REPOSITORY > RESULTS

Abstract

Adaptation to altitude can enhance performance if the hypoxic dose and the frequency of the exposures
are appropriate. While many hypoxic training modalities exist, the protocols utilizing short intermittent
hypoxic exposures are gaining popularity. However, the effects of such novel hypoxic protocols on
athletic performance are unclear. The aim of this thesis was to investigate the effects of selected
hypoxic modalities on performance at simulated altitude and sea level. We undertook three studies that
investigated the effect of hypoxic exposure during exercise (Study I) and rest (Study II & III) on
performance and select hematological and ventilatory responses. Since erythropoietic stimulation is
one of the main objectives of hypoxic manipulations the last study tested a novel protocol inducing
higher relative changes in O2 partial pressure by utilizing successive breathing of hyperoxic and
hypoxic gas mixture. Given that glycoprotein hormone erythropoietin (EPO) is the chief regulator of
endogenous red blood cells production, EPO production was the focal point of Study IV.
Although training in hypoxia has been suggested to improve sea level and altitude performance,
most studies have only evaluated its effect on maximal aerobic capacity in either normoxia or hypoxia.
In Study I we evaluated the effect of the intermittent hypoxic training modality (IHT) on both
normoxic and hypoxic endurance performance and aerobic capacity. Eighteen healthy male subjects
underwent 20 training sessions in either a normoxic (FIO2 = 0.209) or hypoxic (FIO2 = 0.120)
environment. Both the control and IHT group subjects trained at an intensity corresponding to 50% of
peak power output attained in normoxia or hypoxia, respectively. Before, during, upon completion and
10 days after the training period, the aerobic capacity (V̇O2peak) and endurance performance (80% of
V̇O2peak) of the subjects were determined under normoxic and hypoxic conditions. Normoxic V̇O2peak
increased significantly only in the control group whereas hypoxic V̇O2peak did not improve in either
group. The control group exhibited significant improvements in normoxic, but not hypoxic peak power
output and endurance performance, whereas the IHT group only exhibited improvements in normoxic
endurance performance. The tested IHT modality used in Study I had no significant effect on altitude
and sea level performance compared to the same relative intensity normoxic training.
It has also been suggested, that short intermittent exposures to hypoxia at rest can beneficially
effect ventilatory responses and subsequently enhance performance. In Study II we thus investigated
the effects of twenty intermittent hypoxic exposures at rest (IHE) on aerobic capacity and endurance
performance in normoxic and hypoxic conditions. Eighteen healthy male subjects were equally
assigned to either control or IHE group. Both groups performed a 4-week moderate intensity cycling
exercise training. The IHE group additionally performed one hour of IHE prior to the exercise training
sessions. The IHE consisted of seven cycles alternating hypoxic and normoxic air breathing, for 5 and
3 minutes duration respectively. During hypoxic exposures the inspired FIO2 varied from 0.125 to
0.095. The same testing periods as in Study I were used to evaluate V̇O2peak, and endurance
performance in normoxia and hypoxia. Both groups showed similar improvements in normoxic
V̇O2peak, with no changes in hypoxic condition. Both groups increased endurance performance in
normoxia at the post test, whereas only the IHE group preserved this improvement also at the after
testing. The IHE group showed higher levels of minute ventilation at post and after testing compared
to the control. Although IHE did not improve hypoxic performance and aerobic capacity, it can be
beneficial for normoxic performance. In particular, our data show that the addition of IHE to
endurance training has the ability to preserve the improved performance in normoxia longer than
endurance training alone. The underlying mechanism seems to be related to enhancements of
ventilatory responses.
While short hypoxic modalities were shown to have the ability to stimulate ventilatory
acclimatization, the minimal dose required is currently unresolved. Study III investigated the effect of
only four short intermittent hypoxic exposures (SIH) on hypoxic performance, ventilatory responses
and modulation of muscle and cerebral oxygenation. Nineteen healthy male subjects who participated
in the single blind, placebo controlled study were randomly assigned to either the short intermittent
hypoxic (SIH; n = 10) or the control (n = 9) group. Each participant underwent four sessions in a
climatic chamber (4 h·day-1) under either hypoxic (FIO2 = 0.120) or a placebo normoxic (FIO2 = 0.209)
conditions, respectively. Prior to and after the exposures all subjects performed a constant power test
to exhaustion (CP) in normobaric hypoxia (FIO2 = 0.120) at a work load corresponding to 75 % of the
previously determined normoxic VO2peak. Oxygen saturation and minute ventilation were measured
continuously. NIRS was used to monitor cerebral and muscle concentration changes of oxyhemoglobin,
deoxy-hemoglobin and total hemoglobin. Neither group significantly improved the CP
performance time. Despite the unchanged CP time, only the SIH group showed significant increases in
both minute ventilation (+ 15 %) and oxygen saturation (+ 4 %) during the second test. The deoxyhemoglobin
in the vastus lateralis significantly increased following SIH only. No significant
differences were observed between groups and testing periods in cerebral and respiratory muscle
oxygenation. Compared to a placebo control, the SIH significantly altered ventilation and blood
oxygen saturation during intensive hypoxic exercise. The results of this study indicate that four
intermittent hypoxic exposures have the ability to induce some degree of ventilatory acclimatization,
but do not improve maximal aerobic performance in hypoxia.
Study IV investigated a novel breathing protocol that could provide an indirect means for
performance enhancement, through erythropoiesis stimulation. Recent findings suggest that besides
acute and chronic tissue hypoxia, as employed by contemporary hypoxic training modalities, relative
decrements in tissue oxygenation, using a transition of the breathing mixture from hyperoxic to
normoxic, can also augment erythropoietin (EPO) production. This hypothesis, termed the
“Normobaric oxygen paradox” has already been applied in clinical settings. To further clarify the
importance of the relative change in tissue oxygenation on plasma EPO concentration [EPO], and its
possible application for enhancing performance through subsequent oxygen flux augmentation, we
evaluated a novel protocol of successive hyperoxic and hypoxic breathing. Eighteen healthy male subjects were assigned to either IHH (n = 10) or the control (n = 8) group. The IHH group breathed pure oxygen (FIO2 = 1.00) for 1-hr, followed by a 1-hr period of breathing a hypoxic gas mixture (FIO2 = 0.150). The control group breathed air (FIO2 = 0.209) for the same duration (2-hrs). Blood samples were taken just before, after 60 minutes, and immediately after the 2-hr exposure period. Thereafter samples were taken at 3, 5, 8, 24, 32 and 48 hrs after the exposure. During the breathing interventions subjects remained in supine position. There were significant increases in absolute [EPO] within groups at 8 and 32-hrs in the control and at 32-hrs only in the IHH group. No significant differences in absolute [EPO] were observed between groups following the intervention. Relative [EPO] levels were significantly lower in the IHH than in the control group, 5 and 8-hrs following exposure. The tested protocol of consecutive hyperoxic-hypoxic gas mixture breathing did not induce [EPO] synthesis stimulation. Moreover the transient attenuation in relative [EPO] in the IHH group, was most likely due to a hyperoxic suppression. Hence, the IHH protocol does not seem to be a promising tool for erythropoiesis stimulation.
Collectively the findings of the second and the third study show that intermittent normobaric hypoxic applications can potentially have beneficial effects on select physiological indices, without affecting performance. Only IHT did not show any benefits, most probably attributable to the relatively low training workload. Both IHE and SIH have shown that ventilatory adaptation can be expected following 20 or only 4 exposures, respectively. Prolonged retention of the enhanced performance following IHE, 10 days following the protocol cessation, indicates the possibilities of IHE application if exercise training cannot be performed. According to the findings of the final study, the use of hyperoxia as means of increasing the relative change in tissue oxygenation cannot be recommended for enhancing EPO endogenous production. Thus, athletic and clinical applications based on the “Normobaric oxygen paradox” theory are currently unwarranted. In conclusion, while intermittent hypoxia at rest can potentially have beneficial effects on select physiological indices there is no transfer of these benefits that would result in performance enhancement.