1.  Quadriceps ("quads") located in front upper portion of the of the leg, from the hip joint down to the knee joint.

2.  Gluteus ("glutes") located at the back of the pelvis and top of the femur (upper leg bone). 

3.  Hamstrings ("hams") located at the upper rear portion of the leg, from the hip joint down to the knee.

4.  Calves -  Located at the rear lower portion of the leg, from the rear of the knee down to the ankle.

Each muscle plays a specific role in the movement of either the ankle, hip or knee joint.  Now we will take a closer look at the anatomy of each of the major leg muscles. 

Quadriceps

Point of Origin:  Illium (pelvis), femur (upper leg bone)

Insertion:  Tibia (lower leg bone)

Primary Action:  Knee extension, some hip flexion degree (rectus femoris only)

The quadriceps are comprised of the rectus femoris, the vastus lateralis, vastus intermedus, and vastus medialis.  When extending your knee and contract the quadriceps muscle, you can usually see the outline of the vastus medialis (on the inside of the leg), and the vastus lateralis (on the outside of the leg). 

Like the gluteus, the quadriceps are a strong powerful group of muscles.  Their primary role is to extend the knee joint, straightening the leg.  The rectus femoris, which runs up the middle of the quadriceps connects to the pelvis, and is the only quadriceps muscle responsible for both knee extension, and hip flexion.

The quadriceps are usually trained using a combination of compound and isolation exercises like leg extensions, and leg presses.  Any movement that involves straightening the leg will rely on the quadriceps muscle to a large degree.

Exercises:  Leg extensions, squats, leg press, bent legged dead-lift, lunges

Gluteus Maximus

Point of Origin:  Posterior portion of illium (pelvis), sacrum, coccyx

Insertion:  Femur (upper leg bone)

Primary Action:  Hip extension, rotation and abduction

The gluteus maximus, medius and minimus muscles make up what are know as the "glutes".  The glutes are a very powerful muscle, and are in fact the largest muscle in the human body.  The gluteus muscles are responsible for movement of the hip joint, and help make the hip a very strong, stable joint.

The gluteus muscle contract powerfully during any exercise that extends the hip joint, including leg presses, squats, jumps and sprints.  Although the gluteus muscles can be isolated using some machines, they are primarily targeted with compound exercises that also involve the quadriceps muscles.

Exercises:  Squats, leg extension, bent legged dead-lift, lunges

Hamstrings

Point of Origin:  Back of pelvis, sacrum, and lumbar and thoracic vertebrae

Insertion:  Humerous (upper arm bone)

Primary Action:  Adduction, extension, internal rotation, and horizontal abduction of the shoulder joint.

The latissimus dorsi muscle is the largest of all of the upper back muscles.  It is what forms the "V" shape in a well developed back.  It originates in the lower to middle back, and attaches to the upper arm bone.  Any exercises that involve the arms being pulled down bring the "lats" in to a powerful contraction. 

Since most back exercises bring the lats in to play, they are relatively easy to target.  Although exercises like pull-downs and pull-ups isolate the lats the most, they also contract powerfully in any seated or bent over rowing exercise.

Exercises:  Lat pulls, lat pull-downs, pull-ups, barbell pull-overs, barbell or cable rows.

Calves

Point of Origin:  The gastrocnemius originates at the rear lower femur bone.  The soleus originates at the fibula and tibula (lower leg bones)

Insertion:  Heel bone

Primary Action:  Ankle flexion, knee flexion

The calves consist of the gastrocnemus, and lying underneath, the smaller soleus muscle.  The gastrocnemius is what gives well developed calves their "horse shoe" appearance.  While both muscles are primarily responsible for flexing the ankle joint, the gastrocnemius also helps flex the knee to a lesser degree.   

The calves are relatively easy to train, since there are only a handful of exercises that isolate them.  They are normally targeted with isolation exercises  like seated heel raises, which target the soleus muscle, or standing heel raises which target the gastrocnemius muscle.  Any exercise or movement that involves raising your heel off of the ground will involve the calf muscles.

Some bodybuilders believe that if you weren't born with well developed calf muscles, you will never have well developed calf muscles.  This is not entirely true however.  Although your calves may never look like Mike Matarazzo's, training them hard will make them grow.

Exercises:  Standing heel raises, seated heel raises

The Best Weight Training Exercises for the Legs 

There are many different options when it comes to training the legs, since you will be targeting four major muscles.  I suggest you focus primarily on compound exercises as opposed to isolation exercises.

Here are the top muscle building exercises for the legs: 

• Squats - The "king" of all leg exercises.  Primarily targets the quadriceps and gluteals.  Requires excellent balance, as well as core strength.
  

• Lunges - Targets the quadriceps, gluteals, and hamstrings to a lesser extent.
  

• Leg presses - An intense exercise primarily focusing on the quadriceps and gluteuls.  Excellent substitute for squats or lunges.
  

• Stiff legged dead-lifts - The stiff-legged deadlift is an excellent hamstring, and is your best bet if a leg curl machine is unavailable.
  

• Lying leg curls - An isolation exercise for the hamstring muscles.
  

• Standing heel raises - A calf exercise, primarily targeting the larger gastrocnemius muscle.
  

• Seated heel raises - A calf exercise, primarily targeting the smaller soleus muscle.