Adaptations to aerobic training - part IV

 PART I
 PART II
 PART III

What limits aerobic power and endurance performance?

A number of years ago, exercise scientists were divided on what major physiological factor or factors actually limit VO₂max. Two contrasting theories had been proposed.

One theory held that endurance performance was limited by the lack of sufficient concentrations of oxidative enzymes in the mitochondria. Endurance training programs substantially increase these oxidative enzymes, allowing active tissue to use more of the available oxygen, resulting in a higher VO2max. In addition, endurance training increases both the size and number of muscle mitochondria. Thus, this theory argued, the main limitation of maximal oxygen consumption is the inability of the existing mitochondria to use the available oxygen beyond a certain rate. This theory was referred to as the utilization theory.

The second theory proposed that central and peripheral circulatory factors limit endurance capacity. These circulatory factors would preclude delivery of sufficient amounts of oxygen to the active tissues. According to this theory, improvement in VO₂max following endurance training results from increased blood volume, increased cardiac output(via stroke volume), and a better perfusion of active muscle with blood.

Evidence strongly supports the latter theory. In one study, subjects breathed a mixture of carbon monoxide(which irreversibly binds to hemoglobin, limiting hemoglobin’s oxygen-carrying capacity) and air during exercise to exhaustion. VO₂max decreased in direct proportion to the percentage of carbon monoxide breathed. The carbon monoxide molecules bonded to approximately 15% of the total hemoglobin; this percentage agreed with the percentage reduction in VO₂max. In another study, approximately 15% to 20% of each subject’s total blood volume was removed. VO₂max decreased by approximately the same relative amount. Reinfusion of the subjects’ packed red blood cells approximately four weeks later increased VO₂max well above baseline or control conditions. In both studies, the reduction in the oxygen-carrying capacity of the blood – via either blocking hemoglobin or removing whole blood – resulted in the delivery of less oxygen to the active tissues and a corresponding reduction in VO₂max. Similarly, studies have shown that breathing oxygen-enriched mixtures, in which the partial pressure of oxygen in the inspired air is substantially increased, increases endurance capacity.

These and subsequent studies indicate that the available oxygen supply is the major limiter of endurance performance. Saltin and Rowell reviewed this topic and concluded that oxygen transport to the working muscles, not the available mitochondria and oxidative enzymes, limits VO2max. They argued that increases in VO2max with training are largely attributable to increased maximal blood flow and increased muscle capillary density in the active tissues. The major skeletal muscle adaptations(including increased mitochondrial content and respiratory capacity of the muscle fibers) appear more closely related to the ability to perform prolonged, high-intensity, submaximal exercise.

Table below summarizes the expected physiological changes that occur with endurance training. The changes pre- to posttraining in a previously inactive man are compared with values for a world-class male endurance runner.

Expected effects of endurance training in a previously inactive man along with values for a male world-class endurance athlete
	Sedentary male subject
Variables	Pretraining	Posttraining	World Class runner
HRrest(beats/min)	75	65	45
HRmax(beats/min)	185	183	174
SVrest(ml/beat)	60	70	100
SVmax(ml/beat)	120	140	200
Q at rest(L/min)	4.5	4.5	4.5
Qmax(L/min)	22.2	25.6	34.8
Heart volume(ml)	750	820	1.200
Blood volume(L)	4.7	5.1	6.0
Systolic BP at rest(mmHg)	135	130	120
Systolic BPmax(mmHg)	200	210	220
Diastolic BP at rest(mmHg)	78	76	65
Diastolic BPmax(mmHg)	82	80	65
Respiratory
VE at rest(L/min)	7	6	6
VEmax(L/min)	110	135	195
TV at rest(L)	0.5	0.5	0.5
TVmax(L)	2.75	3.0	3.9
VC(L)	5.8	6.0	6.2
RV(L)	1.4	1.2	1.2
Metabolic
(a-ṽ)O2 diff at rest(ml/100ml)	6.0	6.0	6.0
(a-ṽ)O2 diff max(ml/100ml)	14.5	15.0	16.0
VO2 at rest(ml x kg-1 x min-1)	3.5	3.5	3.5
VO2max(ml x kg-1 x min-1)	40.7	49.9	81.9
Blood lactate at rest(mmol/L)	1.0	1.0	1.0
Blood lactate max(mmol/L)	7.5	8.5	9.0
Body composition
Weight(kg)	79	77	68
Fat weight(kg)	12.6	9.6	5.1
Fat-free weight(kg)	66.4	67.4	62.9
Fat(%)	16.0	12.5	7.5

VO_{2  = oxygen consumption}

HR = heart rate

SV = stroke volume

Q = cardiac output

BP = blood pressure

V_E = ventilation

TV = tidal volume

VC = vital capacity

RV = residual volume

(a-ṽ)O₂ diff = arterial-mixed venous oxygen difference

 Long-term improvement in aerobic power and cardiorespiratory endurance

Although an individual’s highest attainable VO₂max is usually achieved within 12 to 18 months or intense endurance conditioning, endurance performance continues to improve with continued training for many additional years. Improvement in endurance performance without improvement in VO₂max is likely attributable to improvements in the ability to perform at increasingly higher percentages of VO₂max for extended periods.

Consider, for example, a young male runner who starts training with an initial VO₂max of 52.0 ml x kg^-1 x min^-1. He reaches his genetically determined peak VO₂max of 71.0 ml x kg^-1 x min^-1 after two years of intense training, after which no further increases occur, even with more frequent of more intense workouts. At this point, as shown in the figure below, the young runner is able to run at 75% of his VO₂max(0.75 x 71.0 = 53.3 ml x kg-1 x min-1) in a 10km(6.2mi) race. After an additional two years of intensive training, his VO₂max is unchanged, but he is now able to compete at 88% of his VO₂max(0.88 x 71.0 = 62.5 ml x kg^-1 x min^-1). Obviously, by being able to sustain an oxygen uptake of 62.5 ml x kg^-1 x min^-1, he is able to run at a much faster race pace.

This ability to sustain exercise at a higher percentage of VO₂max is the result of an increase in the ability to buffer lactate, because race pace is directly related to the VO₂ value at which lactate begins to accumulate.

Factors affecting individual response to aerobic training

Level of conditioning and VO₂max

The higher the initial state of conditioning, the smaller the relative improvement for the same program of training. For example, if two people, one sedentary and the other partially trained, undergo the same endurance training program, the sedentary person will show the greatest relative improvement.

In fully mature athletes, the highest attainable VO₂max is reached within 8 to 18 months of heavy endurance training, indicating that each athlete has a finite maximal attainable level of oxygen consumption. This finite range may potentially be influenced by training in early childhood during the development of the cardiovascular system.

Heredity

The ability to increase maximal oxygen consumption levels is genetically limited. This does not mean that each individual has a preprogrammed VO₂max that cannot be exceeded. Rather, a range of VO₂max values seems to be predetermined by an individual’s genetic makeup, and that individual’s highest attainable VO₂max should fall in that range. Each individual is born into a predetermined genetic window, and that individual can shift up or down within that window with exercise training or detraining, respectively.

Research into the genetic basis of VO₂max began in the late 1960s and early 1970s. Recent research has shown that identical(monozygous) twins have similar VO₂max values, whereas the variability for dizygous(fraternal) twins is much greater. Figure below illustrates this. Each symbol represents a pair of brothers. Brother A’s VO₂max value is indicated by the symbol’s position on the x-axis, and brother B’s VO₂max value is on the y-axis. Similarly in the siblings’ VO₂max values is noted by comparing the x and y coordinates of the symbol(i.e. how close it falls to the diagonal line x=y on the graph). Similar results were found for endurance capacity, determined by the maximal amount of work performed in an all-out, 90 min ride on a cycle ergometer.

Bouchard and colleagues’ concluded that heredity accounts for between 25% and 50% of the variance in VO₂max values. This means that of all factors influencing VO₂max, heredity alone is responsible for one-quarter to one-half of the total influence. World-class athletes who have stopped endurance training continue for many years to have high VO₂max values in their sedentary, deconditioned state. Their VO₂max values may decrease from 85 to 65ml x kg^-1 x min^-1, but this deconditioned value is still very high.

Heredity also potentially explains the fact that some people have relatively high VO₂max values yet have no history of endurance training. In a study that compared untrained men who had VO₂max values below 49 ml x kg^-1 x min^-1 with untrained men who had VO₂max values above 62.5 ml x kg^-1 x min^-1, those with high values were distinguished by having higher blood volume values, leading to higher stroke volume and cardiac output values at maximal rates of exercise. The higher blood volumes in the high VO₂max group were possibly genetically determined.

Thus, both genetic and environmental factors influence VO2max values. The genetic factors probably establish the boundaries for the athlete, but endurance training can push VO2max to the upper limit of these boundaries. Dr. Per-Olof Astrand, one of the most highly recognized exercise physiologists during the second half of the 20^th century, stated on numerous occasions that the best way to become a champion Olympic athlete is to be selective when choosing one’s parents!

Sex

Healthy untrained girls and women have significantly lower VO2max values(20-25% lower) than healthy untrained boys and men. Highly conditioned female endurance athletes have values much closer to those of highly trained male endurance athletes(i.e., only about 10% lower).

High responders and low responders

For years, researchers have found wide variations in improvement of VO₂max with aerobic training. Studies have demonstrated individual improvements in VO₂max ranging from 0% to 50% or more, even in similarly fit subjects completing exactly the same training program.

In the past, exercise physiologists have assumed that these variations result from differing degrees of compliance with the training program. Good compliers should have the highest percentage of improvement, and poor compliers should show little or no improvement – and that certainly is the case. However, given the same training stimulus and full compliance with the program, substantial variations occur in the percentage improvements in VO₂max values of different people.

It is now evident that the response to a training program is also genetically determined. This is illustrated in figure below. Ten pairs of identical twins completed a 20-week endurance training program; the improvements in VO2max, expressed as percentages, are plotted for each twin pair – Twin A on the x-axis and Twin B on the y-axis. Notice the similarity in response for each twin pair. Yet across twin pairs, improvement in VO2max varied 0% to 40%. These results, and those from other studies, indicate that there will be high responders(large improvement) and low responders(little or no improvement) among groups of people who participate in identical training programs.

Results from the HERITAGE Family Study also support a strong genetic component affecting the magnitude of increase in VO2max with endurance training. Families, including the natural mother and father and week for 20 weeks, initially exercising at a heart rate equal to 55% of their VO2max for 35 min per day and progressing to a heart rate equal to 75% of their VO2max for 50 min per day by the end of the 14^th week, which they maintained for the last six weeks. The average increase in VO2max was about 17% but varied from 0% to more than 50%. Figure below illustrates the improvement in VO2max for each subject in each family. Maximal heritability was estimated at 47%. Note that subjects who are high responders tend to be clustered in the same families as those who are low responders.

It is clear that this is a genetic phenomenon, not a result of compliance or noncompliance. One must consider this important point when conducting training studies and designing training programs. Individual differences must be accounted for.

Cardiorespiratory endurance and performance

Many people regard cardiorespiratory endurance as the most important component of physical fitness. It is an athlete’s major defense against fatique. Low endurance capacity leads to fatique, even in the more sedentary sports or activities. For any athlete, regardless of the sport or activity, fatique represents a major deterrent to optimal performance. Even minor fatique can hinder the athlete’s total performance:

• Muscular strength is decreased.
• Reaction and movement times are prolonged.
• Agility and neuromuscular coordination are reduced.
• Whole-body movement speed is slowed.
• Concentration and alertness are reduced.

The decline in concentration and alertness associated with fatique is particularly important. The athlete can become careless and more prone to serious injury, especially in contact sports. Even though these decrements in performance might be small, they can be just enough to cause an athlete to miss the critical free throw in basketball, the strike zone in baseball, or the 20ft(6m) putt in golf.

All athletes can benefit from maximizing their endurance. Even golfers, whose sport is relatively sedentary, can improve. Improved endurance can allow golfers to complete a round of golf with less fatique and to better withstand long periods of walking and standing.

For the sedentary, middle-aged adult, numerous health factors indicate that cardiovascular endurance should be the primary emphasis of training.

The extent of endurance training needed varies considerably from one athlete to the next. It depends on the athlete’s current endurance capacity and the endurance demands of the chosen activity.The marathon runner uses endurance training almost exclusively, with limited attention to strength, flexibility and speed. The baseball player, however, places very limited demands on endurance capacity, so endurance conditioning is not as highly emphasized. Nevertheless, baseball players could gain substantially from endurance running, even if only at a moderate pace for 5km(3.1 mi) per day, three days a week. As a benefit, baseball players would have little or no leg trouble(a frequent complaint), and they would be able to complete a doubleheader with little or no fatique.

Adequate cardiovascular conditioning must be the foundation of any athlete’s general conditioning program. Many athletes in nonendurance activities never incorporate even moderate endurance training into their training programs. Those who have done so are well aware of their improved physical condition and its impact on their athletic performance.