Adaptations to aerobic training – part II

 PART I

Heart rate

Aerobic training has a major impact on heart rate at rest, during submaximal exercise, and during the postexercise recovery period. The effect of aerobic training on maximal heart rate is not as clear.

Resting heart rate

The heart rate at rest can decrease markedly as a result of endurance training. A sedentary individual with an initial resting heart rate of 80 beats/min can decrease resting heart rate by approximately 1 beat/min with each week of aerobic training, at least for the first few weeks. After 10 weeks of moderate endurance training, resting heart rate can decrease from 80 to 70 beats/miin or lower. The actual mechanisms responsible for this decrease are not entirely understood, but training appears to increase parasympathetic activity in the heart while decreasing sympathetic activity. It is important to recognize that several well-controlled studies with large numbers of subjects have shown much smaller decreases in resting heart rate, that is, fewer than 5 beats/min following up to 20 weeks of aerobic training.

Bradycardia is a clinical term indicating a heart rate of fewer than 60 beats/min. In untrained individuals, bradycardia is usually the result of abnormal cardiac function or a diseased heart. Therefore, it is necessary to differentiate between training-induced bradycardia, which is a natural response to endurance training, and pathological bradycardia, which can be a serious cause for concern.

Submaximal heart rate

During submaximal exercise, aerobic conditioning results in proportionally lower heart rates at a given absolute exercise intensity. This is illustrated in figure below, which shows the heart rate of an individual exercising on a treadmill both before and after training. At each specified intensity, indicated here by the speed at which the subject is walking or running, the posttraining heart rate is lower than the heart rate before training. After a six-month endurance training program of moderate intensity, decreases in heart rate of 10 to 30 beats/min are common at the same absolute submaximal workload, the training-induced decrease being greater at higher intensities.

These decreases indicate that the heart becomes more economical through training. In carrying out its necessary functions, a trained heart performs less work(lower heart rate, higher stroke volume) than an unconditioned heart at the same absolute workload.

Maximum heart rate

A person’s maximal heart rate(HR_max) tends to be stable and typically remains relatively unchanged after endurance training. However, several studies have suggested that for people whose untrained HR_max values exceed 180 beats/min, HR_max might be slightly lower after training. Also, highly conditioned endurance athletes tend to have lower HR_max values than untrained individuals of the same age, although this is not always the case. Athletes over 60 years old sometimes have higher HR_max values than untrained people of the same age.

Interactions between heart rate and stroke volume

During exercise, the product of heart rate and stroke volume provides a cardiac output appropriate to the intensity of the activity being performed. At maximal or near-maximal intensities, heart rate may change to provide the optimal combination of heart rate and stroke volume to maximize cardiac output. If heart rate is too fast, diastolic filling time is reduced, and stroke volume might be compromised. For example, if HRmax is 180 beats/min, the heart beats three times per second. Each cardiac cycle thus lasts for only 0.33s. Diastole is as short as 0.15s or less. This fast heart rate allows very little time for the ventricles to fill. As a consequence, stroke volume may decrease at high heart rates at which filling time is compromised.

However, if the heart rate slows, the ventricles would have longer to fill. This has been proposed as the reason highly trained endurance athletes tend to have lower HR_max values: their hearts have adapted to training by drastically increasing their stroke volumes, so lower HR_max values can provide optimal cardiac output.

Which comes first? Does increased stroke volume result in a decreased heart rate, or does a lower heart rate result in an increased stroke volume? This question remains unanswered. In either case, the combination of increased stroke volume and decreased heart rate is a more efficient way for the heart to meet the metabolic demands of the body, especially during exercise. The heart expends less energy by contracting less often but more forcefully than it would if contraction frequency were increased. Reciprocal changes in heart rate and stroke volume in response to training share a common goal: to allow the heart to pump the maximal amount of oxygenated blood at the lowest energy cost.

Heart rate recovery

During exercise, heart rate must increase to increase cardiac output to meet the blood flow demands of active muscles. When the exercise bout is finished, heart rate does not instantly return to its resting level. Instead, it remains elevated for a while, slowly returning to its resting rate. The time it takes for heart rate to return to its resting rate is called the heart rate recovery period.

Following a period of training, as shown in the figure below, heart rate returns to its resting level much more quickly after exercise than it does before training. This is true after standardized submaximal exercise as well as after maximal exercise.

Because the heart rate recovery period is shortened after endurance training, this measurement has been proposed as an indirect index of cardiorespiratory fitness. In general, a more fit person recovers faster after a standardized rate of work than a less fit person, so this measure may have some utility in field settings when more direct measures of endurance capacity are not possible or feasible. However, factors other than training can also affect heart rate recovery time. For example, exercise in hot environments or at high altitudes can prolong heart rate elevation. Some people undergo a stronger sympathetic nervous system response during exercise than others, and this also could prolong heart rate elevation.

The heart rate recovery curve is a useful tool for tracking a person’s progress during a training program. But because of the potential influence of other factors, it should not be used to compare between individuals.

Cardiac output

While stroke volume increases, heart rate generally decreases at rest and during exercise at a given absolute intensity.

Because the magnitude of these reciprocal changes is similar, cardiac output at rest and during submaximal exercise at a given exercise intensity does not change much following endurance training. In fact, cardiac output can decrease slightly. This is likely the result of an increase in the (a-ṽ)O₂ difference(reflecting greater oxygen extraction by the tissues) or a decrease in the rate of oxygen consumption(reflecting an increased mechanical efficiency). Generally, cardiac output matches the oxygen consumption required for any given intensity of effort.

Maximal cardiac output, however, increases considerably at maximal exercise intensity in response to aerobic training, as seen in the figure below, and is largely responsible for the increase in VO₂max. This increase in cardiac output must result from an increase in maximal stroke volume, because HR_max changes little, if any. Maximal cardiac output ranges from 14 to 20L/min in untrained individuals and from 25 to 35L/min in trained individuals, and can be 40L/min or more in highly conditioned endurance athletes. These absolute values, however, are greatly influenced by body size.

Blood flow

Active muscles need considerably more oxygen and nutrients than inactive ones. To meet these increased needs, more blood must be delivered to these muscles during exercise. With endurance training, the cardiovascular system adapts to increase blood flow to exercising muscles to meet their higher demand for oxygen and metabolic substrates. Four factors account for this enhanced blood flow to muscle following training:

• Increased capillarization of trained muscles
• Greater recruitment of existing capillaries in trained muscles
• More effective blood flow redistribution from inactive regions
• Increased blood volume.

To permit increased blood flow, new capillaries develop in trained muscles. This allows the blood flowing into skeletal muscle from arterioles to more fully perfuse the active tissues. This increase in capillaries usually is expressed as an increase in the number of capillaries per muscle fiber, or the capillary-to-fiber ratio. Table below illustrates the differences in capillary-to-fiber ratios between well-trained and untrained men, both before and after exercise.

Muscle fiber capillarization in well-trained and untrained men
	Capillaries per mm2	Muscle fibers per mm2	Capillary-to- fiber ratio	Diffusion distance
Well trained
Preexercise	640	440	1.5	20.1
Postexercise	611	414	1.6	20.3
Untrained
Preexercise	600	557	1.1	20.3
Postexercise	599	576	1.0	20.5

Diffusion distance is expressed as the average half-distancce between capillaries on the cross-sectional view expressed in micrometers.

Not all capillaries are open at any given time in tissues, including muscle. In addition to new capillarization, existing capillaries in trained muscles can be recruited and open to flow, which increases blood flow to muscle fibers. The increase in new capillaries with endurance training and increased capillary recruitment combine to increase the cross-sectional area for exchange between the vascular system and the metabolically active muscle fibers. Because endurance training also increases blood volume, shifting more blood into the capillaries will not severely compromise venous return.

A more effective redistribution of cardiac output also can increase blood flow to the active muscles. Blood flow is directed to the active musculature and shunted away from areas that do not need high flow. Blood flow can increase to the more active fibers even within a specific muscle group. Armstrong and Laughlin demonstrated that endurance-trained rats could redistribute blood flow to their most active tissues during exercise better untrained rats could. The researchers used radiolabeled microspheres, radioactive particles that are injected into the bloodstream. The distribution of these microspheres was then determined by using a Geiger counter, which measures of the amount of radioactive material throughout the area of interest(in this case exercising muscle). The total blood flow to the hindlimbs did not differ between the trained and untrained rats during exercise. However, the trained rats distributed more of their blood to the most oxidative muscle fibers, effectively redistributing the blood flow away from the glycolitic muscle fibers. These findings are difficult to replicate in humans because of the technical difficulty(microspheres cannot be used in humans) of measuring blood flow to specific muscle types, as well as the fact that human skeletal muscle is a mosaic with mixed fiber types among individual muscles.

Finally, the body’s total blood volume increases with endurance training, providing more blood to meet body’s many blood flow needs during endurance activity.

Blood pressure

Following endurance training, arterial blood pressure is reduced at a given submaximal exercise intensity; but at maximal exercise capacity, systolic blood pressure is increased and diastolic pressure is decreased. Resting blood pressure in response to endurance training does not change significantly in healthy subjects but is generally lowered in borderline or moderately hypertensive individuals. This reduction occurs in both systolic and diastolic blood pressure. Drops in blood pressure average approximately 6 to 7mmHg for both systolic and diastolic pressure in hypertensive subjects and slightly less in borderline hypertensives. The mechanisms underlying this reduction are unknown.

Although resistance-type exercise can cause large increases in both systolic and diastolic blood pressure during lifting or heavy weights, chronic exposure to these high pressures does not elevate resting blood pressure. Hypertension is not common in competitive weightlifters or in strength and power athletes. In fact, a few studies have even shown that resistance training may lower resting systolic blood pressure. Hagberg and coworkers followed a group of borderline-hypertensive adolescents through five months of weight training. The subjects’ resting systolic blood pressures decreased significantly. These reductions were somewhat greater than those resulting from endurance training.

Blood volume

Endurance training increases blood volume, and this effect is larger as training intensity increases. Furthermore, the effect occurs rapidly. This increased blood volume results primarily from an increase in plasma volume, but there is also an increase in red blood cells. The time course for the increase of each of these is quite different.

Plasma volume

The increase in plasma volume is thought to result from two mechanisms. The first mechanism, which has two phases, results in increases in the amount of plasma proteins, particularly albumin. Plasma proteins are the major source of osmotic pressure in the vasculature. As plasma protein concentration increases, so does osmotic pressure, and fluid is reabsorbed from the interstitial fluid into the vasculature. During an intense bout of exercise, proteins leave the vascular space and move into the interstitial space. They are then returned in greater amounts through the lymph system. It is likely that the first phase of rapid plasma volume increase is the result of the increased plasma albumin, which is noted within the first hour of recovery from the first training bout. In the second phase, protein synthesis is turned on(upregulated) by repeated exercise, and new proteins are formed. With the second mechanism, exercise increases the release of antidiuretic hormone and aldosterone, hormones that cause increased reabsorption of water and sodium in the kidneys, which increases blood plasma. That increased fluid is kept in the vascular space by the oncotic pressure exerted by the proteins. Nearly all of the increase in blood volume during the first two weeks of training can be explained by the increase in plasma volume.

Red blood cells

An increase in red blood cell volume with endurance training also contributes to the overall increase in blood volume, but this increase is an inconsistent finding. Although the actual number of red blood cells may increase, the hematocrit – the ratio of the red blood cell volume to the total blood volume – may actually decrease. Figure below illustrates this apparent paradox. Notice that the hematocrit is reduced even though there has been a slight increase in red blood cells. A trained athlete’s hematocrit can decrease to a level where the athlete appears to be anemic on the basis of a relatively low concentration of red cells and hemoglobin.

The increased ratio of plasma cells resulting from a greater increase in the fluid portion reduces the blood’s viscosity, or thickness. Reduced viscosity may facilitate blood movement through the blood vessels, particularly through the smallest vessels such as the capillaries. One of the physiological benefits of decreasing blood viscosity is that it enhances oxygen delivery to the active muscle mass.

Both the total amount(absolute values) of hemoglobin and the total number of red blood cells are typically elevated in highly trained athletes, although these values relative to total blood volume are below normal. This ensures that the blood has more than ample oxygen-carrying capacity. The turnover rate of red blood cells also may be higher with intense training.