Introduction

Introduction
VO 2max is primarily limited by the maximal capacity for O 2 delivery to the working muscles (1-3) and
maximal oxygen uptake (VO 2max ) sets the upper limit for aerobic power production (2). An increase in both
VO 2max and maximal cardiac output (Q max ) is seen following few weeks of exercise training (4). These
improvements are likely due to a multitude of factors but it is clear that 70-85 % of VO 2max is limited by Q max
(2, 5). The current consensus that VO 2max is restricted by the capacity of the cardiorespiratory system to
deliver oxygen to the working muscles was stipulated by Hill and Lupton almost 100 years ago (2). Hill and
colleagues introduced the conceptual understanding of VO 2max being limited by cardiac output (Q) as the
primary parameter explaining the individual variability in VO 2max (6). Although disputed (7, 8) this concept
has been confirmed numerous times in a wide variety of studies (9, 10) acutely manipulating Q in either
direction, which alters VO 2max accordingly. Today, classic textbook material refers to VO 2max as being
governed by Q max (11, 12) and it is well described that endurance exercise training increases VO 2max . Saltin
and co-workers demonstrated this in their classical study of bedrest and exercise training, where Q
explained the majority of the difference in VO 2max between the trained an untrained state (figure 1).
However, the mechanism responsible for increased Q max is not clear and remains largely unexplored (4).
Several adaptations to endurance exercise training has been associated with increased Q max and VO 2max
such as improved heart function/structure (13) and increased blood volume (BV) (14).
Figure 1. Changes in VO 2max and contribution of Q following either bedrest or exercise. The increased VO 2max following training is
largely driven by a higher Q. Reproduced from Saltin et al. 1968 (4).
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Adaptations in blood volume following exercise training
Blood Volume (BV) represents the sum of red cell volume (RCV) and plasma volume (PV). With exercise
training PV increases within the first hours to days, whereas RCV expansion usually occurs gradually over
weeks to months. Figure 2 illustrates the changes in vascular volumes in response to exercise training. PV
has been reported to increase with as much as 10% within the first 24 hours (14). As a result haematocrit
(Hct) will initially decrease but is later restored as a new equilibrium occurs with the gradually increasing
RCV. The increase in BV following weeks of endurance exercise training amounts to 8-10% and seems to be
similar across age and gender (14). The exercise training induced increase in BV may increase Q max by
means of the Frank-Strling mechanisms, i.e. by facilitating venous return and thereby stroke volume (SV).
Acute manipulations of BV have been demonstrated to influence Q max in both untrained and trained
individuals (14-16). A higher BV as well as total hemoglobin mass (Hb mass ) partly explains why well trained
athletes have a higher Q max compared to their untrained peers. Untrained individuals with a naturally high
Q max and VO 2max are likewise characterized by a higher BV compared to their peers with lower VO 2max (17).
Figure 2. Time course of vascular volumes in response to exercise training for BV (filled circles), PV (open circles) and red cell
volume (open squares). Shortly upon commencing endurance exercise training PV rapidly increases. As a result total BV is also
increased. After 10-20 days PV starts decrease as red cell volume slowly increases and creates a new equilibrium at a higher total
BV compared to baseline. Reproduced from Sawka et al. (18).
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Structural adaptations to the heart following exercise training
Elite endurance athletes displays a higher VO 2max explained by a high Q max and compliant myocardium with
a large end-diastolic volume compared to untrained individuals (3). Moreover, the heart structure of
athletes is characterized by a higher left ventricular mass, volume and wall thickness compared to
untrained individuals (19). Thus, it is likely that changes to the myocardium after endurance training is
partly responsible for increasing Q max and thereby VO 2max . However, longitudinal training interventions are
inconsistent in their findings of myocardial adaptations and currently the literature is divergent on
structural changes within the heart in response to endurance training.
Both left (LV) and right ventricular (RV) enlargement is seen in endurance athletes (20, 21) along with
increased LV diastolic filling. Enhanced LV filling during exercise is most likely due to an enhanced relaxation
phase despite a high heart rate (20). Diastolic filling rate during maximal exercise has been shown to be
significantly faster in trained compared to untrained individuals BV (18). When normalizing BV between
these two groups the differences were diminished but still statistically different. Hence, it is not unlikely
that increases in BV partly explain the improvements in Q max after endurance training.
Little data exist on heart structural adaptations and the literature from training interventions in untrained
individuals is divergent. Six days of endurance exercise training does not seem able to alter resting LV
function or structure (22), whereas 6 weeks of endurance exercise training increase LV mass by
approximately 29 g to 134 g in total (23). In contrast to this, 6 months of endurance training increased LV
mass by considerably less of 9 g to 122 g in all (24). Thus, some structural adaptations may occur but the
magnitude and time perspective is different across studies. So far, the results are inconclusive regarding
structural adaptations in normal healthy individuals after a period of endurance training.
One aim of this project was to discriminate between the importance of increased BV and cardiac structural
adaptations for the training induced increase of Q max .
Altitude training
Exercise training is not the only stimulus known to increase BV. Heat acclimatization for example results in
elevated PV (14) and it is universally accepted that hypoxia is a main stimulant of erythropoiesis (25, 26)
and thus altitude is a popular training environment among elite endurance athletes seeking to increase
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performance. In the following, the possibility that altitude training increases red blood cell volume and
hence exercise performance in elite athletes will be discussed.
Adaptations to altitude
The ability to deliver oxygen to working muscles is a major determinant for endurance performance.
Although the oxygen uptake at anaerobic threshold may be a stronger predictor of performance, VO 2max is
also correlated to endurance exercise performance (3). As altitude increases, the partial pressure of
inspired oxygen (piO 2 ) decreases resulting in lowered arterial saturation (SaO 2 ). Initially the defense against
a lowered SaO 2 is partly maintained by a rapid decrease in PV (27) resulting in an increase in hemoglobin
concentration ([Hb]).
Figure 3. Changes in plasma and urine erythropoietin from seven healthy volunteers in response to 26 days stay at 3454 m. Filled
circles represents plasma concentrations while open circles represents urine concentrations. Values are mean ± sem. Reproduced
from Lundby et al. (26).
Accelerated renal erythropoietin (EPO) release and serum concentrations is seen 90 minutes upon hypoxic
exposure (25) and peaks around 24-48 hours (28, 29) after which a decline towards, but still above, sealevel values is seen (26, 30, 31). The time course of plasma EPO concentrations in response to 3454m
altitude in seven healthy volunteers are depicted in figure 3. Elevated plasma and urine concentrations are
evident the first day at altitude and remain above baseline level until day 26 at altitude. The magnitude of
plasma or serum EPO concentrations is dependent on the level of hypoxia (25) and possibly with an altitude
of > 2100m being critical for maintaining this sustained increase for at least 24 hours (32).
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The decrease in PV in response to hypoxic exposure results in lowered total BV. Given adequate exposure
(both time and altitude) BV will increase to or above baseline levels. This is partly due to a rise in PV and
increase in RCV. Collectively, this will result in an elevated BV at a higher [Hb] compared to baseline (figure
4).
Figure 4. Data from four subjects from the Silver Hut expedition (33). Changes (%) in [Hb], Red cell volume, plasma volume and
blood volume after: (a) 19 weeks between 4000m and 5800m, (b) after further three to six weeks at 5800m and (c) after a further
9-14 weeks at or above 5800m. Reproduced from West et al. (34)
While the haemoconcentration due to suppressed PV levels may at least partially restore O 2 delivery at a
given submaximal workload, exercise near maximal capacity is compromised by the decrease in total BV
and reduced Q. A primary factor adding to a lower O 2 delivery is exercise induced artierial hypoxemia
(EIAH) (35). Consequently, VO 2max is not only decreased approximately 1% for every 100m above 1500m of
altitude (36) but starts to decline already from 300m of altitude in well-trained athletes (37). The sigmoidal
shape of the oxyhemoglobin dissocation curve indicates that the effect of altitudes lower than 1500m
exerts a negligible effect on arterial O 2 and thus VO 2max , and indeed VO 2max is decreased approximately 1%
for every 100m above 1500m of altitude (36). However, well-trained endurance athletes may experience a
reduction in VO 2max from much lower altitudes which seems associated to a more pronounced reduction in
SaO 2 at maximal exercise (37, 38).
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Altitude training modalities
Since total BV and [Hb] correlates well with VO 2max and exercise performance, elite athlete and coaches
have incorporated altitude training into their training regimes for decades. Early altitude studies date back
to the start of the 18th century (39) but especially since the Olympic Games in 1968 in Mexico City the
interest from athletes and coaches in altitude training increased markedly. During the Games, African
middle to long distance runners originating from moderate altitude areas performed excellent compared to
their low-lander competitors. Extensive interest in the physiological adaptation and benefits of altitude
training for sea-level performance has been evident during the last fifty years. The classical altitude training
concept of living and training at altitude, Live High – Train High (LHTH) has been examined numerous times
with somewhat different conclusions (40). Although the literature is inconsistent on efficacy on classical
altitude training for sea-level performance, coaches has long recommended it to improve performance
(41).
In the late 90’s Levine and Stray-Gundersen introduced the concept of “living high – training low” (LHTL)
(42). As VO 2max is decreased with increasing altitude, training efficiency may be compromised. The LHTL
modality facilitates an increased erythropoiesis by residing at altitude while enabling to athletes to
maintain an adequate training stimulus by descending to lower altitude for training sessions. Since the
introduction of LHTL, coaches, athletes and scientists have regarded this approach superior to classical
altitude training for sea-level performance (42). The efficacy of LHTL has been confirmed by several
investigators (31, 43, 44) following the study by Levine and Stray-Gundersen. However, this modality has
been disputed recently (45, 46) following the first double blinded placebo controlled study using
normobaric hypoxia (47).
Today, a wide selection of hypoxia generators, altitude training facilities and research into different training
modalities allows for vast opportunities in different altitude training modalities. In addition to the above
mentioned altitude training modalities, intermittent hypoxia training (IHT) or a so-called Live Low – Train
High (LLTH) approach has also been suggested to improve performance by inducing peripheral tissue
hypoxia which may result in a greater training response and adaptations in the skeletal muscle tissue
relevant for performance (48). Others have not been able to confirm these findings (49) and the effect of
hypoxia to induce an increase in RCV seems most prudent for performance improvements (50). A relevant
question in this regard is whether altitude training is relevant for athletes who already possess a high
Hb mass (51).
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Due to the inconsistent findings in LHTH studies and widespread application, the second aim of this thesis
was to investigate whether classical altitude training can increase Hb mass , BV and performance in olympic
level athletes.
Altitude training and the Athlete Biological Passport
The effects of altitude training on haematological values may pose a challenge in an anti-doping context.
One approach used by the World Anti-doping Agency (WADA) to determine potential doping misuse is the
longitudinal monitoring of individual [Hb] and reticulocyte percentage (%ret), referred to as the Athlete
Biological Passport (ABP). Abnormal values herein may be indicative of doping, and since the
haematological values used may all be influenced by altitude exposure (52) it is valid to consider whether
altitude training should be considered a confounder for the ABP. Of these parameters athletes can only be
sanctioned on abnormal values of [Hb] or OFF-score, a parameter derived from the relationship between
[Hb] and %ret ([Hb] (g· l-1) – 60 · я (%ret)). Currently, the ABP is based on a statistical model (the so-called
adaptive model) able to generate a range within which the expected physiological value is likely to be. An
example from the adaptive model software is given in figure 5. Initially, the thresholds are based on values
obtained from large population studies, but are narrowed as more samples from the individual athlete are
introduced. The ABP was introduced in 2009 by the World Antidoping Agency (WADA) and first
implemented by the Union Cycliste Internationale (UCI). The original blood models to detect misuse of
erythropetic agents was proposed by Australian researchers (53) and further developed by the
incorporation of the adaptive model by a group from the Swiss Laboratory for Doping Analyses (54).
Evidently, the introduction of the ABP has caused a normalization of reticulocytes in the professional
cycling peloton (55) and the UCI has yet to loose a case opened on basis of abnormal ABP values. Today,
more than 35 anti-doping organisations have employed the ABP. However, very little is known about the
impact of hypoxia on the parameters in the adaptive model which is underlined by several discussions on
the subject (56-58). Evaluation of hametalogical parameters in earlier models preceeding the adaptive
model, indicate that hypoxia could be a confounder. As many athletes undertake altitude training or use
other hypoxic training methods, this could potentially be an important confounder for the ABP. Elite cyclist
living and training at altitude for 20-23 days displayed an increase in OFF-score after returning to sea-level
(59). The same phenomenon was evident in different groups of elite rowers (60) and swimmers (61) after
three weeks at 2440m and 2320m, respectively. Thus, altitude training has the capability to cause
haematological fluctuations which is yet unexplored by the adaptive model. A third aim of this thesis was to
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determine the effects of 3-4 weeks of classical altitude training in relation to the individually generated
references in the ABP.
Figure 5. Examples of normal and abnormal values analyzed in the Athlete Biological Passport software for A: control subject not
doped. B: experimental subject doped by autologous blood transfusions. Reproduced from Pottgieser et al. 2011 (62). Arrows
pointing down marks withdrawal of 500 ml of whole blood while arrows pointing up marks infusion of of 280 ml of RCV.
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