Aerobic Fitness

The best way to be prepare for any form of wilderness venture is to be in the wilderness on a regular basis. However, for most persons who are not professional river or mountain guides, it is not possible to be active in these terrains every day. Thus, we need to improvise and incorporate physical training into our busy schedules so that when we enter the wilderness, we are prepared.

The concepts of aerobic fitness are similar for champion and recreational athletes. The parts of the “engine” are the same; it is the quality and fine-tuning that are different. Three essential characteristics are maximum oxygen consumption (max), lactate or anaerobic threshold, and efficiency. These factors are interrelated in a way that results in effective performance, and each is trainable. The interrelationships result in improved endurance, the most important overall factor for enjoyment and survival in the wilderness.³⁴

Maximum Oxygen Consumption

Oxygen consumption () is defined by the Fick equation:

Cardiac output is equal to heart rate multiplied by stroke volume. Oxygen extraction is the difference between the content of oxygen of the arterial and mixed venous blood (i.e., the amount of oxygen that is used as blood traverses tissue beds). The metabolic response of exertion is limited by cardiac output and the limits of oxygen extraction, both of which can be modulated with training. The role of max and its various considerations are discussed by Levine.⁴⁰

max is the fingerprint of an individual’s physiology. It is a reproducible marker of fitness in an individual that varies depending on training, altitude, and illness. The many genetic factors (i.e., polygenic) that contribute to a person’s max make it highly unlikely that any one individual could be endowed with all of these genes.⁶⁰ One’s max is influenced by both inherited and environmental factors.⁸ What remains to be explained is the observation that, among sedentary subjects in family groups who were maintained on a supervised aerobic exercise program for 20 weeks, there was great variability in how much max could be improved⁷ (Figure 97-2). The improvement in max ranged from negative values to 30% improvement, and these various levels of improvement were grouped in family clusters. Further data from this series of studies looked at age, race, gender, and initial fitness and found that all subjects experienced gains in max, but with a great deal of variability and little correlation among the aforementioned factors that contributed to those gains. It is clear that there are limits in training to improve max. In other words, a “normal” individual with a max of 42 cc/kg/min may be able to improve his or her max to the high 40s cc/kg/min but will never be able to approach the 75 to 85 cc/kg/min range of high-performance middle- to long-distance athletes, who chose their parents well.

FIGURE 97-2 Distribution of 481 subjects by classes of increase (δ) in maximum oxygen consumption (max) as compared with baseline levels.

(From Bouchard C, An P, Rice T, et al: Familial aggregation of VO(2max) response to exercise training: Results from the HERITAGE Family Study, J Appl Physiol 87:1003, 1999.)

What parts of one’s aerobic capacity can be trained? Considering the Fick equation, it becomes apparent that an increase in cardiac output, improved extraction of oxygen, or both will improve max. In fact, both things happen, but it is clearly the heart that can be trained more by increasing its stroke volume and improving its muscular strength.^20,25 Thus, the heart rate necessary to achieve an appropriate cardiac output for any given metabolic rate is lower in the trained state as compared with the untrained state. Although maximum heart rate does not change with training, resting and submaximal heart rates are lower and can be used as simple markers to monitor training. Although the elements of oxygen extraction somewhat improve, the heart’s stroke volume conveys increased ability to perfuse large volumes of muscle mass such that, with training, there are increased capillary and mitochondrial densities and optimization of the components of oxidative metabolism.^3,24,29,30

It is fascinating to put human physiology in perspective with the rest of the animal kingdom. Normal humans in the age range of 20 to 40 years have a max somewhere around 40 cc/kg/min, and accomplished endurance athletes have one in the range of 70 to 85 cc/kg/min; alternatively, some large mammals have extraordinarily high aerobic capacities. For instance, horses have max values that range from 134 cc/kg/min in standardbred horses² to 160 cc/kg/min in thoroughbreds.^38,43 The North American pronghorn antelope is said to have values as high as 300 cc/kg/min. Although thoroughbred horses were bred several hundred years ago to be great aerobic athletes, the antelope’s evolutionary strategy is to have exercise capabilities that optimize its chance of preserving the small family groups that live on an open plain full of predators (i.e., the pronghorn can run sustainably at 80.5 km/hr [50 mi/hr]).

Does max correlate with being able to go faster, last longer, jump higher, climb faster, or survive better in the wilderness? The answer is “yes and no.” Certainly, the high-performance endurance athlete needs to have a large aerobic capacity, but, even in this group, there is heterogeneity in max and performance. This indicates that there are other components of physical characteristics that translate into endurance and performance and that are also influenced by training. Most athletic events attract athletes that share certain phenotypic characteristics that, as with animals in nature, result in some homogeneity; in addition, among people who venture into the wilderness—including even among elite high-altitude climbers—there is a great deal of phenotypic heterogeneity. Regardless of the lack of a strong correlation between max and performance in the wilderness, there is one precept that is sacrosanct: the body must translate energy expenditure into sustainable and efficient mechanical output.

Sustainable Threshold

Exercising at the highest possible sustainable workload results in the best individual performance. The point in progressive exercise above which the level of intensity cannot be sustained has been given many names. Anaerobic, ventilatory, and lactate thresholds are the most commonly used terms, although none clearly defines the phenomenon well. At any given point of training or health, the threshold is fairly reproducible. The term lactate threshold (LT) will be used for sake of this discussion. It is important to understand that the LT—more than max—can be trained to move to a higher level of intensity; this translates into a functional increase in endurance and performance, whether in athletic endeavor or wilderness adventure.

The onset of unsustainable work intensity essentially involves a shift of fuel supply within the cell. At workloads below the LT, free fatty acids are the primary oxidative fuel. Above the LT, when the oxidative turnover of free fatty acids cannot keep up with the demand for adenosine triphosphate, glycolysis occurs. Muscle glycogen is broken down as fuel, with lactic acid being produced at a rate beyond the body’s ability to use it.^33,49 Blood lactate levels correlate with intensity of work and thus are inversely correlated with the duration of a competitive event (Figure 97-3).

FIGURE 97-3 An original figure redrawn from data from Sahlin and colleagues (Figures 1 and 2, p. 46) showing the linear relationship between the amount of muscle lactate and pyruvate as compared with muscle pH. Data are combined from different exercise intensities and different durations of recovery after exercise to exhaustion.

(From Robergs RA, Ghiasvand F, Parker D: Biochemistry of exercise-induced metabolic acidosis, Am J Physiol Regul Integr Comp Physiol 287:R502, 2004; and Sahlin K, Horris RC, Nylind B, et al: Lactate content and pH in muscle samples obtained after dynamic exercise, Pflügers Arch 367:143, 1976.)

For example, a 10,000-m runner may have only a slightly elevated blood lactate level as compared with the resting level as he or she slowly depletes muscle glycogen; alternatively, an 800-m runner will have a markedly elevated blood lactate concentration at the end of the race, because glycolytic signaling is invoked early at high levels of exertion. With sustained aerobic training, use of free fatty acids, which is abundant, is shifted to higher intensities and functionally spares muscle glycogen. The point at which lactate starts to rise in the blood is quite variable, but usually occurs at about 60% of max in untrained individuals; in highly trained individuals, this point may come at 75% to 85% of max. In the trained individual, this difference is due both to improved convection of oxygen with increased capillary density as well as to distribution of muscle fiber types with improved oxidative efficiency.

Lactate has often been portrayed as the culprit that leads to fatigue. However, two misconceptions about this need to be corrected. First, as long as there is blood flow, the LT is actually not associated with mitochondrial hypoxia or anoxia. Anaerobic metabolism is not occurring. Convection of oxygen to the cell, and diffusion gradients from the blood across the cell membrane into the cytosol and from the cytosol into the mitochondria, are adequate to supply oxygen for oxidative phosphorylation. Second, it is not accumulation of lactic acid that causes muscle fatigue or pain during exhaustive exercise. More likely, muscle fatigue is accumulation of the associated hydrogen ion when progressively increasing amounts of pyruvate being delivered to the mitochondria cannot undergo oxidation, thus leading to the generation of lactic acid and the associated hydrogen ion.

Malleability of the Lactate Threshold

Understanding the top end of the body’s physiology is only the beginning of understanding the translation of energy potential into endurance, efficiency, and performance. This next section stresses the importance of sustainable work, which is the key to engaging in any wilderness endeavor. Sustainable work is defined as the level of exertion that can be sustained for many minutes, hours, or days. It is an intensity of exertion that is below one’s LT. Levels of intensity above the LT are reserved for more explosive events in sport or flight of less than a few minutes’ duration. Examples in track and field are the 100- to 1500-m events. In the wild, the short spurt of energy exerted by a cheetah to capture prey is above the animal’s LT and not sustainable, which is why the gazelle sometimes wins.

The ability of a muscle to sustain work is related to its oxidative capacity. This capacity is quite malleable, and depends on the level of the muscle’s activity while it is engaged.^19,28 Among high-level athletes, oxidative capacity can be several-fold greater than among untrained individuals. Functionally, then, high-level athletes have inherently high max levels, and can perform sustainable work at a much higher percentage of their maximum capacity. For example, an international cyclist may have a maximum work capacity of 550 watts and be able to sustain 450 watts of work during an hour-long hill climb. This is an extraordinary level of work output. A more usual and quasi-sedentary individual may have a maximum workload of 200 watts and be able to sustain 50% to 60% of that work intensity, which is considered to be “normal.”

In highly specialized athletes such as cyclists, the muscle mass involved in the effort has been shown to be progressively recruited in a way such that the oxidative stress is balanced and shared.^13,15 As much as 25% of the cyclist’s muscle mass can be spared on a rotating basis, which reduces the oxidative stress of muscle fibers, thus prolonging the onset of the LT. This phenomenon may perhaps be a way to acquire more endurance, delay fatigue, and promote efficiency. Furthermore, with a finite fuel supply, this strategy would preserve glycogen stores and delay the onset of glycolysis (and thus the production of lactate).

Functioning “at the edge” of performance requires delicate juggling of aerobic and anaerobic metabolism. This success translates into activities like running an efficient marathon, where 10% of the activity may be anaerobic, or being able to hike as quickly as possible out of the high mountains to effect a rescue for a fallen colleague without collapsing from fatigue.

The crux of cellular oxidative metabolism is convection of oxygen to the cell by the circulation, diffusion of oxygen across the cell membrane into the cytosol, and then diffusion of oxygen into the mitochondria. The actual diffusion gradient necessary to get oxygen to the mitochondria is on the order of 2 to 3 mm Hg at each of these steps.⁵² Thus, perfusion rather than hypoxemia per se is a limiting factor. Therefore, one of the most important adaptive steps is augmenting blood flow through angiogenesis of the microcirculation. In this regard, in two studies, highly trained cyclists and triathletes with comparable values of max were exercised at 88% of their maximum aerobic capacity until fatigue.^13,14 There were two patterns that showed a shorter and longer time to fatigue. The athletes with more endurance had a substantially greater capillary density than did the athletes who fatigued earlier, despite comparable maximum aerobic capacities. Because both groups were highly trained, it is not clear whether, in certain athletes, there is some inherent propensity for greater signaling of angiogenesis that comes from training. The authors speculated that this augmented perfusion may be important not only for the convective phase of oxygen but also for providing a greater volume of the effluent portion of metabolic byproducts. Another study looked at subtle factors that contribute to fatigue at very high levels of exercise and found that very small changes in energy expenditure when a person is at exercise intensities of greater than 80% of max can lead to rapid onset of fatigue.⁴⁴ Therefore, it is critical for an athlete—whether on the field or in the wilderness—to know the location of his or her “edge” so there is some reserve for optimally finishing an activity.

Training Effect on the Lactate Threshold

Much has been written since the late 1970s about plasticity of the LT. It behooves any athlete to be able to perform at the highest percentage of his or her maximum aerobic capacity. One of the first studies to look at the effect of aerobic training on the LT involved nine sedentary men who performed 9 weeks of supervised endurance training for 45 minutes per day for 4.1 days per week.¹⁶ There was a comparable untrained control group. The exercise group increased its LT by 44% expressed as absolute , and 15% expressed as max. max also increased 25%. Maximum work rate increased 28%, with decreases in the ventilatory equivalent seen at submaximal levels of work. The volume of work was similar in the test group, so the study did not answer the following questions (Figure 97-4): How much volume is necessary to induce these changes? If some work is good, is more or less better?

FIGURE 97-4 Training moves the lactate threshold to a higher level of sustainable work.

The focus of studies then became the effect of volume versus intensity of work on the aforementioned variables, all of which had important implications for performance. As understanding of endurance training expanded, there was emerging and ongoing interest in the effects of other types of training, such as interval training (IT), which for many years had been a standard training technique for athletes. IT can take many forms, but is usually described as a series of intense training bouts above the LT, interspersed with recovery periods. Middle distance and endurance athletes perform some level of endurance (i.e., below LT) training every day and then add IT sessions 2 to 3 days per week. Historically, with IT, the time and volume of training have been thought to be able to be markedly reduced. The thinking has been that very intense exercise levels signal greater changes in muscle oxidative capacity, which otherwise would not be stimulated by endurance-oriented aerobic training. Most studies have been done in athletes who were already engaged in active training, so one of the questions that arose was whether IT could add further benefits to performance for athletes who were already performing at a highly trained level.

Several studies have looked at a number of variations on the IT theme and its effect on performance, LT, serum lactate, time to exhaustion, muscle physiology, and so forth. One study enrolled seven trained male distance runners and added 3 days of intense levels of training (i.e., >95% heart rate maximum) per week for 8 weeks.¹ The results showed no change in max, but there was improvement in 10,000-m times, increased time to exhaustion on a set treadmill pace and incline, decreased serum lactate concentrations at 85% and 90% maximum heart rate, and correlations of the decrease in lactate with improvements in performance times. In another study among recreationally active young males, a mere six bouts of four to seven “all-out” Wingate tests spread out over 2 weeks (with recovery days in between) resulted in a 100% increase in cycle endurance time, with muscle biopsies showing a 26% increase in muscle glycogen and a 38% increase in citrate synthase, both markers of muscle oxidative capacity¹⁰ (Figures 97-5 and 97-6).

FIGURE 97-5 Muscle glycogen concentration measured in resting biopsy samples obtained before and after a 2-week sprint training protocol. Values are given as mean ± standard error of the mean for 8 subjects. dw, Dry weight. *p < 0.05.

(From Burgomaster KA, Hughes SC, Heigenhauser GJ, et al: Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans, J Appl Physiol 98:1985, 2005.)

FIGURE 97-6 Cycle endurance time to fatigue before and after a 2-week sprint training protocol (training group; Sit) or equivalent period without training (control; Con). Values are given as mean ± standard error of the mean for 8 subjects. Individual data are also plotted for all subjects in each group. *p < 0.05.

(From Burgomaster KA, Hughes SC, Heigenhauser GJ, et al: Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans, J Appl Physiol 98:1985, 2005.)

These two studies provide an interesting contrast in that intense training improved the already trained athletes somewhat and the modestly trained recreational athletes a great deal.

Another study divided 16 active young males into intense IT and endurance training groups that were followed for 2 weeks. The time commitment for the two groups was 2.5 hours and 10.5 hours, respectively. Training results for both groups were similar in that they showed improved time trial times and similar changes in markers of oxidative capacity and buffering capacity in muscle biopsy samples after the training intervention²³ (Figure 97-7).

FIGURE 97-7 750-kJ cycling time trial performance before (Pre) and after (Post) 6 sessions of sprint interval training (SIT) or endurance training (ET) over 2 weeks. *p ≤ 0.05 as compared with pretraining values (main effect for time). The lines denote individual data for 8 subjects in each group.

(From Gibala MJ, Little JP, van Essen M, et al: Short-term sprint interval versus traditional endurance training: Similar initial adaptations in human skeletal muscle and exercise performance, J Physiol 575:901, 2006.)

In a study of a somewhat similar design and intent with more outcome variables, this same investigative group showed comparable improvements in performance, endurance time, and oxidative markers in muscle biopsies⁹ (Figure 97-8). After 4 weeks of speed IT in a group of healthy recreationally active subjects, similar findings resulted when compared with those of an endurance group, and the muscle-capillary–to–fiber ratio was similar in both groups.³¹

FIGURE 97-8 Muscle glycogen concentration measured at rest and during cycling exercise that consisted of 60 minutes at 65% peak oxygen consumption before (Pre) and after (Post) 6 weeks of sprint interval training (SIT) or 6 weeks of endurance training (ET). Values are given as mean ± standard error of the mean (n = 10 per group). dw, Dry weight. *Main effect for condition (p < 0.05) such that post-training (Post) > pretraining (Pre). ^†Condition (Pre and Post) × Time (0 and 60 minutes) interaction (p < 0.05) such that Post 60 minutes > Pre 60 minutes in both groups.

(From Burgomaster KA, Howarth KR, Phillips SM, et al: Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans, J Physiol 586:151, 2008.)

Thus, training intensity was heralded as an important part of overall training.¹³ From a practical standpoint, for busy lives, efficiency of training may be an important consideration; thus, this is a significant finding.

These studies are only a few of the better examples of a large body of literature looking at endurance versus intense or interval styles of training. For wilderness activities, the lessons are crucial. Endurance training in the classic sense is the core of training philosophy, but the adventurer also wants to be able to perform at the highest sustainable level that can be created by following a training routine that is reasonable and realistic (i.e., compatible with one’s other life responsibilities). If it can be accommodated, inclusion of 2 to 3 days of IT per week can be an important addition to endurance training.

From a practical standpoint, how does one perform IT? First, if one has never done that type of training, it is important to realize that it is an intensity of exercise that is not particularly comfortable. Thus, when exercising for minutes at a time above LT, at about 40 seconds into the exercise interval, it will feel like it is time to stop. One should design an IT session and stick to it, especially through the last few seconds. Intervals can easily be incorporated into one’s usual routine. Always include an aerobic warm up and cool-down period. Whether on a bike or on foot, on a trail or road, find a modest hill that takes 1 to 2 minutes to “sprint” up, then slowly jog down and repeat. Start out with a single interval, and then, over the course of a few weeks, build up to 10 intervals. Improvement and familiarity with the new territory of anaerobic training will come rapidly. Start out with one interval day per week, and then increase to two interval days per week over a couple of months. It is felt important to not train above LT every day, because adequate recovery time is essential.

There are a couple of ways to achieve recovery between intervals. One can arbitrarily state that there will be a certain number of intervals of 1 to 2 minutes duration, with 2 to 3 minutes between intervals assigned for recovery. For instance, one can run 400-m intervals and walk 200 m between each of them. Whatever one does, my best advice is to stick to it compulsively. A more physiologic way to recover is to use a heart rate monitor. Make sure that, at the end of each interval, near-maximum heart rate is attained. Maximum heart rate is a person’s actual maximum heart rate rather than the “220 beats/min minus age” number that is often used but is only vaguely accurate. Determination of maximum heart rate is actually not as easy as it sounds, because most people do not reach it in a reproducible manner. The heart rate at a true anaerobic interval could be considered maximum heart rate, or one can do a formal cardiopulmonary exercise test during which a trained observer can look for the heart rate at a plateau of max. At the end of each interval, one should walk or cycle slowly until a certain desired post-recovery heart rate is achieved, at which point the next interval may begin. Determining the recovery heart rate will likely take some trial and error. It should be defined as the heart rate after recovery from which the next interval can be done at very close to the previous pace. For example, one may run 400-m intervals and reach a maximum heart rate of 180 beats/min and then undergo a recovery walk. Then, for example, when a heart rate of 110 beats/min is reached, the next interval is started. There are obviously much less rigorous ways to perform interval work, but engaging in the creative design process will make one’s workouts more fun and varied.

Efficiency of Movement

In the final step to understanding movement over ground or water, energy must be turned into work with some degree of efficiency (i.e., using the biomechanics of the body to optimize the energy generated by oxidative metabolism). Most of that biomechanical efficiency is inborn. There are many athletes who are so efficient that their excellent performance may be achieved with a less-than-elite aerobic capacity. By the same token, there are many individuals with prodigious max levels and high LTs with suboptimal biomechanics such that their transformation of energy to movement prohibits them from performing at an elite level. This is contrasted with high-altitude mountaineers who have high (but not extraordinary) levels of , whose efficiency of movement and high LTs somehow allow them to move quickly and efficiently for hours.

A fascinating example of efficient energy expenditure comes from a study at 4700 m in Tibet, where the authors did maximal exercise testing for 17 Tibetans native to the area and 14 recently migrated Han Chinese. Although the Han had higher max values than did the Tibetans (i.e., 36 versus 30 cc/kg/min), the Tibetans generated significantly higher work output (i.e., 176 versus 150 watts) at those maximum levels, LTs at higher percentage of max (i.e., 84% versus 62%), and lower blood lactate concentrations.²² There is ongoing speculation as to whether these characteristics are genetic or adaptive; however, regardless of the mechanism of these differences, the Tibetans appear to be ideal work machines at high altitude, and they are able to perform more efficient work with less energy expenditure. Thus, the question arises: Can we train ourselves not only to be more fit but also more efficient (i.e., the perfect adaptation for the wilderness adventurer)?

There are a number of studies demonstrating that—with aerobic, anaerobic, and resistive training—modest improvements in work efficiency can be attained. In a previously cited study,³¹ speed endurance training in competitive runners resulted in 5.7% to 6.6% lower oxygen consumptions at three set levels of speed on a treadmill compared with endurance-trained runners. In another training study in runners comparing endurance and interval training for 4 weeks,⁶ many of the measured variables, including max, were unchanged. The most notable finding was that, at maximum energy expenditure (which was unchanged between the two groups), velocity was significantly higher in the runners who had used IT, suggesting greater running economy. Although the topic of exposure to intermittent hypoxia will be discussed more later in this chapter, it is worthy of brief mention to note that, in one study, college track athletes were randomly assigned to 29 days of low-altitude normoxia, constant simulated high altitude (3000 m), or nocturnal hypoxia. Although there were no changes in max, hemoglobin, endurance, or LT, the athletes exposed to intermittent nocturnal hypoxia showed about 5% improvement in running economy at a high race speed. The mechanisms for these improvements are only speculative, but some insight may be gained from a study looking at work efficiency in highly trained cyclists. The investigators looked at work related to caloric expenditure and found a positive correlation with type I (aerobic) muscle fiber types taken from thigh muscle biopsies.¹⁵ Of note was the large range (i.e., 32% to 76%) of type I fibers in these athletes. One of the potential mechanisms of energy cost saving with intense training of any type may be a decrease in ventilatory demand and thus the work of breathing, which, although modest, may be critical to performance.²¹ The addition of explosive strength to normal endurance training in competitive runners as compared with ongoing endurance training resulted in improved 5000-m times, which correlated with the improvement in running efficiency. It has even been suggested that ongoing training and competition by a repeat Tour de France winner have resulted in improved efficiency.¹³ Although the improvements in efficiency are modest in most of these studies, such improvements may be critical not only to competitive athletes but also to survival in a wilderness environment.

Effect of High Altitude on Exercise

The next two sections of this chapter cover the effects of altitude on exercise as well as the positive and negative aspects of training at high altitude (Figure 97-11). Both topics have important implications for wilderness travel, much of which occurs in the cold, thin air. Chapter 1 covers the broad topics of high-altitude acclimatization and illness.

FIGURE 97-11 Mt Ama Dablam in the Khumbu region of Nepal.

(Courtesy Robert B. Schoene.)

Anyone who has sojourned quickly to 3000 m or higher cannot exert themselves without dyspnea and exercise limitation. With acclimatization over time, the limitations discovered at 3000 m can be diminished or eliminated.²⁷ At altitudes of 3500 to 8000 m, sea-level performance can not be attained despite prolonged adaptation. Although the fraction of oxygen in the earth’s atmosphere is constant at 0.2093, the barometric pressure and thus the content of oxygen decreases as one ascends. For example, with some variation depending on the weather, the barometric pressure at sea level is approximately 760 mm Hg, whereas, at the summit of Mt Everest (as measured by Chris Pizzo on October 23, 1981), the barometric pressure is 253 mm Hg. Thus, at the summit, there is approximately one-third of the amount of oxygen available for aerobic activity as compared with sea level.⁵⁹ With less oxygen available in the air, there is a decrease in oxygen availability at each step of the delivery of oxygen from air to the lungs to the blood to the tissues and mitochondria. As a climber ascends, his or her max decreases^55,58(Figure 97-12), and the speed of ascent decreases.

FIGURE 97-12 Maximum oxygen (O₂) consumption plotted against inspired partial pressure of oxygen (PO₂). The present data () are contrasted with measurements () made previously by Pugh.48a Note that, although the curve derived from the data in the present study is only slightly shifted to the left, because of the steepness of slope, gain in maximum oxygen uptake at extreme altitudes is substantial.

(From West JB, Boyer SJ, Graber DJ, et al: Maximal exercise at extreme altitudes on Mount Everest, J Appl Physiol 55:688, 1983.)

One of the limiting factors is the increased work of breathing that occurs for any given level of energy expenditure. For example, the ventilatory equivalent is almost four times greater at 6300 m than it is at sea level.^11,53 Thus, blood flow necessary to perfuse the muscles of respiration is stolen from the muscles of locomotion. Furthermore, because of a diffusion limitation of oxygen from air to the blood at high altitude, the higher one ascends, the greater the amount of oxygen desaturation with exercise (Figure 97-13).

FIGURE 97-13 Arterial oxygen (O₂) saturation against work rate for four conditions studied. Note the steeply falling oxygen saturation as work rate was increased when the inspired partial pressure of oxygen was very low. This can be explained by diffusion limitation of oxygen transfer across the blood–gas barrier.

(From West JB, Boyer SJ, Graber DJ, et al: Maximal exercise at extreme altitudes on Mount Everest, J Appl Physiol 55:688, 1983 and Sutton JR, Reeves JT, Wagner PD, et al: Operation Everest II: Oxygen transport during exercise at extreme simulated altitude, J Appl Physiol 64:1309, 1988.)

The body goes through a complex series of adaptations that do their best to optimize delivery and use of oxygen, despite less availability. The breadth of acclimatization involves a progressive increase in ventilation that is immediate and goes on for weeks; an improvement in ventilation and perfusion match in the lungs and thus gas exchange, which occurs over the course of several hours; an increase in oxygen-carrying capacity as a result of an increase in red blood cell production through erythropoiesis that occurs within 10 to 14 days; and an improvement in tissue oxidative capacities over a number of weeks to months.

For persons going to high altitudes, the most important thing to remember is that everyone’s rate of adaptation at each of these steps is different, which results in a range of time for adaptation. Thus, when on a trek, everyone with enough time to do so should adapt well and be equal in terms of acclimatization and fitness; however, it is difficult to predict who will be a slow versus fast adapter. Sometimes the individual who feels less well at the beginning may be the best acclimatized at the end.

In terms of training for high-altitude ventures, other than being generally fit, there is not much a lowlander can do to prepare. Chapter 1 suggests rates of ascent to minimize the chances of getting altitude illnesses. These rates are similar to those needed for people to gain acclimatization and fitness. The most common mistake is to take too little time for a trip to the Himalayas or the Andes—or even a weeklong ski trip to Colorado. A rigid schedule that does not allow sufficient time for acclimatization is dissatisfying at best and dangerous at worst. Having been pre-exposed to high altitude before going on a challenging trek is always beneficial, but usually not feasible or practical.⁴⁶ Such issues have become important considerations for military personnel, who may need to ascend rapidly for certain operations.⁴⁵

Hypoxic Training

Interest in training at high altitude for low-altitude athletic events has not abated over the last two decades. What started as a positive assumption turned into an area of intense research to try to answer whether living or training at high altitude is beneficial for low-altitude competition. Some of these principles apply not only to competitive events but also to travel and adventure to high altitudes. The purpose of altitude training and intermittent hypoxic training is to induce some physiologic adaptation from hypoxia that is presumably beneficial to endurance performance.

When it finally dawned on trainers and coaches that full-time hypoxic exposure might not be beneficial, a number of methods were tried to get “enough” hypoxic exposure without having too much. Levine and Stray-Gundersen⁴¹ set out on a prolonged series of experiments in accomplished runners. The study design included four groups: (1) athletes who lived and trained at low altitude (LLTL); (2) those who lived high and trained high (LHTH); (3) those who lived low and trained high (LLTH); and (4) those who lived high and trained low (LHTL). Part of the rationale for such a design was to test the hypothesis that training at high altitude does not allow an athlete to achieve as intense a training effect as he or she could achieve at lower altitude. The altitude exposure of the LHTL group was 4 weeks of 20 hours per day at 2500 m. The upshot from these series of studies was that the athletes who improved their max and 5000-m times were in the LHTL group, and this improvement correlated with those who had an erythropoietic response: the increase in max corresponded precisely with the increase in oxygen-carrying capacity. However, even in the LHTL group, there were responders and nonresponders, which were differentiated by improvements in 5000-m race time. The responders’ improvements corresponded with whether they had an erythropoietic response. Levine contends that there is a threshold (i.e., specific hypoxic dose) that is necessary to inducing these responses and that, in a number of previous studies, the dose may not have been long or high enough.⁴² In his discussion, he reminds the reader that the mechanism of response to hypoxia is signaled by hypoxia-inducible factor-1α and that this protein is one of the most evanescent proteins of gene transcription described in the body: when the hypoxic stimulus is removed, hypoxia-inducible factor-1α disappears within milliseconds. Thus, all subsequent gene transcription ceases, and no further growth factors (e.g., erythropoietin) are stimulated. The hypoxic exposure must be intense and sufficiently prolonged to keep the cycle going. Not everyone responds, and the gains are minimal among those who do. So is it all worth it?

Many athletes think it is worth it. The urge to excel has led to “hypoxic sleeping tents” that can be purchased. With such a tent, the athlete can sleep in his or her bed and dial in progressive altitudes for weeks of “natural acclimatization and blood doping.” This trend has led to a number of studies of intermittent hypoxia (i.e., normobaric or hypobaric) to see if effects similar to those obtained at true altitude could be found. Julian and colleagues³⁵ exposed athletes to progressive normobaric hypoxia (i.e., a fraction of inspired oxygen of 0.11%) for 2 weeks for 70 minutes each day and found no changes in performance or erythropoietic markers. Using normobaric hypoxia (i.e., a fraction of inspired oxygen of 14.5%) for 6 weeks, Zoll and colleagues⁶¹ divided runners into two groups: LLTL and LLTH. They looked at performance (i.e., max and time to exhaustion) and muscle biopsies to evaluate markers of hypoxic transcription and oxidative phosphorylation. With the LLTH design, the researchers found a modest improvement in max (5%), a remarkable improvement in time to exhaustion (26%), and substantial increases in the transcription factors and markers of oxidative phosphorylation. Gore and colleagues²⁶ exposed athletes to a simulated altitude of 4000 to 5500 m for 3 hours per day on 5 days per week for 4 weeks and found an increase in erythropoietin but no increase in markers of red-cell production. With a similar design, this same research group studied swimmers and found no improvement in swimming times with the intermittent hypoxia.⁵⁰ They concluded that the “hypoxic dose” was not adequate enough.

Not all intermittent hypoxic studies have been negative. Katayama and colleagues³⁶ exposed a small group of runners to a simulated altitude of 4500 m for 90 minutes per day three times per week for 3 weeks. In the hypoxic group, they found improvements in running time and time to exhaustion as well as lower levels at submaximal exercise levels, with no other changes in hemodynamic or hematologic variables. Thus, the authors contended that this type of exposure led to an improvement in running efficiency. However, they did not offer give any insight into the mechanism of improvement.

So, are all of these manipulations much ado about nothing? There seems to be a benefit in performance for some athletes who are “responders,” but clearly the “dose” and duration of the hypoxic exposure have to be adequate. That dose is a tedious one to attain, and such efforts may only be worthwhile for highly competitive athletes. As for those who venture into the mountains, remember to go slowly and enjoy the scenery.