Impact of Mild Hypohydration on Muscle Endurance, Power, and Strength in Healthy, Active Older Men : The Journal of Strength & Conditioning Research

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Impact of Mild Hypohydration on Muscle Endurance, Power, and Strength in Healthy, Active Older Men

Goulet, Eric D.B.1,2; Mélançon, Michel O.3; Lafrenière, David1; Paquin, Jasmine1; Maltais, Mathieu2; Morais, José A.4

Author Information
Journal of Strength and Conditioning Research: December 2018 - Volume 32 - Issue 12 - p 3405-3415
doi: 10.1519/JSC.0000000000001857
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Abstract

Introduction

The mechanisms underlying the impact of hypohydration on muscle performance are still unclear (20). However, an appealing hypothesis is that hypohydration may alter neuromuscular functions. A recent meta-analysis by Savoie et al. (35) has demonstrated that hypohydration of 3% body mass decreases muscle endurance by 8%, anaerobic power by 6%, and strength by 6%. The mean age of volunteers who participated in the studies included in this meta-analysis was 24 ± 2 years. One of reasons to hypohydration affects neuromuscular performance is an impaired excitation-contraction coupling (5).

The muscle endurance, power, and strength reduction is one of the main characteristics of the aging process, a phenomenon known as dynapenia (7). It has been associated with both mortality and physical disability even when adjusting for sarcopenia (7). Several mechanisms possibly contribute to dynapenia. One of them relate to the impairment in the excitation-contraction coupling processes (7). More specifically, reduced homeostatic capacity for intracellular calcium movement has been demonstrated to reduce contractile dysfunction in the aging muscle (4,43).

It has been established that the control of thirst and renal maximal urine-concentrating ability are both impaired in hypohydrated older adults (28). Therefore, when challenged by hypohydration, regulation of fluid homeostasis may become significantly compromised or delayed in older persons. Hence, older adults experiencing daily (e.g., induced by physical exercise or heat exposure) or chronic (e.g., induced by polypharmacy or illness) body water losses may be vulnerable to the development of mild (i.e., 1% body mass loss) (25) chronic hypohydration (24).

Given that hypohydration could impair muscle performance by decreasing excitation-contraction coupling, and that one of the mechanism underlying dynapenia is an impairment in the excitation-contraction coupling process, we wonder whether mild hypohydration could contribute to exacerbate the effect of dynapenia. Whether hypohydration impairs muscle performance in older adults is unclear and, to our knowledge, has not been studied.

The goal of this study was to examine how mild hypohydration of 1% body mass impacts muscle endurance, power, and strength in healthy, active older men. Findings of this study will be useful not only for scientists but also for health practitioners, athletic trainers and dietitians involved in the care of older adults.

Methods

Experimental Approach to the Problem

After a preliminary visit, participants took part in (a) 3 familiarization trials where they accustomed themselves with all exercise procedures and; (b) 2 experiments, which consisted of a passive exposure to heat while euhydration was being maintained or hypohydration of 1% body mass induced, followed by a 1-hour passive recovery period and then an exercise testing session where muscle endurance, power, and strength were assessed. On average, participants underwent a familiarization trial every 5 days. The experiments were spaced by 7–8 days and always started at the same time of day in the morning (09:00 hours) using a randomized, crossover, and counterbalanced study design.

Subjects

Eight nonheat acclimatized, healthy, nonobese and aerobically trained (6) active older men participated in this investigation. Their physical, physiological, and anthropometric characteristics are shown in Table 1. To be eligible for the study, participants had to meet the following inclusion criteria: (a) age range between 60 and 75 years; (b) body mass stable (±2 kg over the past 6 months), nonobese and free of illness or other health problems and; (c) currently training ≥4 h/wk. Potential participants were excluded if they (a) were intolerant to heat exposure; (b) were smoking; (c) were hypertensive (≥140/90 mm Hg); (d) had previously had a hip, thigh, or leg injury; (e) have had a cerebrovascular accident and; (f) were taking medication that could alter fluid and electrolyte balance. All experimental procedures, which were approved by the University of Sherbrooke Institutional Review Board, were explained to the participants before they gave their written and informed consent to take part in this study.

T1
Table 1.:
Physical, physiological, and anthropometric characteristics of participants.*

Procedures

Figure 1 depicts the time-course of the experimental procedures.

F1
Figure 1.:
Schematic representation of the time course of the experimental procedures. BM = body mass; BP = blood pressure; CB = capillary blood; E = enter; EX = exit; GT = gastrointestinal temperature; HR = heart rate; HRS = heart rate sensor installation; MR = mouth rinse; US = urine sample; USG = urine-specific gravity; SP = subjective perceptions; VB = void bladder; WI = water intake. * = blocks of heat exposure were repeated until participants achieved a loss of 1% body mass; ** = starting from the second block of heat exposure; *** = measured only at the end of the last block of heat exposure.

Arrival at Laboratory

Participants voided their bladder in a graduated urinal, provided a midstream urine sample for measurement of urine-specific gravity with a portable refractometer (PAL-10S; Atago, Bellevue, WA, USA) and had their body mass measured (BX-300 +; Altron Systems, West Caldwell, NJ, USA), which was taken as the baseline, euhydrated body mass from which hypohydration was calculated.

Pre-exposure

Immediately after entering the room (30° C, ∼30% humidity), participants put on a heart rate sensor (T-31 Polar electrode, Polar USA) and sat quietly for 20 minutes. At the end of the pre-exposure period, participants provided a capillary blood sample, voided their bladder, had their body mass assessed, and measurements of heart rate (Polar Vantage NV, Polar USA), gastrointestinal temperature (CorTemp Data Recorder, HQ Inc) and perception of thirst (1–11 scale: 1 = no thirst; 11 = extreme thirst, (8)), heat (7-point scale, 1: too cool; 7: too hot, (9)), nausea (5-point scale, 1: none; 5: extreme, (10)), and dizziness (5-point scale, 1: none; 5: extreme, (10)) were taken.

Blocks of Heat Exposure

Wearing only shorts or underwear, participants completed blocks of heat exposure consisting of 20 min inside a 115 × 65 × 80 cm head-out, infrared-heated portable chamber (ProHealth Sauna, Canada) (42–43° C, ∼40% humidity) followed by a recovery period of 5 min outside the portable chamber. This cycle was repeated until participants had accumulated a loss of body mass of 1%. While inside the chamber, measurements of heart rate, perceived thirst, heat stress, nausea, and dizziness were taken at min 18. During the recovery periods, participants voided their bladder and measurements of body mass, gastrointestinal temperature and blood pressure were taken. Starting from the second block of heat exposure, participants received an amount of water (provided at gastrointestinal temperature) equivalent to that lost during the preceding heat exposure (euhydration condition [EUH]) or rinsed their mouth (to reduce dry mouth and thirst sensation) with 25 ml of water (hypohydration condition [HYPO]) 2 minutes after re-entering the portable chamber. When the targeted loss of body mass was reached (after the last block of heat exposure), participants provided a capillary blood sample, voided their bladder, collected a midstream urine sample for urine-specific gravity assessment, and body mass was measured.

Recovery Period

Participants passively recovered in a seated position for 60 minutes in a room held at 20° C, with ∼30% humidity. At the end of the recovery period, participants provided a capillary blood sample, voided their bladder, collected a midstream urine sample for urine-specific gravity assessment, were weighed and had their gastrointestinal temperature measured. They were then provided water in an amount equivalent to the loss of body mass incurred during the recovery period, such to restore euhydration or the 1% body mass loss.

Preliminary Visit

During the preliminary visit, participants' anthropometric characteristics were determined and resting and maximal heart rate, blood pressure and peak oxygen uptake (peak) were measured. Postvoid body mass was measured to the nearest 50 g with a digital medical scale (Seca 707, Seca, Germany), height with a wall stadiometer and fat mass and fat-free mass with dual-energy X-ray absorptiometry (Lunar Prodigy, GE Healthcare, Madison, WI, USA). Resting heart rate and blood pressure were measured after 5 minutes of seated rest with an automatic sphygmomanometer (Welch Allyn, Skaneateles, NY, USA). Mean arterial blood pressure was computed as

Peak and maximal heart rate were measured on a computerized, speed-independent, cycle ergometer (Ergoline ER 900, Jaeger, Germany) using an Ergocard (Medisoft, Belgium) expired gas analysis system that had been calibrated with gases of known concentration. Participants began to cycle at a workload of 0.70 watts·kg−1 body mass for 1-minute, after which the load was step-incremented by 0.30 kg body mass·min−1 until exhaustion. Peak power output was measured with the following formula:

Familiarization Trials

The purpose of these trials was to (a) familiarize participants with the procedures and exercises that would be used to measure the various muscle performances during the subsequent experiments; (b) determine the coefficient of variation (CV) of the different performance outcomes, and; (c) minimize any learning effect. With the exception of the tests used to determine 1 repetition maximum (1RM) seated leg-press extension strength and 1RM seated leg-curl flexion strength during the first familiarization trial, all 3 familiarization trials were performed in the same exact manner and duplicated the sequence of exercises that would be used during the experiments. It has been shown that the learning effect usually becomes negligible after 2 familiarization trials (18). To better understand and interpret our results from a practical and clinical perspective, we added a third familiarization trial to determine the CV of the different muscle performance tests between the second and third familiarization trial. At the end of the last familiarization trial, participants were provided with a food log, as they would have to record all food and fluid intake 24 hours before the first experiment to replicate this diet before the second experiment and were instructed on how and when to ingest the gastrointestinal temperature sensor (CorTemp Ingestible Core Temperature Sensor; HQ Inc., Palmetto, FL, USA).

Pre-experimental Procedures

Participants were asked to refrain from dietary supplement intake 48 hours before the experiments. Furthermore, 24 hours before experiments participants were required to (a) refrain from training; (b) replicate the diet they had consumed and fluid they had drunk before the first experimentation; and (c) refrain from consuming diuretic substances. Participants were asked to ingest the gastrointestinal temperature sensor, along with 250 ml of tap water, 10 hours before arriving at the laboratory for the experiments (34). Finally, the night before the experiments, participants were requested to drink 500 ml of tap water 2 hours before bedtime and go to bed at identical times and, on the day of the experiments, ingest 500 ml of tap water 2 hours before their arrival at the laboratory and then to remain fasted.

Hematocrit and Plasma Volume Changes Measurements

Capillary blood was obtained from participants (after they had been seated for at least 10 minutes) by pricking their index or middle finger with a high volume lancet (Unistik 3 Dual; Owen Mumfort, Oxfordshire, United Kingdom). After the first drop of blood was removed, ∼120 μl of blood was collected with 2 heparinized microcapillary tubes, which were then centrifuged at 10,000g for 5 min. Plasma volume changes were computed from hematocrit values with the method proposed by Van Beaumont (41).

Exercise Testing Periods

After the recovery period and before undertaking the testing period, participants completed a 5-min cycling bout at a self-selected intensity and a standardized 5-min stretching period. Then, participants underwent, in this particular order, (a) 2, 30-second chair stand tests; (b) 3 grip strength tests on both the left and right hand; (c) 2 maximal isometric tests on a seated leg-press machine; (d) 1 muscle endurance test to exhaustion on a seated leg-press machine; (e) 1 muscle endurance test to exhaustion on a seated leg-curl machine; (f) 1 muscle endurance test to exhaustion on a seated leg-press machine; (g) 1 muscle endurance test to exhaustion on a seated leg-curl machine and; (h) a 30-second Wingate test.

Thirty-Second Chair Stand Test

The 30-second chair stand test is a reliable and valid indicator of lower-body strength in active older adults living in the community (19). Participants started the test from a standing position with arms crossed at the wrists and held against the chest. The participants were asked to sit and stand from a 40-cm high chair as quickly and safely as possible for 30 seconds. A period of 2 minutes separated each test. The total number of time participants seated on the chair was taken as the measure of performance for each test, with the final overall performance representing the cumulative work achieved in both tests. The between visit CV for the 30-second chair stand test was 0.8%. There was a 3-minute recovery period between the 30-second chair stand test and the grip strength test.

Grip Strength Test

It has been shown that grip strength is a predictor of future disability (1) and correlates with leg-press and upper-body force in healthy older adults (29,31). Grip strength was measured from a standing position using a Smedley digital hand dynamometer (Baseline Evaluation Instruments, China) held along the body (33), with 0° angle flexion at the elbow and shoulder (40). The height of the grip was individually standardized and to prevent slipping, participants wore medical latex gloves. The participants were asked to produce a maximal gripping force as fast as possible with minimal movements at the arm and shoulder level. The participants performed 3 tests with each hand, alternating every minute between the left and right hands. The final performance was taken as the sum of force achieved by the right and left hand. The between visit CV for the grip strength test was 2.6%.

Lower Limbs Maximal Isometric Test

The maximal isometric test was performed on a seated leg-press machine (knee angle of 90–100°, standardized for each participant) using a force plate (Vernier, USA) tightly fixed on the footrest plate. The participants' feet height and width on the force plate were standardized. Participants were asked to produce a maximum of force in the shortest time possible. Two tests were performed, each separated by a 3-minute rest period. The final performance was taken as the cumulative force achieved in both tests. The between visit CV for the lower limb maximal isometric test was 1.8%. Participants were allowed a 5-minute recovery period before undergoing the next exercise.

Lower Limb Muscle Endurance Tests to Exhaustion

Muscle endurance of the lower limbs was tested on a seated leg-press and leg-curl machine using an individually standardized position. Participants were asked to perform each exercise at 60% of 1RM (12), at a standardized speed (imposed by a metronome at 50 BPM) and until exhaustion. Participants underwent 2 seated leg-press and leg-curl exercises performed in an alternate manner, with 5 minutes of rest separating each exercise. The measure of performance was taken as the sum of work (repetitions) achieved during both the leg-press and leg-curl exercises. The between visit CV for this muscle endurance test was 3%. Participants took a 10-minute recovery period before undergoing the Wingate test.

Determination of 1RM measurement (performed during the first familiarization trial) began with an exercise-specific, standardized warm-up protocol after which participants were asked to lift a load they thought best corresponded to 80% of their 1RM. Then, depending on the ease with which the load was raised, adjustments of 2.5–15% were made until 1RM was achieved. Attempts were separated by a 3-minute rest period with, on average, approximately 5 attempts needed to achieve 1RM.

Wingate Test

Before undergoing the Wingate test, which was performed on a Monark 874-E (Monark Exercise AB, Sweden) cycle ergometer, participants completed a (a) 5-minute warm-up period consisting of 3 minutes of pedaling at a load of 2% body mass followed by a 5-second sprint and; (b) 2-minute pedaling at a load of 3% body mass followed by a 10-second sprint. After a 2-minute rest period, participants started pedaling as fast as possible for 30 seconds against a load corresponding to 7.5% of body mass (23). Participants remained seated throughout the test. The measure of performance was taken as the highest power output produced over a 1-second period. The between visit CV for anaerobic power during the Wingate test was 5%.

Statistical Analyses

All statistical analyses were performed with the IBM SPSS Statistics software (version 21; NY, USA). Normality of data distribution was assessed with a Shapiro-Wilk test. Normally distributed data were analyzed using either paired t tests, 1-way repeated-measures analysis of variance (ANOVA) or 2-way (treatment × time) repeated-measures ANOVA, with Greenhouse-Geisser corrections when sphericity was violated. Abnormally distributed data (length of time taken to reach the targeted body water loss in each condition) were tested with a Wilcoxon signed ranks test, using the Asymptotic test to calculate significance. Because not all participants could produce urine at all collection points, all urine-related variables were analyzed using linear mixed-effects modeling. When significant treatment or interaction effects were detected, multiple pairwise comparisons were performed and corrected with the false discovery rate procedure. The practical significance and clinical implication of the effect of hypohydration on the various performance outcomes was determined with magnitude-based inference statistics (17). Magnitude-based inference is an intuitive statistical approach relying on where the confidence interval derived from a p value lies in relation to a clinical or practical threshold value for a substantial or worthwhile effect rather than the null value (17). The smallest acceptable worthwhile change in any muscle performance outcomes between the hypohydrated and euhydrated condition was set at 0.5 x performance test CV (17). The qualitative probabilistic terms were assigned using the following scale (17): 0.5–4.99%, very unlikely; 5.0–24.99%, unlikely, 25.0–74.99%, possibly; 75.0–94.99%, likely; 95.0–99.49%, very likely; 99.50% +, almost certainly (16). The study's sample size was calculated based on a CV of 4.9% (personal communication, (14)) for the measurement of absolute anaerobic power during the Wingate test. On the basis of such a CV, a power analysis (α = 0.05, β = 0.2) indicated that 8 participants would provide sufficient power to detect a change of 6% in anaerobic power between conditions. Because not all participants completed the same number of blocks of heat exposure, data measured during that time-period are reported as a percentage of the total number of blocks completed. The threshold for statistical significance was set at 95% (α ≤ 0.05). Results are presented as mean ± SD.

Results

State of Hydration of Participants at Arrival to the Laboratory

Participants were adequately and similarly hydrated before each trial, as supported by the nonsignificant differences in urine-specific gravity (1.005 ± 0.008 g·ml−1 vs. 1.008 ± 0.005 g·ml−1, p = 0.14), urine production (233 ± 156 ml vs. 187 ± 125 ml, p = 0.25), and body mass (76.8 ± 13.5 kg vs. 77.0 ± 13.4 kg, p = 0.08) between the euhydrated and hypohydrated conditions.

Heat Exposure Duration, Hydration State, and Plasma Volume Changes

The time to achieve the targeted loss of body mass (1%), which takes into account the times outside the chamber (recovery periods), was 94.9 ± 17.6 and 96.9 ± 18.0 minutes (P = 0.51) for the euhydrated and hypohydrated condition, respectively. As per study design, the euhydrated group started the exercise testing period with their euhydrated body mass, while the hypohydrated group with a loss of body mass of 1%. A time, group, but no interaction (p = 0.31) effects were observed for the change in urine-specific gravity between conditions (Figure 2). Before starting the exercise testing period, urine-specific gravity was 1.012 ± 0.006 g·ml−1 with the euhydrated condition, compared with 1.019 ± 0.006 g·ml−1 with the hypohydrated condition (p = 0.02). There was a time, but no treatment or interaction effects in the change in plasma volume across conditions. From previous heat exposure to before start of the exercise testing period, plasma volume increased by 0.6 ± 3.7% in the euhydrated group, compared with 0.3 ± 3.5% in the hypohydrated group (p = 0.22).

F2
Figure 2.:
Urine-specific gravity at arrival at the laboratory, after the last block of heat exposure and before exercise testing while being euhydrated and hypohydrated. * = time effect; ¶ = group effect. Results are mean ± SD.

Heart Rate, Blood Pressure, and Gastrointestinal Temperature

As demonstrated in Table 2, heart rate increased during heat exposure (p < 0.01), but no group or interaction effects were observed. Systolic and diastolic blood pressures were not significantly impacted by heat exposure. In fact, mean arterial blood pressure during heat exposure was, on average, 89 ± 5 mm Hg with the euhydrated condition, compared with 90 ± 8 mm Hg for the hypohydrated condition (p = 0.84). As depicted in Figure 3, gastrointestinal temperature significantly varied over time (P < 0.01), but no group or interaction (p = 0.60) effects were observed between hydration conditions. Gastrointestinal temperature immediately before starting the exercise period was 0.23 ± 0.29° C higher than the preheat exposure value in the euhydrated condition, compared with 0.33 ± 0.25° C in the hypohydrated condition, a change which was not significantly different from baseline values in neither hydration conditions (p = 0.30).

T2
Table 2.:
Perceived thirst, perceived heat stress and heart rate immediately before heat exposure (0%), during heat exposure (50 and 75%) and immediately postheat exposure (100%) while being euhydrated and hypohydrated.*†
F3
Figure 3.:
Gastrointestinal temperature immediately before heat exposure (0%), during the blocks of heat exposure (50 and 75%), after the last block of heat exposure (100%) and before exercise testing while being euhydrated and hypohydrated. * = time effect. The percentages of the total number of heat exposure blocks completed by participants are represented by 0, 50, 75, and 100%. Results are mean ± SD.

Subjective Perceptions

A time, interaction (p < 0.01) but no treatment effects were observed in the change in perceived thirst across hydration conditions (Table 2). At the end of heat exposure, perceived thirst was higher while being hypohydrated (4.6 ± 1.5 arbitrary units [AU]) than euhydrated (2.9 ± 1.5 AU) (p = 0.01). Perceived heat stress increased over time (p < 0.01), but no treatment or interaction effects were observed between conditions (Table 2). Perceived dizziness and nausea remained at 1 AU throughout heat exposure and were unaffected by hydration state.

Muscle Performance

To provide a quick and valuable reference to the readers, Table 3 reports the p values and magnitude-based inference statistics associated with the results of each test of muscle performance.

T3
Table 3.:
Alpha values and magnitude-based inference statistics associated with the results of each muscle performance test.

Thirty-Second Chair Stand, Grip Strength, and Lower Limb Maximal Isometric Force Tests

Figure 4 demonstrates the impact of hydration state on 30-second chair stand (A), grip strength (B), and seated leg-press maximal isometric force (C) performances. There was no significant difference between hydration conditions in the total amount of repetitions completed during the 30-second chair stand test (HYPO: 63.3 ± 8.5 reps; EUH: 64.0 ± 9.1 reps, Δ% = −1.0 ± 4.4%). The total amount of force achieved during the grip strength test did not differ between hydration regimens (HYPO: 221.5 ± 22.3 kg; EUH: 226.9 ± 21.3 kg, Δ% = −2.4 ± 4.1%). Hypohydration had a nonsignificant effect on seated leg-press maximal isometric strength. While being hypohydrated, participants accumulated a maximal isometric force of 3993 ± 532 N, compared with 3970 ± 483 N when they were euhydrated (Δ% = 0.5 ± 5.6%).

F4
Figure 4.:
Impact of the euhydrated and hypohydrated condition on 30-second chair stand (A), grip strength (B) and seated leg-press maximal isometric force (C) performances. Results are mean ± SD.

Lower Limb Muscle Endurance Tests to Exhaustion

As depicted in Figure 5, being hypohydrated did not significantly reduce the total number of repetitions completed until exhaustion while combining the cumulative leg work achieved during the seated leg-press and leg-curl exercises (HYPO: 124.6 ± 15.0 reps; EUH: 137.4 ± 29.8 reps, Δ% = −7.5 ± 11.2%).

F5
Figure 5.:
Impact of the euhydrated and hypohydrated condition on cumulative work achieved during the seated leg-press and seated leg-curl exercises. Results are mean ± SD.

Wingate Test

How hypohydration impacted absolute (A) and body mass-corrected (B) anaerobic power is illustrated in Figure 6. Compared with euhydration, hypohydration significantly reduced absolute (584 ± 126 W vs. 561 ± 122 W, Δ% = −3.9 ± 4.3%, p = 0.04) and relative (7.5 ± 1.1 W vs. 7.3 ± 1.3 W, Δ% = −3.9 ± 4.3%) anaerobic power. This observation remained significant despite correcting for lower limbs muscle mass (p = 0.05).

F6
Figure 6.:
Impact of the euhydrated and hypohydrated condition on absolute (A) and body mass-corrected (B) anaerobic power. * = significant difference between conditions. Results are mean ± SD.

Discussion

The goal of this study was to examine the impact of mild hypohydration (1% body mass) on muscle endurance, power, and strength in healthy, nonobese active older men. To the best of our knowledge, this is the first study examining the effect of hypohydration on muscle performance in older adults. This is a topic of interest because dynapenia is an inevitable characteristic of aging, under certain circumstances older persons are vulnerable to the development of mild chronic hypohydration, and decreased muscle performance is linked with hypohydration in young individuals. This combination led us to believe that, potentially, mild body water loss, as can be observed with chronic hypohydration, could also be detrimental to muscle exercise capacity in older adults and exacerbate the effect of dynapenia. Although findings were not statistically significant, magnitude-based inference statistics indicate that from a clinical and practical standpoint the 30-second chair stand, grip strength, and lower limb muscle endurance tests are possibly or likely negatively impacted by mild hypohydration. The impact of mild body water loss on lower limb muscle power was statistically significant and from a clinical and practical point of view, likely to be important. We interpret these findings to suggest that mild hypohydration could worsen the impact of dynapenia. Our results may therefore have important implications for scientists, clinicians, kinesiologists, and dietitians.

Hypohydration was induced using repeated blocks of passive exposure to hot ambient temperatures such to incur a hypertonic hypohydration mimicking the type of hypohydration that older adults would experience when drinking insufficient fluids (15). The prevalence of hypertonic hypohydration may be as high as 60% among community-dwelling older adults (38,39). However, gastrointestinal temperature was not different between hydration regimens before exercise onset and had returned to within 0.3° C of baseline value, which is inside the normal daily core temperature fluctuation (22). Hence, the use of such dehydrating protocol enabled us to successfully isolate the impact of hypohydration from that of increased core temperature because of heat exposure (20,35), while at the same time providing findings with high external validity.

Our findings show that the 30-second chair stand test as well as the grip strength test, which respectively provide an index of lower limb strength (19) and overall/upper body strength (31), are possibly and likely negatively impacted by hypohydration. We are aware of no other study that examined the influence of hypohydration on the 30-second chair stand test. Besides, sauna-induced hypohydration of 1.8%, body mass has been shown in young adults to nonsignificantly impair grip strength by 4% which, in terms of magnitude, is in close agreement with what we observed (13). Unfortunately, these authors did not analyze nor discuss their observation from a clinical perspective. Because the 30-second chair stand test and the grip test relate to measure of strength which could potentially limit older persons in their daily life activities (e.g., climbing stairs, getting out of a chair or bath tub, opening a jar, lifting and holding grocery bags, grabbing a bottle, etc.), these findings highlight the implications that maintaining an adequate hydration may confer to the quality of life of older persons.

Surprisingly, the effect of hypohydration on lower limbs maximal isometric strength was unclear, which is not in line with what we observed for the other indices of muscle strength. However, several other studies conducted in young adults have reported an unclear effect of hypohydration on lower limb maximal strength, although the level of hypohydration assessed was 2-to 5-fold higher than that in this study (3,11,21,26,27,37,42). Nevertheless, Savoie et al. (35) concluded in their meta-analysis that hypohydration of 3% body mass reduces lower limb muscle strength in a considerable manner (i.e., 6%). Altogether, our results highlight the interesting possibility that in older individuals, mild hypohydration may only impede isometric force production for small muscle masses. This needs to be verified in future studies.

We observed that lower limbs muscle endurance (−7.5%) and anaerobic power (−3.9%) are both likely impaired by mild hypohydration, which agrees with results of Savoie et al.'s (35) meta-analysis indicating that the effect of hypohydration on muscle endurance (−8.2%) is almost certainly very important, whereas that on anaerobic power (−5.8%) likely important. These findings are significant, since an earlier onset of muscle fatigue or impairment in anaerobic power can both have a major impact on mobility in older adults, which is critical for maintaining independent functioning. Indeed, a lack of muscle endurance may increase the difficulty in, or ultimately prevent, performing physical tasks such as walking several hundred meters, climbing long hills or 10–20 stairs in a row or remaining on feet for prolonged period while shopping, for example. Moreover, muscle power has emerged as a very important predictor of functional limitations in older adults, since it has been shown to decline earlier and more rapidly than muscle strength with advancing age (32). Finally, both a lack of muscle power (2) and endurance (36) have been associated with increased risks of falls in community-dwelling older adults with a history of falls or with impaired mobility. Altogether, our findings suggest that by contributing to likely improve muscle endurance and power, adequate hydration in older persons may help maintain physical functioning and enhance quality of life.

This study has limitations that need to be taken into consideration when interpreting findings. First, the impact of hypohydration was evaluated in healthy, aerobically trained older men accustomed to the stress of hypohydration. Had the study been conducted in nontrained sedentary older adults, it is possible that the impact of hypohydration on muscle performance would have been more pronounced (35). Second, a hypohydration of only 1% body mass was induced, since our goal was to replicate a state of body water loss often experienced by older adults (24). However, it is very likely that had the hypohydration state been greater, the impact on muscle performance would have been more important (35). Finally, per research design, participants were not blinded from the hydration treatments they received. Because of the general belief among the population that exercising while being hypohydrated may impair exercise capacity and physiological functions (35), it cannot be excluded that part of the effect of hypohydration on muscle performance may have been due to a nocebo effect (30).

We evaluated the impact of hypohydration on muscle performance in older adults. This question was relevant since (a) under particular scenarios older adults are vulnerable to mild, chronic hypohydration; (b) muscle performance declines with aging and, when too severe, leads to physical disability and; (c) in young adults hypohydration has been shown to impair muscle exercise capacity. Our results indicate that hypohydration of as low as 1% body mass could impair muscle strength, endurance, and power in healthy, active older men. These findings have 2 important implications. They (a) suggest that hypohydration may exacerbate dynapenia and; (b) act as a further incentive encouraging older adults to sustain adequate hydration in the sake of maintaining, or preventing the decline in, physical functioning. Future studies are needed to study the impact of hypohydration in persons with impaired mobility or at risk for falls.

Practical Applications

Older persons experiencing daily body water losses may be susceptible to the development of mild, chronic hypohydration. In fact, their capacity to replace fluid loss through sweat or urine may be jeopardized or significantly delayed since the control of thirst and renal maximal urine-concentrating ability are both impaired in hypohydrated older adults. Results of this study demonstrate that hypohydration of as low as 1% body mass could impair muscle endurance, strength, and power in older persons. These findings are important, since aging is accompanied by dynapenia and therefore it cannot be excluded that inadequate hydration in the aged persons could act to exacerbate this condition. Hence, health practitioners, athletic trainers, and dietitians involved in the care of older adults should take this new information into consideration and advise older individuals that maintaining adequate hydration is of major importance because it could potentially act to (a) reduce the impact of dynapenia; (b) improve quality of life and; (c) contribute to enhance the effect of resistance training in those who have integrated this type of exercise into their life.

Acknowledgments

Sincere thanks are due to all participants for their outstanding contribution to this research project. Authors are grateful for the technical assistance of Dr. Daniel Tessier. This study was made possible through a research grant provided by the Réseau Québécois de la Recherche sur le Vieillissement (RQRV). The authors declare no conflict of interest.

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Keywords:

aging; dynapenia; exercise; dehydration

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