Prior to the next exercise session, rapid recuperation of an already exercised athlete is very important. A major component of such recovery is the restoration of the body's energy store during rest, but the duration of the rest period is a limiting factor on the effectiveness of recuperation. Fat and carbohydrate are the principal substrates that supply adenosine triphosphate, an energy storage and transport chemical, to skeletal muscle. If a body does not get sufficient rest after a period of high-intensity exercise, the body will not have sufficient energy available to adequately power the body, and the body will become fatigued, reducing the effectiveness of the power application. Previous research into exercise has focused on exercise intensity and energy partitioning during exercise or on dietary intervention to increase muscle glycogen levels [12345].

Although indirect bio-indices for substrate utilization and energy expenditure, such as substrate oxidation rate or substrate plasma level, are available [67], the appropriate conditions for recuperation and substrate recovery after exercise have not been fully elucidated. It has been reported that physical training leads to a metabolic adaptation of the exercising muscles to ensure maintenance of an adequate energy supply as well as an acceptable antioxidant defense system status [89]. Trained muscles use less glucose and conserve carbohydrate storage; in contrast, they use more fat, increase fat oxidation, and have an upregulated antioxidant defense system [1011]. Most fat-source energy requirements of muscle cells during exercise are met by the supply of free fatty acid in plasma and free fatty acid derived from intramuscular triglyceride [12]. The effect of an exercise period on energy substrate utilization may continue for many hours into the subsequent rest period, and during that period, resting lipid metabolism activity is altered [13]. To obtain the best possible recuperation results, for both beginners and advanced athletes, different rest periods between exercise sessions may be needed.

This study investigated the effects of different exercise period-to-rest period ratios on the restoration of stored energy sources in tissue and on oxidative system status in rats that underwent previous exercise training and those that did not receive exercise training. The analysis sought to determine the minimum exercise-to-rest ratio for the restoration of stored energy and recuperation of the oxidative system.

The results of this study show that animals can benefit from a 0.5:1 to 1:1 exercise-to-rest period ratio to restore muscle energy stores and allow their oxidative defense system to recuperate. Moreover, the results show that a previous moderate exercise training period is helpful for replenishing energy stores but is not effective in the recovery of anti-oxidative status.

Although energy is provided by carbohydrates, lipids, and proteins, only carbohydrates and lipids provide the major portion of the total energy supply during exercise. During exercise, carbohydrate sources include blood glucose, muscular, and hepatic glycogen. Lipid sources include plasma FFA, intramuscular TG, and adipocyte TG [15]. The restoration of these energy stores after exercise is critical prior to the next exercise session. In particular, depletion of carbohydrate stores in liver and muscle is related to fatigue in animals. In this study, in the training group animals, plasma glucose levels were increased significantly in the ER2:1 group and were similarly and significantly high in the ER1:1 and ER0.5:1 groups. In contrast, the liver and muscle glycogen levels were high in the ER2:1 group, decreasing to levels similar to the NE groups in the ER1:1 and ER0.5:1 groups. In the non-training animal groups, these carbohydrate energy sources tended to show similar patterns, but the changes were not statistically significant due to the groups' large standard deviations.

Over time, training does increase the body's physiological capacity for both work and recovery. Exercise training helps maintain good blood flow to and from the muscles. However, without the adaptation on optimizing carbohydrate energy balance by exercise training, glycogen re-synthesis might be delayed by slowing the delivery of glucose into tissue due to the failed integrity of the glucose transport process in some animals [16], and glycogen re-synthesis might show large individual variation in non-training animals. Restoration of muscle glycogen stores supports the recovery of endurance capacity for exercise. It has also been reported that the presence and action of carbohydrate metabolism regulators other than insulin can initiate metabolic adaptations [17]. Thus, moderate exercise training, as used in this study, can increase the physiological capacity for restoration of carbohydrate stores during a rest period, and a 2:1 exercise-to-rest period ratio might be sufficient for the replenishment of carbohydrate energy sources for a subsequent exercise session.

In this study, the plasma FFA level was highest in the ER2:1 group but was also high in the ER0.5:1 group. Liver TG level was unchanged in the ER1:1 group but did increase in the ER0.5:1 group of training animals. However, there were no significant differences among NE and ER groups, regardless of exercise-to-rest period ratio, in the non-training animals. Endogenous triacylglycerol is an important energy source for exercise, and fat oxidation is increased during moderate intensity exercise. The training-induced increase in fat oxidation is primarily a product of intramuscular triacylglycerol stores [18]. Exercise training enhances mobilization of FFA from fat depots and increases fat use as a consequence of the increased activity of peripheral tissue enzymes, which are in charge of fat oxidation [19]. For the regulation of the plasma FFA supply to peripheral tissues, the flow of FFA from adipose tissue is predominant and this FFA supply may be extended if the rate of mobilization exceeds the rate of lipolysis in the post-exercise recovery period [2021]. It is also reported that 24-hour fat oxidation is increased, even under energy-balanced conditions, if the exercise-induced transient energy deficit is significant [22]. However, there is a report indicating that the lipid oxidation rate in the post-exercise period rate is not accompanied by an increase in plasma FFA [23]. Thus, the post-exercise lipid oxidation rate may be regulated both by the FFA supply and by energy source selection in tissues, and a higher plasma FFA level after resting in an animal in training may favor fat oxidation by increasing substrate availability.

Although there is some disagreement on the actual contribution of muscle TG as an energy source during exercise [24], it is generally accepted that the main fat energy sources are plasma FFA and FFA derived from intramuscular TG during exercise [12]. Although there is a report that the muscle TG pool functions as an important energy source during submaximal exercise [25], and there is some change in intramuscular TG pool size even after a single exercise session [26], it has been suggested that a concurrent increase in lipid oxidation, intramuscular TG utilization, and FFA storage associated with training could enable the use of intramuscular TG as an alternative energy source to glycogen as it is quickly available within the tissues [27]. The observation that muscle TG level decreased in the ER2:1 group and slightly increased in the ER1:1 group (although not to that of the NE group in training animals) suggests that the body starts to catabolize muscular TG during exercise and replenishes it during rest. Thus, a lower muscle TG level after rest in a training animal might be the result of an increased utilization of muscle fat in order to conserve carbohydrate storage after rest following exercise training. Protein also contributes as an energy source when endogenous carbohydrate stores start to be depleted. Thus, the level of protein in training animals in this study was expected to be increased in plasma and decreased in muscle during rest. However, the muscle TG levels of the ER groups were not different from that of the NE group, and the protein content after rest in non-training animals was similar at all exercise-to-rest period ratios. This unexpected result might be due to metabolic adaptation designed to achieve sufficient energy substrate levels that may not be fully achieved in the absence of exercise training.

Exercise training enables an animal to adapt to stress during exercise and induces a homeostasis that is associated with the increased aerobic metabolism [2829]. SOD activity of the non-training (training) animals in this study was increased (decreased) in the ER2:1 group, decreased (increased) in the ER1:1 group, and increased (decreased) again in the ER0.5:1 group, but the differences were not significant. Catalase activities in the ER groups of both training and non-training animals were not different from those in the NE group at all exercise-to-rest period ratios. The altered SOD levels in the ER1:1 group could be attributed to the substantial increase in MDA level among the non-training animals in the ER1:1 group. This suggests that an ER0.5:1 ratio may be considered a minimum exercise-to-rest period ratio to allow animals to recuperate their anti-oxidative status for the next exercise session in non-training animals. In the training animal group, catalase activity levels and MDA concentrations of the three ER groups were not different from those of NE group at all exercise-to-rest period ratios. These results are consistent with other reports showing that catalase activity was unaffected by exercise-induced oxidative stress in animals in training groups [1114]. Thus, the effectiveness of the anti-oxidative defense system might be enhanced by training. As a consequence, exercise-induced oxidative stress could be offset during exercise in training animals and might have resulted in the absence of significant differences in catalase activity levels and MDA concentrations among the NE and ER groups.

The results of this study suggest that animals need an exercise-to-rest period ratio of 0.5:1 to 1:1 in order to restore muscle energy sources and allow the anti-oxidative defense system to recover. Therefore, for athletes that are beginning exercise training, a proper rest period after exercise or between workouts is needed to restore energy source capacity and recover anti-oxidative system capability. However, for the already exercise-trained athlete, an appropriate rest period between workouts is helpful for replenishing energy stores but is ineffective in the recuperation of anti-oxidative status. As this study was designed to assess changes over the short term, the results of this study cannot be used to elucidate the chronic effects of different exercise-to-rest period ratios on the restoration of energy sources following exercise.