Some of the major muscles of the thigh on both the anterior (the quadriceps femoris group), and posterior (the hamstring group) sides are two-jointed muscles. In essence, they have an action at the knee joint and an action at the hip joint.
Thus to develop these muscles fully you must stretch and stabilize one end so that the other end is capable of contracting with greater force. The thigh muscles also have combined actions. Because of this it is necessary to work them in more than one action and to take the other actions into consideration when doing various exercises.
You should also not ignore the muscles on the inside and outside of the thigh since they work together with the anterior and posterior thigh muscles. Because of this, how you do the exercises for the thigh is very important.
When doing thigh exercises it is very important to protect the knee joint during execution of the exercises. For example, if you begin leg extensions with the knee bent greatly, the pressure on the knee joint is very high. If done for extended periods of time this exercise can irritate the cartilage of the knee bones and cause damage.
In addition, some exercises such as the squat, are heavily criticized. Now the major majority opinion is in favor of squats. It is one of the best exercises for the knee joint.
It should also be noted that some individuals have slightly different insertions of thigh muscles which allows them to use the muscle in different ways.
For example, certain muscles in some people assist in knee flexion while on other people they assist in knee extension. This may explain why some bodybuilders and athletes experience greater or more rapid growth.
The anterior thigh
The muscles on the anterior thigh consist mainly of the quadriceps femoris muscle group. These muscles consist of the rectus femoris, a straight (vertical) muscle crossing the hip joint and the knee joint. It has an action of hip flexion and knee extension. The remaining three muscles are vasti muscles, more specifically the vastus medialis, lateralis and intermedius.
One of the major functions of the vastus medialis is to maintain the patella in its groove, and to conteract the vastus lateralis which pulls the patella to the outside. The two muscles balances each other so that the patella stays in its groove.
When there is an imbalance in these muscles the patella is pulled to either side causing irritation of the cartilage and eventually the possibility of chondromallacea. The vastus intermedius does not seem to have any distinct role. But, all of these muscles act in concert to achieve a common end (leg extension).
Because of the oblique fibers of the vastus medialis and lateralis, they play an important role throughout the entire range of knee extension. However, they are not very powerful in the last 15 degrees of motion. At this time, their major role is to keep the patella in its groove, i.e., not allow the patella to move out to the side. Also, the vasti muscles come into play at different phases of leg extension.
Anterior thigh exercises.
The squat.
One of the best exercises to strengthen the quadriceps femoris muscle group is the squat. To most effectively and safely execute this exercise, it is important that your feet remain flat on the ground and the spine in the neutral (slightly arched) position in all phases of the movements. The trunk should incline forward approximately 45 degrees at the lowest point in the descent.
This positioning provides the greatest balance both of the body and the forces acting on the various joints. Keep in mind that there are three joints involved which act as a safety valve for each other. They all assume a portion of the loads which are modified by your trunk and leg actions.
You can lower yourself to the thigh level position or lower as long as you maintain the slightly arched position of the lumbar spine. If you begin to loose the arch (lower spine becomes rounded) it creates a highly dangerous situation. You should avoid the rounded position at all costs. Thus, the correct depth of your squat is determined by your ability to maintain the slightly arched position, while keeping the feet flat on the floor.
It should be mentioned here that if you go sufficiently deep you will also involve the hamstring and gluteus maximus muscles. They usually come into play at approximately the thigh level position or slightly below. However, since the quadriceps is most active in the last 35 degrees of motion it is not necessary to go into a very deep squat for strength and mass.
The leverage in the knee joint when you are in a deep squat is poor and the muscles cannot work very effectively in raising you. But as the leg and trunk straightens the quadriceps becomes much stronger. Thus, working in the last 90 or less degrees of motion is usually most effective.
Squat variants.
Working through 90 degrees of motion is especially important if you do exercises such as the leg press, Hack squat or Smith machine squat. In each of these variants of the squat, it is important that you do not exceed the 90 degree angle in the knee. Going much beyond can be injurious not only to the knees, but also to the spine.
Also, when doing the Hack or Smith machine squats it is very important that you place the feet approximately 12 inches in front of the body. This positioning is needed so that when you reach the bottom position there will be a right angle in the knees and in the hips.
If the feet are placed directly under the hips as in the squat, the amount of flexion in the knees can be excessive and cause injury. In addition, it requires exceptional amounts of flexibility in the hip joint in order to do these squat variants safely. This flexibility is not possessed by many individuals.
This is part one of the full article which was first published in Muscle and Fitness in the 1980s.
For more information see Biomechanics and Kinesiology of Exercise