The knees are the largest joints in the body. They're also one of the most commonly reported areas of injury. So put on the brakes before breaking into a sweat and consider whether or not your chosen workout has the potential to exacerbate prior injuries, especially if you've had knee problems in the past. While you needn't feel restricted to certain workouts, it's wise to be mindful of the amount exertion you use and to pay special attention to using correct form.
If you're looking to strengthen the muscles around your knees, Pilates is a great place to start. There are group classes as well as private instruction. The base of the workout is high-intensity and low-impact. Make no mistake: "Low-impact" does not mean "light workout"; Pilates can be some of the toughest training around. Expect to see noticeable changes almost immediately. Your core will get stronger, helping to keep your body in alignment (helpful for keeping proper form in other workouts). Because knee pain often comes from other imbalances in the body, Pilates can help to build symmetrical muscles while also increasing flexibility.
Perhaps you want to work out on your own before joining a fitness class or hiring a trainer. You can purchase resistance bands (Amazon has a large variety to choose from, with prices starting at $5), and turn to YouTube for your personal workout guide. The video "5 Resistance Band Exercises for Knee Pain" from Onnit Academy is a great way to start a routine that can help you feel confident about working your way back up to tougher exercises.
While many fitness experts advocate the power of squats, if you decide to engage in two- or one-legged lunges or squats, make sure to pay close attention to your body and how it reacts. If you feel even the slightest twinge of pain, do not push yourself. Ease back, and nix that from your plan for now. Pushing through the pain will hurt you immensely in the long run -- and the same goes for running. Listen to what your body is telling you. Does running downhill put stress on your knee, but jogging on a rubber track is perfectly fine? Do what feels good, and avoid what causes pain.
A great alternative to running is using an elliptical machine: a no-impact cardio workout in which you can cover long distances using both forward and backward motion.
Swimming laps is another great way to get your heart rate up without causing any harm to your body. You can begin to strengthen your hamstrings and quads without the shooting pain that might occur in an overused knee from the impact of running.
Stretching is just as important as the workout itself. Whichever stretch you're engaging in, do not push to go further into the stretch if you feel any discomfort. Instead, keep your focus on longer holds and less straining, allowing yourself to gradually ease more deeply into a position.
Yoga can be a great option for a gentle (yet intense) workout. However, classes can bring out a competitive side in all of us, so make sure you aren't trying to out-stretch your neighbor. Keep your eyes to yourself and your form. Some instructors will try to push their students to go into the stretch more, so talk to your instructor before class. Let him or her know that you need to keep it light, and they'll be happy to help you find adjustments for poses that might be too straining. In hot yoga, heated rooms (usually around 80 to 90 F, sometimes hotter) help to keep your body loose and increase flexibility. This is a pro and a con, as it can lead you to feel more flexible than you really are. Be careful, and err on the side of caution.
If yoga isn't your thing, try simple stretches at home. For a standing hamstring stretch, stay balanced by keeping your back to the wall or lightly holding the back of a chair with one hand. Or try a sitting hamstring stretch while watching TV. Fully extend one leg on the couch, toes pointed upward, while keeping the other leg down, foot on the ground. Lean toward the outstretched leg until you feel it in your hamstrings. Keep your core engaged to keep your back straight.
Another good option is the leaning wall calf stretch: One foot is closer to the wall; one foot is back. Both legs are straight. Tilt your upper body into the wall, and land your hands on it before alternating sides.
For more information on gentle stretches, as well as visual aids, see "The 7 Best Stretches for Knee Pain" on Self.com.