## 4 Discussion
### Changes in the Number of Repetitions for Each Set of Rest Interval Conditions
In addition, heart rates after exercise with the 2- and 3-min rest interval conditions were maintained or increased, and the heart rates after exercise with 4- and 5-min rest interval conditions gradually increased. This suggests that heart rate (contraction frequency) does not increase during the 65% IRM bench press exercise and that the increase in stroke volume begins at the 2-min level. However, it is worth noting that this response of no increase in heart rate (contraction frequency) and an increase in stroke volume occurred across rest interval conditions and set progression, but there were no significant differences in maximum heart rate levels. After that point, it is thought that the heart rate increased immediately after exercise due to the increase in the number of repetitions and the increase in accumulated metabolic stress. Conversely, there was a significant difference 'before rest' and 'after rest' is right, and there was a significant difference in the rest interval condition and the interaction effect between the sets. This is thought to indicate high metabolic stress levels and low physiological recovery levels in relatively short rest interval conditions. As the set progressed at the same time, the ratio of energy required for recovery was higher than that with long rest interval conditions due to the energy being used for muscle contraction [25,26]. The average heart rate recovery rate (%) according to rest interval condition was 16.9 \(\pm\) 2.2% in the 30-s rest interval condition, 27.7 \(\pm\) 2.83% in the 1-min rest interval condition, 40.2 \(\pm\) 3.9% in the 2-min rest interval condition, 43.2 \(\pm\) 5.2% in the 3-min rest interval condition, 43.3 \(\pm\) 3.4% in the 4-min rest interval condition, and 53.1 \(\pm\) 4.1% in the 5-min rest interval condition, while the heart rate recovery rate in the relatively long rest interval (3, 4, and 5 min) conditions was higher than the short rest interval (30 s, 1 min, and 2 min) conditions. Figures 7 and 8 show the percentage of heart rate recovery (%) and the percentage of reps (%) between sets, by rest interval condition and set, after performing 1 set of exercises in each condition. The percentage of reps between sets for the 30-s rest interval condition was 35%, 23%, 19%, 16%, 15.5%, and 15.5%. The percentage for the 1-min rest interval condition was 48.5%, 33.7%, 26.2%, 22.3%, 21.3%, and 21.3%; that for the 2-min rest interval condition was 64.4%, 47.8%, 42.9%, 39%, 36.1%, and 35.6%; that for the 3-min rest interval condition was 73%, 56.4%, 49.5%, 40.7%, 41.2%, and 36.8%; that for the 4-min rest interval condition had 83.3%, 69.1%, 59.3%, 57.8%, 53.9, and 49.5%; finally, that for the 5-min rest interval condition had 88.9%, 76.7%, 70%, 62.6%, 58.3%, and 52.4%. The percentage of heart rate recovery (%) during set progression across the six rest interval conditions also increased in the longer rest interval conditions, as did the percentage for exercise performance (reps) level. Afterward, as the set progressed, the ratio of the number of repetitions increased as the heart rate recovery ratio between the rest interval conditions increased. This difference can be explained by a previous study which reported that the relatively long rest interval between sets during resistance exercise can allow a great deal of exercise [28] because there is enough time for the body to supplement its energy compared to the short rest interval; this is considered to indirectly imply the difference in metabolic stress between each rest interval condition. A decrease in relative muscle strength compared to absolute muscle strength results from the short rest interval conditions' low heart rate recovery rate, which also means that metabolic stress accumulation [25,26] is relatively high as the set progresses. This is different from the results for relatively long rest interval conditions, which are thought to have decreased exercise performance (in terms of the number of repetitions) from the start of the set.