The purpose of this study was to identify the effects of three exercise modalities, in which EE was homogenized, on cardiorespiratory function and energy metabolism during EPOC and the difference in EPOC according to modality. As per the design, EE during the three exercise sessions did not show a significant difference between the exercise types. This means that the exercise program of this study was properly configured, and there was no difference in exercise volume in CE, IE, and AE, indicating that the measurement was performed correctly. During exercise, CHO was highest in IE, and FAO was highest in CE. However, both cardiorespiratory function and energy metabolism were significantly higher in AE than in CE and IE during EPOC. As a result, during the full session, including exercise and EPOC, AE showed a higher metabolic benefit from EPOC than the other two exercise modalities.
Oxygen demand increases rapidly at the beginning of exercise, but the supplied VO2 does not meet the oxygen demand, resulting in an oxygen deficit while the supplied oxygen reaches a steady state [40]. During exercise, VO2 shows an elevated steady state to support the increased energy demand according to exercise intensity. Immediately after exercise, oxygen demand should decrease to a resting state, but EPOC appears as a compensatory action for an oxygen deficit that occurs early in exercise [41, 42] and can be mainly divided into the alactacid and lactacid components during exercise. The alactacid component is related to the resynthesis of phosphocreatine used in muscle during exercise, oxygen storage in muscle and blood, and an increase in heart rate and respiratory rate after exercise. The lactacid component is associated with lactate elimination, elevated body temperature, elevated hormone levels, and elevated heart and respiratory rates after exercise [43]. In general, EPOC shows a quantitative correlation with exercise intensity and is reported to increase linearly as the exercise duration increases at an exercise intensity of more than 50% maximal oxygen uptake (VO2max) [44]. In particular, it has been reported that exercise intensity affects both the duration and size of EPOC, while exercise time only affects EPOC duration [45].
In this study, the difference in cardiopulmonary function and energy metabolism among the three exercise sessions showed that RER was significantly higher in IE (0.98 ± 0.04 mL/min) than in CE (0.93 ± 0.05 mL/min) and AE (0.94 ± 0.06 mL/min). Accordingly, CHO was significantly higher in IE (30.23 ± 7.86 mL/min) than in CE (27.00 ± 7.49 mL/min) and AE (28.41 ± 7.11 mL/min). The reason IE showed more carbohydrate oxidation than CE and AE seems to be due to its higher-intensity nature, comprising repeated high-intensity exercise and recovery exercise as compared to CE, comprising continuous moderate-intensity exercise sessions, and AE with divided exercise sessions [36]. CE (3.43 ± 1.98 mL/min) showed significantly higher FAO than IE (2.02 ± 1.24 mL/min) and AE (3.18 ± 2.05 mL/min). CE and AE included exercises with the same intensity, but FAO appeared higher in CE. In the case of AE, these results were obtained by dividing the exercise into three separate times, and oxygen deficit occurred three times. This appears to have resulted in the consumption of more CHO and less FAO [34].
Regarding EPOC, the catecholamine response stimulated by exercise affects EPOC during moderate-intensity or high-intensity exercise. In particular, among the catecholamines, the increase in epinephrine and norepinephrine, which affect energy substrate mobilization, is a potential stimulator of mitochondrial respiration after exercise, resulting in lipolysis, and it has been reported to contribute to increased oxidation [46–48]. In addition, high-intensity exercise can cause glycogen depletion, which can increase the size of the EPOC through lactate, hydrogen ion (H+) accumulation, and glycogen resynthesis, and the size of the EPOC appears in proportion to the O2 deficit [49]. In particular, EPOC contributes to fat oxidation by increasing adenosine triphosphate (ATP) and oxygen consumption through glycogen resynthesis after exercise [22, 50, 51]. As high-intensity exercise continues, mitochondrial activity and oxygen consumption increase in muscle cells to generate more ATP [52] however the amount of ATP gradually decreases during exercise and is resynthesized during the recovery period to provide additional energy [53].
In a study comparing the EPOC for CE and IE, Islam et al. [54] compared the EPOC and FAO after CE and IE in eight healthy men. CE included 30 min of running at 65% VO2max intensity, and IE included 30-sec of running and 4-min of rest at > 100% VO2max intensity for a total of four times. During EPOC, VO2 was significantly higher in IE (42.8 ± 3.6 L) than in CE (38.8 ± 3.7 L), and FAO was significantly higher in IE (0.115 ± 0.026 g/min) than in CE (0.047 ± 0.018 g/min). Kristian et al. [55] compared the EPOC of CE and IE for walking exercises with similar oxygen consumption in 10 participants with type 2 diabetes. Exercise intensity was designed as follows: CE included 60 min of exercise at 73% VO2peak intensity; IE included a total of 60 min of exercise by repeating 54% VO2peak and 89% VO2peak intensity every 3 min. As a result, the total EPOC size was larger in IE (8.4 ± 1.3 L) than in CE (3.7 ± 1.4 L), but carbohydrate and lipid oxidation rates were not different between IE (1.54 ± 1.03 and 3.06 ± 0.20 mg/kg/min) and CE (9.24 ± 0.92 and 3.26 ± 0.28 mg/kg/min). Nevertheless, looking at the oxygen consumption in the end of EPOC, there was a significant difference between IE (271 ± 10 mL/min) and CON (252 ± 9 mL/min) but no difference with CE (259 ± 9 mL/min) and CON, confirming that the EPOC period of IE can last longer than that of CE. Greer et al. (56) compared the EPOC in isocaloric CE and IE using a cycle ergometer in 10 healthy men. CE was performed for approximately 43 min at approximately 39% VO2peak intensity, and IE was performed for approximately 44 min by repeating 30 sec of exercise at 90% VO2peak and 120 to 180 sec of rest. As a result, energy consumption during EPOC was higher in IE (62 ± 7.2 kcal/30 min) than in CE (50 ± 5.3 kcal/30 min). In this study, when CE and IE were compared during EPOC, IE had significantly greater HR_sum, VE_sum, VO2_sum, VCO2_sum, CHO, FAO, and EE during EPOC based on higher exercise intensity and a greater O2 deficit during exercise than CE. This means that when performing an exercise with homogenized energy consumption, IE has a shorter exercise time than CE but a larger EPOC, that is, energy consumption after exercise, which is consistent with the results of previous studies [54–56].
AE, which divides exercise, causes an O2 deficit several times and accumulates. It has been reported that accumulated O2 deficit affects the size of EPOC by inducing the oxidation of more energy substrates to compensate for energy consumption after exercise [57, 58]. In addition, lipolysis (an increase in free fatty acids and glycerol) in adipose tissue during moderate-intensity aerobic exercise is enhanced when repeated workouts of same intensity and duration [59]. Stich et al. [60] confirmed the effect of repeated bouts of aerobic exercise on glycerol and catecholamine concentrations in seven male participants. For exercise, 60 min of exercise and 60 min of rest were repeated twice at an intensity of 50% VO2max using a bicycle ergometer. The glycerol concentration at the end of the 2nd exercise (1,126 ± 298 µM) was significantly higher than that at the end of the 1st exercise (728 ± 159 µM). In addition, the plasma catecholamine concentration that induces the mobilization of the energy substrate was confirmed in this study. As a result, norepinephrine increased significantly in the first and second exercise sessions (35.417 ± 4.743 pg/mL and 37.425 ± 3.950 pg/mL), but did not increase any more during repetitive exercise. On the other hand, epinephrine was significantly higher during the second exercise (10.502 ± 1.73 pg/mL) than in the first exercise (2.927 ± 2.07 pg/mL).
In a study comparing the EPOC for CE and AE, Goto et al. [61] confirmed the difference in the effects of CE and AE on fat oxidation during EPOC in nine male participants. The exercise sessions were CE performed at 60% VO2max for 30 min and AE performed at 60% VO2max for 10 min of exercise and 10 min of rest, for a total of three times using a cycle ergometer. It was reported that the contribution of fat to total energy consumption during EPOC was significantly greater in AE (77.6 ± 2.7%) than in CE (62.1 ± 5.7%). Darling et al. [62] compared EE during EPOC for CE and AE in 20 male participants. The exercise sessions were CE performed at 70% VO2max for 30 min of walking exercise and AE performed at 70% VO2max for 10 min of walking exercise, three times and at 3-h intervals. Energy consumption during exercise was higher in the CE (409 ± 67 kcal) group than in the AE (462 ± 67 kcal) group. On the other hand, EE during EPOC was greater in the AE (105 ± 13 kcal) group than in the CE (83 ± 9 kcal) group, and consequently, in total EE by exercise, AE was greater than CE. Additionally, Jung et al. [36] confirmed the difference in EPOC size according to CE, IE, and AE in nine female participants. The exercise session was CE performed at 60% VO2max for 30 min and IE performed at 80% VO2max for 2 min, followed by 40% VO2max for 1 min, and 80% VO2max for 3 min, repeated six times for a total of 26 min. For AE, 10 min of exercise at 60% VO2max intensity were repeated three times at 1-h intervals, and a cycle ergometer was used for all exercises. As a result, the EPOC period was longer in IE (42.44 ± 14.06 min) and AE (45.00 ± 14.31 min) than in CE (25.22 ± 15.06 min), and the energy consumption during EPOC was in the following order: AE (88.57 ± 43.03 kcal/min), IE (63.54 ± 21.39 kcal/min), and CE (38.81 ± 23.06 kcal/min).
It has been reported that EPOC size appears to be proportional to the increase in exercise intensity [53]. In other words, during high-intensity exercise, the amount of oxygen required at the beginning of exercise increases rapidly, and the amount of oxygen deficiency increases accordingly, which seems to affect EPOC as a result [43]. Thornton and Potteiger [63] reported that high-intensity exercise results in a larger EPOC than moderate-intensity exercise. AE causes an O2 deficit to accumulate, thus increasing the size of EPOC [33]. Therefore, in the difference between oxygen deprivation and EPOC according to the three different exercise treatments in this study, IE appeared larger than CE, and AE appeared larger than CE and IE (O2 deficit: 1,050.91 ± 245.37 mL/min, 1,346.53 ± 283.93 mL/min, 3,386.73 ± 868.89 mL/min; p < 0.001 in CE, IE, and AE respectively, and EPOC: 5,239.12 ± 1,833.12 mL/Rec, 6,578.96 ± 2,347.63 mL/Rec, 1,3259.97 ± 3,849.27 mL/Rec; p < 0.001 in CE, IE, and AE respectively). The correlation between O2 deficit and EPOC according to the exercise trials also showed a significant correlation with CE, IE, and AE (R = 0.377; p < 0.05, R = 0.308; p < 0.05, R = 0.558; p < 0.001 for CE, IE, and AE, respectively). As a result, it was confirmed that the O2 deficit caused by high-intensity exercise not only affects the size of the EPOC but also that the accumulation of the O2 deficit due to AE affects the size of the EPOC.
In this study, AE showed significantly greater cardiopulmonary function and energy metabolism than CE and IE did during EPOC. This means that AE showed a significantly larger EPOC based on more compensation systems because of the accumulation of O2 deficit by exercise repeated three times. This was confirmed through the correlation between the O2 deficit due to exercise and EPOC. These results show that even if the exercise is performed in short 10-min increments, as it was in this study, EPOC can be maximized based on a repetitive O2 deficit to increase oxidation of energy metabolism, which is consistent with the results of previous studies [36, 60–62].
Since 1995, the US Physical Activity Guidelines have recommended that physical activity be accomplished by accumulating short exercise sessions throughout the day [64]. Exercise that accumulates short workouts throughout the day is more attractive and can encourage exercise participation than traditional long-duration workouts; if short exercises can be accumulated for 10 min each, it becomes easier to achieve the recommended 30 min of moderate-intensity exercise as a daily amount of physical activity [65]. As is well known, regular aerobic exercise not only prevents metabolic and cardiovascular diseases but also reduces their risk of death [66, 67]. In addition to regular exercise, there are health benefits that can be obtained by simply replacing sedentary time with shorter periods of activity [67, 68]. Altena et al. [69] reported that accumulated physical activity is effective in lowering triglyceride concentrations after a meal, which is important because a high postprandial triglyceride concentration increases the risk of cardiovascular disease [70]. Murphy et al. [57] reported that AE could contribute to cardiovascular health and the normalization of blood pressure by increasing fat oxidation as well as maximizing EE through EPOC. Taken together, AE and EPOC can contribute to lipid oxidation and energy consumption and are considered an important form of exercise that lowers the barrier to participation in exercise for individuals who want to maintain or lose weight.