Less than half of American adolescents are aerobically fit — about 40 percent, according to a new report from the American Heart Association. These young people have what is known, in medical terms, as good cardiorespiratory fitness, referring to the body’s ability to supply oxygen to muscles to produce energy during physical activity. The AHA report says that cardiorespiratory fitness is a key indicator of physical fitness and overall health, and that aerobically fit youths will probably have longer, healthier lives. The 60 percent of adolescents who lack healthy cardiorespiratory fitness — which the AHA says is half of boys and two-thirds of girls ages 12 to 15 — are at higher risk for heart disease, high blood pressure and Type 2 diabetes. The report also links good aerobic fitness with academic achievement, better mental health and an improved sense of self worth. The report says young people’s cardiorespiratory fitness has been declining for several decades, not just in the United States but worldwide, as obesity and sedentary time have increased and time spent on vigorous physical activity has decreased. Today, as much as 75 percent of an adolescent’s waking hours are sedentary, according to research cited in the AHA report. Suggested ways to improve cardiorespiratory fitness include regularly running, swimming, dancing, jumping rope or participating in high-intensity sports, such as basketball or soccer.