Abstract
he aim of the present study was to test the effectiveness of carbohydrate (CHO) feeding supplemented every 2.5-km, as in of cial races, on the performance, rating of perceived exertion (RPE), and glycaemia during a 10-km intermittent training workout in elite open-water swimmers. A randomized crossover design was used. Participants completed two 10-km intermittent training sessions (20 500-m). The relative velocity was expressed in percentage of a single 500-m. Glycaemia was monitored by continuous glucose monitoring. Participants had to ingest either 1 L of tap water (WAT; 0.50 L h 1 ) or 120 g of CHO in the form of 8% solution (60 g h 1 ). The 15-point RPE scale was used during the trials. A two-way ANOVA for repeated measures was performed (p< 0.05). The relative velocity of each 500-m was not signi cantly different between the two trials. No signi cant differences emerged in the relative velocity of the last 500-m between trials. Average RPE was not statistically different between the two trials (11 3 in WAT and 12 3 in CHO). In the last 500-m, glycaemia was signi cantly higher in the CHO trial (5.92 0.47 mmol L 1 in CHO; 5.61 0.61 mmol L 1 in WAT). CHO ingestion did not improve performance or affect RPE during a 10-km intermittent training in elite open-water swimmers. Keywords:endurance; performance; nutrition 1. Introduction Competitive swimming includes 17 pool individual events from 50-m to 1500 m (21 s to approximately 15 min) and three open water
trial (5.92 0.47 mmol L 1 in CHO; 5.61 0.61 mmol L 1 in WAT). CHO ingestion did not improve performance or affect RPE during a 10-km intermittent training in elite open-water swimmers. Keywords:endurance; performance; nutrition 1. Introduction Competitive swimming includes 17 pool individual events from 50-m to 1500 m (21 s to approximately 15 min) and three open water events: 510 and 25-km, lasting from less than 1 h to approximately 6 h. Independently, distance elite swimmers perform approximately 5580 km per week [13] and frequently participate in multiple high-volume training sessions per day [3]. Open-water swimming races include three of cial race distances: 5, 10, and 25-km, but only the 10-km is an Olympic event since 2008 [4]. The 10-km race is an event that lasts around 1 h 50 min for the best male and 1 h 56 min for the best female swimmers. Environmental challenges (unpredictable waves, tides, and currents) may have an in uence on the effective distance covered by swimmers and also on their nutritional strategies [5]. Currently, both the shorter (510-km) and the longer (25-km) open-water events are completed on a loop course (normally of 2.5-km), and at the end of each loop oating or stationary feeding stations are positioned. Accredited handlers are allowed to pass solids and liquids through long poles with a cup or delivery vessel attached [6]. However, contrary to running, feeding for swimmers is unpractical. First, the feeding stations are positioned off the race Sports2018,6, 147; doi:10.3390/sports6040147
Sports2018,6, 147 2 of 11 line, therefore, swimmers are forced to deviate from the optimal line and from drafting; and secondly, athletes normally roll on their back once they collect their cup, and then roll back over and this might bring about a decrease in swimming speed [6]. Because the duration and intensity of a 10-km open water race corresponds to the theoretical limits of glycogen storage [6], athletes are aware of the importance of carbohydrate (CHO) supplementation during the race [7]. The ergogenic effects of CHO feeding during continuous and intermittent endurance exercise have been consistently demonstrated in numerous studies [812] and reviews [1318]. During prolonged strenuous endurance tasks, there is a drop in the total carbohydrate oxidation rate, depletion in skeletal muscle glycogen, and a decline in blood-glucose availability to the contracting muscle, probably secondary to liver glycogen depletion [15]. Speci cally, the high volume training adopted by elite swimmers can signi cantly deplete muscle glycogen stores [19], and carbohydrate intake during a workout can provide additional fuel to support performance in a particular session [6,20]. Ingestion of carbohydrates during prolonged endurance exercise produces several bene cial effects, such as an attenuation in central fatigue, a better maintenance of CHO oxidation rates, muscle glycogen sparing, changes in muscle metabolite levels, a reduction of stress hormones and in ammatory cytokines, and a better maintenance of excitation-contraction coupling [13]. Costill et al.[19] demonstrated that swimmers who did not increase carbohydrate intake in response to an increased training volume were unable to complete ef ciently the programmed training compared to swimmers who increased carbohydrate intake to maintain muscle glycogen stores. The ergogenic effect of glucose supplementation is often ascribed to a higher glucose uptake by the exercising muscles, thereby allowing a suf cient carbohydrate oxidation late in exercise when muscle glycogen levels are low [9,11]. Moreover, CHO ingestion has been shown to reduce the rating of perceived exertion (RPE) during continuous endurance exercise [8,2123]. However, the current nutritional guidelines utilized by open-water swimmers are extrapolated from other sports with a similar duration and physiological requirements [5,6,24]. In fact, the vast majority
suf cient carbohydrate oxidation late in exercise when muscle glycogen levels are low [9,11]. Moreover, CHO ingestion has been shown to reduce the rating of perceived exertion (RPE) during continuous endurance exercise [8,2123]. However, the current nutritional guidelines utilized by open-water swimmers are extrapolated from other sports with a similar duration and physiological requirements [5,6,24]. In fact, the vast majority of studies have been performed with endurance athletes (runners or cyclists) performing continuous exercise, and very little research is available regarding nutritional practices [24] and fuel utilization [18] during training and competition in highly trained athletes and, speci cally, open-water swimmers [5]. Therefore, the aim of the present study was to test the effectiveness of CHO feeding supplemented every 2.5-km, as in of cial races, on performance, RPE, and glycaemia during a 10-km intermittent training workout in elite open-water athletes. It was hypothesized that CHO supplementation guidelines, extrapolated from other sports with a similar duration, may be suitable for swimmers, with the main effect of improving performance and reducing RPE. 2. Methods 2.1. Participants Ten elite open-water swimmers (6 males and 4 females) volunteered to take part in this study. Volunteers mean age, height, and weight were 22 5 years (23.5 5 males and21 2 females),1.76 0.05 m (1.78 0.04 males and 1.73 0.06 females), 70.5 7 kg (72.50 6 males and65.33 6 females ) respectively. The inclusion criteria required participants to have performed at least one international open-water competition (World Cup, European championships, World championships, or Olympic Games), the ability to swim 5000-m in a 50 m swimming pool within 55 and 60 min, respectively, for male and female participants, and no history of metabolic diseases. Participants were informed about the purpose and procedures of the study, which was approved by the local ethical committee (code CARD2018/10) in accordance with the Declaration of Helsinki. All athletes provided written consent before participation.
Sports2018,6, 147 3 of 11 2.2. Experimental Design Participants completed two main trials, each separated by 2 days of rest, and the order was randomized to counteract order effects. Both trials were conducted under similar environmental conditions, in a 25-m indoor swimming pool, with a water temperature of 27 C. The day before the rst trial, a continuous glucose monitoring system (CGMS) was applied on the left side of the lower back, allowing a correct calibration of the system. Participants were asked to consume a high-carbohydrate diet (810 g kg 1 per day [6,25]) during the 2 days prior to the rst trial as they would normally do before a race. Food and drink intake were recorded by a personal diary and the diet was replicated before the second trial. Athletes arrived in the swimming pool at 8.00 a.m., following an 89 h sleep. Each participant consumed his or her regular pre-race breakfast at least 2 h before the trial. After a standard warm up (30-min), athletes performed a 10-km intermittent training session, divided in 20 sets of 500-m. This is a typical session utilized by the coach, several times during a season, to monitor training progression; in fact, although elite open-water swimmers perform the majority of their training below the rst ventilatory threshold, about 23% is performed at higher intensities [1]. Therefore, athletes were all familiarized with this type of training. The participants were divided in four time-groups, with different xed restart times in each set (5:50; 6:00; 6:15; 6:30 min:s). The coach de ned the rest time through a single 500-m (500max) performed 1 month prior to the rst trial. Participants were instructed to perform the rst 9.5-km of training at an intensity between 80% and 90% of their 500max and the last 500-m as fast as possible, replicating a typical open-water swimming race strategy [26,27]. During both sessions, verbal encouragements, technical recommendations, and feedback on split times were given by the coach to all athletes. Each athlete swam in his or her own lane and next to another athlete within the same time-group. In order to evaluate
their 500max and the last 500-m as fast as possible, replicating a typical open-water swimming race strategy [26,27]. During both sessions, verbal encouragements, technical recommendations, and feedback on split times were given by the coach to all athletes. Each athlete swam in his or her own lane and next to another athlete within the same time-group. In order to evaluate the performance and reduce the velocity variability in the female and male participants, the relative velocity was expressed as a percentage of personal 500max(%-500max). On each occasion, participants were asked to ingest either tap water (WAT) or a solution of water plus carbohydrate (CHO). During the trials, swimmers drank three times every 2.5-km in both conditions in order to simulate feeding zones during the race. In the WAT trial, the swimmers ingested approximately 1.5 L of tap water, ~0.47 L each time (~0.50 L h 1 ), as indicated by the guidelines to prevent dehydration status [28]. In the CHO trial, the swimmers ingested 120 g of carbohydrate in the form of 8% solution (glucose:fructose ratio of 1:1; Enervitene Sport Cheerpack, Enervit©, Milan, Italy) with an ingestion rate of 60 g h 1 (~0.47 l of water plus 40 g of carbohydrate each time), as indicated by guidelines for endurance exercise of the same time duration [6,14,15]. The 15-point Rating of Perceived Exertion Scale (RPE) [29] was used to assess perceived exertion during the trials. The scale ranges from 6 to 20, with verbal-anchors ranging from no effort to maximum effort. RPE was administered every 1000-m and after training through a clearly visible poster. All athletes had been familiarized with the RPE scale before the rst trial. 2.3. Continuous Glucose Monitoring Glycaemia was monitored by a Continuous Glucose Monitoring System (CGMS ® iPRO TMMedtronic©, Northridge, CA, USA). Participants were instructed on the use of the device and were asked to measure capillary blood glucose four times daily using a personal glucometer (Contour ® Next Link, Bayer, Germany). These measurements were used to calibrate the sensor. Interstitial glucose was continuously measured in the subcutaneous tissue every 5 min during the trials.
(CGMS ® iPRO TMMedtronic©, Northridge, CA, USA). Participants were instructed on the use of the device and were asked to measure capillary blood glucose four times daily using a personal glucometer (Contour ® Next Link, Bayer, Germany). These measurements were used to calibrate the sensor. Interstitial glucose was continuously measured in the subcutaneous tissue every 5 min during the trials. The accuracy and validity of the CGMS during exercise was assessed in previous studies [30,31], and has been shown to be waterproof. The device was applied on the left side of the lower back, in order to allow all swimmers the possibility to do their normal freestyle ip turn during both trials.
Sports2018,6, 147 4 of 11 2.4. Statistical Analysis Data are presented as mean standard deviation (SD). The con dence interval at 95% of the difference of the means (95% CI), Hedge's G effect size (g), and observed power were indicated when appropriate. All statistical analysis was performed using the statistical software, PASW statistics 22 (SPSS Inc., Chicago, IL, USA). All data were tested for normal distribution using a Shapiro-Wilk test and the sphericity was checked with the Mauchly's test. A two-way ANOVA for repeated measures was used to assess the different effects of CHO and WAT on RPE, glycaemia, and velocity. When a signi cant F-value was achieved, Bonferroni adjustment procedure was performed to locate the pairwise differences. Where appropriate, comparison of variables between two conditions was conducted by using a Student'st-test for paired samples. The level of signi cance was set atp 0.05. 3. Results 3.1. Performance The absolute and relative velocity expressed as a percentage of 500maxis reported in Table. No signi cant differences emerged in the relative velocity between the trials (F(1,9) = 0.002,p= 0.963, 95% CI [ 0.626, 0.601], observed power 0.050, g = 0.019; Table). No order effect was evident between the rst and second trial (F(1,9) = 0.511,p= 0.493, 95% CI [ 0.408, 0.785], observed power 0.511,g = 0.180). Pacing was not different between the two trials (F(1,9) = 0.000,p= 1.00; Figure). The relative velocity from the rst to the last 500-m (t(9) = 0.579,p= 0.577, CI [ 5.082, 3.010]; 3.59 3.41% in CHO and 4.63 4.08% in WAT) and from the 19th to the last 500-m (p= 1.000; 0.86 1.76% in CHO and1.80 2.37% in WAT) were not signi cantly different between the two trials. No signi cant differences emerged in the relative velocity of the last 500-m between the two trials.Sports 2018, 6, x FOR PEER REVIEW 4 of 11 2.4. Statistical Analysis Data are presented as mean ± standard deviation (SD). The confidence interval at 95% of the difference of the means (95% CI), Hedge’s G effect size (g), and observed power were indicated when appropriate. All statistical analysis was performed
velocity of the last 500-m between the two trials.Sports 2018, 6, x FOR PEER REVIEW 4 of 11 2.4. Statistical Analysis Data are presented as mean ± standard deviation (SD). The confidence interval at 95% of the difference of the means (95% CI), Hedge’s G effect size (g), and observed power were indicated when appropriate. All statistical analysis was performed using the statistical software, PASW statistics 22 (SPSS Inc., Chicago, IL, USA). All data were tested for normal distribution using a Shapiro-Wilk test and the sphericity was checked with the Mauchly’s test. A two-way ANOVA for repeated measures was used to assess the different effects of CHO and WAT on RPE, glycaemia, and velocity. When a significant F-value was achieved, Bonferroni adjustment procedure was performed to locate the pairwise differences. Where appropriate, comparison of variables between two conditions was conducted by using a Student’s t-test for paired samples. The level of significance was set at p ≤ 0.05. 3. Results 3.1. Performance The absolute and relative velocity expressed as a percentage of 500 max is reported in Table 1. No significant differences emerged in the relative velocity between the trials (F(1,9) = 0.002, p = 0.963, 95% CI [−0.626, 0.601], observed power 0.050, g = 0.019; Table 1). No order effect was evident between the first and second trial (F(1,9) = 0.511, p = 0.493, 95% CI [−0.408, 0.785], observed power 0.511, g = 0.180). Pacing was not different between the two trials (F(1,9) = 0.000, p = 1.00; Figure 1). The relative velocity from the first to the last 500-m (t(9) = −0.579, p = 0.577, CI [−5.082, 3.010]; 3.59 ± 3.41% in CHO and 4.63 ± 4.08% in WAT) and from the 19th to the last 500-m (p = 1.000; 0.86 ± 1.76% in CHO and 1.80 ± 2.37% in WAT) were not significantly different between the two trials. No significant differences emerged in the relative velocity of the last 500-m between the two trials. Figure 1. Median and interquartile ranges of the velocity (m . s −1 ) maintained during each km in the CHO and WAT
= 1.000; 0.86 ± 1.76% in CHO and 1.80 ± 2.37% in WAT) were not significantly different between the two trials. No significant differences emerged in the relative velocity of the last 500-m between the two trials. Figure 1. Median and interquartile ranges of the velocity (m . s −1 ) maintained during each km in the CHO and WAT trials. Boxplot lower and upper whiskers represent the minimum and maximum value, respectively. Table 1. Velocity data in CHO and WAT trials. CHO WAT 500 max Average 1st 19th 20th Average 1st 19th 20th Velocity (m·s −1 ) 1.46 ± 0.05 1.44 ± 0.04 1.48 ± 0.07 1.50 ± 0.07 1.46 ± 0.06 1.44 ± 0.04 1.48 ± 0.08 1.51 ± 0.08 1.60 ± 0.06 %-500max (%) 87.14 ± 3.33 86.28 ± 2.54 88.64 ± 4.21 89.39 ± 4.32 87.16 ± 3.61 86.20 ± 2.60 88.63 ± 4.72 90.20 ± 4.67 Males 12345678910 1.3 1.4 1.5 1.6 1.7 km Velocity (m·s −1 ) CHO WAT Figure 1. Median and interquartile ranges of the velocity (m.s 1 ) maintained during each km in the CHO and WAT trials. Boxplot lower and upper whiskers represent the minimum and maximum value, respectively.
Sports2018,6, 147 5 of 11 Table 1.Velocity data in CHO and WAT trials. CHO WAT 500 max Average 1st 19th 20th Average 1st 19th 20th Velocity (m s 1 ) 1.46 0.05 1.44 0.04 1.48 0.07 1.50 0.07 1.46 0.06 1.44 0.04 1.48 0.08 1.51 0.08 1.60 0.06 %-500max (%) 87.14 3.33 86.28 2.54 88.64 4.21 89.39 4.32 87.16 3.61 86.20 2.60 88.63 4.72 90.20 4.67 Males Velocity (m s 1 ) 1.49 0.05 1.47 0.03 1.51 0.08 1.52 0.08 1.50 0.05 1.47 0.03 1.53 0.06 1.56 0.05 1.64 0.04 %-500max (%) 89.01 2.82 87.80 1.56 90.43 4.51 90.76 4.78 89.34 3.02 87.83 1.83 91.60 3.61 93.20 3.15 Females Velocity (m s 1 ) 1.41 0.03 1.41 0.03 1.44 0.03 1.46 0.05 1.40 0.02 1.40 0.02 1.41 0.01 1.43 0.03 1.55 0.03 %-500max (%) 84.35 1.68 84.01 1.95 85.94 1.77 87.35 2.94 83.89 1.06 83.76 1.24 84.18 0.88 85.70 1.94 500max= seasonal best time on 500-m;%-500max= velocity expressed as % of 500max. 3.2. Rating of Perceived Exertion RPE did not show a signi cant main effect between the two conditions (F(1,9) = 1.922,p= 0.199, 95% CI [ 1.158, 0.278], observed power 0.237, g = 0.333), and order effect between the rst and second trial (F(1,9) = 4.378,p= 0.066, 95% CI [ 0.049, 1.249], observed power 0.464, g = 0.319). The average RPE was 11 3 and 12 3 in the CHO and WAT trials, respectively (Table). A signi cant main effect was observed in the time course in both conditions (F(9,81) = 36.158, p< 0.001, observed power 1.00; Figure); RPE increased signi cantly over time across both conditions with the highest value observed at the end of the exercise. The percentage increment of RPE from the rst to the last 500-m was not signi cantly different between the two trials (t(11) = 0.469,p= 0.579, 95% CI [ 3.010, 5.082]; 70.27 37.01% in CHO and 74.99 47.61%).Sports 2018, 6, x FOR PEER REVIEW 5 of 11 Velocity (m·s −1 ) 1.49 ± 0.05 1.47 ± 0.03 1.51 ± 0.08 1.52 ± 0.08 1.50 ± 0.05 1.47 ± 0.03 1.53 ± 0.06
500-m was not signi cantly different between the two trials (t(11) = 0.469,p= 0.579, 95% CI [ 3.010, 5.082]; 70.27 37.01% in CHO and 74.99 47.61%).Sports 2018, 6, x FOR PEER REVIEW 5 of 11 Velocity (m·s −1 ) 1.49 ± 0.05 1.47 ± 0.03 1.51 ± 0.08 1.52 ± 0.08 1.50 ± 0.05 1.47 ± 0.03 1.53 ± 0.06 1.56 ± 0.05 1.64 ± 0.04 %-500max (%) 89.01 ± 2.82 87.80 ± 1.56 90.43 ± 4.51 90.76 ± 4.78 89.34 ± 3.02 87.83 ± 1.83 91.60 ± 3.61 93.20± 3.15 Females Velocity (m·s −1 ) 1.41 ± 0.03 1.41 ± 0.03 1.44 ± 0.03 1.46 ± 0.05 1.40 ± 0.02 1.40 ± 0.02 1.41 ± 0.01 1.43 ± 0.03 1.55 ± 0.03 %-500max (%) 84.35 ± 1.68 84.01 ± 1.95 85.94 ± 1.77 87.35 ± 2.94 83.89 ± 1.06 83.76 ± 1.24 84.18 ± 0.88 85.70 ± 1.94 500max = seasonal best time on 500-m; %-500 max = velocity expressed as % of 500max. 3.2. Rating of Perceived Exertion RPE did not show a significant main effect between the two conditions (F(1,9) = 1.922, p = 0.199, 95% CI [−1.158, 0.278], observed power 0.237, g = −0.333), and order effect between the first and second trial (F(1,9) = 4.378, p = 0.066, 95% CI [−0.049, 1.249], observed power 0.464, g = 0.319). The average RPE was 11 ± 3 and 12 ± 3 in the CHO and WAT trials, respectively (Table 1). A significant main effect was observed in the time course in both conditions (F(9,81) = 36.158, p < 0.001, observed power 1.00; Figure 2); RPE increased significantly over time across both conditions with the highest value observed at the end of the exercise. The percentage increment of RPE from the first to the last 500-m was not significantly different between the two trials (t(11) = −0.469, p = 0.579, 95% CI [−3.010, 5.082]; 70.27 ± 37.01% in CHO and 74.99 ± 47.61%). Figure 2. Median and interquartile ranges of RPE reported during each km in the CHO and WAT trials. Boxplot lower and upper whiskers represent the
Description
This study evaluates the impact of carbohydrate supplementation on swimming performance.