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