The influence of swimming performance in triathlon : implications for training and competition

Triathlon is a sport that involves three different exercise modes, swimming, cycling and running, each of which contributes differently to the triathlete’s performance. The objective of the present study was to analyze the factors that determine performance in swimming and how this can be used in triathletes’ training and competitions. We searched articles on triathlon swimming using the following keywords in Portuguese: “triathlon,” “triatleta,” “performance na natação,” and their translations into English when necessary. Swimming is considered the most intense phase during a triathlon event. In short triathlon races, the intensity of this phase seems to be higher than the anaerobic threshold. The split time of this first phase seems to correlate positively with final time of the short races, which is not true for longer races. During the short and Olympic triathlon, the pacing strategy adopted at the swimming phase seems to directly influence the final outcome of the race. Thus, the planning of training loads within the three exercise modes of triathlon must take into consideration the distance and format of the races to achieve success.


INTRODUCTION
Post-exercise hypotension (PEH), characterized by a decrease in blood pressure (BP) to values below those measured at rest before exercise 1 , has been studied for decades in different modalities of physical activities 2,3 .PEH is seen as an important non-pharmacological resource in the treatment of systemic hypertention 3 because the performance of a single physical exercise session may result in a BP decrease that may last for up to 23 hours after exercising, when compared with values on a control day without physical exercise 1 .
Numerous mechanisms are associated with PEH, and they are associated with both cardiac output and peripheral vascular resistance.Some of these important mechanisms are the reduction of blood volume, the activation of the cardiac efferent and afferent sympathetic nervous system and of skeletal muscle ergoreceptors, as well as the production of vasopressin, noradrenalin, rennin and angiotensin II (AngII) in addition to the increased release of adenosine, blood lactate, potassium and nitric oxide (NO) 1,[4][5][6] .The complexity of BP regulation 7 suggests that PEH has a multifactorial origin 8 , also affected by ethnic and genetic factors.
According to Agarwalet al. 9 , genetic mutations, among other physiological and behavioral factors, may affect the capacity to maintain adequate BP and, consequently, favor hypertension and even interfere with PEH.Additionally, the association between the current worldwide epidemics of hypertension and modern lifestyles seems clear, particularly because of high salt intakes and sedentary habits, among other behaviors (also called environmental factors), as well as due to the interaction between these variables and ancestral susceptibility 5,7,10 , represented by ethnicity and genetics.These factors have become the object of recurrent studies in the last decades, particularly because such susceptibility may affect BP-related phenotypes.
Therefore, considering the multiplicity of factors widely studied and associated with the hypotensive effects of physical exercise, the possible effects of individual, unchangeable characteristics on these responses should be investigated.This study reviewed studies about ethnic and genetic factors and their associations with PEH.Our data search was based on studies retrieved from the following databases, available in the Internet: Medline, SciELO and Portal Capes.The following keywords, both in English and Portuguese, were combined in various ways: post-exercise hypotension, human beings, ethnicity, genetic polymorphism, rennin-angiotensin-aldosterone system, ethnicity and blood pressure.Data used in the review were collected only from studies published in national and international journals, and results of studies (dissertations and theses) not yet published in peer-reviewed journals were not included.

POsT-exeRCIse hyPOTeNsION aND eThNIC faCTORs
The tendency towards elevated BP is assigned, in part, to human evolution.Adaptation to climate, first in Africa and then in the rest of the world, may have led to the current patterns of susceptibility to hypertension.
According to Young 7 , populations in hot and humid environments are more susceptible to hypertension than those that live in cold environments because the propensity to retain salt and water may raise BP, as heat loss is fundamental in hot environments.However, as large amounts of water and salt are lost in transpiration, hypovolemia occurs and there is a decrease in the capacity to dissipate heat.Therefore, individuals genetically adapted to a high appetite for salt and the increase of the renal conservation of sodium are better adapted to hot environments due to evolution.
Based on that, evidence suggests that genetic susceptibility may have expanded due to human evolution, as well as a result of population migrations and miscegenation between different ethnicities, which favored elevated BP values 7 and may be associated with PEH.Moreover, studies about associations based on miscegenation were first published for hypertension phenotypes 11 , and epidemiological analyses conducted in the United States consistently pointed to a greater risk of hypertension among the African American population 12 .
The variations in BP responses due to physical exercise may also be associated with ethnicity, both in its acute and subacute forms.Brandon and Elliott-Lloyd 13 found a decrease in BP after 16 weeks of walking (subacute effect) 14 among both African American and white American women, and the greatest variation was found in the group of white women.In contrast, Pescatello et al. 15 found a BP reduction after moderate exercise (acute effect) only in white women with hypertension, with no BP fall among white women with normal BP and BP elevations among African American women with hypertension or normal BP.
A study with white and African American men with normal BP found lower values of systolic BP (SBP) at rest, as well as a smaller reduction of SBP after acute exercises of moderate to high intensity before and after training at 55-75% VO 2 max for African American men with a specific mutation of the angiogenin gene 16 .However, Headley et al. 17 found that young African Americans with normal BP that practiced aerobic exercises of moderate intensity had a prolonged reduction of post-exercise SBP, a results that was probably associated with the changes in cardiac output and total peripheral resistance.
In addition, a recent study conducted with 22-year-old individuals detected a significant reduction in diastolic BP and mean BP in the 15 minutes after exercise, among both African American and white individuals, without any significant difference between the two groups.However, their study also analyzed both central and peripheral vascular stiffness, greater in the group of African Americans, a finding that led the authors to discuss the possible association between stiffness and reduction in sensitivity to β2 adrenoreceptor 18 .
According to Muszkat 19 , three polymorphisms identified for β2 adrenoreceptor blocked it and, consequently, affected vasodilatation.The frequency of these polymorphisms varies according to ethnicity and is greater among black people than among European whites.Finally, Gainer et al. 20 reported that the different genotypes of ACE insertion/deletion (I/D) polymorphism also interacted with ethnicity to define the impact of the endothelium-dependent vascular response of bradykinin.
Important to mention is the fact that most studies about different ethnicities and BP after physical exercise, or even after training, were conducted using observation and/or self-report questionnaires about skin color and ethnicity.No specific laboratory techniques seem to have been used to confirm the estimates of individual genomic origins.For that purpose, it would be necessary to use informative markers of ethnicity selected from literature according to the differences in allelic frequencies and the European, African and American Indian parental populations.
The summary of the studies discussed in this topic, which investigated ethnicity and the reduction of post-exercise BP values, is shown in Box 1.

POsT-exeRCIse hyPOTeNsION aND geNeTIC faCTORs
Studies with families, twins and model organisms estimate that 30% to 60% of the variability found in BP phenotypes are explained by genomic variation 21,22 .According to a review conducted by Arora and Newton-Cheh 23 , the heritability of high BP may be found in 50% to 60% of the population.According to Cowley 24 , hundreds of BP-related phenotypes and genomic regions have been mapped for human beings and rats.However, the total number of genes and alleles that contribute to the hypertension phenotype, as well as other BP-related phenotypes, remains unknown.The identification of genes segregated in a Mendelian way in cases of familial hypertension and, more recently, the genomic scanning techniques based on single nucleotide polymorphisms (SNP) have both made important contributions to the study of BP-related phenotypes 25 .
According to Binder 25 , candidate genes should be selected for investigation according to the physiological system implicated in BP regulation.The rennin-angiotensin-aldosterone system (RAAS), one of the components of the complex network that regulates BP, has been extensively studied, as well as aldosterone synthesis and the genes that express the proteins of this system, such as rennin, angiotensinogen, angiotensin II (AngII) type 1 receptor and angiotensin converting enzyme (ACE) 9,26 .
However, few studies evaluated the association between genetic mutations and essential hypertension [27][28][29][30] .Blanchard et al. 27 analyzed the interaction of three RAAS polymorphisms (I/D ACE, A/C AT 1 R and W/C Int2) with BP for 14 hours after low and moderate intensity exercises in adult men with borderline normal BP or hypertension (stage 1).Surprisingly, significant differences in essential hypertension were only found after low intensity exercises (40% VO 2 max) for SBP and DBP of the volunteers that had three or more of the alleles theoretically favorable to BP increases.Moreover, Pescatello et al. 28 stressed that the BP response after aerobic exercises may be modulated by the interactions between dietary calcium intake, exercise intensity, and RAAS polymorphisms.More marked SBP decreases were found in the group that had a low dietary calcium intake, as well as after the performance of light (40% VO 2 max) and moderate (60% VO 2 max) intensity exercises for the groups with greater and lower risk of heart disease due to genotype combinations, which referred to the same gene mutations studied by Blanchard et al. 27 .
Pescatello et al. 30 also found substantial differences in hypotensive responses nine hours after aerobic exercises performed at light and moderate intensities by men with stage 1 hypertension.About 60% of the individuals had a reduction in systolic BP after moderate exercise, regardless of the presence of alpha-adducin gene polymorphisms (Gly460Gly vs. Gly460Trp).However, careful analysis revealed that, once more, the difference between the effects of genotype interaction and exercise on BP responses are defined at lower intensities (40% VO 2 max).Therefore, BP reductions of 83% and 56% may be recorded for individuals with the Gly460Trp and Gly460Gly genotypes.According to the authors, the best explanation for these results is found in the balance between sodium volume and the vasodilation-vasoconstriction status after stimulation generated during the two exercise sessions (light and moderate), in the case of the polymorphism under study .
The balance mentioned by the authors may also be affected by other factors associated with specific genotypes.For example, a study conducted by Tanriverdi et al. 32 found greater blood flow due to endothelium-dependent vasodilation in athletes with genotype II of the ACE I/D polymorphism, probably due to the greater concentration of bradykinin and the bioactivity of nitric oxide.The ACE I/D polymorphism refers to the absence or presence of an ALU sequence of 287 nitrogen-containing base pairs in intron 16 of chromosome 17 31 .Recently, results of a number of Master's and Doctorate studies conducted in Brazil, although unpublished in journals, suggest a possible association of the DD genotype, or even the D allele of the I/D polymorphism with a weaker hypotensive response after physical exercise.
However, it seems that genetic polymorphism mapping, according to published studies, may facilitate the identification of people that respond better to BP decreases, particularly after low intensity aerobic exercises, which may act as a useful treatment against hypertension.This discovery has great importance for the development of physical activity programs to reduce BP values among individuals with hypertension.The magnitude of the exertion tested by Pescatello et al. 30 is compatible with a low intensity walk, which is usually enjoyable and favors adherence 33 .Therefore, their findings support the idea that genetic information may and should be taken into consideration when prescribing exercises for individuals with borderline normal BP or with hypertension.
All information described here clearly stresses the complexity of the mechanisms of BP control and requires further investigation.The possible interaction between environmental factors and genetic analyses should also be discussed.Environmental factors are usually characterized by intermediate phenotypes susceptible to human intervention along life 26 .Guedes and Guedes 34 reported that the levels of physical activity and cardiorespiratory aptitude are inversely correlated with BP at rest and with the amount and distribution of body fat, which are associated with possible increases of the risk of heart diseases.
Therefore, environmental factors, and even ageing alone 36 (not discussed in this review), affect the functioning of the arterial wall, vascular resistance and, consequently, BP control 5,10 .Such characteristics may affect the magnitude and duration of essential hypertension and favor the appearance of systemic hypertension.
In summary, gene mutations seem to play a role in essential hypertension, even though few polymorphisms have been studied and there are practically no studies using genomic scanning.However, these studies are important for the better understanding of the interaction between genetics and essential hypertension, as well as the relevance of adequate control of the variables associated with environmental and ethnic factors to avoid spurious associations between polymorphisms and BP reductions after physical exercise.However, adequate control of ethnicity, using estimates of individual genomic history, remains an obstacle in studies conducted in this area.

fINaL CONsIDeRaTIONs
This review of the literature demonstrated that both BP regulation and the mechanisms associated with essential hypertension have a multifactorial origin and are affected by environmental, ethnic and genetic factors.
Environmental factors act upon endothelial functioning, the maintenance of the vascular tonus and, consequently, BP regulation.Ethnicity reveals that human evolution may favor the elevated BP standards found in Modernity, and studies suggest that black individuals have an unfavorable tendency to have essential hypertension when compared with white individuals.
Finally, genetic studies with human beings suggest that there is a possible effect of some polymorphisms on essential hypertension.Further studies should investigate those possibilities, and the selection of candidate genes should be based on the physiological systems implicated in BP regulation.Attention should be drawn to the mutations associated with the genes involved in the rennin-angiotensin-aldosterone system, which may favor a greater decrease of BP, particularly after low intensity aerobic exercises performed by adult men with borderline normal BP or with hypertension.

Box 1 .
Summary of the studies about ethnicity and acute and chronic effects of physical exercise on BP F: female; M: male; W. white; B: black; BP: blood pressure; SBP: systolic blood pressure; DBP: diastolic blood pressure; MBP: mean blood pressure; ↓ : decrease; >: greater