Most efficient way to strengthen your hamstrings : hamstring curl on the ball (you can also use a chair instead of the ball)
Start with this :http://www.youtube.com/
He performs the same exercise as above very well please watch : http://www.youtube.com/watch?v=nqX6e7vN2lA
Once you are able to do 3x12 reps then you can progress to this version.
Progression : http://www.youtube.com/watch?v=czf-1snzG2c
More information about the hamstring muscles:
The hamstring is a group of three muscles forming the posterior thigh muscles: semitendinosus, semimembranosus and biceps femoris long & short head.
The hamstrings cross and act upon two joints - the hip and the knee.
Semitendinosus and semimembranosus extend the hip when the trunk is fixed; they also flex the knee and medially (inwardly) rotate the lower leg when the knee is bent.
The long head of the biceps femoris extends the hip as when beginning to walk; both short and long heads flex the knee and laterally (outwardly) rotates the lower leg when the knee is bent.
The hamstrings play a crucial role in many daily activities, such as, walking, running, jumping, and controlling some movement in the trunk. In walking, they are most important as an antagonist to the quadriceps in the deceleration of knee extension.
Prevent. Perform. Recover.
Equinox Health Clinic