The benefits of CHO ingestion during prolonged (>90 min) continuous exercise have been well documented (11). The literature is consistent in showing that CHO feedings can delay fatigue in a variety of exercise protocols by maintaining blood glucose and in some cases sparing muscle glycogen levels, both being important sources of energy for working muscles. In addition, the notion that CHO ingestion influences perceptions of exertion during prolonged cycling exercise has been well documented in the recent literature (8,20,29).
To our knowledge, research has yet to examine the influence of CHO ingestion on affective states (“how” a person feels) during prolonged cycling. As noted earlier, previously the focus has been on “what” a person feels, as measured by the RPE scale. Therefore, in accordance with the assertion of Hardy and Rejeski (19) that the RPE on its own provides limited information about the subjective experiences of individuals during exercise, “what” and “how” a person feels were examined during a prolonged exercise session. This was facilitated by the administration of the Borg (7) RPE scale, a measure of “what” a person feels, and the Feeling Scale (FS) (19) as the measure of “how” a person feels.
The administration of both scales in the present study provides a more encompassing representation of the subjective exercise experience because affect and perceived exertion are not isomorphic constructs (19). In support of this assertion are previous studies (1,19) that have reported that the correlations between the FS and RPE scale change across a range of exercise intensities, but generally never exceed a moderate level (approximately −0.6, leaving a large proportion of the variance in each unaccounted for). Furthermore, a linear response to incremental exercise is often reported using the RPE scale, which differs from the curvilinear responses elicited using the FS (14). It is not surprising that a linear relationship is often reported because the Borg RPE scale was originally designed to facilitate linearity of RPE values with HR (6). This supports the importance of administering additional subjective measures during exercise studies because, as stated previously (19), two individuals may report the same perception of exertion on the RPE scale (e.g., “hard,” 15), but this may be accompanied by feelings of pleasure in one individual and displeasure in another. One can also distinguish affect from perceived exertion based on the neuroanatomical and neurophysiological mechanisms involved as they differ beyond the level of the brainstem. It appears that the thalamus–insula–somatosensory cortex forms the basis of perceived exertion (10) and the thalamus–amygdala–nucleus accumbens axis forms the basis of affective responses (9,22). Therefore, even though the relationship between CHO ingestion and RPE has been documented previously, such conceptual differences support the need for further investigations into the role of CHO ingestion on subjective states during exercise.
A number of studies have been reported in the literature that have examined the influence that glucose-containing drinks exert on affective states at rest (2,3). Benton and Owens (3) concluded that higher blood glucose levels (that were within the normal range) were associated with lower reported tension. Negative affect has also been associated with low blood glucose in insulin-dependent diabetics (17) and in individuals experimentally manipulated into a hypoglycemic state (15). It has also been reported that CHO ingestion resulted in improved affective states during prolonged training periods in field hockey players (21). Two recent studies (30,32) have considered the relationship between CHO feedings and affect among other factors during intermittent high-intensity exercise. However, these investigations by Welsh et al. (30) and Winnick et al. (32) focused on specific states, such as fatigue, and inconsistent patterns were reported. Few studies have examined the possible link between CHO feedings and affective states during exercise, which is unfortunate because whether one feels good or bad during exercise is clearly an important factor. Indeed, it could determine the outcome of a competitive event and also impact on task persistence (1). Therefore, given the relationships previously documented in the literature, it was hypothesized that affective states may be influenced by CHO ingestion during prolonged exercise. Based on studies at rest, we speculated that the effects might be due to changes in blood glucose level and therefore activation of the autonomic nervous system in an attempt to return blood glucose concentration to normal levels (2). It was also hypothesized that, in accordance with previous studies investigating CHO ingestion and RPE during cycling exercise (8,13), an attenuation in RPE would be observed following CHO ingestion.
Despite more than three decades of research concerning the exercise–affect relationship, there is, and continues to be, a void in the literature on prolonged exercise. There is a strong advocacy that exercise makes people “feel better”; however, aside from a study by Acevedo et al. (1), affective changes during and following prolonged constant-paced exercise have not been identified. Indeed, Acevedo and colleagues (1) reported that ratings of pleasure progressively declined during a 2-h constant-paced run at 70% V̇O2max, whereas RPE progressively increased. This is in contrast to the positive affect that Morgan (24) suggests is experienced by some runners after the first hour of a run (based on anecdotal accounts). The present study, although focusing predominantly on the effect of CHO ingestion, also enables a further examination of such discrepancies in prior research.
Therefore, the purpose of this study was to examine the effects of CHO ingestion and prolonged (i.e., 120 min) exercise on pleasure–displeasure and perceived exertion.
This study was performed as part of an investigation examining the role of CHO ingestion on immune measures, the data from which have already been published (4).
Nine endurance trained males (mean ± SEM; age 25 ± 2 yr; height 191 ± 4 cm; body mass 76.8 ± 2.8 kg; V̇O2max, 64.7 ± 2.7 mL·kg−1·min−1) volunteered to participate in this study. All participants were fully informed of the nature and purpose of the study before signing a statement of informed consent. The study had the approval of the ethical advisory committee of Loughborough University.
Measures of affect and perceived exertion.
The FS (19) was used as a measure of the affective dimension of pleasure–displeasure. Commonly used for the assessment of affective responses during exercise, it is an 11-point single-item bipolar rating scale. The scale ranges from −5 to +5. Anchors are provided at the 0 point (“neutral”) and at all odd integers, ranging from “very good” (+5) to “very bad” (−5). Participants were asked to rate how they felt at that particular moment. The FS was administered preexercise, every 15 min throughout the prolonged cycle, upon cessation of exercise, and 5, 15, and 30 min postexercise (Fig. 1). The RPE scale (7) was used as a measure of perceived exertion during exercise and was administered every 15 min during the trial (Fig. 1). The scale ranges from 6 to 20, with anchors ranging from “very, very light” to “very, very hard.” The FS was presented first, followed by the RPE scale.
Maximal oxygen uptake was estimated by means of a continuous incremental exercise test on an electrically braked cycle ergometer (Lode Excalibur, Groningen, The Netherlands) to volitional fatigue. From the V̇O2–work relationship, the work rate equivalent to 70% V̇O2max was interpolated. Please refer to Bishop et al. (4) for further details of the preliminary procedures.
Participants reported for each experimental trial following an overnight fast of between 10 and 12 h. This was to ensure that participants began each trial with an empty stomach, thus eliminating any negative effect that a previous meal might have had, both on exercise metabolism and gastric emptying. For the 48 h before the experimental trial, participants refrained from any strenuous physical activity. Participants consumed their normal diet, but weighed and recorded food eaten in a food diary during the 48 h before the first trial. This diet was then replicated during the corresponding period before any other further trials. This information was later analyzed using COMP-EAT (Nutrition Systems, London). In addition, caffeine and alcohol were also prohibited during the 48 h before the trial because both have been found to have transient effects on affective states (27).
In a randomized, double-blind, counterbalanced design, participants completed two exercise trials, each separated by at least 7 d. On each occasion, participants consumed either a carbohydrate (6.4%) solution (CHO) flavored with lemon or a placebo solution (PLA) that was artificially sweetened and flavored with lemon. Each drink was identical in flavor and appearance, and participants were not aware of the content of the drinks in each trial. These solutions were ingested immediately before exercise (5 mL·kg−1 body mass), every 15 min during (2 mL·kg−1 body mass), and 5 min postexercise (5 mL·kg−1 body mass).
Participants reported to the laboratory at 8:00 a.m. on each occasion following an overnight fast. Upon arrival, they responded to the FS. They then emptied their bladders before body mass (in shorts only) was measured. Participants were then seated quietly for 15 min after which a blood sample was taken from an antecubital vein by venipuncture. They then performed a 2-h cycle ergometer ride on an electronically braked ergometer (Lode Excalibur) (Fig. 1). Participants began cycling at 70% V̇O2max and, although the intensity was monitored, they completed the exercise without continual adjustment of the resistance. The FS was administered at 15-min intervals. Expired air samples were obtained using the standard Douglas bag method at 20, 50, 80, and 110 min during the exercise bout to determine exercise intensity. A paramagnetic oxygen analyzer (Servomex 1420B, Crowborough, UK) and an infrared carbon dioxide analyzer (Servomex 1415B) were used along with a dry gas meter (Harvard Apparatus, Edenbridge, UK) for determination of V̇O2 and V̇CO2. HR were recorded every 15 min during exercise using short-range telemetry (Sportester®, Polar Electro, Kempele, Finland).
Upon cessation of the exercise task, participants immediately responded to the FS. Participants once again responded to this measure 5, 15, and 30 min postexercise. Further blood samples were taken immediately postexercise and at 1 h postexercise. No food was consumed during this period, and fluid ingestion was as prescribed.
Blood samples were collected into monovette tubes (Sarstedt, Leicester, UK), containing lithium heparin and were centrifuged at 1500 × g at 4°C for 10 min within 15 min of sampling. The plasma obtained was immediately stored at minus 70°C for later analysis for glucose and cortisol using hexokinase (No. 16-50 Kit, Sigma, Poole, UK) and 125I radioimmunoassay (ICN Pharmaceuticals, Costa Mesa, CA) methods, respectively. Radioactivity was measured using an automated gamma counter (Cobra II, Packard Instruments Co. Inc.). For further details of the analysis procedures, the reader is referred to Bishop et al. (4).
A series of two-way ANOVA for repeated measures on two factors (experimental condition and sampling time) was used to examine the affective, physiological, metabolic, and perceived exertion data. For the FS ratings, separate ANOVA were conducted on the pre- to postexercise time points (pre, post 0, post 15, post 30) and during-exercise time points (from 15 to 120 min). This approach allowed direct comparisons during exercise between the FS and RPE scale. Significant main effects were further analyzed using paired t-tests and the Bonferroni adjustment for the number of pairwise comparisons was employed. Greenhouse–Geisser epsilon corrections were used when the sphericity assumption was violated. Statistical significance was set at the 0.05 level, apart from the Bonferroni analyses. Values are presented as means (SEM).
A nutrient analysis of the 2-d food records before each of the two exercise sessions revealed no differences in the energy intake and nutrient composition between conditions. The mean energy intake of the participants was 2322 kcal·d−1, with the proportion of energy being 64.1% ± 3.7% from CHO, 19.6% ± 3.0% from fat, and 17.3 ± 0.8% from protein.
Physiological responses to the exercise protocol.
HR and %V̇O2max did not differ between trials with the mean V̇O2max during exercise being 74.2 ± 1.3% in the CHO trial compared with 75.2 ± 1.4% in the PLA trial, thus demonstrating that the participants were exercising at the same relative exercise intensity in both conditions. HR during exercise ranged from 158 to 170 bpm.
Plasma glucose concentration was higher immediately postexercise in the CHO trial (6.1 ± 0.3 mmol·L−1) than the PLA trial (5.4 ± 0.3 mmol·L−1), and there was a significant interaction for treatment × time (F(2, 16) = 7.6; P < 0.05). This showed that in the CHO trial, blood glucose concentration increased from preexercise to immediately postexercise, whereas in the PLA trial, the concentration remained stable from preexercise to post 0, and by 1 h postexercise had fallen to a concentration lower than that observed preexercise (Table 1).
Plasma cortisol concentrations increased by 11% during the CHO trial (528 ± 23 to 588 ± 74 nmol·L−1) and by 24% (576 ± 46 to 811 ± 153 nmol·L−1) in the PLA trial, but this increase across both trials was not statistically significant due to large variability (Table 1). Overall, there was a main effect for trial, with values being higher in the PLA trial (F(1, 8) = 8.7; P < 0.05) compared with the CHO trial.
Analysis on the FS revealed that during exercise, an overall main effect for treatment was observed (F(1, 8) = 8.5; P < 0.05), but not time. Overall ratings of pleasure were higher during the CHO trial, compared with the PLA trial. Pleasure ratings became more positive and were maintained throughout exercise in the CHO trial, whereas in the PLA trial, ratings became less positive (Fig. 2). Analysis of the pre- to postexercise changes revealed a significant interaction effect (F(4, 32) = 2.77; P < 0.05) with ratings of pleasure being higher overall in the CHO trial compared with the PLA trial, and pleasure improved from preexercise to 15 min postexercise in the CHO trial only (Fig. 2).
Rating of perceived exertion.
RPE increased over time (F(7, 56) = 21.1; P < 0.001) across both conditions. There was an interaction of treatment × time (F(7, 56) = 2.6; P < 0.05), with RPE being significantly lower only at 75 min in the CHO trial compared with the PLA trial (Fig. 3).
The correlations between the FS and RPE scale were weak. All correlations were of negative direction; however, only one was significant (90 min, PLA trial; −0.75, P < 0.05). The remainder displayed low to moderate (range: −0.02 to −0.63), nonsignificant correlations, demonstrating that the FS and RPE are conceptually distinct constructs.
In the present study, we observed that ratings of pleasure–displeasure changed as a result of prolonged cycling and that CHO supplementation appeared to influence this response. Repeated in-task assessment of pleasure–displeasure led to the observation that CHO ingestion noticeably influenced “how” participants felt during and following exercise as measured by the FS. This study highlights the importance of assessing this psychological parameter in addition to the often administered RPE scale because “what” the participants felt was reduced by CHO ingestion to a much lesser extent. Furthermore, we found no significant correlations between the FS and RPE scale over the course of the trial, supporting the assertion that affect and perceived exertion are not isomorphic constructs (19).
Based on previous studies at rest, we hypothesized that affect would be positively maintained in the CHO trial. Our data supported this hypothesis as pleasure–displeasure was observed to shift in opposite directions when the CHO and PLA trials were compared, with participants in the CHO trial reporting a more pleasurable exercise experience. Indeed, the ingestion of CHO during exercise prevented the observed reduction in pleasure noted in the PLA trial (Fig. 2). We also hypothesized that CHO ingestion during exercise would attenuate RPE, but this was limited to the 75th minute of exercise. Consequently, future studies should continue to employ both scales in order to obtain a more complete understanding of the subjective exercise experience.
Although this study was not designed to uncover the mechanisms by which CHO feedings improve affect, a number of mechanisms have been previously described at rest (2) and the findings of this study could offer support for one of the propositions during exercise. The overall differences during exercise in pleasure–displeasure between the CHO and PLA trials could be linked to the physiological changes observed following the 120-min cycle. Immediately postexercise, blood glucose concentrations were higher (P < 0.05) following the CHO trial. This is consistent with findings that low blood glucose concentrations are associated with negative affective states (15,17). Although participants in the PLA trial did not become hypoglycemic, glucose concentrations were markedly lower. However, the adequacy of this mechanism in explaining the link between CHO ingestion and affect during exercise remains to be elucidated. In addition, there was a main effect for trial on plasma cortisol concentration, with overall values being higher following the PLA trial than the CHO trial. This is consistent with the findings of Utter et al. (29) who reported lower cortisol concentrations during the later stages of a 2.5-h bout of cycling at approximately 75% V̇O2max. According to Morgan et al. (23), such findings may indicate that the PLA trial was more stressful, both perceptually and physiologically. Cortisol is often secreted in response to emotional stress and unpleasant sensations (26) and the reduction in pleasure noted in the PLA trial would support this suggestion. Rudolph and McAuley (28) have emphasized that the role that cortisol might play in affective responses to acute exercise has received minimal attention. Indeed, a number of studies have reported an association between increased cortisol levels and increases in negative affective states (16) and further investigations may be warranted.
As highlighted, the relationship between CHO ingestion and RPE during prolonged exercise has been well documented (8,20,29), and findings indicate that CHO availability attenuates RPE during the latter stages of exercise (29,31). This study offers tentative support for such findings as RPE was lower 75 min into exercise compared with the PLA trial. However, such a reduction was not sustained and it is not clear why the effect was limited to 75 min into exercise. In agreement with previous studies involving cycling (12), blood glucose concentration was influenced by CHO supplementation, with higher concentrations noted following exercise in the CHO trial. Although blood samples were not collected during exercise, they were collected as soon as the exercise bout was complete and therefore still offer partial support for the assertion by Coggan and Coyle (12) that, during cycling exercise, reductions in RPE following CHO ingestion may be a function of elevated circulating levels of blood glucose.
This study was also designed to determine the impact of prolonged exercise on the temporal dynamics of pleasure–displeasure. Unlike the RPE scale, FS ratings can be bidirectional and research is consistently observing something of a “rebound” pattern immediately following exercise that has induced negative responses during exercise (1,5,18). The present study offers further support for this phenomenon (Fig. 2). It appears that the trajectory of pleasure–displeasure during and following exercise exhibits two distinct phases. The first phase involves the decline during exercise and the second phase an improvement following exercise (1,5,25). In the PLA trial, if only pre- to postexercise changes had been considered, the decline in reported pleasure during the exercise bout would have been masked. We also found that prolonged cycling elicited positive affective changes 15 min postexercise, but this was restricted to the CHO trial only. Therefore, it appears that nutritional intervention during prolonged exercise may be a key determinant of the exercise experience and should be considered in future studies.
In summary, our results suggest that CHO ingestion can attenuate the reductions in pleasure that occur when only fluid and electrolytes are ingested. Therefore, athletes, coaches, and sport and exercise scientists can extrapolate from this study the observation that CHO ingestion during exercise also influences the athlete’s psychological state, as assessed by the FS, adding to the literature on physiological, performance, and exertional effects. RPE was also reduced in the CHO trial, but only at 75 min. In sum, these results show that CHO ingestion exerts differential effects on affect and perceived exertion and emphasize the importance of assessing not only “what” but also “how” a person feels during exercise.
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