Lactate Threshold and Endurance Training

The ability of the cardiopulmonary system to deliver adequate amounts of oxygen to the exercising muscles at the beginning of exercise may be insufficient because of the time lag required to make proper cardiovascular adjustments. During this time, therefore, the muscles respire anaerobically and a "stitch in the side"—possibly due to hypoxia of the diaphragm—may develop. After numerous cardiovascular and pulmonary adjustments have been made, a person may experience a "second wind" when the muscles are receiving sufficient oxygen for their needs.

Continued heavy exercise can cause a person to reach the lactate threshold, which is the maximum rate of oxygen consumption that can be attained before blood lactic acid levels rise as a result of anaerobic respiration. This occurs when 50% to 70% of the person's maximal oxygen uptake has been reached. The rise in lactic acid levels is due to the aerobic limitations of the muscles; it is not due to a malfunction of the cardiopulmonary system. Indeed, the arterial oxygen hemoglobin saturation remains at 97%, and venous blood draining the muscles contains unused oxygen.

The lactate threshold, however, is higher in endurance-trained athletes than it is in other people. These athletes, because of their higher cardiac output, have a higher rate of oxygen delivery to their muscles. As mentioned in chapter 12, endurance training also increases the skeletal muscle content of mitochondria and Krebs cycle enzymes, enabling the muscles to utilize

Table 16.11 Changes

in Respiratory Function During Exercise

Variable

Change

Comments

Ventilation

Increased

In moderate exercise, ventilation is matched to increased metabolic rate. Mechanisms responsible for increased ventilation are not well understood.

Blood gases

No change

Blood gas measurements during light and moderate exercise show little change because ventilation is increased to match increased muscle oxygen consumption and carbon dioxide production.

Oxygen delivery to muscles

Increased

Although the total oxygen content and PO2 do not increase during exercise, there is an increased rate of blood flow to the exercising muscles.

Oxygen extraction by muscles

Increased

Increased oxygen consumption lowers the tissue PO2 and lowers the affinity of hemoglobin for oxygen (due to the effect of increased temperature). More oxygen, as a result, is unloaded so that venous blood contains a lower oxyhemoglobin saturation than at rest. This effect is enhanced by endurance training.

Table I6.I2

Altitude

Blood Gas Measurements at Different Altitudes

Arterial PO2 Percent Oxyhemoglobin (mmHg) Saturation

Arterial Pco2 (mmHg) 2

Sea level

9D-9S

96%

4D

I,S24 m (S,DDD ft)

7S-8I

9S%

32-33

2,286 m (7,SDD ft)

69-74

92%-93%

3I-33

4,S72 m (IS,DDD ft)

48-S3

86%

2S

6,D96 m (2D,DDD ft)

37-4S

76%

2D

7,62D m (2S,DDD ft)

32-39

68%

I3

8,848 m (29,D29 ft)

26-33

S8%

9.S-I3.8

Source: From P. H. Hackett et al., "High Altitude Medicine" in Management of Wilderness and Environmental Emergencies, 2d ed., edited by Paul S. Auerbach and Edward C. Geehr. Copyright © I989 Mosby-Yearbook. Reprinted by permission.

Source: From P. H. Hackett et al., "High Altitude Medicine" in Management of Wilderness and Environmental Emergencies, 2d ed., edited by Paul S. Auerbach and Edward C. Geehr. Copyright © I989 Mosby-Yearbook. Reprinted by permission.

more of the oxygen delivered to them by the arterial blood. The effects of exercise and endurance training on respiratory function are summarized in table 16.11.

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