There are several traditional asanas (postures) which keep all the big and small joints of the human body in a normal and healthy condition. Our muscles have property to stretch and contract. Many of them work in pairs to accommodate all proper or improper movements carried out by a human body.
When you practice different postures they strengthen different group of muscles with self weight. There are standing, sitting, forward/backward bending, twisting, inverted and relaxing postures. You should practice them in a proper sequence. Even Suryanamaskar given under Reduce Weight is a good sequence of postures. Practice few PranayamKriya. All the Yoga postures practiced in a sequence regularly bring flexibility in the body. And you feel experience the lightness in the body.
The cleaning of the body and mind takes place, which makes you aware of the inside of the body. Keep your diet and daily routine under control.
Experience this yourself