Almost every fine, powerful pair of legs in existence today is the result of the squat.
You should do many sets of from 10 to 15 or 20 reps. The number of sets you can work up to will depend on your energy and ability. It may be as high as seven sets, or you may be able to obtain sufficient results with only two sets. Always perform your squats with a flat back. Use as heavy a weight as you can for the required number of reps. Follow the methods of deep breathing as advised under the paragraph on breathing. You won’t have to concentrate on it to the extent you do when trying to gain weight or build a large chest. You will find that you need considerable rest between sets as this is a very strenuous program.
If you want to develop the lower part of your leg more than the upper part then you should squat with the heels on a block of wood or you can squat on the toes with your back against the sliding post. Keep feet close to the post. Sit erect and go straight up and down.
For general thigh development, however, with equal development for the front and back of the thigh, nothing can equal the regular flat footed squat with the back flat, or even arched a little.
Three workouts per week is ample and two is often best. Leg specialization is hard work and requires lots of energy, necessitating lots of rest and sleep, with a full diet.
Due to differences in body mechanics, many people have struggled to complete the back squat with the correct form required for optimal growth.
Indeed, which is why I value the history of fitness so much… it teaches variation!