TO THE EDITOR: The proposal by our well-respected colleagues (2) to introduce a new metric-incorporating the altitude elevation and the total exposure duration, termed "kilometer hours"-for better describing the "hypoxic dose" is decidedly a step forward. By only quantifying the "external" stress, this metric presents several limitations: It suggests a linear relationship between altitude elevation and saturation decrease [but the Fick curve is curvilinear (3)] or that it applies to all athletes irrespectively of their training background [but elite endurance athletes suffer the largest decrease in V O 2max (1)], altitude experience [but elite athletes who have had previous hypoxic exposure better adapt to hypoxic condition (4)], or type of hypoxia [but hypobaric vs. normobaric hypoxia induces larger desaturation (5)].The large intersubject variability in the physiological responses to a given "hypoxic dose" implies that the magnitude of the stimulus rather than the altitude elevation should instead be considered. We therefore propose a new metric based on the sustained duration at a given arterial saturation level. Hence, desaturation levels in normoxia (exercise-induced arterial hypoxemia) or in hypoxia (3) predict the decrement in V O 2max in hypoxia and therefore the˙amplitude of the "hypoxic stimulus." This metric termed "saturation hours" is defined as %·h ϭ (98/s -1) ϫ h ϫ 100, where s is the saturation value (in %) and h the time (in hours) sustained at any second level.Practically, with the development of new sport gears incorporating the oximeter inside the textile, this metric will readily be measured without any disturbances to individuals.TO THE EDITOR: are to be congratulated for their "kilometer hours" (km·h) approach predicting increasing response along with increasing hypoxic dose during altitude training. Previous literature has clearly shown that both endurance training and hypoxic exposure as such can increase hemoglobin mass (Hb mass ), and the responses to their doses are individual. Wehrlin et al. (5) with a 1,080 km·h (24 days 18 h/day, 2,500 m) showed an average 5.3% increase in Hb mass

Endurance exercise performance in hypoxia may be influenced by an ability to maintain high minute ventilation () in defense of reduced arterial oxyhemoglobin saturation. Inspiratory muscle training (IMT) has been used as an effective intervention to attenuate the negative physiological consequences associated with an increased , resulting in improved submaximal-exercise performance in normoxia. However, the efficacy of IMT on hypoxic exercise performance remains unresolved. Purpose: To determine whether chronic IMT improves submaximal-exercise performance with acute hypoxic exposure. Methods: A total of 14 endurance-trained men completed a 20-km cycling time trial (TT) in normobaric hypoxia (fraction of inspired oxygen [FiO2] = 0.16) before and after either 6 wk of an IMT protocol consisting of inspiratory loads equivalent to 80% of sustained maximal inspiratory pressure (n = 9) or a SHAM protocol (30% of sustained maximal inspiratory pressure; n = 5). Results: In the IMT group, 20-km TT performance significantly improved by 1.45 (2.0%), P = .03, after the 6-wk intervention. The significantly faster TT times were accompanied by a higher average (pre vs post: 99.3 [14.5] vs 109.9 [18.0] L·min−1, P = .01) and absolute oxygen uptake (pre vs post: 3.39 [0.52] vs 3.60 [0.58] L·min−1, P = .010), with no change in ratings of perceived exertion or dyspnea (P > .06). There were no changes in TT performance in the SHAM group (P = .45). Conclusion: These data suggest that performing 6 wk of IMT may benefit hypoxic endurance exercise performance lasting 30–40 min.

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Ventilation is higher at any submaximal workload in hypoxia as compared to normoxia. Whether or not training the respiratory muscles helps to improve exercise performance in hypoxia is unclear. Purpose: To determine if improvements in ventilatory strength with chronic inspiratory muscle training (IMT) improves 20km cycling time trial (TT) performance in hypoxia (FIO2 = 16.1%). Methods: Thirteen highly-trained men were pair-matched based on pre-exercise values of maximal inspiratory pressure (MIP) and randomly placed into either a sham (n = 5, V O2max = 61.7 ± 2.0 ml•kg -1 •min -1 ) or an IMT (n = 8, V O2max = 63.5 ± 3.4 ml•kg TT heart rate and SpO2 were unchanged in both groups post-training. Conclusion: In a small cohort, IMT-induced improvements in respiratory muscle strength which resulted in greater ventilation and oxygen uptake during a 20km time trial in hypoxia. IMT should be explored as a useful strategy for improving the quality of cycle exercise training and/or endurance exercise performance at altitude. Chapter I: IntroductionAthletes competing in a hypoxic environment experience a worsening of exercise performance. One of the main causes is a decline in arterial oxygenation which causes a decline in maximal oxygen consumption. The body compensates for the decline in arterial oxygenation by increasing ventilation, which increases work of breathing. During exercise in normoxia, it is estimated that the inspiratory muscles consume 10 to 15 percent of total body oxygen uptake (1).However, in hypoxia, the oxygen demand of the respiratory muscles increases an additional 20 to 30 percent (2). In response to an increase in work of breathing, the inspiratory muscles increase their oxygen supply by shunting blood away from the working muscle and to the inspiratory muscles (15). Therefore, this causes a decrease in locomotor muscle power production and increases the rate of fatigue.There have been many different theories and techniques used to attenuate the effects of altitude on exercise performance. Inspiratory muscle training (IMT) has been shown to increase performance during a cycling time trial of 20 km or longer in a normoxic environment (20, 34). 7Interestingly, to our knowledge, there has only been one study examining the effects of IMT on performance in hypoxia. In this study by Downey et al., IMT increased oxygen diffusing capacity and arterial oxygen saturation while decreasing ventilation, cardiac output and inspiratory muscle fatigue during a treadmill time to exhaustion test (TTE) in hypoxia (8). Based on these results one would expect exercise performance in hypoxia to benefit from IMT, but there was no significant difference in the time to exhaustion test on a treadmill between the IMT and sham training groups. A lack of significant difference in exercise performance in hypoxia in the Downey et al. study may be due to the specific performance trial used not being robust enough to detect differences. The duration, for the experimental and control group, was 9.7 ± 1.5 min and 7.4 ± 1.2 m...