Why do workouts work?

LiveScience features a fascinating article on workouts:

As weightlifters know, the more that people use their muscles, the stronger they become. And unused muscles do not remain preserved; neglect causes them to waste away, or atrophy... NASA needs to know [why workouts work]. Astronauts in space exercise furiously to keep fit. Even so, their muscles tend to weaken... To find out [which type of space workout is best], Baldwin's group gave laboratory rats a workout by activating the rodents' leg muscles with painless electrical stimulation. They tested three types of exercise: muscle contraction, muscle lengthening, and isometric, where the muscle exerts a force while remaining the same length... ...after 12 sessions, all three types of workout tended to provide about the same amount of muscle growth... In addition to measuring overall muscle mass--how "buff" [lean] were the rats? ...To their surprise, Baldwin's team found that while isometric exercises did prevent leg muscles from withering, they did not stop a decline in the amount of contractile proteins in those muscles. The muscle was actually degrading on the molecular level...

LiveScience: Why do workouts work?