The hamstrings are one of those muscle groups that get under-worked due to lack of knowledge.  Most people assume they ONLY perform knee flexion. So they do all the gym machines that flex the knee and think they have successfully worked them out.
WRONG.
The hamstrings are complex. So I have a few points to make. This will rock your world. If you read these 4 points and start applying them to your hamstring workouts, you will notice a huge difference in muscle development, I promise!
Point #1 They also perform hip extension (bring the leg backward). So if you want to work the hamstrings in their shortest position, you must do hip extension + knee flexion. Â This means that if you have poor quadricep flexibility, you won’t be able to hit the hamstrings to their full potential. So work on quadricep stretches please! (Our Limber100 program has some of the top 100 yoga poses for flexibility, with a quad lengthening program as well! You can get it in the shop)
Point #2 They require a high load aka a whole lotta weight with low reps (<10) for maximal growth. So crank up the weight people!
Point #3 They are made up of 3 muscles: semitendinosus (medial), semimembranosus (medial), and biceps femoris (lateral with long & short head). So to hit them all, you want to do some of the exercises with hip internal rotation (feet inward), some with hip external rotation (feet outward), and some with the hips in neutral.
Point #4 To lift heavier, it is best to dorsiflex (flex) the ankles during the concentric movement and plantar flex (point) during the eccentric movement. Why? Because the gastrocs cross the knee joint but the fibers are so short that they struggle to do knee flexion and plantar flexion at the same time. They like to do just one at a time. So you want to strengthen the hamstrings using as much weight as possible, right? So knowing that the gastrocs are stronger with knee flexion when the ankle is dorsiflexed, you should do the concentric hamstring curl with dorsiflexion so that the gastrocs can help the hamstrings lift a heavier load. Then during the eccentric return, you should plantarflex so the hamstrings are more isolated and have to slowly return on their own.  So an easy way to remember: Bend & Flex. See the video at the end of this post for an example.
[this concept can be reversed if your goal is to lift lighter but isolate the hamstrings on the concentric motion]
Here are some of my favorite Hamstring Exercises:
- Standing Leg Curls machine
- Seated Leg Curls machine
- Barbell Good Mornings
- Reverse Hyperextension Bench holding weight
- Stiff leg Deadlifts
- Romanian Deadlifts
- Lying Leg Curls (see video below)
[youtube]http://www.youtube.com/watch?v=hlk-gO_GQqM[/youtube]