Interval training encompasses an array of exercise intensities and work-to-rest ratios to elicit a range of physiological responses (6,20,23). For example, typical interval training sessions reported in the literature have elicited between 90 and 95% maximal heart rate (HR) at fixed or self-selected speeds (12,25), 90-95% maximal oxygen consumption (27,28), and ratings of perceived exertion (RPE) of 16 on the Borg scale (27). Interval training has subsequently been used to improve both aerobic and anaerobic components of fitness (1,2).
Intermittent exercise, similar to that undertaken in interval training, elicits greater carbohydrate metabolism and blood-lactate values when compared to continuous exercise matched for total energy expenditure (7). Furthermore, intermittent exercise can also result in decreases in blood pH (20) similar to those observed after fatiguing high-intensity exercise (e.g., 120% max power) (19). Methods to offset fatigue during intermittent exercise may therefore be useful in increasing an athlete's ability to repeat high-intensity exercise bouts or maintain a given exercise intensity for longer. Being able to increase the duration of each repetition during an interval training session or the number of repetitions undertaken would provide a greater training stimulus and potentially, greater training adaptations.
One method that has been examined to offset fatigue and prolong high-intensity exercise performance is the ingestion of sodium bicarbonate (16,24). The majority of studies in this area have examined individual sprint efforts (13,18,29), repeated short-duration sprints (8,15), or field-based activities (3,10). More recently, sodium bicarbonate ingestion has been examined during prolonged exercise (9,17,21). These recent studies have shown significant alkalosis and greater blood-lactate concentrations during prolonged high-intensity exercise (60 minutes, 67-70% maximal oxygen uptake) (9) and increases in power output during both a 1-hour maximal cycle ergometer test (17) and during prolonged intermittent high-intensity exercise (21). However, no study has reported whether the ingestion of sodium bicarbonate improves exercise capacity after interval type training.
The literature focusing upon ingestion of sodium bicarbonate on performance is equivocal (24). However, where means for group data are presented, this masks both the number of participants who may have improved performance after ingestion of sodium bicarbonate and those who did not improve performance. Applying a more individualized approach to performance effects may be of more importance to the coach and athlete, particularly because it is imperative to have individualized training plans and goals, including various nutritional supplements. Therefore, the aim of this study was to determine the effects of sodium bicarbonate ingestion upon intermittent high-intensity running akin to that used in interval training sessions. A secondary aim was to examine individual performance responses to ingestion of sodium bicarbonate. We hypothesized that ingestion of sodium bicarbonate would result in greater blood-lactate concentration, greater pH, and improved running performance when compared to placebo; however, this may be more pronounced in some individuals.
Experimental Approach to the Problem
The study was designed to simulate interval training type exercise. The intensity of the exercise undertaken and work-to-rest ratios were consistent with previous studies using intermittent exercise models (7,20,23,26). A performance aspect, involving a run to exhaustion, was undertaken to determine the exercise capacity of the participants at a speed relative to their maximal performance after the intermittent exercise bout. This would also allow the potential for improved exercise capacity in a team sport or training situation to be assessed. The protocol was undertaken after either a sodium bicarbonate or control solution of conventional doses with participants acting as their own controls. Oxygen consumption and heart rate were taken as conventional makers of exercise intensity and for comparison to previous protocols in the literature. Blood lactate and parameters reflecting acid-base status (pH, bicarbonate concentration) were measured to determine the physiological status of the participants. Furthermore, blood lactate is often used by coaches and sport scientists in to assess adaptations to training and training zones.
Eight healthy men (mean ± SD age 20.3 ± 1.0 years, height 1.77 ± 0.07 m, weight 70.1 ± 9.0 kg, and maximal oxygen uptake [o2max] 55 ± 5 ml·kg−1·min−1, maximal HR 195 ± 8 b·min−1) volunteered to take part in the study that had received University Ethics Committee approval. Participants played intermittent type sports twice a week (total ∼4 h·wk−1) and competed at University level. Testing took part over a 3-week period in January and was considered to be during the ‘off season’ for their sport. All participants were briefed as to the risks of the research before being given the opportunity to provide written informed consent. All participants subsequently gave informed consent to participate. Participants completed a health screen questionnaire before each test.
Participants were required to visit the laboratory on 3 occasions. The first visit involved an incremental treadmill test for the determination of o2max and the velocity at which o2max occurred. Two further trials involved performing repeated high-intensity running after ingestion of either a sodium bicarbonate (NaHCO3) or placebo solution (sodium chloride; NaCl). Trials were undertaken in a randomized and counterbalanced order. Before testing, participants were asked to refrain from strenuous exercise for 24 hours, consume a similar diet for 48 hours, fast for 6 hours, and to avoid caffeinated products and alcohol for 12 hours. Participants were asked to drink plenty of fluids during the fasting period to ensure they arrived at the laboratory in a hydrated state. Although tests were not performed on the same day each week, all were performed at the same time of the day and separated by at least 4 days.
On the first visit to the laboratory, subjects completed an incremental treadmill test for o2max and velocity at o2max (v-o2max). The protocol involved running on a treadmill (Powerjog UK, GX200 UK, Cranlea UK, Birmingham, United Kingdom) at an initial speed of 8 km·h−1 for 5 minutes after which the treadmill was increased by 1 km·h−1 every minute (20,23). Expired gas samples were collected for 45 seconds in the final minute of the test using the Douglas bag technique. Douglas bag contents were analyzed for fractions of oxygen and carbon dioxide (Servomex 1440, Cranlea Ltd.), minute ventilation (Harvard dry gas meter, Cranlea Ltd.) and expired gas temperature (RS Supplies 206-3722, Cranlea Ltd.). Minute ventilation (eSTPD), oxygen consumption (o2), carbon dioxide production (co2), and respiratory exchange ratio (RER) were subsequently calculated. The v-o2max (16.3 ± 2.0 km·h−1) was used to determine the speeds employed in the intermittent exercise bout and performance tests. Heart rate was continually monitored (Polar Heart Rate Monitor, Kempele, Finland).
One hour before beginning the intermittent exercise trials, participants ingested either a NaHCO3 or NaCl solution within a 5-minute period. The NaHCO3 dose was 0.3 g·kg·body mass−1, which has been shown to aid performance and minimize the possible side effects associated with NaHCO3 ingestion (8,16,21). The NaCl solution was administered with a dose of 0.045 g·kg·body mass−1 (21). Both the treatment and placebo were mixed with water (400 mL) and low calorie orange cordial (100 mL) and matched for taste. Participants then sat quietly in the laboratory for the remainder of the pre-exercise period.
The intermittent exercise protocol involved 20 × 24-second runs separated into 4 exercise bouts consisting of 5 runs each. Each sprint was undertaken at a speed corresponding to 100% v-o2max (26). The first 4 sprints of each block were followed by 36 seconds of passive recovery (6,20,23), whereas the fifth was followed by 60-second passive recovery to facilitate ease of blood sampling. Because of the high exercise intensity and short recovery duration, this is unlikely to have affected the lactate kinetics between each run. Each recovery period consisted of the participants standing passively with their feet astride the treadmill belt. One minute after the cessation of the intermittent protocol, a run to exhaustion at 120% v-o2max (19.5 ± 2.0 km·h−1) was performed.
Fingertip capillary blood samples were taken preingestion, 30- and 60-minute postingestion (−60, −30, and 0 minutes), after runs 5, 10, 15, and 20 of the intermittent exercise protocol and at the end of the performance test. After collection, blood samples were put on ice until analysis at the end of the exercise protocol. A 100 μL sample was analyzed for blood pH and blood bicarbonate concentration ([HCO3 −]) (ABL5 Radiometer, Copenhagen, Denmark), and an 80 μL sample was analyzed for blood lactate concentration in triplicate ([Bla]; Analox GM7, Surrey, United Kingdom). Respiratory gases were collected over a 60-second period using the Douglas bag technique on 4 occasions. Samples were collected 10 seconds before the fifth run of each exercise block. Each gas collection therefore included a 24-second run and 36-second passive rest to provide an indication of the mean oxygen consumption during the test and overall exercise intensity rather than the energy expenditure of each specific run and recovery period. Heart rate was continually monitored. Ratings of perceived exertion (Borg scale) (5) were recorded before each blood sample during intermittent exercise and the performance test.
Blood parameters, oxygen consumption, HR, and RPE data were analyzed via 2-way analysis of variance (ANOVA; trial × time) with repeated measures on both factors. Significance was taken at the level of p ≤ 0.05. Where significance was achieved, Tukeys' post hoc analysis was undertaken by calculating the difference required between means for significance at the level of p ≤ 0.05 (30). Individual regression equations were generated for comparisons between pH, [Bla], and [HCO3 −]. The resulting gradients and intercepts for each trial and maximal exercise parameters at exhaustion were analyzed by paired t-tests. Correlations between variables were analyzed via Pearson's correlation. The statistical power for each 2-way ANOVA interaction ranged from >0.80 ([HCO3 −] and pH) to 0.37 ([Bla]) and less than 0.10 for HR, RER, RPE, and performance time. Effects sizes (Eta2) were generally low for interactions (<0.2). Intraclass correlations for all variables were high (0.824-0.967). To examine the individual performance responses more specifically, 2 approaches were taken. Firstly, individual performance times were plotted for each trial. Secondly, the difference between performance trials for each participant against and the average time for both trials were plotted. The latter approach enabled specific improvements and decrements in performance to be assessed in relation to performance duration.
Blood pH at rest and during intermittent exercise and performance is shown in Figure 1. A significant trial × time interaction was observed (p = 0.027). Blood pH was significantly lower during the NaCl trial when compared with NaHCO3 postingestion throughout exercise and after performance (p < 0.05). Before ingestion, pH values were similar (7.40 ± 0.03 vs. 7.40 ± 0.02, for NaCl and NaHCO3, respectively; p > 0.05) increasing to 7.45 ± 0.02 for the NaHCO3 trial. Based on Tukey post hoc analysis requiring a difference of 0.07 pH units for significance, this increase approached significance. No changes in blood pH were observed during the ingestion period for NaCl. At 5 minutes of intermittent exercise, blood pH tended to decrease from values observed at 0 minute in both trials (7.41 ± 0.04 and 7.35 ± 0.04, for NaHCO3 and NaCl, respectively; p > 0.05) returning to pre-exercise levels for the remainder of the intermittent protocol. Values then decreased at the end of the performance trial (7.37 ± 0.04 and 7.29 ± 0.04 for NaHCO3 and NaCl, respectively; p < 0.05).
A significant trial × time interaction was observed for blood [HCO3 −] (p = 0.036, Figure 2). Blood [HCO− 3] was greater during the NaHCO3 trial when compared to the NaCl trial at every time point from −30 minutes (p < 0.05). As a result of sodium bicarbonate ingestion, [HCO− 3] increased from 25 ± 1 mmol·L−1 at rest to 30 ± 2 mmol·L−1 after 30 minutes (p < 0.05) and 32 ± 1 mmol·L−1 after 60 minutes (p > 0.05). No change from resting values was observed after ingestion of NaCl (25 ± 2, 25 ± 2 and 26 ± 1 mmol.L−1, respectively; p > 0.05). By 5 minutes of intermittent exercise, blood [HCO3 −] decreased in both trials (27 ± 2 and 22 ± 2 mmol·L−1 for NaHCO3 and NaCl, respectively; p < 0.05) continuing to do so until the end of exercise (25 ± 3 and 20 ± 3 mmol·L−1, respectively; p < 0.05). On the cessation of performance, blood [HCO3 −] decreased further when compared to values at the end of the exercise in both trials (21 ± 1 and 16 ± 2 mmol·L−1 for NaHCO3 and NaCl trials, respectively; p < 0.05).
A significant main effect was observed between trials (p = 0.013) for [BLa] (Figure 3). Blood lactate concentration increased from resting values to 3.5 ± 1 mmol.L−1 during NaHCO3 and 3.4 ± 1.3 mmol.L−1 during NaCl (p < 0.05) at 5 minutes of intermittent exercise. Values then remained similar until the end of intermittent exercise bout. After the performance trial, [Bla] increased to 8.3 ± 1.2 mmol·L−1 during NaHCO3 and 6.8 ± 1.2 mmol.L−1 during NaCl (p < 0.05).
No differences were observed between trials for o2, E, RER, HR, or RPE. The mean o2 for each trial was similar throughout exercise representing 62.0 ± 6.0% o2max for NaHCO3 and 62.6 ± 4.2% o2max for NaCl at 20 minutes of intermittent exercise. Heart rate at the end of the 20-minute intermittent protocol was 169 ± 13 and 170 ± 11 b·min−1 for NaHCO3 and NaCl, respectively, reaching 193 ± 8 and 193 ± 9 b·min−1, respectively at exhaustion (p > 0.05). Ratings of perceived exertion increased with time (main effect; p < 0.05) but was not different between trials reaching 16 ± 2 at 20 minutes in both the NaHCO3 and NaCl trials. On cessation of the performance, run values reached 20 ± 1 and 19 ± 1, respectively (p < 0.05).
There was no significant difference in performance time or distance covered between treatments (78 ± 22 vs. 75 ± 22 seconds and 414 ± 102 vs. 402 ± 109 m for NaHCO3 − and NaCl, respectively) (Figure 4). A plot of the difference in performance time between trials (NaHCO3-NaCl) and mean time to exhaustion is shown in Figure 5. The 2 participants with shortest performance duration (43.0 and 52.5 seconds) also demonstrated the least difference between trials (2 and −1 seconds, respectively). Of the remaining participants, 3 demonstrated an improvement and 3 demonstrated a reduction in performance after ingestion of sodium bicarbonate. The data points for those participants reporting gastrointestinal distress are shown as crosses.
Significant differences were observed for blood pH, [Bla], and [HCO3 −] between trials at exhaustion (p < 0.05). A negative correlation was observed between [Bla] at the end of the 20-minute intermittent exercise period and performance time (r = 0.61, p < 0.05) but not between performance time and [Bla] at volitional exhaustion. The relationships between [Bla], pH, and [HCO3 −] are shown in Figures 6-8. Differences between trials were observed for the intercepts of [Bla] vs. pH and [Bla] vs. [HCO3 −] (p < 0.05) but not for pH vs. [HCO3 −]. No differences were observed between trials for the gradients of each relationship (p > 0.05). Significant correlations were observed between performance time achieved in both trials (r = 0.77; p < 0.05) and between performance duration during each participant's first and second exercise trial (r = 0.81; p < 0.05).
The primary finding of the present study was that although buffering capacity was enhanced after ingestion of sodium bicarbonate in all participants, high-intensity running performance of the group after a high-intensity intermittent running protocol was not improved. However, a range of individual responses including improvements, no improvement, and decrements in performance were observed.
The increases in blood [HCO3 −] and pH after ingestion of NaHCO3 were similar to those previously observed for the dosage employed (19,21,22). During both exercise trials, blood pH decreased by similar amounts after the first 5 runs. After this point, pH gradually returned to pre-exercise levels in both trials. This is in contrast to previous work examining sodium bicarbonate ingestion and prolonged high-intensity intermittent exercise where a greater decrease in pH during exercise was observed, most likely because of maximal sprints being undertaken and resulting in a greater acid-base disturbance (21). To restore pH to pre-exercise values, blood [HCO3 −] decreased significantly during the initial stages of the intermittent exercise bout reaching steady state between 10 and 20 minutes. Consequently, blood pH was maintained during repeated high-intensity running but at the expense of the bicarbonate reserve.
Although blood [HCO3 −] decreased during intermittent exercise, both blood pH and [HCO3 −] were still maintained at greater levels throughout exercise and performance after ingestion of NaHCO3. The increased buffering capacity is emphasized in Figures 6 and 7 where differences between the intercepts of the individual regressions equations for [BLa] against both pH and [HCO3 −] are shown. However, the relationship between pH and [HCO3 −] was not altered. Ingestion of sodium bicarbonate therefore increases the buffering capacity to a higher level rather than altering the buffering process per se. At exhaustion, blood pH remained greater for the NaHCO3 trial than during NaCl. Messionier et al. (19) observed similar blood pH values to those in the present study after exercise at 120% peak power output to exhaustion for sodium citrate and control trials but no differences in muscle pH values. Consequently, muscle pH was unlikely to have been a limiting factor in the present study.
Blood-lactate concentration was similar throughout both intermittent exercise bouts before performance at approximately 3.5 mmol·L−1. Previous studies of sodium bicarbonate ingestion before prolonged continuous or intermittent high-intensity exercise have demonstrated greater [Bla] (9,11,21) during the treatment trial as have protocols involving short duration or repeated sprints (8). The reason for no differences in [Bla] may be because of the exercise intensity. Previous studies involving the same work:rest duration but at intensities of 120% o2max have reported [BLa] values of approximately 6 mmol·L−1 (6,20,23). Therefore, it is possible that the exercise undertaken in the present study did not elicit sufficient lactic acid production or accumulation to warrant increased lactate and proton transport from the muscle to the blood. The benefit of sodium bicarbonate ingestion may therefore be greater for interval training eliciting greater muscle and blood-lactate concentrations. Although differences in [BLa] were not elicited, it is important to note that the HR and RPE values achieved were representative of interval training type exercise previously reported (12,25,27). Furthermore, the same relationship between performance and blood lactate before the performance trial was observed as in previous studies eliciting greater [BLa] (23).
Although no differences were observed between performance trials for the group, individual differences in performance were noted. For example, those participants with the shortest performance times demonstrated little change in performance with ingestion of sodium bicarbonate. These performance times were less than 60 seconds in duration and coincide with sodium bicarbonate ingestion not improving performance where the duration is of less than 60 seconds (16,18). These participants may also have simply been exercising at too high an intensity to allow a significant duration of exercise to be completed. Of the remaining participants, 3 demonstrated improved performance with sodium bicarbonate ingestion, whereas 3 performed worse. Messionier et al. (19) reported that the greatest improvements in performance with alkalosis occurred mainly in those participants with lower work capacities who also demonstrated the lowest concentrations of lactate and proton transporters (i.e., monocarboxylate transporters). Participants demonstrating greater work capacities were considered to be less dependent upon the muscle content of these proteins with other adaptations of improved muscle function, most likely greater muscle capillary density, facilitating greater washout of lactate and protons from muscle.
Ibanez et al. (14) suggested that improvements in performance may not occur unless there is a difference in blood lactate between trials of more than 2 mmol·L−1, although this study utilized the ingestion of sodium citrate rather than bicarbonate. In the present study, those participants performing better after ingestion of NaHCO3 actually demonstrated a smaller flux of lactate during the NaHCO3 trial when compared to the control trial. Conversely, those participants whose performance worsened with ingestion of NaHCO3 demonstrated a greater flux of blood lactate in the NaHCO3 trial. This would also appear to be in contrast to the findings of Messionier et al. (19) regarding the lactate and proton transport characteristics of participants benefiting from NaHCO3 ingestion. However, this study differed from the current study with respect to the protocol undertaken, the blood-lactate values elicited and potential performance benefits. From the current study, it is clear that participants can be grouped as those responding to and not responding to the ingestion of NaHCO3. Further research should focus upon examining physiological differences characterizing such groups.
Four of the participants reported gastrointestinal distress before commencing the exercise protocol. Of these participants, one demonstrated no difference in performance after NaHCO3 ingestion (−1 second), one demonstrated a decrease in performance (−16 seconds), and 2 showed improved performance (10 and 26 seconds). As those participants performing well in the NaCl trial also performed well in the NaHCO3 trial, and there were no improvements in performance because of the order of testing, performance decrements are unlikely to be a result of gastrointestinal distress per se. As noted earlier, it is possible that improvements in performance with ingestion of NaHCO3 relate to the absence of lactate and proton membrane transporters in the exercising muscle. As a result, any attempts to improve interval training ability or performance with NaHCO3 ingestion should be individually based.
A significant inverse relationship was observed between blood lactate concentration after 20 minutes of interval type exercise and subsequent running performance time. This has been observed previously (20) and shows that the higher the blood-lactate concentration before the start of performance, the shorter the performance duration. This is an interesting concept for future research into bicarbonate ingestion and performance because [Bla] is usually greater after bicarbonate ingestion (11,21). Therefore, if interval training is undertaken, which elicits potentially greater [Bla] after NaHCO3 ingestion, this may theoretically impair running performance. However, if higher exercise intensities do elicit a benefit from bicarbonate ingestion in the form of a greater flux of lactate and H+ ions out of the muscle cell, this may allow greater anaerobic energy production to occur within the muscle cell (4) and possibly improve running performance. Examining the effects of sodium bicarbonate ingestion on a range of exercise intensities may therefore shed light on how NaHCO3 may affect performance.
The results of the present study suggest that if sodium bicarbonate ingestion is to be undertaken to try and enhance high-intensity interval training, then an individualized approach should be undertaken. Those athletes being able to perform for longer than 60 seconds at a given exercise intensity may benefit but not exclusively. Therefore, the intensity of interval training and the duration of exercise bouts undertaken after ingestion of sodium bicarbonate or the duration of the specific performance trial should be considered. Furthermore, the presence of gastrointestinal distress, although uncomfortable for the athlete, may not necessarily offset the potential performance benefits. Although the current study only examined one type of interval training, exercise intensities eliciting greater blood-lactate concentrations may benefit from sodium bicarbonate ingestion.
The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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Keywords:© 2010 National Strength and Conditioning Association
interval training; sprinting; pH; blood lactate; buffer capacity