The ergogenic effects of glucose ingestion either before (29) or during (21) sustained (>60 minutes) bouts of exercise are well documented (26). However, the effect of glucose supplementation on performance of shorter duration (<60 minutes) is inconsistent, with only a limited number of studies reporting some improvements in performance (1,13,15,27,28), wherein 2 of these studies had a duration greater than 50 minutes (1,15). Additionally, although the study by Lee et al. (13) demonstrated improved performance during multiple short-duration (2 × 30-second efforts interspersed with 10 × 10-second efforts) cycling bouts after ingestion of carbohydrate, this benefit was ascribed to improved performance in the first 30 seconds' effort only.
With respect to the role of carbohydrate supplementation in resistance training and force output, the literature is equally conflicting. Some studies have reported a benefit in time to exhaustion tasks (∼16 minutes vs. 29 minutes, placebo vs. carbohydrate [PL vs. CHO]; 50% Maximum Voluntary Contraction [MVC] (27,28)) and performance over multiple resistance training sessions (8), whereas others observed no improvements in either performance (12,14,25) or perceived exertion (24) with dietary carbohydrate manipulation or acute carbohydrate ingestion. Given the ingestion of carbohydrate has other potential benefits (e.g., promoting an anabolic environment (23)) and has not previously been associated with decrements in performance, the ingestion of carbohydrate is still generally recommended for resistance training (7,19).
More recently, studies have demonstrated that a carbohydrate mouth rinse at regular intervals can stimulate central motor drive and reduce perceived exertion during exercise (4,6). Specifically, the presence of carbohydrate in the mouth was shown to facilitate corticomotor output and increase maximal voluntary force (6). This provides an additional previously unrecognized mechanism by which endogenous glucose may improve exercise performance. Based on the current knowledge, we would anticipate the ergogenic effects of endogenous glucose to occur either (a) shortly after the ingestion of glucose in response to stimulation of glucose-sensitive receptors in the oral cavity (6,10), or (b) when blood glucose concentration peaks, thereby increasing total availability of glycolytic substrate (21) and/or regulating muscle activity, specifically by altering electrical properties of the muscle membrane (5,11), which is associated with increased maximum dynamic force (11). To our knowledge, no previous research has assessed changes in force output after glucose ingestion with respect to time. Because multiple potential mechanisms explaining the ergogenic role of glucose exist and time to peak blood glucose concentration after ingestion of glucose varies between individuals, it seems prudent to establish whether force output may alter as a function of time after glucose intake. Thus, the purpose of this study was to determine whether the ingestion of glucose was associated with greater force output during maximal isokinetic contractions and whether this is altered with time from ingestion. We hypothesised that there would be a moderate, albeit significant increase in force output in response to glucose ingestion, and this would coincide with peak blood glucose concentration.
Experimental Approach to the Problem
After the initial visit and familiarization session, the experimental trials were completed using a crossover, double-blinded experimental design. Allocation to treatment (CHO or PL) occurred by assigning de-identified participant codes to a computer-generated randomized number list (consisting of ones and twos; counterbalanced) by an individual not involved in the testing session (T.J.F.). Participants were instructed to consume their regular diet on each day before participation and to avoid physical activity. All testing was conducted in the morning (07:00–10:00 hours) after an overnight fast (>12 hours) and was kept consistent between trials.
Participants (11 males, 6 females; height: 175.2 ± 8.1 cm; weight: 69.5 ± 9.6 kg) were young (22.1 ± 3.9 years), lean (body mass index [BMI]: 22.5 ± 2.0 kg·m−2; %body fat [%BF]: 14.3 ± 8.0), and recreationally active (>150 min·wk−1 of physical activity). All participants had resistance training experience in the previous 6 months and were free from illness at the time of testing. The exclusion criteria for study participation were existing diabetes mellitus (type 1 or 2); pregnancy; BMI > 30 kg·m−2; medications known to alter glucose concentration; and previous or current injuries and conditions, which may be exacerbated as a result of study participation (assessed through the Exercise and Sports Science Association Pre-Exercise Screening Tool). Participants were recruited to this study through local advertisement, and written informed consent was provided by all subjects prior to participation. All aspects of the study were approved by the University's Human Research Ethics Committee in accordance with National Statement on Ethical Conduct in Human Research, 2007.
At least 3 days before the first testing session, participants attended a familiarization session that also included collection of anthropometric data, including height, weight, and %BF (3-site skinfold method (17)). For the familiarization, participants were then fitted to the isokinetic dynamometer (HUMAC NORM; CSMi, Stoughton, MA, USA) in accordance to manufacturer instructions and provided some practice trials (≥5 sets of 3 repetitions, with ≥2 sets at maximum effort) using the participants' perceived dominant leg. The back rest was adjusted to create a hip joint angle of 100° from flexion, and all trials were performed at a knee-angle speed of 60°·s−1. The range of motion was set at 10° from anatomical extension to 100° from anatomical extension while the contralateral limb was secured at 90°. These settings were recorded and kept consistent between trials.
Bipolar adhesive surface electrodes (Ag-AgCl; Duo-Trode, Kent, WA, USA) were placed over the muscle bellies of the vastus medialis and vastus lateralis for assessment of motor recruitment using surface electromyography (EMG) Telemyo DTS (Noraxon, Scottsdale, AZ, USA). Participants then completed a standardized warm-up (2 sets of 3 repetitions at 50 and 75% maximum effort); all repetitions during the warm-up and subsequent trials were performed at 60°·s−1. A finger-stick blood sample was then taken for assessment of blood glucose (Accu-Chek glucometer; Roche Diagnostics Australia, Castle Hill, NSW, Australia) concentration. All measures were performed in duplicate; where these values differed by more than 20%, a third sample was taken. Participants then performed a 3 repetition maximum (3RM) followed by ingestion of either the PL or CHO drink. The CHO drink consisted of 75 g glucose (Glucodin powder) dissolved in 280 ml of water and 20 ml of a green artificially sweetened (predominantly sucralose; 4 kJ per 10 ml undiluted solution) cordial. The PL drink consisted of 260 ml of water and 40 ml of the same green artificially sweetened cordial. The drinks were prepared by an individual not directly involved in the data collection, with those conducting data collection remaining naive to the condition. The drinks were provided in nontransparent drinking containers, and participants asked to ingest the solution in 2 minutes. Blood glucose, EMG, and isokinetic force were then recorded at 5, 15, 30, 45, 60, 75, and 90 minutes from ingestion of the solution. Blood glucose was consistently recorded 1 minute before the force and EMG recordings. Participants were then asked to recall their dietary intake the day before the first testing session (24 hours recall) and asked to replicate this diet on the day preceding the next testing session.
After 7 days, participants then returned to the laboratory and performed the identical study protocol with the exception of ingestion of the alternative drink (CHO or PL). Compliance to a similar diet and restriction of physical activity for the 24-hour period preceding the testing was determined through verbal report from participants.
Force was calculated in 2 ways: (a) as the maximum peak force attained during the 3 repetitions (MaxPeak), and (b) the average force produced during the single repetition that resulted in the greatest peak force (MeanRep). The raw EMG signal was processed using a custom MATLAB (The Mathworks, Natick, MA, USA). Initially, the signal was band-pass filtered using a fourth-order Butterworth filter at 20 and 500 Hz. Subsequently, the signal was full-wave rectified and a linear envelope was created using a 6 Hz low-pass fourth-order Butterworth filter. Finally, the data were normalized to the maximum EMG recorded in the baseline trial. The mean normalized EMG was then calculated for each of the concentric phases of the isokinetic exercise. Finally, these values were average to provide as estimate of the muscle activation across the 3 phases.
Data are presented as mean values ± SD, unless otherwise noted. Treatment effects were estimated using separate, random-intercept linear mixed models for each outcome variable (glucose concentration, force output, EMG data). Condition (CHO, PL) and time (pre, 0, 5, 15, 30, 45, 60, 75, 90 minutes) were modeled as fixed effects. The hypothesis of interest was the condition by time interaction, which we examined with pairwise comparisons of the estimated marginal mean values. To explore whether MaxPeak or MeanRep force output was different at either 5 minutes or at the time point corresponding to peak-glucose concentration, separate repeated-measures (time: pre, 5 minutes; time: pre, force at peak-glucose concentration) analyses of variance were conducted. The glycemic excursion was calculated as the absolute difference between the peak-glucose concentration and the blood glucose concentration measured at baseline. Effect size (Cohen's d) calculations were performed to assess the magnitude of difference within experimental trials (d ≤ 0.2, small; 0.5–0.79, moderate; ≥0.8, strong). All data analyses were performed using IBM SPSS package (version 21). Significance was set at α ≤ 0.05.
Ingestion of glucose resulted in a rapid and significant increase in blood glucose concentration, which remained significant until the completion of the 90-minute testing period (Figure 1). The mean glycemic excursion in response to glucose ingestion was 4.01 ± 1.18 mmol·L−1 (95% confidence interval [CI] preglucose [4.83–5.25]; 95% CI peak-glucose [8.51–9.59]), indicating a very strong effect (d = 5.03) of ingestion on blood glucose. The time to peak-glucose concentration varied between participants, ranging from 30 to 60 minutes (30 minutes: n = 11; 45 minutes: n = 5; 60 minutes: n = 1) after the ingestion of glucose.
There were no significant differences in force when compared as either MaxPeak (p = 0.567) or MeanRep (p = 0.843). When force output was adjusted for respective baseline values, there was no significant interaction but a significant main effect of condition (Figure 2). The force data corresponding to the glucose condition were extracted and explored further using univariate analysis (Figure 3). There was no difference in either the MaxPeak (p = 0.252; d = 0.076) or MeanRep (p = 0.217; d = 0.095) 5 minutes after ingestion of glucose. Likewise, there were no differences in MaxPeak (p = 0.337; d = 0.084) or MeanRep (p = 0.703; d = 0.037) when the time point corresponding to the maximum glucose concentration was compared with baseline force data.
In agreement with the force data, there were no significant differences in the EMG data corresponding to either the MaxPeak or MeanRep (both p > 0.955), although there was a significant main effect of condition (Figure 2). No significant differences were observed when the EMG was expressed relative to the force output during the MeanRep (p = 0.948).
The purpose of this study was to determine whether the ingestion of glucose would enhance force output during maximal isokinetic contractions and whether this would occur in a time-dependent manner. The main finding of this study was that ingestion of carbohydrate provided no clear benefits to force output during an isokinetic 3RM performance, despite a significant increase in blood glucose concentration. Indeed, when assessing the effect of condition on force output (Figure 2), participants performed better during placebo than glucose ingestion, which may be explained by a slight increase in force output over time during the placebo condition while force output slightly declined over time during the glucose condition. Similar changes were observed in the EMG (Figure 2) and as a consequence, there was no difference in the force:EMG ratio response to glucose ingestion.
Although the findings of this study are contrary to the stated hypothesis, closer inspection of the available literature casts some light on these findings. The studies by Wax et al. (27,28), which demonstrated significant improvements in performance with carbohydrate consumption during a time to exhaustion task used a very different protocol to the one adopted in this study. Their protocol consisted of repeated 20-second isometric contractions at 50% MVC followed by 40 seconds of rest until exhaustion. As a consequence, the average exercise duration was 16.0 ± 8.1 minutes and 29.0 ± 13.1 minutes during the PL and CHO trials, respectively (27), demonstrating a very large effect of the carbohydrate ingestion (d = 1.2). Another study investigating the role of carbohydrate ingestion during a time to fatigue task found no significant difference (CHO vs. PL) in either the number of successful sets (3.5 ± 3.2 vs. 3.5 ± 2.7), repetitions (20.4 ± 14.9 vs. 19.7 ± 13.1), or total work (29.9 ± 22.3 kJ vs. 28.6 ± 19.5 kJ) performed in the squat exercise (5 repetitions per set) at an intensity of 85% 1RM (12). Possible explanations for the differences observed between the studies of Wax et al. (27,28) and Kulik et al. (12) may stem from the type of muscular contractions adopted. In particular, isometric contractions at 50% of MVC are expected to partially occlude blood supply (2) and therefore increase the reliance on anaerobic metabolism, specifically through glycolysis. As such, glucose availability may have become a limiting factor to performance in the study of Wax et al. Additionally, participants in the study of Kulik et al. ingested the carbohydrate supplement immediately preceding the exercise and then every other successful set of squats; although in the study of Wax et al., participants ingested the carbohydrate every 6 minutes during exercise. Whether the timing of carbohydrate ingestion may have contributed to the differences observed between studies, or whether altering the timing or pattern of ingestion (i.e., minimum of 15 minutes pre-exercise to ensure endogenous glucose appearance in blood) influenced results within studies, has not previously been investigated and is therefore unknown.
To examine whether a time-dependent change in force output in response to glucose ingestion occurs, we assessed force output at 5 minutes after glucose ingestion and at the time point corresponding with peak-glucose concentration. The 5-minute post–glucose ingestion time point was based on a study demonstrating increased corticomotor excitability and maximal voluntary force with the presence of carbohydrate in the mouth (6). This research builds on previous work demonstrating reduced perceived exertion and improved exercise performance (3,10,18,20) in endurance events when carbohydrate (typically in the form of glucose or maltodextrin) was rinsed in the mouth. In contrast to our hypothesis, we observed no difference in maximal voluntary force at 5 minutes after glucose ingestion, despite the liberal statistical approach (within-condition univariate analysis). Indeed, the calculated effects (d < 0.1 for all) were interpreted as small within the context of this study design. This finding being similar to what was observed by Painelli et al. (16), where no differences in 1RM was observed after a carbohydrate mouth rinse. Likewise, in contrast to our a priori hypothesis, there were no differences in any force parameters measured at the time point corresponding to the maximum glucose concentration (Figure 3).
The rationale for inclusion of EMG in this study relates to the potential mechanisms for the expected increase in performance with glucose ingestion. Research on the ergogenic effects of glucose during a range of exercise tasks has now extended beyond simply acting as an energy substrate. Indeed, a number of studies now suggest that glucose may alter the electrical properties of the muscle fiber membrane (5,11,22) and that this is independent of entry into the glycolytic pathway. Based on these previous findings, the authors of this study speculated that the force:EMG ratio would be altered at the time point corresponding with peak-glucose concentration. However, there were no changes in the EMG either when assessed in isolation (Figure 2) or as a ratio (force:EMG ratio).
Previous research identified improved performance during isometric time to exhaustion tasks with glucose supplementation (27,28), although this benefit of glucose did not translate to improved performance during dynamic contractions (12). Moreover, exercise-induced glycogen depletion of muscle fibers has been associated with a decrement in maximal muscular strength during a single dynamic contraction (9). Here, we sought to determine whether previous inconsistencies in findings are a result of a time-dependent effect of glucose supplementation, with a potential benefit of glucose only occurring at the corresponding peak in blood glucose concentration. Results in this study however have demonstrated no benefit for carbohydrate ingestion during performance of maximal force efforts. This is likely due to an adequate supply of additional energetic substrates (e.g., muscle glycogen, high-energy phosphate compounds) to meet the energetic demands of a maximal effort, and the other proposed ergogenic mechanisms of glucose supplementation not playing a significant role during this type of task. This is the first study, to the authors' knowledge, to examine maximal force output in response to glucose ingestion over time. Although this study adopted an isokinetic testing protocol to appropriately address the study's aims, the findings from this study are expected to be transferable to other modes of strength training and testing; although this may be the focus of future studies.
There is limited research assessing the role of glucose supplementation on maximal force output. Although some research supports the ingestion of glucose before resistance-based exercise, these studies have typically focussed on delaying the onset of fatigue during sustained submaximal efforts, as opposed to enhancing maximal voluntary force capacity. The results of this study clearly demonstrate that ingestion of glucose does not improve performance of maximal voluntary force during isokinetic leg extensions. In addition, the results of this study demonstrate that force output did not change at any time point after glucose ingestion, despite a significant increase in blood glucose concentration. The ingestion of glucose is therefore not expected to provide any immediate performance benefits to resistance-based exercise training.
The authors thank the work of the undergraduate research team (D. Bates, S.B. Baldock, T. Burton, X. Hand, J.A. Hofferberth, M.E. Noakes, M. Vibert) who helped in the data collection. T. J. Fairchild is in receipt of a McCusker charitable grant that helped defray the costs of the study and publication.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
carbohydrate; MVC; strength; dynamic; contraction