Just when you thought you had the perfect excuse not to exercise -- and that excuse being a lack of time -- there comes research to invalidate your time-crunch claims.

A recent Canadian study discovered that six sessions of just four to seven full exertion sprints over two weeks improved fitness test performance by ten percent. According to the study, these short, but intense, workouts increased levels of citrate synthase -- markers of muscle tissue's ability to utilize oxygen and boost physical output.

High-intensity training is perfect for people who don't have the time in their schedule for longer endurance-based workouts. And, it doesn't come at the expense of your results. Numerous studies have demonstrated that high-intensity training actually burns more fat than longer exercise sessions performed at a low or moderate effort level.