## 4 Discussion  
### Changes in the Number of Repetitions for Each Set of Rest Interval Conditions  
The number of repetitions and the speed of performance during resistance exercises show a strong inverse correlation [22]. The exercise performance is relatively fast in terms of low-intensity load compared to high-intensity load, but as the number of repetitions increases, the metabolic stress also increases, and the speed decreases [9]. When the rest interval conditions between sets are performed at 75% of the athlete's maximum bench press weight, previous studies on the number of repetitions according to exercise intensity and rest interval conditions [23] reported that the number of repetitions from two sets after 1 set decreased due to an increase in metabolic stress when the rest interval conditions were performed at 1, 3, and 5 min. In a previous study [24], it was reported that it is impossible to continue with 5 sets at 15 RM when the rest intervals between sets are fixed at 30 s, 1 min, and 2 min during resistance exercise. The results of this study were consistent with previous studies, wherein the short rest interval condition results (30-s and 1-, 2-, and 3-min rest intervals) were lower than for the long rest interval conditions (4- and 5-min rest intervals). However, considering the fact that there are no previous studies that have suggested six rest interval conditions during resistance training, the change and ratio of the number of repetitions shown in Tables 2 and 3 and in Figures 3 and 4 are judged to indicate the difference in exercise performance and exercise amount between rest interval conditions during 65% 1 RM resistance training. In addition, the short rest-interval condition performance was due to the lack of time allowed for physiological recovery, such as for energy re-compensation [25; 26] and oxygen transfer in the muscle, compared to the long rest interval conditions after performing the sets. It is thought that this reduction in energy utilization ability and the accumulation of metabolic stress caused by this pattern decreased the number of repetitions significantly as the sets progressed. As a result of post-comparison according to sets and set intervals, the number of repetitions according to set interval conditions decreased statistically significantly from 30 s to 1, 2, and 3 min, while the 4- and 5-min conditions showed similar levels. This is judged to indicate the level of energy replenishment [25; 26], as well as the level of metabolic stress [27; 28; 29] for each rest interval during resistance exercise at 65% 1 RM intensity. Therefore, it was found  
Figure 8: Heart rate recovery rate (%) after exercise and rests between sets, shown by rest interval. that energy replenishment through physiological recovery was relatively higher in the long rest interval conditions (4- and 5-min rest intervals) compared to the short rest interval conditions. In addition, it is believed that this is because the shorter rest intervals had less effect on the exertion and reduction of mechanical tension [18] due to the metabolic stress caused by exercise performance. Therefore, it is thought that the decrease in the number of repetitions was different as the sets progressed. This suggests that the performance of sets with short rest intervals between the sets can cause high metabolic stress levels, which can affect muscle endurance improvement. However, based on the assumption that incomplete recovery and performance in the next exercise performance are prerequisites for endurance training improvement, the 30-s and 1-min rest interval conditions yield low mechanical tension efficiency due to metabolic stress [27; 28; 29; 30; 31; 32] is low. In this case, there is a limit to the expected muscle endurance improvement. Therefore, it is thought that a 2-3-min rest interval between sets during a 65% 1 RM bench press exercise can be suggested for muscular endurance training.