maintain performance using the typical reserve of resources acknowledges the imbalance of existing physical or mental resources and task demands [9] and implies performance decrements without making them synonymous with the fatigue state.

1.2 Dimensions

Fatigue is a complex and multi-dimensional construct. When examining the literature, it is possible to find works that explain the theoretical distinctions between such dimensions and empirical works that demonstrate their effects. However, no subjective assessment tool yet identified has addressed all of the following dimensions. It is the intention of this project to create a psychometric instrument capable of quantifying and being sensitive to the levels of these different types of fatigue.

Active versus Passive Fatigue

Dependent on the type of task, fatigue may manifest from either active or passive activity. Active fatigue is the result of prolonged task performance necessitating continuous effortful processing and motoric execution [10]. Passive fatigue, on the other hand, results from prolonged monitoring performance (which taxes limited attentional resources) wherein operators are rarely, if ever, required to provide an overt response [10].

Cognitive versus Physical Fatigue

Cognitive fatigue is characterized as a deficit in mental resources (namely, attention) due to their expenditure on task performance, and the consequent inability to maintain performance [11]. Physical fatigue, in contrast, is an inability to continue physical work or performance to a required level due to prolonged use of the muscles [12, 13].

Acute versus Chronic Fatigue

Acute fatigue is characterized as a transient and temporary state experienced by healthy operators as the direct result of taking part in effortful work [14, 15]. In contrast, chronic fatigue is a long-term condition (or even illness) that persists as a feeling and influence on performance despite adequate rest [16].

1.3 Types of Response

Just as there are diverse dimensions and sources (task, environment, etc.) of fatigue, so too are the responses many and varied. In nature, these responses may be categorized as objective or subjective responses to the fatigue experience. Both types hold significant implications for performance outcomes as well as operators’ state, health, and well-being. Ideally then, the proposed psychometric assessment method attempts to gauge both such responses.

Objective Responses

Objective indicators of a fatigued state include sleepiness, alertness, activity level, muscle tension, and muscular capacity. Polysomnography can objectively measure physiological correlates of restful sleep that can be predictive of operator state [17]. Activity trackers can record activity and inactivity levels over time in an easy, non-intrusive fashion [18]. Such actiwatches can also track unintended sleep. Finally, researchers have established a battery of muscular strength measures to gauge physical fatigue [19].

Subjective Responses

Subjective responses to fatigue include various cognitive and affective reactions. A representative though inexhaustive list of such responses include perceived tiredness, sleepiness, lethargy, and exhaustion [20]. Operators’ subjective experiences of fatigue are commonly measured via visual analog scales and/or valid and reliable questionnaires using Likert-type items. Typically, however, the psychometric options address only a single dimension or symptom of fatigue; for example, the Karolinska Sleepiness Scale [21]. As a result, the goal of the present project is to generate a valid, reliable, and widely applicable subjective fatigue assessment instrument capable of assessing the many causes, dimensions, and responses of fatigue.

2 Methodology

From the survey of all extant subjective fatigue assessment scales, commonalities were distilled in terms of concepts addressed (e.g., feelings of lassitude), subjective symptomatology (e.g., ‘heavy’ eyes), and subjective performance degradation (e.g., meta-cognitive recognition of increased error rate), as well as specific interrogatories and queries. This procedure took the form of assessing 1,033 identified articles derived from keyword searches and reference list searches from all the major scientific databases. The extension beyond the typical human factors and ergonomic databases was necessary as fatigue assessment is practiced in highly disparate constituencies (e.g., airspace operations, clinical psychological assessments, etc.). For the key terms, exclusionary terms, and modifiers, please see Table 1.

Table 1. Key terms, exclusionary terms, and modifiers.
Full size table

From this survey, 89 key subconstructs were identified that contribute to the latent construct of ‘fatigue’, which were embodied in 1,698 questionnaire items. These collective identifications have been summated into an extended survey instrument in order to conduct critical factor identification. This inquiry is currently in progress and will form the foundation of an accompanying meta-analytic study of fatigue effects and assessment.

3 Results

From the above elicited data, the authors identified 90 currently developed scales which provide substantive assessments of subjective fatigue state. Eighty-nine subconstructs related to fatigue embedded within these 90 identified scales were reviewed. The results compare and contrast the validity, reliability, and applicability of such scales alongside recommendations for practitioners who employ them. Moreover, the subconstructs for fatigue were analyzed and categorized to reflect its multifaceted nature in the construction of the more comprehensive proposed scale. The dimensions of the authors’ own developing scale include i) active versus passive fatigue; ii) acute versus chronic fatigue; iii) subjective versus objective responses (mental, physical, and emotional dimensions), as well as iv) the contributions of diverse other sources of fatigue (i.e., task, environment, etc.).

Next steps include analyzing the 1,698 items to remove duplicates, re-phrasing items to allow for maximum applicability in terms of administration and use (i.e., generic or specifiable system terms), assessing internal consistency, and determining the nature of responses (i.e., visual analog scale versus Likert-type scale). Items must also be generated if an acceptable exemplar cannot be identified or refined from existing scales. This latter process may need to occur multiple times given the dearth of existing scales addressing some of the key dimensions of fatigue.

Assuming a Likert-type scale, items will be positively worded with some being reverse-coded to ensure the integrity of responses. A selection of items will therefore be determined for each dimension of fatigue. Each of these subsections will then be amalgamated into the complete battery for fatigue assessment. This completed questionnaire will then undergo multiple validation studies to ensure the internal consistency of all items, and that each item loads onto its proposed sub-construct associated with the latent construct of fatigue.

4 Discussion

Creating an effective worker fatigue assessment instrument does not only consist of item identification, selection, grouping, administration and verification of validity and reliability. There are also ergonomic issues involved with its administration. In the present project, the authors have created a fatigue ‘app’ which employs minimalist interface design principles to garner both objective (e.g., time-of-day of administration) and subjective (e.g., specific item response levels), in order to produce a fatigue ratio scale ranging from 0–100. These anchors connote no discernible presence of fatigue and the ceiling being intolerable fatigue. Some present concerns involve questions as to whether individuals can effectively respond at the 100 level, or whether increasing levels on the ratio scale actually inhibit accurate responding. Efforts are also underway in the process of identifying salient thresholds on the scale; that is, providing easily recognized cognitive anchors for meaningful values (e.g., 50).

5 Conclusions

Subjective assessment of fatigue holds important promise for ergonomic applications. Yet, current methods, derived from multiple disciplines and domains, provide no simple unified scale that is easily applicable in real-world circumstances. Here, the authors provide a roadmap to such scale development and report on the progress of this effort thus far.

To date, the authors have combed the relevant literature from multiple disciplines, including human factors/ergonomics, to establish what psychometric instruments are currently in use for the dynamic assessment of operator fatigue. Herein, the strengths and limitations of those instruments have been discussed as well as propositions for their improvement, particularly with regard to their utility to practitioners. Given the nature of evolving human-machine systems and ever more autonomous adaptive automation, fatigue may well be one of, if not the, most pressing human performance issue of the future. The successful quantitative assessment of fatigue’s effects on performance and operators’ cognitive and affective states is therefore not only integral to the design of future human-machine systems; it is also critical for determining the efficacy of targeted countermeasures. A valid, reliable, and flexible instrument for the subjective assessment of fatigue across multiple tasks, environments, and operational domains is therefore desperately needed, and this work represents the first steps in the construction and validation of this valuable psychometric instrument.