You cannot lengthen muscle (aside from surgically). The length of your muscle belly is determined genetically.
Muscle lengthening means to stretch the muscles so as to relieve pressure. By lengthening the muscles, the whole body gets longer, straighter and more erect. Stretching should be done everyday and it is essential if you work out in a gym. By lengthening your muscles you will be able to become more flexible and perform much better. You will not suffer any muscle pains in your body. There are many stretching exercises you can do. You can pull out your arms and legs and try reaching your left hand to the right and leg and right hand to the left leg. Cycling and swimming are known as the best stretching exercise. Sports like tennis also help in a lot of muscle lengthening. If you are too lazy to do exercises, you can also get a massage. The masseur will help you release all tension in the muscles.