One of the main foundations of Parkour is the Natural Method of Georges Hébert ("Méthode naturelle d'éducation physique, virile et morale"), which Raymond Belle followed and passed on to his son, David Belle.
As a fusilier marin (~ French Marines), Georges Hébert traveled the world at the beginning of the 20th century and marveled at the simple physical fitness of primitive peoples. He soon started to incorporate his observations into a physical education method which he used to train sailors, children, Olympic athletes and even women.
His method focused on basic, practical movements: walking, running, jumping, climbing, swimming, lifting, throwing and defending. He invented the term Quadrupédie and developed quadrupedal movement, which shocked the men of his times even more than his teaching of women. Yet, his method laid the principles of today's military exercises such as the obstacle course, and has received a lot more interest recently for its links with Parkour.
Unfortunately, most of Georges Hébert's works are out of print and only available in French. One of his earliest books surfaced on Google Books and was "fandubbed" into English by Gregg from Hawaii PK and myself. You can get a copy of the translation (and links to the original) here. There is still a group of followers of the Méthode Naturelle in Belgium, and a pretty cool derived method called MovNat. You can also find a lot of discussion in this APK / Hawaii PK forum, and a new website from Germany.
To me, the Méthode Naturelle has a few very important training guidelines:
- Train regularly: better to train half an hour every day than four hours every Sunday; better to train a little every week than a lot when summer comes.
- Train everything: rather than focus on highly specialized moves and techniques related to a single sport, it is better to train a variety of techniques and sports. This is why parkour is a good fit for the Natural Method, as it includes many different types of movements and skills.
- Monitor your performance: whether you count a number of repetitions, track the weights you lift, the scaling of the climbing walls, or even keep a mental list of things you can and cannot do in your parkour training, it is important to know what you are capable of and how much that changes with your training, so you can adjust and improve faster.
- Focus on form: understanding the moves, their biomechanics and physics, is essential to progress in training. By paying attention to the details and working on them, your movements become more and more fluid, natural, effortless.