HIIT - high intensity interval training.
Maximum effort for a short period, typically a minute or less. Followed by a rest period typically 3 times or longer, and repeating the exercise again for a number of sets.
This type of exercises have been shown to produce high levels of HGH post workout. One big physiological difference between a person at young & old age is their level of growth hormones. So in a way HIIT exercises can help people stay lean and young.