## 1 Introduction
### Research Objectives  
The aforementioned studies indicate the characteristics of fatigue and tension in various aspects. The impairment process of ATCOs resulting from fatigue and tension and the effects on air traffic safety may be as follows: first, the fatigue state or tense state of ATCOs occurs due to the work environment or personal factors; second, since the ATCOs are in an at-risk state, their physical characteristics change, and their abilities are impaired; finally, their working performance decreases, adversely affecting air traffic safety. Studies on the causal factors can provide guidance for reducing fatigue and tension by managing these factors. Research on the effects of the states on physical characteristics and abilities provides a clear understanding of how these states cause high-risk situations in air traffic control, and an analysis of the relationship between the states and the working performance would help to develop timely countermeasures to maintain air traffic safety.  
The focus of this study is on the effects of fatigue and tension on the physical characteristics and abilities of ATCOs. Although the effects have been analyzed in some studies, most only considered a single aspect of the characteristics and did not determine the influence mechanisms of fatigue and tension on abilities clearly. In this study, the physical characteristics and abilities of ATCOs were both considered, and multiple indicators were investigated simultaneously to analyze the effects of fatigue and tension and their influence mechanisms. Influence mechanisms refer to the influence process and mode of the states on physical characteristics and abilities, which are helpful to clearly understanding how the two states impair safety performance of air traffic controllers. Because of the high risk of fatigue and tension states on actual work conditions, it was impractical to design an experiment and collect data in a real work setting. Fortunately, air traffic control simulators can provide a simulated environment. In simulated experiments, almost every factor can be designed and the interference of undesired factors is minimized. Air traffic control simulators have frequently been used and validated in a variety of applied and basic research programs [54]. They have played an important role in some studies about ATCOs [55; 56]. In addition, young ATCOs are the primary members of the frontline staff in China [7]. Compared with older controllers, they may show worse work performance under the impairment of fatigue or tension because of less work experience. It was also demonstrated that young ATCOs were more likely to be tense than older ATCOs [7]. Furthermore, almost all controllers in China are male. Therefore, young male ATCOs were recruited to conduct simulated control tasks in a simulated environment under the influence of different states, and their physical characteristics and abilities were evaluated in a laboratory. The objectives of this study are the following: to (1) analyze the effects of fatigue and tension on the physical characteristics and abilities of ATCOs and (2) explore the influence mechanisms of the two at-risk states on the abilities of ATCOs. The results of this study are expected to provide useful references for the assessment of fatigue and tension states in ATCOs, development of management methods for at-risk states, and quantification of the effects of fatigue and tension on the behavior and ability of ATCOs.