Aerobic endurance training improves aerobic capacity by 5% to 25% in previously untrained, healthy adults. The magnitude of improvement is primarily dependent upon the initial level of physical fitness. The lower the fitness level, the greater the gain from aerobic training.

The improvement in aerobic capacity is the result of several physiological adaptations that increase the body's production of energy. First, adenosine triphosphate (ATP), the actual unit of energy for muscular contraction, is increased because organelles within the cells that use oxygen to produce ATP increase in number and size. These organelles are the mitochondria and are often referred to as the cell's powerhouse. Second, an increase in the enzymes located within the mitochondria that accelerate the production of ATP occurs. Third, cardiac output and blood perfusion of the muscles performing the work increase. Finally, training facilitates and increases the extraction of oxygen by the exercising muscles. These are some of the major adaptations that combine to enhance aerobic endurance.

Aerobic capacity is limited by heredity and is finite. This represents a substantial influence regarding a given individual's potential for aerobic performance. If those who inherit the genetic potential for endurance events also train diligently, they become capable of exceptionally high levels of performance. However, diligent training with an average genetic potential results in average or slightly above average performance. Only a select few inherit the ability to produce world class endurance performances. The majority of people are in the average category, but all can achieve their aerobic potential with training.

Aerobic capacity reaches a peak after 6 months to 2 years of steady endurance training. At this point, it levels off and remains unchanged for a number of years, even if training is intensified. However, aerobic performance continues to improve with harder training, because a higher percentage of the aerobic capacity can be maintained for a longer period. For example, 6 months of appropriate training may allow you to jog 3 miles at 60% of your aerobic capacity. Another year of harder training may allow you to run 3 miles at 85% of your capacity. Capacity has not changed during this time, but physiological adaptations have occurred that enable the body to function at progressively higher percentages of maximum capacity.

Aerobic capacity does decrease with age, but an excellent longitudinal study indicates that it declines more slowly in physically active subjects as compared with sedentary subjects. The subjects were middle aged men whose average age at the inception of the study was 45 years. They were involved in consistent physical training for the next 23 years, and at the end of that time, they experienced only one third of the aerobic decline that was measured in a non exercising control group. In addition, their body weights decreased by an average of 8 pounds, and blood pressure did not increase with age, which often occurs among sedentary people.

The effects of training persist as long as training continues. Fitness developed through years of continuous training can be lost in months if training is interrupted or discontinued. Subjects who suspended training for 84 days after 10 years of active participation experienced a significant decline in aerobic capacity after 3 weeks of inactivity and returned to pretraining levels in most fitness parameters by the end of the study. The exceptions to complete reversal were muscle capillary density and mitochondrial enzymes, which remained 50% higher than levels measured in sedentary control subjects. This study indicated that the results of inactivity are variable and affect some systems more quickly than others. Physical decline cannot be prevented with physical inactivity.

These include the rockport fitness walking Test, the 3 mile walking Test, the 1.5 mile run/walk test, and a 3 minute bench stepping test. each is accompanied by norms, so you can compare your performance against the standards.

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