Across all trials, the mean change in HR for every additional 1% ΔBML was 3 ± 2 b·min−1. For trials where subjects arrived euhydrated, the mean change in HR for every additional 1% ΔBML was 3 ± 5 b·min−1. For the trials in which subjects arrived hypohydrated, the mean change in HR for every additional 1% ΔBML was 3 ± 2 b·min−1. Analyzing fixed and variable intensity trials (time trials or graded exercise tests), the change in HR for every 1% ΔBML was 4 and 1 b·min−1, respectively. Change in HR for every 1% ΔBML for cycling and running only trials was 3 ± 2 and 3 ± 2 b·min−1, respectively.
The results of this review indicated that HR is increased with increasing levels of dehydration. Our analysis revealed that HR was increased by an average of 3 b·min−1 for every 1% ΔBML. The increases in HR as a result of the increased levels of dehydration coincide with the results of Montain and Coyle (30) who showed that the level of dehydration was directly correlated with the factors affecting cardiovascular strain; HR is increased and SV is subsequently decreased. The increase in cardiovascular strain as a result of exercise-induced dehydration has been shown to decrease performance (3,37).
Results from our review showed that there were no differences in the change in HR for every 1% ΔBML for subjects arriving either euhydrated or hypohydrated before exercise. The effects of cardiovascular drift have been shown to be in proportion to the degree of dehydration and hyperthermia (17,19,30). For example, if there were 2 athletes performing exercise in the heat with 1 athlete arriving euhydrated and the other athlete arriving hypohydrated, there would be no differences in the magnitude of cardiovascular drift (increase in HR and decrease in SV), if they both lost the same amount of body water during exercise. This might explain why there were no differences in HR between those arriving euhydrated and hypohydrated from our results. When examining performance from the studies included in our review, it is evident that arriving in a hypohydrated state has detrimental effects on performance, thus suggesting hydration status as a key determinant of subsequent exercise performance.
When examining the differences in HR with increasing levels of dehydration in cycling and running trials, our results showed that there were no differences between either exercise modes. This is in support of previous literature (34), which showed that there were no differences in HR between cycling and running, despite greater decreases in cardiac output and SV during cycling. These authors attribute the changes in cardiac output and SV to other mechanisms not associated with the tachycardia occurring during exercise that has been previously examined (15). It can be suggested by our results that the mode of exercise does not affect the changes in HR with increasing levels of dehydration during exercise in the heat.
Studies that required subjects to perform exercise using a time trial or graded exercise protocol saw negligible differences in HR with increasing levels of dehydration (4,7,17,25,45). The small differences in HR between subjects who are greater dehydrated vs. those less dehydrated during variable intensity exercise is most likely because of sensory input: the combination of exercise in the heat and dehydration increase overall physiological strain (9). In other words, when compared with exercise in a euhydrated state, levels of dehydration exceeding ∼2% BML in real-world situations (time trials, and so on) will result in similar exercising HR at the cost of reduced exercise intensity and thus poor performance. During a race, for example, a runner who is dehydrated will have an overall slower finish time over a runner who is hydrated, although both runners are performing at their maximum capacity. This is due to the physiological strain placed on their bodies, in that HR will be higher at a lower intensity in the more dehydrated runner; so although they may perceive themselves performing maximally, they will be running at a slower pace and be unable to pace themselves evening during the duration of the race (7,41).
Increasing levels of dehydration during prolonged exercise, especially in the heat, have profound effects on exercise performance. The American College of Sports Medicine (36) and National Athletic Trainers' Association (6) recommend that minimizing fluid losses during exercise (<2%) will assist in attenuating any decreases in exercise performance. In addition, there is supporting evidence recommending that minimizing fluid losses before and during exercise will enhance performance during prolonged exercise (8,13,33). This current review supports the aforementioned recommendations in that the trials where fluid was replaced, subjects exercised at lower HR intensities than in trials where fluid was restricted (2,4,7,14,17,28). It is suggested that if increases in HR, occurring during cardiovascular drift, are minimized during similar stress or exercise intensity, an athlete will be able to exercise longer before reaching fatigue.
This review was not without limitations. For the purposes of this review, we focused on exercise in the heat. Although we found that HR was increased at increasing levels of BML, we cannot generalize this across all situations because we did not take into account the temperate and cold environmental conditions. Second, all but 3 of the studies were performed in a controlled-laboratory setting and all but 5 studies had subjects exercise at a fixed exercise intensity. To be able to generalize these results during real-world athletic competitions, additional research needs to be conducted analyzing the changes in HR at increasing levels of BML in the field setting. Nonetheless, the presented studies support the notion that HR is increased with increased BMLs during exercise.
The evidence presented in this review may be useful to both athletes and coaches. The results indicated that there is an additional increase in HR for every 1% change in BML at given exercise intensities during exercise in the heat. This increase in HR with increasing levels of dehydration can cause a decline in athletic performance, not only during training but also during competition. For example, an athlete arriving for a practice session or competition will have an increased HR of about 6–10 b·min−1 during exercise if they are 2–3% dehydrated. This increase in HR exacerbates cardiovascular drift and the level of perceived fatigue, thus decreasing exercise performance, especially during endurance events and exercise in the heat.
Beginning exercise in a euhydrated state and minimizing fluid losses throughout exercise will attenuate the increases in cardiovascular strain and increase overall performance. Athletes who begin exercise in a euhydrated state and minimize fluid losses throughout exercise will be able to exercise at a higher intensity and have a lower perception of exertion and fatigue. As a result, athletes will be able to achieve greater gains during training to help maximize athletic performance during competition. In addition, maintaining an appropriate level of hydration during exercise in the heat is of particular importance because the physiological stresses of heat exposure are responsible for performance decrements.
1. Armstrong LE, Maresh CM, Gabaree CV, Hoffman JR, Kavouras SA, Kenefick RW, Castellani JW, Ahlquist LE. Thermal and circulatory responses during exercise: Effects of hypohydration, dehydration, and water intake. J Appl Physiol 82: 2028–2035, 1997.
2. Arnaoutis G, Kavouras SA, Christaki I, Sidossis LS. Water ingestion improves performance
compared with mouth rinse in dehydrated subjects. Med Sci Sports Exerc 44: 175–179, 2012.
3. Barr SI. Effects of dehydration on exercise performance
. Can J Appl Physiol 24: 164–172, 1999.
4. Below PR, Mora-Rodríguez R, González-Alonso J, Coyle EF. Fluid and carbohydrate ingestion independently improve performance
during 1 h of intense exercise. Med Sci Sports Exerc 27: 200–210, 1995.
5. Buono MJ, Wall AJ. Effect of hypohydration on core temperature during exercise in temperate and hot environments. Pflugers Arch 440: 476–480, 2000.
6. Casa DJ, Armstrong LE, Hillman SK, Montain SJ, Reiff RV, Rich BS, Roberts WO, Stone JA. National athletic trainers' association position statement: Fluid replacement for athletes. J Athl Train 35: 212–224, 2000.
7. Casa DJ, Stearns RL, Lopez RM, Ganio MS, McDermott BP, Walker Yeargin S, Yamamoto LM, Mazerolle SM, Roti MW, Armstrong LE, Maresh CM. Influence of hydration
on physiological function and performance
during trail running in the heat. J Athl Train 45: 147–156, 2010.
8. Cheuvront SN, Carter R III, Sawka MN. Fluid balance and endurance exercise performance
. Curr Sports Med Rep 2: 202–208, 2003.
9. Cheuvront SN, Kenefick RW, Montain SJ, Sawka MN. Mechanisms of aerobic performance
impairment with heat stress and dehydration. J Appl Physiol 109: 1989–1995, 2010.
10. Coyle EF, González-Alonso J. Cardiovascular drift during prolonged exercise: New perspectives. Exerc Sport Sci Rev 29: 88–92, 2001.
11. Coyle EF. Physiological determinants of endurance exercise performance
. J Sci Med Sport 2: 181–189, 1999.
12. Crandall CG, González-Alonso J. Cardiovascular function in the heat-stressed human. Acta Physiol (Oxf) 199: 407–423, 2010.
13. Von Duvillard SP, Braun WA, Markofski M, Beneke R, Leithäuser R. Fluids and hydration
in prolonged endurance performance
. Nutrition 20: 651–656, 2004.
14. Ebert TR, Martin DT, Bullock N, Mujika I, Quod MJ, Farthing LA, Burke LM, Withers RT. Influence of hydration
status on thermoregulation and cycling hill climbing. Med Sci Sports Exerc 39: 323–329, 2007.
15. Fritzsche RG, Switzer TW, Hodgkinson BJ, Coyle EF. Stroke volume decline during prolonged exercise is influenced by the increase in heart rate. J Appl Physiol 86: 799–805, 1999.
16. Fujii N, Honda Y, Hayashi K, Kondo N, Nishiyasu T. Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men. J Appl Physiol 105: 1509–1518, 2008.
17. Ganio MS, Wingo JE, Carrolll CE, Thomas MK, Cureton KJ. Fluid ingestion attenuates the decline in VO2peak associated with cardiovascular drift. Med Sci Sports Exerc 38: 901–909, 2006.
18. González-Alonso J, Mora-Rodríguez R, Below PR, Coyle EF. Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise. J Appl Physiol 79: 1487–1496, 1995.
19. González-Alonso J, Mora-Rodríguez R, Below PR, Coyle EF. Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise. J Appl Physiol 82: 1229–1236, 1997.
20. González-Alonso J, Mora-Rodríguez R, Coyle EF. Supine exercise restores arterial blood pressure and skin blood flow despite dehydration and hyperthermia. Am J Physiol 277: H576–H583, 1999.
21. González-Alonso J, Mora-Rodríguez R, Coyle EF. Stroke volume during exercise: Interaction of environment and hydration
. Am J Physiol Heart Circ Physiol 278: H321–H330, 2000.
22. Huggins R, Martschinske J, Applegate K, Armstrong L, Casa D. Influence of dehydration on Internal body temperature changes during exercise in the heat: A Meta-analysis. Med Sci Sports Exerc 44: 791, [date unknown].
23. Ishijima T, Hashimoto H, Satou K, Muraoka I, Suzuki K, Higuchi M. The different effects of fluid with and without carbohydrate ingestion on subjective responses of untrained men during prolonged exercise in a hot environment. J Nutr Sci Vitaminol 55: 506–510, 2009.
24. Judelson DA, Maresh CM, Anderson JM, Armstrong LE, Casa DJ, Kraemer WJ, Volek JS. Hydration
and muscular performance
: Does fluid balance affect strength, power and high-intensity endurance? Sports Med 37: 907–921, 2007.
25. Kenefick RW, Cheuvront SN, Palombo LJ, Ely BR, Sawka MN. Skin temperature modifies the impact of hypohydration on aerobic performance
. J Appl Physiol 109: 79–86, 2010.
26. Kenefick RW, Maresh CM, Armstrong LE, Riebe D, Echegaray ME, Castellani JW. Rehydration with fluid of varying tonicities: Effects on fluid regulatory hormones and exercise performance
in the heat. J Appl Physiol 102: 1899–1905, 2007.
27. Kenefick RW, O'Moore KM, Mahood NV, Castellani JW. Rapid IV versus oral rehydration: Responses to subsequent exercise heat stress. Med Sci Sports Exerc 38: 2125–2131, 2006.
28. Lopez RM, Casa DJ, Jensen KA, DeMartini JK, Pagnotta KD, Ruiz RC, Roti MW, Stearns RL, Armstrong LE, Maresh CM. Examining the influence of hydration
status on physiological responses and running speed during trail running in the heat with controlled exercise intensity. J Strength Cond Res 25: 2944–2954, 2011.
29. Montain SJ, Coyle EF. Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume. J Appl Physiol 73: 903–910, 1992.
30. Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol 73: 1340–1350, 1992.
31. Montain SJ, Sawka MN, Latzka WA, Valeri CR. Thermal and cardiovascular strain
from hypohydration: Influence of exercise intensity. Int J Sports Med 19: 87–91, 1998.
32. Moran DS, Montain SJ, Pandolf KB. Evaluation of different levels of hydration
using a new physiological strain index. Am J Physiol 275: R854–R860, 1998.
33. Murray B. Hydration
and physical performance
. J Am Coll Nutr 26: 542S–548S, 2007.
34. Nassis GP, Geladas ND. Cardiac output decline in prolonged dynamic exercise is affected by the exercise mode. Pflugers Arch 445: 398–404, 2002.
35. Pichan G, Sridharan K, Gauttam RK. Physiological and metabolic responses to work in heat with graded hypohydration in tropical subjects. Eur J Appl Physiol Occup Physiol 58: 214–218, 1988.
36. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 39: 377–390, 2007.
37. Sawka MN, Coyle EF. Influence of body water and blood volume on thermoregulation and exercise performance
in the heat. Exerc Sport Sci Rev 27: 167–218, 1999.
38. Sawka MN, Montain SJ, Latzka WA. Hydration
effects on thermoregulation and performance
in the heat. Comp Biochem Physiol Part a Mol Integr Physiol 128: 679–690, 2001.
39. Sawka MN, Young AJ, Francesconi RP, Muza SR, Pandolf KB. Thermoregulatory and blood responses during exercise at graded hypohydration levels. J Appl Physiol 59: 1394–1401, 1985.
40. Sawka MN, Young AJ, Latzka WA, Neufer PD, Quigley MD, Pandolf KB. Human tolerance to heat strain during exercise: Influence of hydration
. J Appl Physiol 73: 368–375, 1992.
41. Stearns RL, Casa DJ, Lopez RM, McDermott BP, Ganio MS, Decher NR, Scruggs IC, West AE, Armstrong LE, Maresh CM. Influence of hydration
status on pacing during trail running in the heat. J Strength Cond Res 23: 2533–2541, 2009.
42. Turkevich D, Micco A, Reeves JT. Noninvasive measurement of the decrease in left ventricular filling time during maximal exercise in normal subjects. Am J Cardiol 62: 650–652, 1988.
43. Wingo JE, Casa DJ, Berger EM, Dellis WO, Knight JC, McClung JM. Influence of a pre-exercise glycerol hydration
Beverage on performance
and Physiologic function during mountain-bike races in the heat. J Athl Train 39: 169–175, 2004.
44. Wingo JE, Ganio MS, Cureton KJ. Cardiovascular drift during heat stress: Implications for exercise prescription. Exerc Sport Sci Rev 40: 88–94, 2012.
45. Wingo JE, Lafrenz AJ, Ganio MS, Edwards GL, Cureton KJ. Cardiovascular drift is related to reduced maximal oxygen uptake during heat stress. Med Sci Sports Exerc 37: 248–255, 2005.
46. PEDro Scale [Online]. Physiotherapy evidence database [date unknown]. Available at: http://www.pedro.org.au
. Accessed at August 2013.