In Chapter VI ("On the Road") of Running Through the Wall: A Guide for the Serious Runner Raymond Bridge observes:

... The whole idea of training is to stress the body, breaking down some tissue and pushing your muscles to work hard so that they will be stimulated to grow back stronger, better coordinated, and able to deal with still greater stress. This is a cycle like the tides, not a linear progression like climbing a ladder. There are two equally essential parts to the whole, and if either is removed training won't take place. The first part is the stress, which is essential as a stimulus to the body. The second part is the recuperation stage, during which the actual rebuilding takes place. If you remove the stress, the body won't bother to get stronger. If you remove the rest phase, it won't be able to. Though there are many cycles operating at different rates in the body, it has been shown time and again that rebuilding after a hard workout normally requires more than a day. This means that hard workouts should be alternated with easy days during which the body actually strengthens itself.

(cf. Bill Bowerman's advice quoted in 2006-08-19 - Shaky Ladder 'Speedwork': "... That's all training is. Stress. Recover. Improve....") - ^z - 2008-01-17

(correlates: NoSweat, RespectTheDistance, StrongCoffee, ...)