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Rapid Glycolysis (Lactic Acid System)

('.biolysis hsts carbohydrates primarily in tin- form of muscle

glycogen as a fuel source. When glycolysis is rapid, the pathways

ili.it normally use oxygen to make energy arc circumvented in

favor of other, faster yet less efficient paths that do not requireoxygen. As a result, only a small amount of AIT is produced

anaerobically, and lactic acid is produced as a by-product of the

reaction

For many years, lactic acid was considered to he the waste

product caused by inadequate oxygen supply lactic acid limited

physical activity by building up in muscles anil lead ing to fatigue

and diminished performance. Since the early i')80s. there has been

a fundamental change in thought, and evidence now shows that a

limited oxygen supply is not required for tactic acid production,

Lactate is produced ami used continuously under fully aerobic

conditions. Ibis is referred to as the cell-to-cell lactate shuttle in

which lactate serves as a metabolic intermediate tying together

glycolysis (as an end product) and oxidative metabolism.

Once lactic acid is formed, there are two possible venues it canlake. Hie first involves conversion into pyruvic acid and

subsequently into energy (ATP) under aerobic conditions (see

"Aerobic Oxidation System" section below). The second involves

hepatic gluconeogenesis using lactate to produce glucose, which is

known as the Cori cycle

Anaerobic oxidation starts as soon as high-intensity exercise

begins and dominates for approximately V/i to 2 minutes (see

figure i s i ) it would fuel activities

such as middle distance sprints (400 . 600-. and 800-m runs] or events

requiring sudden hursis ol energy such as weight lift ing.

Although glycolysis is considered an anaerobic patfr it can

readily participate in the aerobic metabolism w oxygen is

available and is considered the first step in aerobic metabolism of

carbohydrates.2611,1108

Oxidation System

The final metabolic pathway for AIT production comb: two complex

metabolic processes: the Krebs cycle and electron transport chain.

The aerobic oxydation sys resides in the mitochondria. It is capable

of using carbo drates, fat, and small amounts of protein to produce

ene (ATP) during exercise, through a process called oxidati

phosphorylation. During exercise, this pathway uses o~ gen to

completely metabolize the carbohydrates to prodir energy (MP),

leaving only carbon dioxide and water byproducts. The aerobic

oxidation system is complex requires 2 to ? minutes to adjust to a

change in exer-intensity (see figure 18-1). It has an almost unlimited

a ity to regenerate AIT, however, limited only by the am-of fuel and

oxygen that is available to the cell. Maxi oxygen consumption,

also known as Vi);max. is a me~ of the power of the aerobic energy

system, and is gen~ regarded as the best indicator of aerobic

fitness. J6-,0J'1

All the energy-producing pathways are active du~ most types of

exercise, but different exercise types p greater demands on

different pathways 'Ihe contribu* of the anaerobic pathways

(creatine phosphate and glycolysis) to exercise energy metabolism

is inv related to the duration and intensity of the activity, shorter

and more intense the activity, the greater the tribution of anaerobic

energy' production, whereas longer the activity and the lower the

intensity, the greaufl the contribution of aerobic eneigy prodiuiion.

In general," carbohydrates are used as the primary fuel .it the

onset ofj exercise and during high-intensity work. Hut during prqj

longed exercise of low to moderate intensity (longer 30 minutes), a

gradual shift from carbohydrate toward increasing reliance on fat

as a substrate occurs. The grea" amount of fat use occurs at about

60% of maximal aero capacity (VD.m j l )26.101,101

Cardiovascular Exercise

Cardiorespiratory Physiology

The cardiorespiratory system consists of the heart, lung and blood

vessels, 'iTic purpose of this system is the del" cry of oxygen and

nutrients to the cells, as well as removal of metabolic waste

products to maintain the int nal equilibrium.70-10VIM

Cardiac Function

Heort Rote

Normal resting bean rate (IIR) is approximately 60 to bcats/min. IIR

increases in a linear fashion with the ' rate and oxygen uptake

during exercise. The magnitude

IIR response is related to age. body position, fitness, type activity,

the presence of heart disease, medications, blood volume, andenvironmental factors such as tcmpei.uuK

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