To our knowledge, this is the first study to investigate the two doses of post-exercise recovery CHO intake (30 and 60 g∙h− 1) compared to placebo on repeated sport-specific kicking performance across a simulated taekwondo competition day. In contrast to our hypothesis, CHO intake, independent of dose, failed to enhance total kicks, total successful kicks, and percentage of successful kicks compared to placebo.
Taekwondo is characterized by repeated bursts of high-intensity activities interspersed with brief rest periods. Oxidative phosphorylation contributes the greatest amount of ATP re-synthesis (~ 58–66%); primarily to support low-intensity actions and rest intervals , while non-oxidative energy systems (i.e., PCr) and anaerobic glycolysis contribute ~ 26–30% and 4–5%, respectively [2, 30]. Non-oxidative phosphorylation predominantly supports explosive actions, such as kicking, punching, explosive advances, and evasions during a match . Santos et al. reported that a single high-intensity action lasting 1 second was followed by 2–8 s of step or pause . Further, similar to our taekwondo test, a 1:2 effort-pause ratio during a taekwondo youth Olympic match was observed . As such, we modified the Frequency of Speed Kick test with 1:1 effort-pause ratio in taekwondo  to more closely simulate the effort-pause of a typical taekwondo match. Additionally, the use of this effort-pause ratio was able to improve Taekwondo-related performance .
CHO storage (i.e. muscle and liver glycogen) and oxidation play an important role during prolonged low-intensity and short duration high-intensity exercise performance . CHO ingestion before and/or during exercise delays peripheral fatigue (i.e., glycogen depletion) and enhances exercise performance . Ingestion of 30–90 g CHO∙h− 1 in a dose dependent manner enhances exercise capacity, potentially through maintenance of BG/glycogen and allowing greater CHO availability and utilization for anaerobic and aerobic glycolysis during exercise > 1 h . The rate of glycogen depletion during high-intensity exercise also appears to be an important factor associated with metabolic fatigue . As such, CHO ingestion during acute high intensity exercise has been shown to improve performance. For example, CHO ingestion enhanced short duration repeated sprints performance [14, 34]. However, these findings were in contrast to our results, possibly suggesting nutrient timing, duration and intensity of exercise, and recovery time intervals may modulate the ergogenic effects of CHO supplementation.
Taekwondo matches are comprised of intermittent high intensity burst of activity followed by periods of low intensity exercise or rest. During a tournament day these matches are repeated three to five times interspersed with passive rest (ranging from ~ 30 min to 3 h) . We hypothesized that CHO ingestion immediately following each sport-specific kicking test would maintain kick performance and euglycemia, across repeated tests. However, in contrast to our hypothesis, there was no statistical differences for percentage of successful kicks, total successful kicks and total kicks across the five tests with either a high (60 g∙h− 1) or low dose (30 g∙h− 1) of CHO compared to PLA. Within combat sports and competition setting, there is limited evidence pertaining to CHO supplementation and performance and presently it is difficult to compare across studies due to considerable methodological differences, such as different assessments, participant’s characteristics, and various types of CHO and dosages used. Nevertheless, previous research suggests that CHO ingested during short recovery periods can augment subsequent exercise capacity and glycogen storage in endurance athletes [7, 35]. In this regards, two recent studies reported that subsequent endurance exercise was improved after 1.2 g∙kg− 1∙h− 1 CHO was ingested during a 2–4 h recovery period followed by an intermittent high intensity aerobic test  and time to exhaustion at 70% VO2max . In the present study, an absolute dose of CHO was used for all participants. This dosing strategy was selected due to the relatively short recovery time and the practically of an absolute dose. Furthermore, the kick test and the repetitions of the test were selected to simulate a typical competition day, thus enhancing the external validity. In support of our results, Podlogar et al. 2020 failed to show significant improvements in subsequent exercise performance when either glucose alone or co-ingestion of glucose and fructose were compared to placebo, despite higher CHO oxidation rates following the ingestion of multiple transportable CHOs . In one of the few studies utilizing taekwondo athletes, no improvements in kick performance with a CHO mouth rinsing protocol in both fed or fasted states were found . Collectively, despite a likely enhancement of CHO exogenous and decreasing endogenous CHO oxidation after two doses of CHO drink , it appears that the test used in this study could not possibly results in enough muscle glycogen depletion which may justify the lack of kick performance improvement during five repeated intermittent kick tests with 1:1 effort-pause ratio.
In the present study BG was assessed since hypoglycemia is related to metabolic fatigue during exercise and is associated with impaired performance. Hypoglycemia is defined as a BG lower than 63 mg∙dL− 1 (3.5 mmol∙L− 1) . In the current study, both CHO doses were able to significantly elevate BG assessed after each test (high dose: first test 92.27 ± 7.5 mg/dL and last test 100.27 ± 13 mg/dL; low dose: first test 92.81 ± 5.5 mg/dL and last test 106.90 ± 11.08 mg/dL) compared to placebo (P ≤ 0.05). However, there was no indication of hypoglycemia in the PLA condition (first test 90.90 ± 8.1 mg/dL and last test 88.81 ± 5.6 mg/dL). This avoidance of hypoglycemia was likely attributed to the standardized breakfast (1 g∙kg− 1 CHO) provided 3 h prior to the first test, besides the liver glycogenolysis and gluconeogenesis, which may have provided sufficient fuel to maintain the glycaemia during the exercise protocol . Non-glucose substrates required for gluconeogenesis, such as glycerol and lactate are elevated during a simulated taekwondo tournament  due to the higher secretion of catecholamines  and cortisol . In agreement with our finding, no cases of hypoglycemia were reported (6.1 6 ± 0.8–5.6 ± 1.7 mmol/L from first to fourth combat) in ten elite taekwondo players during a tournament across 4 matches after consuming a pre-exercise CHO meal containing 2.7 to 3.7 g∙kg− 1 .
CHO ingestion did not alter RPE compared to PLA, which is consistent with previous research [36, 40]. Lastly, a mild to severe GI discomfort adverse event was reported following the high-dose of CHO ingestion (60 g∙h− 1) in one participant. These events are possibly related to the lower rate of gastric emptying following high intensity exercise thorough the splanchnic blood flow decrement (41).
This study also has several limitations. First, the task used in the present study, while taekwondo-specific, did not replicate all the demands and requirements of a real match with an opponent. Further, we only assessed kick performance, which is only one technique among several used in taekwondo. In addition, more research is needed to apply less or more than 45 min of recovery time between matches, as a match in a real tournament are separated by 30 min to 3 h. Future research may consider using an accelerometer attached to wireless sensors for measuring speed and force of each kick. Further, we did not assess muscle glycogen or measure CHO oxidation, which would be of interest in future research. Also, we only used male taekwondo athletes with a relatively low sample size without performing a power analysis. Therefore, future research examining female athletes along with a priori power analysis is required. Lastly, it is worth investigating the effect of CHO supplementation after acute weight loss during a competition day on combat athletes’ performance, which is a situation that muscle glycogen is likely depleted.
In conclusion, CHO ingestion, independent of dose, failed to enhance kicking performance in a simulated competition day in trained taekwondo athletes.