Introduction
Dehydration has been linked to impaired exercise performance (2–4,15,17,23). Reduced fluid balance is associated with decreased stroke volume and cardiac output, elevated heart rate (HR), and central nervous system function which may collectively result in elevated lactate concentration ([La]) during exercise (3,4,9,13,15,17). Furthermore, these factors, associated with premature fatigue, may be critical to optimizing performance (5,17).
Maximal oxygen consumption (V̇o2peak) is an accepted assessment of aerobic fitness, proposed to set the upper limit for endurance performance capacity (20). However, [La] responses may be more sensitive and, therefore, more effectively predict endurance performance success (12). The utility of [La] response for prescription (from [La] threshold [LT]), prediction of performance capacity or assessment of training program efficacy is challenged when confounding factors are introduced. Although research is limited, hypohydration may influence [La] response. Moquin and Mazzeo (16) found the LT, a well-accepted physiological metric, occurred at a lower workload and lower percent V̇o2peak as a result of hypohydration, although no elevation in [La] at baseline or at V̇o2peak were observed. Likewise, Logan-Sprenger et al. (15) found higher [La] at various time points across 120 minutes of cycling (∼65% V̇o2peak) when dehydration occurred through fluid restriction (vs. replacement) during exercise.
Fluid restriction during exercise may compromise performance; however, insufficient fluid replacement during recovery may lead to subsequent exercise bouts being initiated in a hypohydrated state particularly when training daily or completing multiple bouts in a given day. We have shown initiating an exercise bout hypohydrated (i.e., inadequate “prehydration”) to negatively impact repeated 40 yd sprint performance and associated HR and perceptual responses (10). A paradigm of systematic dehydration across time would not only augments hydration at the beginning of exercise but also results in greater relative levels of acute dehydration as fluid is lost during exercise. This fluid deficit could impair performance.
Although effects of hypohydration on LT have been examined (9,13,16), maximal trials do not effectively mimic typical daily training vs. constant-load exercise. Logan-Sprenger et al. (15) examined effects of dehydration on [La] response during cycling at a clamped resistance. Lactate was significantly elevated during dehydration trials; however, dehydration was induced through in-task fluid restriction. Effects of inadequate prehydration on [La] during constant-load exercise are not well understood. This study examined effects of hypohydration from previous day heat exposure on [La] responses and physiological and perceptual measures during 40-minute constant-load cycling.
Methods
Experimental Approach to the Problem
To assess the potential influence of hydration on blood lactate concentration response during constant load exercise, a within-subjects study was completed. Subjects completed constant load cycling trials while well-hydrated and following passive heat exposure leading to hypohydration. Passive heat exposure was completed the evening before cycling trials.
Subjects
Recreationally active men (n = 9) reporting ≥2 hours per week of aerobic physical activity completed a health screening including the Physical Activity Readiness Questionnaire (PAR-Q) providing information allowing stratification using ACSM guidelines (1) according to known risk factors. Before data collection, participants (age range: 19-26 years old) signed a written informed consent describing requirements and risks of participation. All participants were 18 or older. Exclusion criteria were as follows: any contradiction to physical activity participation based on PAR-Q, stratification other than “low risk,” and aged younger than 19 or older than 55 years. All procedures were approved by the University of North Alabama's Institutional Review Board for the Protection of Human Subjects. After screening, height was measured to the nearest cm with a stadiometer (Detecto, Webb City, MO, USA), and mass was measured to the nearest 0.1 kg with a digital scale (Tanita Corporation, Japan). Body fat percentage was estimated using Lange skinfold calipers (Cambridge, MD, USA) and a 3 site method (chest, abdomen, and thigh) (19).
Procedures
V̇o2peak Trial
After descriptive data, participants completed a peak oxygen consumption (V̇o2peak) assessment on a cycle ergometer (Monark Ergomedic 828E; Monark AB, Varberg, Sweden). Seat height was adjusted to achieve a slight bend of the associated leg with the pedal at the 6 o'clock position. Handlebars were adjusted for personal preference. Each participant was fitted with an air-cushioned facemask (Vacumed; Vacumetrics, Ventura, CA, USA) and a HR monitor (Polar Electro, Kempele, Finland) at the level of the sternum. As a warm-up (WU), participants completed 3 minutes of pedaling (60 rev·min−1) with no resistance. After the WU, power output was increased 30 W·min−1 until the participant reached volitional exhaustion, cadence could not be maintained, or investigators deemed it unsafe to continue. Metabolic data were collected using a Vacumed Vista mini cpx measurement system (Ventura, CA) with Turbofit software (Vacumed) providing metabolic data (20-second mean values) every 10 seconds. The system was calibrated before each test with a gas of known composition and a 3-L syringe (Hans Rudolph, Kansas City, MO, USA). Heart rate was measured every min using a HR monitor (Polar Electro). Perceptual estimations (rating of perceived exertion [RPE]) relative to overall feelings were recorded every minute using the Omni RPE scale (22). Participants were instructed that a rating of 1 corresponds to “very easy,” whereas 10 is considered “maximal” on the Omni scale.
Hydrated Trial
Between 24 hours and 7 days after V̇o2peak trial, participants reported to the laboratory between 0700 and 0800 to complete a constant-workload cycling trial in a euhydrated state (HYD). In preparation for HYD, participants were instructed to refrain from consuming alcohol 24 hours before reporting and to consume 500 ml of water (provided) at 1,500, 1,800, and 2,100 the day before to ensure euhydration with additional fluid consumption encouraged but not recorded. Consumption of prescribed fluids was verbally verified. On arrival, participants voided and provided a urine specimen. Weight was then measured using a digital scale (Tanita Corporation), and urine specific gravity (USG) was measured using a manual refractometer (Atago, Tokyo, Japan). A rectal thermocouple (Physitemp RET-1; Physitemp Instruments Inc., Clifton, NJ, USA) inserted 8 cm beyond rectal sphincter and linked with a Physitemp Thermalert TH-8, Clifton, NJ, to monitor core rectal temperature (Trec) during cycling. A Polar HR monitor (Stamford, CT) was worn to assess HR response. Participants (30 W, 60 rev·min−1) WU 5 minutes and then estimated subjective feeling of recovery using a perceived recovery status (PRS) (Laurent, et al. (14)). With cadence maintained, workload was then increased to an individualized power output approximating 60–80% of V̇o2peak, calculated from the V̇o2peak trial. This workload was sustained for 40 minutes with a cool down period of 5 minutes (30 W, 60 rev·min−1) followed by 5-minute passive recovery (seated on the bike). Heart rate, overall RPE, and Trec were recorded every 10 minutes. Capillary blood samples were taken from the fingertip at the end of WU, 10, 20, 30, 40, and 50 minutes and analyzed for [La] using a YSI 1,500 Sport Lactate Analyzer (Yellow Springs Instruments, Yellow Springs, OH, USA), calibrated according to manufacturer's instructions before each trial using 5 mmol·L−1 standard.
Dehydrated Trial
Participants reported to the laboratory ∼1,800–1,900 to initiate a dehydration procedure. Efforts were made to ensure dehydration (DEH) and HYD occurred on the same day of the week. On arrival, participants were instructed to void and insert a Physitemp rectal thermistor for assessment of Trec before being weighed (wearing shorts only) using the Tanita scale. Dehydration was achieved by passive submersion in a hot (∼40° C) water bath. Participants entered the water to the neckline and sat passively. Every 30 minutes participants exited the water, toweled themselves dry, and were weighed and returned to the water bath. This was continued until ∼2.5% loss of body mass was achieved. Participants were given a 500 ml bottle of water for consumption (ad lib) that evening after dehydration with instructions to drink no other fluids. This procedure was designed to result in a net loss of ∼2.0% body mass while not restricting fluid intake completely. Participants then completed the constant-workload cycling bout in the hypohydrated state (DEH) the same as HYD (described above). DEH and HYD were counterbalanced to control for ordering. Participants were instructed to eat a light meal approximately 1,600–1,700 on the day before their first constant-load cycling trial. Using the food record, they were instructed to replicate that meal (contents and time) for HYD and DEH. Session RPE (SRPE) was estimated approximately 15 minutes after the conclusion of HYD and DEH.
Statistical Analyses
An a priori power analysis set at 0.80 determined that 9 participants were needed to determine statistical significance. This was based on a difference of mean values of 1.0 [La] with a pooled SD of 1.0 when incorporating an α of 0.05 and a β of 0.20. Mean values and SDs were calculated for descriptive variables. To compare dependent measures between HYD and hypohydrated trials, series of 2 (trial) × 5 (time point) repeated measures analysis of variance was completed. A Tukey's Least Significant Difference post hoc was used when necessary. Perceived recovery status (PRS) and USG (DEH vs. HYD) were compared using paired t-tests. Results were considered statistically significant at p ≤ 0.05.
Results
Descriptive characteristics of participants are presented in Table 1. Table 2 displays resting measures and postexercise SRPE between trials with associated p values. Significantly greater values were observed for DEH for postexercise SRPE and USG with resting HR approaching significance (p = 0.06). Also, before cycling, participants reported significantly lower (i.e., less well recovered) PRS for DEH vs. HYD (Table 2).
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Table 1.:
Descriptive characteristics (n = 9).*
Table 2.:
Comparison between DEH vs. HYD for resting HR, session RPE, urine specific gravity, and perceived recovery status.*†
There were significant main effects for trial and for time point for [La] (p = 0.02), HR (p = 0.04), Trec (p = 0.04), and RPE (p = 0.02) with no significant interactions observed. Follow-up T-tests at specific time points showed [La] significantly greater for DEH at 10, 30, and 40 minutes with difference at 20 minutes approaching significance (p = 0.10) (Figure 1). Heart rate was significantly greater for DEH at 20, 30, and 40 minutes with the difference at 10 minutes approaching significance (p = 0.08) (Figure 2). Rectal temperature was significantly higher for DEH at 30 and 40 minutes (Figure 3). Rating of perceived exertion was significantly elevated for DEH at 20, 30, and 40 minutes (Figure 4) with no significant interactions between variables. Because of missing data, analyses were conducted for SRPE and USG with n = 7 and for PRS with n = 8.
Figure 1.:
Lactate concentration (mmol·L−1) between trials at specific time points. Values are mean ± SD. *p ≤ 0.05 DEH vs. HYD. DEH = dehydration; HYD = hydrated.
Figure 2.:
Heart rate response (b·min−1) between trials at specific time points. Values are mean ± SD. *p ≤ 0.05 DEH vs. HYD. DEH = dehydration; HYD = hydrated.
Figure 3.:
RPE estimation during exercise between trials at specific time points. Values are mean ± SD. *p ≤ 0.05 DEH vs. HYD. DEH = dehydration; HYD = hydrated; RPE = rating of perceived exertion.
Figure 4.:
Core temperature (° C) between trials at specific time points. Values are mean ± SD. *p ≤ 0.05 DEH vs. HYD. DEH = dehydration; HYD = hydrated.
Discussion
Hypohydration may impair exercise performance (3,4,15,17,23). Insufficient fluid intake during the recovery period before the next exercise bout, whether later in a day or the following day, may lead to hypohydration at the initiation of subsequent exercise bouts. With daily training, risk of systematically reduced hydration status across time is magnified. Inadequate prehydration could exacerbate progressive dehydration during exercise, potentially compounding anticipated performance decrements associated with fluid imbalance. Compared with acute responses from dehydration during exercise (sweat loss and fluid restriction), physiological responses to inadequate prehydration are less well understood. This study examined the influence of insufficient prehydration on physiological responses during constant-load cycling.
Current results indicate inadequate prehydration resulting from a single bout of passive dehydration the previous evening, leads to increased [La] during cycling at a clamped workload. Other dependent measures including resting HR, exercise HR, and Trec support concomitant elevated physiological strain in the current paradigm. Significantly elevated USG coupled with fluid restriction (other than 500 ml water) verify the effectiveness of the current passive dehydration/moderate fluid restriction protocol used to establish a fluid deficit at the initiation of exercise. Outside a paradigm in which athletes attempt to “target a specific body weight before performance,” a passive hot water bath the evening before a morning exercise bout would not be an anticipated practice. However, failing to adequately rehydrate after exercise is a concern and we would suggest that imposing a fluid deficit via exercise (coupled with fluid restriction) would have potentially magnified negative responses compared with passive dehydration particularly with regard to perceptual responses. Much research has focused on physiological responses and performance impairments linked with insufficient fluid replacement during exercise. However, comparatively little is understood with respect to physiological responses associated with inadequate prehydration.
Previous research has examined effects of hypohydration on the LT. Moquin and Mazzeo (16) found LT occurred at a significantly lower relative percent of V̇o2peak after ∼2.6% dehydration (via fluid restriction) (LT = 65.5 ± 1.8%) vs. a euhydrated state (LT = 72.2 ± 1.1%). In that study, LT occurred at a lower absolute power output as well and time to exhaustion was significantly shorter after dehydration. Authors attributed the LT shift to elevated epinephrine concentrations associated with dehydration. Kenefick et al. (13). also found LT occurred at a lower absolute exercise intensity after exercise-induced dehydration to ∼3.9% body mass. By contrast, Papadopoulos et al. (18). concluded that a hot, humid environment resulted in a downward shift of LT similar to Moquin and Mazzeo (16), but that LT was unaffected by dehydration (2–4% via fluid restriction). Although contradictory, these studies have extended the understanding of the LT response to dehydration. However, LT is commonly measured during a graded exercise test (GXT) in which workload is progressively increased to volitional exhaustion. Compared with studies on LT, the current paradigm of constant-load exercise more accurately simulates daily exercise training. To that end, current results concur with previous studies (13,16) showing elevated [La] concentrations are associated with hypohydration.
Mechanisms responsible for elevated [La] concentration in the current study are speculative. Epinephrine was not measured and, therefore, cannot be reliably charged as in previous work (16). Some (16) did not assess core temperature. However, others (18) found LT occurred at higher tympanic temperature during trials in the heat. Febbraio et al. (7) found muscle [La] and glycolytic rate higher during exercise of 40 minutes when completed in hotter environment. In the current study, core temperature (Figure 4) was higher at every time point (significantly at 30 and 40 minutes) after dehydration. It is plausible that elevated core temperature was linked with greater rate of oxygen-independent glycolysis with [La] as the end product. However, no direct measures supporting this notion were collected in the current study and further research is, therefore, warranted.
Rating of perceived exertion presents a convenient, subjective estimation of exercise intensity (1) mediated by a myriad of factors (21). It is indicative of impending fatigue during a GXT to exhaustion (1). Rating of perceived exertion is often associated with factors reflecting increased exercise intensity or workload (21) although it may diverge from physiological factors (i.e., [La]) during prolonged constant-load exercise (11). Rating of perceived exertion was not measured in Moquin and Mazzeo (16) or Papadopoulos et al. (18). However, Kenefick et al. (13) found LT to occur at a lower workload and RPE after 3.9% dehydration. That observation differs from current results regarding the RPE-workload relationship. Kennefick et al. (13) appear to have observed a systematic reduction in workload and RPE with dehydration although the RPE-workload relationship may have been unchanged. By contrast, current results show elevated RPE at a clamped workload (Figure 3) after dehydration. Current discordance with previous work (13) is not easily explained; however, perceptual measures would be expected to be sensitive to magnified physiological strain resulting from a fluid deficit which is demonstrated in the current study.
Perceptual measures in the current study also show individuals perceived a lack of recovery and greater overall effort relative to the entire exercise bout. PRS (Table 2) and SRPE (Table 2) were both sensitive to hypohydration. The PRS scale was developed by Laurent et al. (14) with the initial study showing individuals were capable of predicting their pending performance outcome using subjective responses linked with feelings of recovery. That study created paradigms in which recovery level would vary greatly. Feelings of inadequate recovery in the current study are remarkable in that the procedure followed to induce a fluid deficit was passive (water bath) and subjects received a full night of rest before performance trials. Even so, participants reported feeling significantly less well recovered (lower PRS estimation). Furthermore, acute (i.e., in task) RPE estimations were significantly higher (Figure 3), and SRPE estimations reflecting the subjective response associated with the overall bout (8) were significantly higher although total work volumes (DEH vs. HYD) were equated. PRS, SRPE, and acute RPE responses indicate perceptual responses are sensitive to latent influence of hypohydration even when induced passively the day before exercise. If a fluid deficit was generated from exercise participation and concomitant fluid restriction, it is plausible that the magnitude of differences between conditions (DEH vs. HYD) would be greater. However, additional research is needed to definitively determine this conclusion.
Inadequate prehydration resulted in elevated HR at rest (Table 2) and during constant-load cycling (Figure 2) and higher core temperature (Figure 4). Mechanistically, stroke volume may have been lower after dehydration, and HR increased in a compensatory manner in attempt to maintain necessary cardiac output for the clamped workload. Although this has previously been discussed relative to cardiovascular drift (6), direct measures of stroke volume and cardiac output were not included in the current study and definitive conclusions would, therefore, be speculative.
An alternative explanation for elevated HR would be that it is coupled with core temperature which was elevated in the current study. Furthermore, hormonal changes could have played a role in HR elevation but no hormone levels were measured in the current study.
Practical Applications
Current results indicate that initiating an exercise bout in a dehydrated state results in significantly elevated [La] at a clamped cycling workload. Concomitant changes were observed for HR and Trec. Perceptual responses (before, during, and after exercise) reflected sensitivity to the fluid deficit incurred in the current study even after a passive dehydration protocol the day before exercise. In addition to significantly greater in-task RPE estimations (Figure 3), participants estimated feeling less well recovered (PRS, Table 2) and that the entire exercise bout was significantly more difficult (SRPE, Table 2). Results indicate that the altered [La] kinetics previously observed in graded exercise testing paradigms is present in constant-load work as well even when dehydration occurs passively the day before exercise. Current (single-trial) testing should be less taxing and promote easier recovery vs. rigorous activity across multiple bouts/days as might be endured during practice sessions typically involved with competitive athletes. Although direct data for progressive dehydration are lacking in the current study, it is plausible that athletes should avoid progressive dehydration across time. Systematic dehydration which may accompany multiple practices in a single day or across several sequential days, may intensify changes in the responses observed in the current study may be intensified. Current results are that athletes should focus not only on hydration immediately before and during activity but also the day(s) before training/competition. Finally, results indicate perceptual ratings including SRPE and PRS may be useful for coaches assessing recovery.
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