First, is anyone else wondering why they call it a hamstring? One possible reason, according to Wikipedia, is that legs of ham used to be hung by a hook through the space between the thighbone and the tendons behind the knee, –the hamstrings. Whatever. In the Los Angeles Times, exercise guru Karen Voight shows us an excellent exercise for "training" our hamstrings without putting much strain on our knees.
Start on all fours. Place your elbows on the floor under your shoulders, palms facing down. Balance on your right knee, making sure it's directly below your right hip. Straighten your left leg behind you and raise it up to hip level. Press out through your left heel and flex your foot. Keep your abdominals pulled in toward your spine and avoid over-arching your lower back. Move your right foot a little to the left. Slowly start to bend your left knee, bringing it down just to the outside of your right calf. Pause for 2 seconds with your knees close together. Straighten your left leg out and repeat. Perform 10 to 20 repetitions on each leg.