Training hamstrings is unlike training any other muscle group because it requires extremely fine tuned technique. It's not merely a matter of push and pull like some other muscle groups. First off, the hamstrings attach at both the hip and the knee. Therefore, doing merely leg curls will not give you the hamstrings that one desires nor will doing only stiff-leg or Romanian deadlifts. So if that is all you are doing for your current hamstring routine, discard it immediately if you want to make any type of progress. It is a good idea to warm up the entire body for a few minutes, preferably on the bike and then follow this up with some stretching. I make warming up and stretching a habit before every training session and so should you. You'll notice a difference in your ability to train immediately. After you're finished warming up, proceed to the lying leg curl machine. You have the option of doing lying leg curls with your feet either together or apart to better emphasize different parts of your hamstrings. Keeping your feet together will emphasize the inner hamstrings while keeping them apart will better stress the outer portion. If you don't believe me, give it a try.
For the sake of confusion, let us begin with feet together because this is more comfortable for most people. Begin with a light set of 15 reps and follow this up with some stretching and flexing of the hamstrings. Next increase the weight slightly and perform another set for easy reps and follow this up with some more stretching and flexing. Now increase the weight even more and perform another 15 reps once again followed by some stretching and flexing. Why is it important to stretch and flex the working muscle? Well, I like doing it because I am able to force blood into the targeted area and thus better "feel" the working muscle. After all, what's motion without emotion? By this point, your hamstrings should be nice and pumped. Now you are ready to work! Adjust the weight to where you can perform with a full range of motion about 15 reps. Afterwards, you will stretch and flex the hamstrings even more. Now add just enough resistance to where you can perform about 12 reps with a full range of motion.
If you are training with the correct intensely, the amount of weight is generally around 10-20 lbs for the average person. Now perform a third and final set using a slightly higher weight for about 10 reps. By this point, your hamstrings should feel as if they do not exist. Don't fret because this is what you want. Now proceed to the dumbbell rack and prepare yourself for Romanian deadlifts. I like performing Romanian deadlifts better than stiff-leg deadlifts because the latter places unnecessary stress on my knees. Also, after a curling motion such as the lying leg curl, I believe it is a good idea to perform a stretching motion which will aid in developing the hamstrings. The trick to performing Romanian deadlifts correctly is to keep your back tight and arched throughout the entire motion and to keep a slight bend in your knees. When you are at the top of the movement, just lower the dumbbells along your body while sticking your glutes out. When you feel the hamstrings stretch, then pull back up with your hamstrings and glutes. Once you learn how to do these correctly, you are in for some serious hamstring development.
Your target rep scheme for the first set is about 15 reps. Grab a pair of dumbbells that will enable you to perform with good form about 15 reps. Afterwards the poundages to where you can only perform about 12 reps. If done at the correct intensity level, this is generally about 10-15 lbs per dumbbell. Now you will do a third and final set with even heavier dumbbells for about 8-10 reps. The trick to this movement is to keep almost perfect form. If your form sucks, and be extremely critical about yourself, drop the weight and get the form down or else you will hurt yourself.
After only 2 exercises, you will feel as if you are about to pass out but you aren't finished yet. You head back over to the lying leg curl machine for a final 2-3 sets. I generally like to perform only 2 sets at the end because I am completely exhausted and my hamstrings feel as if they do not exist. You will perform lying leg curls with your feet positioned apart. You will have to experiment a bit to find where your strength levels stand at this point. Don't be surprised if you have very little strength left. I generally aim for straight sets of 10-15 reps. If I can get 15 reps on the first set, I will increase the weight just a bit so I can reach total failure at about 10-12 reps. If for my first set I can perform 20+ reps, then I will merely treat that set as a "warm up" and perform 2 more sets afterwards. Now you are finished with hamstring training but for how long? Well, it all depends on your training split and if you ever feel like torturing yourself like this again in the near future. Since the choice is yours, I would advise that you choose wisely!
Warm up on bike 5 min. followed by stretching
Lying leg curl(feet together) 1 x 15 x 15 x 15 (warm up)
Lying leg curl(feet together) 1 x 15 x 12 x 10
Romanian deadlifts 1 x 15 x 12 x 10
Lying leg curl(feet apart) 1 x 15 x 12
Pass out from exhaustion!