Define "aerobic exercise."

Aerobic exercise is characterized as sustained physical activity that elevates both heart rate and respiration over a prolonged period. This type of exercise primarily relies on the aerobic energy-generating process, allowing the body to use oxygen efficiently to fuel muscles during extended activities such as running, swimming, or cycling. This sustained effort ensures that the cardiovascular and respiratory systems are actively engaged, improving overall fitness levels, stamina, and endurance.

The other options describe different forms of exercise that do not fit the definition of aerobic exercise. For instance, high-intensity training involves short bursts of activity, which is more anaerobic as it focuses on quick, intense efforts rather than maintaining a lower level of exertion over time. Weightlifting is largely associated with strength training rather than aerobic capacity, as it involves lifting weights for muscle growth rather than sustained cardiovascular exertion. Lastly, exercises performed while sitting down typically do not elevate heart rate and respiration significantly, which is contrary to the principles of aerobic activity.