## 1 Introduction  
Regular exercise has beneficial effects on health [1, 2] and concurrently increases the production of reactive oxygen species (ROS). Although ROS primarily act as messengers in signal transduction for regulating various cellular functions [3], an imbalance in the equilibrium of ROS and antioxidants is proven to cause higher levels of oxidative stress [4, 5]. Previous studies showed that acute exhaustive exercise impaired these health benefits via the rise of ROS and free radical production [6, 7]. The increased levels of markers due to oxidative damage to lipids, proteins, and DNA are often said to be caused by the high intensity of exercises [8]. Oxidative stress instigated by exercise is due to the rise in oxygen consumption (VO2) by 10- to 15-fold above the resting level, leading to the increase in oxygen flux by 100-fold higher than the resting value in the contracting skeletal muscles [9]. The oxygen flux's steep rise is followed by ROS's overproduction [10]. Subsequently, such a high concentration of ROS overwhelms the antioxidant capacity and affects redox balance [6; 8; 9]. Oxidative stress associated with exercise could lead to a disruption in cellular homeostasis, such as muscle fatigue [11; 12], and after performing a high-intensity exercise, cellular apoptosis [13] and muscle damage ensue [14]. However, exercise also produces the necessary stress to stimulate chronic adaptations that are beneficial [15], including the reduction of inflammatory responses [16; 17] and oxidative stress by enhancing the antioxidant system [18]. The degree of oxidation is proportional to the amount of oxidant production contributed by exercise mode, intensity, and duration [19]. Cycling is one of the safest modes of exercise, especially for overweight, obese, and older people for cardiovascular fitness and weight loss training program [20]. It has become popular, especially indoor cycling, with the aid of personal trainers [20], music rhythm [20], and sometimes high energy demand [21]. However, the optimal load (intensity) and duration needed by sedentary adults, especially those unfamiliar with cycling, is still unclear. As suggested by ACSM [22], the standard guidelines for beneficial exercise effects in healthy adults aged 18-65 years are moderate aerobic physical activity for a minimum of 30 min five days per week or vigorous-intensity aerobic activity for a minimum of 20 min three days per week. According to the intensity classification by [PERSON] et al. [23], low-intensity exercise ranges from 37-45% of VO\({}_{2}\)max (equivalent to 57-63% HRmax), moderate intensity exercise ranges from 46-63% of VO\({}_{2}\)max (equivalent to 64-76% HRmax), and high-intensity exercise ranges from 64-90% of VO\({}_{2}\)max (equivalent to 77-95% HRmax). However, a study by [PERSON] and [PERSON] [24] showed that the minimum training-intensity threshold for improving aerobic capacity is at least 45% VO\({}_{2}\)max. At these intensities, the production of reactive oxygen species (ROS) has been associated as being a stimulus to physiological endurance adaptations [15; 25]. The increased ROS production is maintained by a concomitant increase in antioxidant enzymes such as Superoxide Dismutase (SOD), Catalase (CAT) and Glutathione Peroxidase (GPx). However, the evidence is still limited, particularly regarding different intensity levels and duration of cycling on oxidative stress, especially among sedentary adults. Thus, this study aimed to investigate the optimal intensity and time duration limits of cycling exercise for sedentary healthy adults as the oxidative stress responses seem to be dependent on several factors, including age, gender [25], fitness level and psychological stress [26], nutritional intake [27], training status [28; 29], muscle mass involved [24], exercise intensity [27; 30], and duration [27]. The age, body composition, fitness level, and nutrient intake were recorded to minimize the variance between recruited subjects.