Hamstring muscles flex the knee.  Cramps and tightness may be felt in the muscle belly.  Strains of the tendon
can cause pain on the sides of the knee. Flexing the knee and hip and be uncomfortable if not painful.  Common
in sprinters, track and field and other sports that require speed and agility.

The hamstring group of muscles, located at the back of the upper leg, are actually a group of three separate
muscles. The top of these muscles are attached to the lower part of the pelvis  and the bottom of the hamstring
muscles are attached to the lower leg bone just below the knee joint. The technical or anatomical names for
the three hamstring muscles are semimembranosus, semitendinosus and biceps femoris.

The picture below shows the muscles located at the rear of the upper right leg. The three specific hamstring
muscles can be seen on the picture, by looking for the anatomical names located half way down the right hand side.

By far the most common cause of hamstring injuries originates from an imbalance between the quadriceps muscles
(located at the front of the upper leg) and the hamstring muscles (back of upper leg). The quadriceps are a very
large, strong group of muscles which help to extend the leg. These muscles can become so strong that they
overpower the hamstrings, putting a massive amount of tension on the hamstring muscles. Combine strong
quadriceps with weak hamstrings and you have a hamstring injury waiting to happen.

Other factors which contribute to hamstring injuries are a lack of flexibility and poor strength of the hamstring muscles.
Also, when the hamstrings become fatigued or tired they are more susceptible to injuries.

The best preventative measures involve a consistent program of both stretching and strengthening exercises.
Increased flexibility will contribute greatly to the ability of the hamstring muscles to resist strains and injury.