This brings us to No. 3 on the list: HIIT, which stands for high-intensity interval training. This type of training is generally 30 minutes or less and incorporates short, intense intervals that can push your effort level up to 90 to 95 percent of your maximum heart rate. It has long been a mainstay among elite athletes, but has just recently made its way into the time-crunched public. "I think at first we were all afraid that HIIT would lead to increased injuries in the general public, but that has not proven to be true," says Thompson, who is also an associate dean and professor of kinesiology and health at Georgia State University. "There is no more risk of injury than any other exercise."