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00005768-200311000-0001900005768_2003_35_1901_timmons_carbohydrate_11miscellaneous-article< 101_0_17_4 >Medicine & Science in Sports & Exercise©2003The American College of Sports MedicineVolume 35(11)November 2003pp 1901-1907RPE during Prolonged Cycling with and without Carbohydrate Ingestion in Boys and Men[APPLIED SCIENCES: Psychobiology and Behavioral Strategies]TIMMONS, BRIAN W.; BAR-OR, ODEDChildren’s Exercise and Nutrition Centre, McMaster University, Hamilton, Ontario, CANADAAddress for correspondence: Oded Bar-Or, M.D., Children’s Exercise and Nutrition Centre, Chedoke Hospital, Evel Bldg. (4th floor), Sanatorium Road, Hamilton, Ontario, Canada L8N 3Z5; E-mail: baror@mcmaster.ca.Submitted for publication November 2002.Accepted for publication July 2003.AbstractTIMMONS. B. W., and O. BAR-OR. RPE during Prolonged Cycling with and without Carbohydrate Ingestion in Boys and Men. Med. Sci. Sports Exerc., Vol. 35, No. 11, pp. 1901–1907, 2003.Purpose: To examine the effect of prolonged cycling on ratings of perceived exertion (RPE) in boys and men and whether carbohydrate (CHO) ingestion would lower RPE during exercise.Methods: Ten boys (9–10 yr) and 10 men (20–25 yr) cycled for 60 min at ∼70% V̇O2peak on two occasions. In a double-blind, counterbalanced design, a total volume of 24 mL·kg−1 body mass of either a 6% CHO-electrolyte (CT) or flavored water (WT) beverage was consumed intermittently before and during exercise in each trial. Oxygen consumption (V̇O2), ventilation (V̇E), respiratory rate (RR), RPE (Borg’s 6–20 scale), and heart rate (HR) were recorded periodically throughout exercise. Plasma glucose (GLU) was determined before and after exercise.Results: Postexercise GLU was not different between age groups but higher (P < 0.001) during CT (5.6 ± 0.2 mmol·L−1) compared with WT (4.7 ± 0.1 mmol·L−1). CHO ingestion had no effect (P > 0.05) on V̇O2, V̇E, RR, or RPE in either group. RR during exercise was higher (P < 0.01) in boys (39.0 ± 2.2 breaths·min−1) than in men (30.9 ± 1.3 breaths·min−1). HR was slightly higher (P = 0.047) during CT (160 ± 3 beats·min−1) compared with WT (156 ± 4 beats·min−1) and increased less over time (P < 0.01) in boys compared with men. RPE at 5 min of exercise was similar (P > 0.05) between boys (11.8 ± 0.7) and men (12.0 ± 0.7) but increased faster (P < 0.01) over time in boys. The average exercise RPE was higher (P < 0.01) in boys (15.8 ± 0.5) than in men (14.0 ± 0.4).Conclusions: The higher and faster increase in RPE during exercise in boys, compared with men, may reflect a sensitivity to RR that outweighed any effect of CHO ingestion on RPE.The rating of perceived exertion (RPE) during exercise is a subjective indication of physiological cues, such as respiratory-metabolic (e.g., ventilatory drive) or peripheral (e.g., lactate accumulation) and psychological cues (e.g., cognitive style) arising from the activity (20). Borg’s 6–20 RPE scale, constructed to integrate the respiratory-metabolic and peripheral signals of physical strain (4), has been employed extensively in the adult literature with relatively fewer applications to the pediatric population (20). Although child-oriented scales have been developed to study perceptual responses to exercise in children (24,30), Borg’s 15-grade scale is considered to be both valid and reliable for use with children as young as 9 yr old (2,13,16).Compared with adults, children tend to assign a lower (2) or similar (14) RPE to the same relative exercise intensity. It has also been shown (2) that the ratio of RPE to heart rate (HR) tends to be lower in children than in adults, suggesting that for a given physiological strain, children rate exercise to be lighter. However, the majority of studies conducted with children, regardless of the effort scale used, have adopted only short-duration (e.g., ≤20 min) exercise of varying intensity (13). Consequently, there is a clear lack of research comparing RPE over a prolonged period of constant-load exercise between children and adults. Recently, Cheatham et al. (7) found that RPE (Borg’s 6–20 scale) increased faster over 40 min of cycling exercise at the same relative intensity in 10- to 13-yr-old boys compared with 18- to 25-yr-old men. These authors suggested that the boys may not have been accustomed to the exercise employed and, therefore, experienced a greater degree of muscle fatigue reflected by a faster increase in RPE.The relationship between fatigue and RPE has been studied, at least in adults, from an energy perspective. It is well known that the onset of fatigue can be delayed by ingesting carbohydrate (CHO) throughout exercise (8). In adults, the consumption of CHO, compared with water, can also lower RPE during cycling exercise (6,9,26,29). However, the benefit of CHO ingestion seems to become apparent only after ∼60 min of exercise, and although the mechanisms for this effect are not clear, they may be related to maintained levels of plasma glucose (9,19) and higher oxidation rates of the ingested CHO (6,25). We have also shown that glucose ingestion (∼1.4 g·kg−1 body weight) before and during 60 min of cycling at ∼60% V̇O2peak lowered RPE in healthy 13- to 19-yr-old adolescent boys but not in adolescent boys with insulin-dependent diabetes mellitus (22). In contrast, we have also reported that CHO ingestion (∼0.8 g·kg−1 body weight) had no effect on RPE during 50 min of intermittent exercise at ∼50% V̇O2peak in a warm environment (34–35°C, 42–45% relative humidity) in healthy 9- to 12-yr-old children (18). The different exercise intensities, environmental conditions, rate of CHO ingestion, and age of the subjects in our previous studies may explain these conflicting results. To our knowledge, no study has compared the effect of CHO ingestion on RPE in children and adults under identical experimental conditions.Therefore, the purpose of this study was to examine the effects of prolonged (i.e., 60 min) exercise and CHO ingestion on RPE in a group of 9- and 10-yr-old boys and a group of 20- to 25-yr-old men. We hypothesized that, during exercise, RPE would increase faster in the boys compared with the men, and that CHO ingestion would lower RPE in the boys but not in the men. We hypothesized that CHO ingestion would have no effect on RPE in the men because exercise duration was restricted to 60 min.METHODSSubjects.Ten boys and 10 men volunteered to participate in this study approved by the McMaster University Research Ethics Review Board and in compliance with the policy statement of the ACSM regarding the use of human subjects. Our sample size calculation was based on the work of Cheatham et al. (7). The difference in RPE between their boys and men produced an effect size (ES) of 1.25. With this ES, and a desired power of 0.9, we calculated that 10 boys and 10 men would be needed to detect a statistically significant difference, if one existed. Subjects were healthy and recreationally active but not competitive athletes. Table 1 summarizes their physical and fitness characteristics. To determine the pubertal status of each boy, the child and a parent were asked to assess pubic hair development according to the criteria of Tanner (27). Self-assessment of pubertal status according to pubic hair development has been shown to be valid among boys (17). Six boys were at Tanner stage 1, and four boys were at Tanner stage 2. The purpose, procedures, and risks of the study were explained to each subject and a parent, in the case of a child subject. The men signed a written informed consent and the boys assented verbally to participate, with each boy’s parent signing the written informed consent on his or her son’s behalf.TABLE 1. Physical and fitness characteristics of boys and men.Values are means ± SD.V̇O2peak, peak O2 uptake; HRpeak, peak heart rate; POpeak, peak power output.* Significantly different from men, P < 0.05.Preliminary session.This session was completed approximately 1 wk before the first experimental session and served to collect anthropometric data, including height (Harpenden Stadiometer, CMS Weighing Equipment, London), naked body weight (BW; BWB-800, Tanita, Japan), and percent body fat (Bioimpedance-Analyzer-101A, RJL Systems, Clinton Twp., MI). Borg’s 6–20 RPE scale was introduced and explained using instructions previously described (3). Briefly, subjects were told the following: “This scale is a method that allows us to know how hard you feel you are working, and we will show it to you from time to time during the exercise test. There are no right or wrong answers, but you must point to one of the numbers on the list: 6 is the lightest possible effort you can think of and 20 the hardest possible effort. Some of the numbers have words beside them, which are to help you remember what the number means. So, we will show you the scale, and you point to the number that best describes how hard you feel you are working.”Peak oxygen uptake (V̇O2peak) was determined by a maximal exercise test, as previously described (3), on a mechanically braked cycle ergometer (Fleisch-Metabo, Geneva, Switzerland). The pedaling rate remained constant at 60 rpm as work rate was increased every 2 min for the boys and every 3 min for the men until exhaustion, despite strong encouragement. Stage duration was different between groups to account for the fact that children reach steady state faster than do adults (1). V̇O2 and CO2 production (V̇CO2) were recorded continuously using a metabolic cart (Vmax29, SensorMedics, Yorba Linda, CA). HR was recorded continuously during the test using a Polar HR monitor (Polar Vantage XL, Polar Electro, Kempele, Finland).Experimental design.Two counterbalanced and double-blinded experimental sessions were separated by 1–2 wk and were identical except for the drink provided before and during exercise. Subjects were required to cycle at 70% of their individual V̇O2peak for two 30-min periods separated by 5 min of rest. Power output at 70% of V̇O2peak was calculated from a regression equation relating V̇O2 (y-axis) to power output (x-axis). Expired gas was collected for the first 5 min of exercise for each subject on his first experimental trial. Power output was adjusted accordingly to achieve the required V̇O2, and any adjustments made were repeated on their next visit, without the initial gas collection. After the initial 5 min, no additional changes in power output were made. In one trial, each subject consumed a 6% CHO-electrolyte (CT) solution (4% sucrose, 2% glucose, ∼18 mmol·L−1 Na+, ∼3 mmol·L−1 K+), and in the other trial, they drank artificially sweetened water (WT) that was identical in flavor (lemon-lime) and electrolyte concentration but without CHO. A total volume of 24 mL·kg−1 BW was divided equally into six drinks, each consumed at 15-min intervals during each trial. We have previously shown that a similar drinking protocol (i.e., CHO intake of ∼1.4 g·kg−1) raises blood glucose in children and adolescents during prolonged exercise (22). Both beverages were prepared in powder form by the Gatorade Sports Science Institute (Barrington, IL).Experimental protocol.Subjects arrived at the laboratory in the morning (∼0730) after an overnight fast and were given a small standardized breakfast (boys: 125-mL tap water and one slice of toast with sugar-free jam ∼90 kcal; men: twice that amount). We provided a small breakfast to better reflect a nonlaboratory environment and to provide some degree of external validity. After eating and emptying their bladder, a naked BW was taken (BWB-800) to calculate the volume of fluid intake for that session. Subjects then sat quietly for ∼20 min during which time the RPE scale was reintroduced and each subject’s understanding of the scale reconfirmed. After the 20-min rest period, a preexercise blood sample was drawn from an arm vein with a “butterfly” winged infusion set (Terumo, Japan), and the subject consumed his first beverage (4 mL·kg−1 BW). Subsequently, five more drinks of the same volume were consumed every 15 min. Exercise began 30 min after the resting blood sample, with the pedaling rate constant at 60 rpm. HR (Polar Electro) and RPE were recorded every 5 min during exercise. Subjects were blinded to their HR, which was always recorded before administration of the RPE scale. The RPE scale was always out of sight of the subject until the time of administration, and they were asked to assign an overall (i.e., whole body) feeling of exertion, with no attempt to differentiate exertional signals from the legs or chest. The ratio of RPE to HR was used to standardize the subjective indicator of exertion (i.e., RPE) to an objective indicator (i.e., HR). At the 12th and 27th min of each exercise bout, V̇O2 V̇CO2, minute ventilation (V̇E), and respiratory rate (RR) were measured from an expired gas sample collected over 3 min to ensure a steady-state period. For each gas collection, the respiratory exchange ratio (RER) was calculated from V̇O2 and V̇CO2 to determine the rate of whole-body CHO oxidation according to a table of nonprotein respiratory quotients (21). Within 30 s after exercise, a second blood sample was drawn while subjects were seated on the cycle ergometer. For the pre- and postexercise blood samples, whole blood (∼2 mL) was added to an EDTA-containing Vacutainer® (Becton Dickinson, NJ) and centrifuged (2000 ×g for 10 min at 5°C). The supernatant was removed and stored at −70°C for subsequent analysis of plasma concentrations of glucose [glucose] and lactate [lactate] (YSI 2300 L STAT, Yellow Springs Instruments, Yellow Springs, OH). Values were corrected for changes in plasma volume according to the hemoglobin and hematocrit method of Dill and Costill (10).Data analyses.Data are presented as means ± SE unless otherwise stated and were analyzed using a statistical software package (STATISTICA for Windows 5.0, StatSoft, OK). Physical and fitness characteristics of the groups were compared with independent t-tests. Significance for all variables measured repeatedly was determined using a three-way ANOVA with one between factor (group) and two within factors (trial and time). Where appropriate, a Tukey’s HSD post hoc test was used to determine the location of significance among means. The Pearson correlation was used to determine relationships between RPE and physiological variables within each group. The threshold for statistical significance was set at P < 0.05 for all tests.RESULTSExercise intensity.Exercise intensity was not different between CT and WT, respectively, when expressed as a percent of V̇O2peak (70.8 ± 1.2% vs 71.4 ± 1.4%; main effect trial, P = 0.54), peak HR (HRpeak, 79.8 ± 1.1% vs 77.8 ± 1.3%; main effect trial, P = 0.07), or peak power (PP, 52.0 ± 5.2% vs 51.9 ± 5.2%; main effect trial, P = 0.25). There was also no intergroup difference (i.e., boys vs men, respectively) in exercise intensity expressed as %HRpeak (78.8 ± 1.2% vs 78.8 ± 1.2%; main effect group, P = 0.96) or %PP (53.5 ± 4.9% vs 50.4 ± 5.4%; main effect group, P = 0.70). However, V̇O2peak was slightly, but significantly (main effect group, P = 0.01), lower in the boys (68.9 ± 1.5%) than in the men (73.3 ± 1.0%).Respiratory-metabolic responses.V̇O2, V̇E, the ventilatory equivalent of O2 (V̇E/V̇O2), and RR responses are presented in Table 2. CHO ingestion had no effect on any of these variables in either group. V̇O2 (mL·kg−1·min−1) was not different between groups (main effect group; F (1,18) = 0.20, P = 0.66) or trials (main effect trial, P = 0.57) and remained constant over time (main effect time; F (3,54) = 1.73, P = 0.20). V̇O2 was expressed relative to body weight to show that the metabolic cost of exercise was equal between the two groups and is an appropriate normalizing method to compare adults and children. During exercise, V̇E (L·min−1) was significantly (main effect group, P < 0.001) lower in the boys than in the men, whereas V̇E/V̇O2 was significantly (main effect group, P = 0.01) higher in the boys than in the men. During exercise, RR was significantly (main effect group, P < 0.01) higher in the boys than in the men and increased significantly (main effect time, P < 0.001) over time. Post hoc analysis found that the RR at 30, 45, and 60 min were not different from each other but were significantly (P < 0.01) higher than at 15 min.TABLE 2. Respiratory-metabolic responses to exercise in boys and men during flavored water and carbohydrate trials.Values are means ± SE.CT, carbohydrate trial; WT, flavored water trial; V̇O2, oxygen uptake; V̇E, minute ventilation; V̇E/V̇O2, ventilatory equivalent for oxygen; RR, respiratory rate.Main effect for group (V̇E, V̇E/V̇O2, RR), P < 0.05; Main effect for time (RR, values at 30, 45, and 60 min significantly higher than 15 min), P < 0.05.* Significantly different from men, P < 0.05.HR and RPE.Figure 1 shows the HR response to exercise for boys and men in CT and WT. HR increased over time in both groups during both trials (main effect time; F (11,198) = 17.62, P < 0.001) with the increase (i.e., HR drift) in the boys significantly less than in the men (group × time interaction, P < 0.01). The average HR during exercise was not significantly (main effect group, P = 0.66) different between groups. However, the average HR during CT (160 ± 3 beat·min−1) was significantly (main effect trial, P = 0.047) higher compared with WT (156 ± 4 beat·min−1).FIGURE 1— Heart rate response for boys (○, •) and men (□, ▪) during exercise (disregarding 5-min rest between exercise bouts) in flavored water (○, □) and CHO (•, ▪) trials. Values are means ± SE. Main effect for condition, group × time interaction, P < 0.05.CHO ingestion had no significant effect on RPE (main effect trial, P = 0.65) or the ratio of RPE to HR (main effect trial, P = 0.35) during exercise. Data from CT and WT were, therefore, pooled for each group before graphical presentation (Fig. 2). RPE increased over time during exercise in both groups (main effect time, P < 0.001), but the increase was significantly (group × time interaction, P < 0.01) faster in the boys compared with the men (Fig. 2A). Furthermore, the average RPE during exercise was higher (main effect group, P < 0.01) in the boys (15.8 ± 0.5) compared with the men (14.0 ± 0.4). Fig. 2B shows the change in the ratio of RPE to HR during exercise for both groups. Because the increase in HR was faster in the men than in the boys (i.e., group × time interaction), it was appropriate to calculate the ratio of RPE to HR in order to confirm the RPE results. Similarly to RPE, the ratio increased over time (main effect time, P < 0.001), and the increase was faster in the boys compared with the men (group × time interaction, P < 0.001). However, the ratio of RPE to HR failed to achieve a statistically significant group effect (main effect group, P = 0.10).FIGURE 2— A. Ratings of perceived exertion during exercise (disregarding 5-min rest between exercise bouts) in boys (•) and men (○). B. Ratio of ratings of perceived exertion to heart rate during exercise (disregarding 5-min rest between exercise bouts) in boys (•) and men (○). Values are means ± SE. Main effect for group, time and group × time interaction, P < 0.05; *significant difference between boys and men, P < 0.05.CHO oxidation.The rate of whole body CHO oxidation, relative to BW, was significantly (main effect group, P < 0.001) lower in the boys (28.0 ± 1.8 mg·kg−1·min−1) than in the men (35.6 ± 1.4 mg·kg−1·min−1). The average rate of CHO oxidation was not significantly different (main effect trial, P = 0.18) between CT (32.5 ± 1.9 mg·kg−1·min−1) and WT (31.1 ± 1.3 mg·kg−1·min−1) but decreased to a greater extent (trial × time interaction, P = 0.02) in WT than in CT. Post hoc analyses revealed no intertrial differences at any time point, except for the boys at the end of exercise (see below). During CT, CHO oxidation at 15 min of exercise was 29.8 ± 2.5 and 37.6 ± 2.2 mg·kg−1·min−1 in the boys and men, respectively, and remained relatively constant at 28.9 ± 2.4 mg·kg−1·min−1 in the boys (P = 0.99) and at 34.6 ± 1.5 mg·kg−1·min−1 in the men (P = 0.16) by the end of exercise. During WT, CHO oxidation decreased from 29.5 ± 1.3 to 24.9 ± 1.5 mg·kg−1·min−1 in the boys (P < 0.01) and from 38.5 ± 1.7 to 33.0 ± 1.0 mg·kg−1·min−1 in the men (P < 0.001).Plasma glucose and lactate.There was a trial × time interaction (F (1,18) = 21.68, P < 0.001) for [glucose] with preexercise values not different (P = 0.99) between CT (6.3 ± 0.3 mmol·L−1) and WT (6.4 ± 0.3 mmol·L−1). However, postexercise [glucose] was higher (P < 0.001) in CT (5.6 ± 0.2 mmol·L−1) than in WT (4.7 ± 0.1 mmol·L−1). There was no significant difference (main effect group, P = 0.43) in [glucose] between the boys and the men. Pre- and postexercise values for the boys were 6.0 ± 0.2 mmol·L−1 and 5.3 ± 0.2 mmol·L−1 and for the men were 6.7 ± 0.4 mmol·L−1 and 5.0 ± 0.1 mmol·L−1. During both trials, [lactate] increased (main effect time, P < 0.001) with exercise in the boys and men. Postexercise [lactate] was significantly lower (main effect group, P < 0.001) in the boys (1.9 ± 0.2 mmol·L−1) than in the men (3.8 ± 0.5 mmol·L−1) and significantly higher (main effect trial, P < 0.01) in CT (3.0 ± 0.4 mmol·L−1) than in WT (2.7 ± 0.3 mmol·L−1).Correlations.There was no correlation between the final RPE during exercise and postexercise [glucose] (r = 0.09, P = 0.56) or [lactate] (r = −0.12, P = 0.46). When all data points from CT and WT were combined, there was no correlation between RPE and V̇E (r = −0.02; P = 0.89), V̇E/V̇O2 (r = 0.02; P = 0.89) or RR (r = 0.07; P = 0.52) for the boys. Likewise, when data were combined for the men, there was no correlation between RPE and V̇E (r = −0.04; P = 0.73), V̇E/V̇O2 (r = −0.12; P = 0.28), or RR (r = −0.18; P = 0.12).DISCUSSIONWe compared the perceptual responses between boys and men during prolonged cycling performed at a similar relative intensity, with and without the ingestion of CHO. Based on previous findings (7), we expected the boys to rate the effort of the exercise task higher than the men. Our data supported this hypothesis as the boys assigned a higher RPE, using Borg’s 6–20 scale, to 60 min of cycling at ∼70% V̇O2peak than the men (Fig. 2A). We also hypothesized that CHO ingestion during exercise would lower RPE in the boys but not in the men. As expected, there was no effect of CHO on RPE in the men, but contrary to our hypothesis, CHO ingestion also had no effect on RPE during prolonged cycling in our group of boys.In recent years, investigators have created new scales in an attempt to better meet the cognitive functioning of young children (24,30). However, it is generally accepted that children as young as 9 yr old (2,19), with adequate instruction, can understand and respond appropriately to Borg’s original 6–20 scale. We made every effort to ensure that our subjects were familiar with and understood the RPE scale before experimental trials, and, therefore, we feel that our data reflect valid perceptual responses of the boys and the men and not spurious results due to a lack of understanding.Previous research comparing RPE between children and adults, for the most part, has used exercise protocols of short duration (e.g., ≤20 min) and of varying intensity (e.g., ramp-type protocols). These types of designs suggest that children have a lower (2) or similar (14) RPE, compared with adults, for a given relative intensity of exercise. Our findings agree with Mahon et al. (14) in that at 5 and 10 min of exercise, RPE was similar among boys and men (Fig. 2A). However, a recent study by Cheatham et al. (7) showed that, compared with men, boys actually had a higher RPE during moderate intensity exercise lasting 40 min. We extend these findings by showing that the RPE of boys is significantly higher than of men during high-intensity exercise lasting 60 min (Fig. 2A) when performed at 70% V̇O2peak. We also confirm and extend findings (7,14) showing that the increase in RPE during constant-load exercise is faster in boys than in men (Fig. 2A). Our RPE findings are not confounded by group differences in the HR response to exercise (see Results) because the ratio of RPE to HR also responded in the same manner (Fig. 2B). This ratio is a useful index of subjective and objective strain (2) and, in the present study, indicates that exercising at a given physiological strain (e.g., HR) is perceived to be harder by the boys than by the men.The explanation for a higher and a faster increase in RPE during prolonged exercise in children compared with adults is difficult to establish. Respiratory-metabolic factors (e.g., V̇E, V̇E/V̇O2 and RR) believed to mediate respiratory-metabolic exertional signals were not correlated with RPE for either the boys or the men in the present study. Both V̇E/V̇O2 and RR were significantly higher during exercise in the boys than in the men (Table 2). Although RR increased from 15 to 30 min of exercise, it remained constant thereafter. This suggests that children perceive a sustained physiological response (e.g., RR) differently than do adults. A higher RR in the boys, compared with the men, may have resulted in a relatively higher degree of respiratory muscle fatigue, resulting in a higher exertional signal and RPE. However, not differentiating the exertional signal from breathing in this study limits our interpretation of the results, but it may be that RR is a critical factor responsible for the current and previous findings indicating differences in RPE between children and adults during prolonged exercise. Future studies should compare RPE among different age groups exercising at the same absolute RR in order to confirm, or refute, this hypothesis.The perception of effort during exercise can also be influenced by peripheral factors. Lactate accumulation is associated with an increase in RPE (5,6). However, we found that even with a higher RPE during exercise, boys had a lower plasma [lactate] than the men. A lower [lactate] in children, compared with adults, during exercise is a well-established phenomenon (3) even at intensities above ventilatory threshold (14). The frequency of muscle contraction (i.e., pedaling rate) may also influence RPE. Although pedaling rate was kept constant between our boys and men (60 rpm), sustaining this frequency may have resulted in a stronger exertional signal for the boys due to qualitative differences in the working muscles, as previously suggested by Mahon et al. (15). Indeed, we have shown that the co-contraction index (CI; an indication of coactivation of antagonist muscles) of the thigh muscle during treadmill running at ∼70% V̇O2peak is higher in 10- to 12-yr-old children compared with 15- to 16-yr-old adolescents (12). A higher CI in young children implies that they unnecessarily activate their muscles. Whether this occurs during cycling (e.g., to sustain a required pedaling frequency) is not known but may contribute to the higher RPE observed in the present study through an increase in central feedforward commands from the motor cortex. Interestingly, Zanconato et al. (31) found that the O2 cost of high-intensity cycling normalized to the actual work done is higher in children than in adults. Future studies comparing electromyographical responses in children and adults during cycling exercise may provide more insight into this issue. Alternatively, scaling work intensity to leg-muscle mass may also resolve apparent differences between children and adults in the perceptual cues arising from high-intensity cycling.One limitation in this study is that we did not differentiate between respiratory-metabolic (e.g., chest) and peripheral (e.g., legs) RPE. We asked each subject to assign a rating according to their feeling of whole-body exertion. Interestingly, Mahon et al. (15) found that, compared with adults, children reported a higher RPE for their legs than their chest at the time corresponding to their ventilatory threshold during a graded maximal exercise test. However, it has been suggested that at higher exercise intensities, such as in the current study (∼70% V̇O2peak), ventilation becomes a potent sensory signal contributing to overall RPE (23). Our results suggest that, at a high exercise intensity, the perception of breathing dominates the RPE response in children, although this possibility requires further study.In adults, CHO ingestion usually lowers the RPE during cycling exercise if performed longer than ∼60 min (6,9,26,29). Therefore, it was not surprising that RPE was the same during CT and WT for our men, considering that exercise duration was restricted to 60 min (Fig. 2A). However, contrary to our hypothesis, CHO ingestion (∼1.4 g·kg−1) also had no effect on RPE in our boys.We have previously reported that glucose ingestion (∼1.4 g·kg−1) lowered RPE during 60 min of cycling in healthy adolescent boys (22). However, subjects in that study were not blinded to the test beverages, and the glucose trial always followed the water trial. The experimental trials in the current study were double blinded and counterbalanced. Therefore, methodological differences likely explain the contrasting results in these two studies. In another study (18), which was counterbalanced and double blinded, we found that CHO ingestion (∼0.8 g·kg−1) had no effect on RPE during relatively low-intensity exercise performed in a warm environment. Based on our previous (18) and current findings, it seems that CHO ingestion during exercise, lasting 60 min or less in duration, has no effect on RPE in young children, regardless of the intensity or environmental conditions. Our observations are, however, restricted to the laboratory setting and future work describing perceptual responses to more prolonged exercise performed during “real-life” field events (e.g., soccer tournaments, triathlons, etc.) when CHO availability may become limiting is needed. The fact that [glucose] did not reach hypoglycemia in the men is the most likely explanation for why CHO had no effect on RPE in this group. Generally speaking, exercise duration must be greater than 60 min in order to see an effect of CHO ingestion on RPE in adult men (6,9,26,29).That CHO ingestion did not lower RPE during exercise in the boys is somewhat surprising if one considers the mechanisms by which CHO is thought to lower RPE in adults. One mechanism suggested for adults is the high rate of oxidation of the ingested CHO (6,25). In a companion study of the same subjects as in the current experiment, we have shown that the oxidation rate of the ingested CHO, measured with 13C stable isotope and expressed relative to BW, was considerably higher (∼37%) in the boys than in the men (28). Therefore, it seems that this peripheral mechanism believed to lower RPE during exercise in adults, when fed CHO, is not at work in children. Alternatively, RPE may be more associated with muscle glycogen, not glucose supply. During prolonged exercise, fatigue can occur when muscle glycogen becomes very low, but blood glucose remains at euglycemia levels (11), suggesting that the energy status of the muscle cell per se may exert a greater influence on RPE than exogenous substrate availability. If glycogen depletion results in greater motor unit activation due to enhanced central motor outflow commands (i.e., a feedforward mechanism), exertional perceptions may be intensified independent of circulating energy substrates. In our boys, the contribution of endogenous CHO stores (i.e., muscle and liver glycogen) to total energy expenditure during WT was considerably smaller (∼65%) than in the men (∼83%). It may be that due to children’s inherently lower reliance on glycogen for energy during exercise, compared with adults, sparing muscle glycogen utilization further, with exogenous CHO, has a minimal effect on RPE. This may help explain why CHO ingestion had no RPE-lowering effect in our boys.In summary, this study is the first to compare the effects of CHO ingestion on RPE in a group of boys and men exercising under identical experimental conditions. During 60 min of cycling performed at ∼70% V̇O2peak, RPE was higher in 9- and 10-yr-old boys compared with 20- to 25-yr-old men, and the increase in RPE over time was faster in the boys than in the men. CHO ingestion had no effect on RPE in either group. 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