I want to address something that, whether you're active or not, is going to affect you at some point or another: knee pain.
Even if you're not particularly active, think about how you move through life. How do you usually get out of a chair? How do you walk up the stairs? I address this a little in my blog post on squatting. We do a lot of things loaded up on our toes. As such, we are giving our quads little workouts all over the place, and largely ignoring our hamstrings.
This is even more evident when someone starts working out, or takes up a sport. Because we're already so used to having our weight forward, we automatically learn new movements with our weight forward.
Try to Get Some Balance Back
Before you head to the doctor or physical therapist, I challenge you to try some exercises to strengthen the hamstrings, and try to change some of your daily movements to reinforce them. Instead of spending money on appointments, therapies, and medications, spend some time gaining balance and naturally correcting the knees.
Start this one without weight and make sure you can feel the hamstrings working first. Put your hand behind your head (like many do for crunches). Shift your weight way back into the heels. Keeping your back in its neutral S curve and a slight bend in the knees, push your hips back as far as you can without falling over, and bend at the hips.
When you bring your back just past parallel, or you can't maintain your back S curve, you should feel a stretch in your hamstrings. If you can't, shift your body weight and hips back further, and check the curve of your lower back. Don't blame flexibility, I used to be a dancer and and can feel it at this point.
Once you feel that stretch, really focus on squeezing those hamstrings to pull your body back up to standing. Do a few like this, without weight, to get used to the movement and feeling.
To add weight, hold onto anything, heavy books, jugs of water, etc. Hold your weight in front of your legs, and as you bend forward (with the hips pushed back!) the weight should just skim along the front of your legs. I want the weight in front over the feet, not clasped as the chest, so that the work is focused on the hamstrings and not on lower back leverage (that's another workout entirely).
As you get used to feeling the hamstrings working, start to focus on using them in other situations. When you stand up out of a chair or climb stairs, drive through the heels and think about squeezing the hamstrings the extend the hips. If you're squatting down, say putting things away in a cupboard, try to put those heels down early and drive through them to finish standing up.
Of course, the longer the knee pain has been going on, the longer it might take for you to regain the muscular balance needed to reduce the pain. Even if you decide to take a medical intervention route, still try to engage the hamstrings more as adjunct therapy.
Some Other Things You Can Do
Reverse Hyper - You lay your chest on the pad, ankles between the rollers. You use your glutes and hamstrings to pull up on the swing arm, and try to pause at the top without arching your back.
As the name implies, you hit can hit the hamstrings with it.
Variation One- Adjust the pad so that with the feet between the rollers, the pads hit your thigh just below your hip. Hang your body down, keep your back tight and in a neutral S curve, and raise your body up to parallel by pulling against the rollers with your heels. The shape of your back shouldn't change during any point.
Variation Two - Much harder. Adjust the pad so that your knees are just on the inside of the curve's peak. Starting with your body parallel, pull with your heels and raise your body from the knees up to a kneeling position.