Short bouts of maximal intensity exercise (HIT) build high levels of fitness quickly. Canadian researchers found that six sessions of high-intensity interval training on a stationary bike increased muscle oxidative capacity by almost 50 percent, muscle glycogen by 20 percent, and cycle endurance capacity by 100 percent. The subjects made these amazing improvements exercising a mere 15 minutes in two weeks.
A University of Nebraska study found that 30-second-long intervals were better for building endurance and aerobic capacity than intervals lasting three minutes. Short, intense interval training is incredibly effective and builds fitness quickly. (Journal Strength Conditioning Research, 25: 1279-1284)