Exercise developes muscle by working and making the muscles grow to expand
lactic acid that gets released from your muscle when you exercise.
Improve muscle strength not just of the quadriceps
The best way for a 13-year-old girl to get a muscle body is to practice lifting her own weight. With this exercise, you can develop a lean body that is muscular, but not bulky.
Lunges. see: http://en.wikipedia.org/wiki/Lunge_(exercise)
An exercise ball can be used to develop many different muscle groups. Placing one hand on an exercise ball while doing a push up will help develop the pectorial and latimus dorsi muscles. Doing a crunch while seated on an exercise ball will help develop oblique muscles.
roman chair sit ups
The only way to build muscle is with exercise. That being the case, since exercise requires calories, building muscle will also burn off fat.
Fast twitch mustle fibers.
Yes. Studies have demonstrated that even people in their 90's can get stronger from resistance exercise.