## 1 Introduction  
It has been reported that regular exercise has mental health benefits such as reducing depressive symptoms and improving mood and self-esteem [1]. In particular, aerobic exercise is mainly conducive to stress relief and emotional stability, contributing to mental health [2]. In addition, because aerobic exercise has such a powerful effect on depression, some have even referred to it as an antidepressant with no side effects [3]. The psychological benefits of exercise participation vary depending on exercise intensity, duration, and type [4, 5]. For example, exercise intensity is a major influencing factor of affect, in that exercise at a moderate to a vigorous intensity below the ventilatory threshold (VT) or lactate threshold (LT) is known to induce positive affective changes [6, 7, 8, 9]. One can usually maintain a pleasant affect until the VT, but a sudden shift occurs from a pleasant to unpleasant affect upon exceeding the VT. During exercise at the VT level, the affective experience of individuals gradually declines in the direction of displeasure, which bounces back to pleasure once the exercise completes [6, 7, 8, 9, 10]. As such, the affective experience during exercise varies according to the intensity of exercise. Despite a positive affect restored after exercise, the discomfort experienced during the prior exercise can influence exercise adherence or withdrawal in the future [11]. Therefore, the affective point of view does not recommend immediately exposing beginners to exercise at a high intensity because it is likely to induce negative affective responses during exercise. Furthermore, even with the same exercise program, individuals can feel the intensity of exercise differently depending on their physical fitness level. For example, a moderate-intensity exercise of 60% of HRmax for the high-fit group may be equivalent to a vigorous-intensity exercise of more than 90% of HRmax for the low-physical group [12]. However, little research has investigated how the affective benefit of exercise varies depending on the fitness status of an exerciser. In a study examining the affective changes during vigorous-intensity exercise in association with exercise intensity preference, the group that preferred low-intensity exercise exhibited gradually increasing displeasure three minutes after the start of the exercise to the end of the exercise (15 min). The affect returned to the pre-exercise level after the exercise. On the other hand, the group that preferred high-intensity exercise maintained the initial level of affect throughout the exercise, and the pleasure increased after the exercise [13]. These results suggest that the impact of exercise on our affective experience can vary significantly according to one's preference for high-intensity exercise. Considering that high-fit individuals have a higher preference for vigorous-intensity exercise than low-fit individuals [14], it can be predicted that there will be differences in affective changes during exercise depending on physical fitness. Therefore, it is of importance to investigate how physical fitness mediates affective experience associated with exercise. Another important variable that comes into play between exercise and affect is the dynamics experienced when exercising with others. For example, according to the social interaction hypothesis [15] or the psychosocial hypothesis [16], the interaction between people participating in exercise or positive attention from those exercising together is a key factor in affective improvement we receive from exercise. Furthermore, the degree to which exercise increases endorphins is greater when a group is together than alone [17]. A rhythmic exercise accompanied by music may add benefit to the mood-lifting effect [18; 19]. In this regard, considering the dynamic factors involved in the effect of exercise on affect, researchers should conduct studies on the exercise-affect relationship in a real-life exercise field rather than in a laboratory setting.