Sunday, May 23, 2010

METABOLISM DURING EXERCISE - AEROBIC VS. ANAEROBIC

Metabolically, there are two types of exercise, aerobic and anaerobic. Aerobic exercise uses oxygen as energy substrate to metabolize food to adenosine triphosphate (ATP) (see box below, METABOLIC CHANGES DURING AEROBIC AND ANAEROBIC EXERCISE). When the supply of oxygen is no longer sufficient to meet the needs of exercising muscles, anaerobic metabolism begins. In anaerobic metabolism, glucose is converted to ATP without oxygen, and lactic acid is generated as a by-product. A healthy person can perform aerobic exercise for several hours; in contrast, pure anaerobic exercise can only be sustained for a few minutes before severe dyspnea and fatigue set in.

During short bursts of activity, such as sprinting, energy may be obtained only anaerobically. Otherwise, anaerobic metabolism occurs in addition to ongoing aerobic metabolism. Typically, anaerobic metabolism begins approximately midway between resting and maximal oxygen consumption. The point at which anaerobic metabolism begins is called the anaerobic threshold (AT). AT can be identified by a typical pattern of changes in the blood and in expired gases (see the next section).

METABOLIC CHANGES DURING AEROBIC AND ANAEROBIC EXERCISE

During aerobic exercise, both glucose and fatty acids are metabolized. One molecule of glucose utilizes 6 molecules of oxygen and produces 6 molecules of carbon dioxide, for a metabolic respiratory quotient (RQ) of I.O. For fatty acids, 23 molecules of oxygen are used for every 16 molecules of carbon dioxide produced, giving an RQ of 0.71. The average RQ during mild to moderate exercise (before anaerobic threshold) is approximately 0.85.

By contrast, anaerobic metabolism produces only 2 molecules of ATP per molecule of glucose; at the same time 2 molecules of lactic acid are produced, which, when buffered, generate carbon dioxide in excess of that from aerobic metabolism.

AEROBIC METABOLISM

C6H12O6 + 6 02 ----> 6 CO2 + 6 H2O + 36 ATP (RQ = 1.0)
(Glucose)

C16H32O2 + 23 02 ----> 16 CO2 + 16 H2O + 130 ATP (RQ = 0.71)
(Fatty acid)

ANAEROBIC METABOLISM

Glucose + 2 ADP ----> 2 H+ lactate + 2 ATP (Lactic acid)

H+ lactate- + Na+HCO-3 ---> Na+ lactate- + H2CO3

H2CO3 ---> H2O + CO2


Chapter 12: Exercise Physiology

from Pulmonary Physiology in Clinical Practice, copyright 1999 by

Lawrence Martin, M.D.

Energy Supply

Energy supply

a. ATP à ADP + energy

b. Creatine phosphate + ADP à creatine + ATP (anaerobic , alactic)

c. Glucose + ADP à lactic + ATP (anaerobic, lactic)

d. Glucose + oxygen + ADP à carbon dioxide + ATP + water (aerobic, alactic)

e. Fat + oxygen + ADP à carbon dioxide + ATP + water (aerobic, alactic)

Classification of maximum activity of various duration together with energy – supplying system for this activity

duration

Classification

(aerobic/ anaerobic)

Enrtgy supplied by

Observations

1-4 sec

Anaerobic, alactic

ATP


4-20 sec

Anaerobic, alactic

ATP +CP


20 – 45 sec

Anaerobic, alactic

ATP + CP +

High lactate production


+ anaerobic, lactic

Muscle glycogen

45-120 sec

Anaerobic, alactic

Muscle glycogen

with increasing




duration,




decreasing lactate




production

120- 140 sec

Aerobic + anaerobic,

Muscle glycogen

ditto


lactic



240-600 sec

Aerobic

Muscle glycogen

With increasing



+ fatty acids

Duration higher

Etc



Share of fats

Various substrates for energy supply and their characteristics

Substrate

breakdown

availability

Speed of energy production

Creatine phosphate (CP)

Anaerobic, alactic

Very limited

Very fast

Glycogen or glucose

Anaerobic, lactic

limited

fast

Glucose or glycogen

Aerobic, alactic

limited

slow

Fatty acids

Aerobic, alactic

unlimited

sluggish


Reference:

Training Lactate Pulse-Rate by Peter GJM Janssen