Unlike traditional weight building exercises, in yoga your body provides the resistance. While you are not likely going to produce the bulked up muscles of some weight lifters, you will certainly increase your muscle strength.
Many poses in yoga are done very slowly or you stay in the posture for several breaths. It is much more challenging to your muscles to hold a pose or do it slowly than it is to allow momentum to move you through an action. I have been working out with weights for many years so my muscles are used to being strengthened. Yet, I have practiced certain yoga routines during which my muscles screamed for mercy and I actually had to get out of the pose and then resume it because my muscles were so challenged. I don’t often experience this during weight training sessions.
In addition, some balance postures such as Tree Pose and Shoulderstand require enormous muscle control in order to prevent you from falling over. This helps to build and strengthen your muscles.
In weight training, you isolate a specific muscle as you perform an exercise and this leads to a short, tightened muscle. The muscles you develop during yoga are more likely to be elongated, because while you are strengthening them, you are simultaneously lengthening them. You also do not focus on an isolated muscle, but actively recruit the smaller muscle groups as well. You truly work your entire body when you practice yoga.
Practicing yoga can help realign your muscles, so they are more balanced. Since you are not overworking any specific muscle group, you are less likely to get injured.
In addition to all its other benefits, yoga can help you improve muscle fitness. Whether you choose to use it as your primary means of strength training or you want it to supplement your other exercises, yoga can help your muscles grow fit, balanced, and strong.