Janine E Walsh BSc. Hlth & Ex. Sc. ESSAM AEP - Accredited Exercise Physiologist - Exercise Life Skills Consultancy
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Cardiorespiratory Fitness
Health Conditions/Benefits

Cardiorespiratory Fitness

Cardiorespiratory endurance or aerobic fitness can be defined as the ability of the circulatory and respiratory systems to supply oxygen during sustained physical activity. Cardiorespiratory endurance is considered health related because low levels have been consistently linked with markedly increased risk of premature death from all causes, especially heart disease.

There are several major cardiorespiratory changes at rest following exercise training, some examples include:

  • An increase in heart size. The size of the left and right ventricular cavities increases, with proportional increases in the thicknesses of the heart muscle walls and septum. These changes occur gradually over months or years of training

  • A decrease in resting heart rate. The resting heart rate decreases approximately one beat per minute for every 1 to 2 weeks of aerobic training for about 10 to 20 weeks. Further decreases are possible if training volume and intensity are increased. Some of the best endurance athletes in the world have resting heart rates below 40bpm

  • An increase in stroke volume (blood pumped from heart per beat), with a corresponding decrease in rate. For example sedentary people have stroke volumes at rest at about 60ml, whereas those of athletes often measure greater than 100ml

  • An increase in the total blood volume from about 5 liters in sedentary people to 6 or 7 liters in athletes. The 20-25% increase in blood volume is evident in males and females of all ages

In response to regular aerobic training, many significant changes take place in the muscle cells:

  • An increase in myoglobin content. [Myoglobin aids in the delivery of oxygen from the blood to the mitochondria (organelles in the muscle cell that produce ATP for energy)]

  • An increase in the number and size of mitochondria

  • An increase in the concentration of important enzymes in the mitochindria, specifically those of the Krebs cycle and the electron transport system. (These enzymes are involved in the production of ATP from aerobic metabolism)

  • An increase in both the amount of glycogen stored in the muscle and the maximal capacity to oxidize carbohydrates

  • An increased capacity to oxidize fat (from both muscle fat and adipose tissue stores). [The trained person thus oxidizes more fat and less carbohydrate during cardiorespiratory exercise (at an absolute workload), which means less glycogen depletion, less lactic acid accumulation, and therefore less muscle fatigue and greater endurance

  • An increase in the area of slow-twitch fibers. [The aerobic-type muscle fibers are called Type I (red, tonic, slow twitch); the anaerobic-type fibers are called Type II (white, glycolytic, fast twitch). People vary widely in their proportions of slow-twitch and fast-twitch fibers. This proportion is set at birth and remains constant throughout life.]

Other changes that occur in response to regular cardiorespiratory exercise include:

  • A small decrease in total body fat, reduced intra-abdominal fat and a slight increase in lean body weight

  • An increase in HDL cholesterol, a decrease in triglycerides, but little or no change in total serum or LDL cholesterol

  • A greater ability to exercise in the heat

  • An increase in the density and breaking strength of bone, ligaments, and tendons, and an increase in the thickness of cartilage in the joints

  • Reduced resting systolic/diastolic blood pressures

  • Reduced insulin needs, improved glucose tolerance

  • Reduced blood platelet adhesiveness and aggregation

  • Decreased morbidity and mortality- lower incidences of death rates from coronary artery disease, stroke, type 2 diabetes, osteoporotic fractures, cancer of the colon and breast, and gallbladder disease

  • Decreased anxiety and depression

  • Enhanced physical function and independent living in older persons


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