As a pilates instructor I hear this all the time. "Oh I love pilates because it lengthens your muscles," or "weight training shortens and tightens your muscles but pilates makes them longer." While pilates will give you the appearance of longer, lean muscle, it does not actually cause your muscles to grow in length, as this is impossible. Pilates helps create space between the vertebrae as well as improve posture, therefore with pilates training you stand taller, appearing longer. Pilates also helps create lean muscle mass, adding to this effect.
So how exactly does pilates create the "long and lean" look so successfully? The key is in the quality of the movement. What this means is really using your muscle energy to create resistance, and by actually using your muscles instead of momentum. For example, when you see a ballerina whirling and twirling around in the air it seems effortless, but there is an immense amount of energy being exerted, with all movements beginning from the core. Additionally, in pilates there is a focus on eccentric muscle contraction. An eccentric contraction is the “release” part of a movement, like when you lower a weight to its starting position. In other words, the muscle is being strengthened while its being stretched. With pilates, the use of springs, resistance against gravity and slow, controlled movements are what enable eccentric contractions. Last but not least, there are fewer repetitions of exercises. Typically we do anywhere from 5-12 reps of an exercise on each side, therefore working your body hard but not to the point of exhaustion or muscle failure. Don't get me wrong---it's not for wimps! As soon as you are done with your reps with one exercise, you immediately move onto the next so the class is very fluid and you are always actively engaged.
Bottom line, pilates creates a strong core and a flexible spine makes which not only make you feel good, but stand straighter and actually appear taller as well.