to body tissue. A 2005 Cochrane review demonstrated that physical activity interventions are effective for increasing cardiovascular fitness. There are many benefits of cardiorespiratory fitness. It can reduce the risk of heart disease, lung cancer, type 2 diabetes, stroke, and other diseases. Cardiorespiratory fitness helps improve lung and heart condition, and increases feelings of wellbeing. Additionally, there is mounting evidence that CRF is potentially a stronger predictor of mortality than other established risk factors such as smoking, hypertension, high cholesterol, and type 2 diabetes. One study concluded that levels of CRF were associated with deaths earlier than 65 years of age which was not detected in preceding generations, suggesting low CRF might be an emerging risk factor for early death among among US Baby Boomers and Generation Xers. Significantly, CRF can be added to these traditional risk factors to improve risk prediction validity. The American College of Sports Medicine recommends aerobic exercise 3–5 times per week for 30–60 minutes per session, at a moderate intensity, that maintains the heart rate between 65 and 85% of the maximum heart rate. Cardiovascular system adaptations The cardiovascular system responds to changing demands on the body by adjusting cardiac output, blood flow, and blood pressure. Cardiac output is defined as the product of heart rate and stroke volume which represents the volume of blood being pumped by the heart each minute. Cardiac output increases during physical activity due to an increase in both the heart rate and stroke volume. Stroke volume is defined as the percent of blood pumped out of the heart during systole and is a function of the heart's contractility, preload, and afterload. Contractility refers to the strength capacity of the heart muscle during systole. At rest, the basal level of autonomic nervous system activity, the length of cardiac sarcomere, and the metabolite environment of the myofibril determine the vigor with which the heart muscle can contract. Of these, during physical activity, the level of the autonomic nervous system activity is the most modifiable. During the early stages of physical activity, the parasympathetic arm of the autonomic nervous system decreases its rate of firing, followed by an increase in firing rate of the sympathetic nervous system. "Within a second after muscular contraction, there is a withdrawal of vagal outflow to the heart, which is followed by an increase in sympathetic stimulation of the heart. This results in an increase in cardiac