The thighs are the largest and most massive of all muscles of the human body. This is not a secret, even for a person not familiar with anatomy. One only needs to take a look at a bodybuilder or even a person exhibiting moderate muscular development, to be able to tell that probably half of all muscle-mass on the body is concentrated in the thighs.
Not only are the thighs the biggest of al muscle-groups, they’re also a regular ‘Terminator’ of the muscle universe. They can withstand incredible loads and in the same time exhibit quite unbelievable endurance as well. They’re simply unstoppable. This is due to the fact that most people’s thighs are in training almost all day long. Just think about it, they carry one’s bodyweight around almost without a rest for the better part of 12 hours or more. The very fact that makes these muscles the lean mean lifting machine that they are, also makes them exceptionally difficult to train (in comparison to other muscles of the body that is)
Because of the constant wear and tear they’re subjected to, these muscles need not only extra heavy loads but also an increased amount of reps, to be properly stimulated. This of course means there’ a whole lot more effort needed on the part of the bodybuilder, to align the thighs to the rest of his/her physique. Since many bodybuilders - especially the rookies - consider the upper body their utmost priority, and in the same time the body part that gives them most of the satisfaction needed to keep them going, the tendency to neglect proper thigh training is rather wide-spread.
Let us now take a closer look to the individual muscle-groups that make up the thighs.
There are two basic groups (each of them in turn, made up of several muscles) that work opposite each-other and constitute the thigh: the quadriceps and the biceps femoris.
The quadriceps are located in the front of the thigh, they’re extensors, and as such serve to straighten the leg. Executing squats puts strain on these muscles. It is in turn composed of the following muscles: rectus femoris, vastus intermedius, vastus medialis, vastus lateralis.
The factors that bodybuilders need to concentrate on, regarding the quadriceps are about the same as with every other muscle-group, only they’re a bit harder to obtain for the above mentioned reasons: great muscle mass and awesome definition (all muscles we queued up before should be clearly visible) viewed from the side there should also be a clear separation between the front part of the thigh, and the biceps femoris.
The biceps femoris are the muscles that curl one’s leg back. This group is also made up of several muscles.
The biceps femoris are important in side poses on stage for professional bodybuilders and they help shape out that round look the thigh needs to have looked-at from the side.
Mass and thorough separation are what bodybuilders should look to achieve. Every head of the muscle should be clearly visible from the back.
Other muscles of the thighs: The tensor fasciae latae, and the sartorius.
Famous bodybuilders renowned for their outstanding leg development are Flex Wheeler and Lee Priest, but before you say we should’ve mentioned Tom Platz before and above anyone else, you ought to know we left him last on purpose. Tom Platz is a special case, a bodybuilder who developed his leg muscles before anything else, it was the top priority for him. He was exactly the opposite of a typical rookie bodybuilder who puts upper-body development before everything else. He actually had to pull hard so that his upper body should reach the level of development he had already long achieved in his legs. Yes, indeed, as far as thigh development goes (especially the lower part of the quadriceps) there is definitely no equal to Tom Platz.
The exercises that can be done to achieve thigh development, are grouped according to the part of the thigh they target. Thus we’ve taken the liberty of categorizing them as follows:
Exercises for the quadriceps:

1) Squats executed with a barbell
2) Squats done till half-way through the motion
3) Squats done with the barbell held on the shoulders in the front
4) Squats done on the leg-press machine
5) Lunges executed with a barbell
6) Extensions executed on a machine

Exercises for the biceps femoris:

1) Curls executed on a machine
2) Curls done on a machine while in a standing position
3) Deadlifts done in a stiff standing position