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Lunges are are the perfect exercise for stretching and strengthening the muscles and ligaments in the knee, including the MCL. By strengthening this entire area of your knee, you put less strain on your MCL. Try doing lunges regularly to keep your MCL healthy.  To perform a lunge, stand with your feet slightly wider than shoulder-width apart. Keep your back straight, your tummy tucked in and your eyes looking straight ahead. Take a big step forward with your right foot, bending the knee until your thigh is parallel with the floor. The knee should not extend past the tips of your toes. Bend the back knee until it is almost (but not quite) touching the floor. Hold this position for 10 seconds, then return to the starting position and repeat with the other leg. Leg curls are another great exercise for strengthening both the MCL and hamstrings (a set of four muscles at the back of the thigh which are essential for flexing the knee). These exercises are even more effective if you place a resistance band or ankle weight around your ankles as you perform them. Here's the correct way to do them:  Lie face down on an exercise mat, with your legs stretched out behind you. Place an ankle weight or resistance band around your ankle, then slowly lift up your right foot until your heel touches your behind. Slowly lower your right foot back to the floor. Remember to keep your hips pressed into the mat at all times — if you raise them, you risk straining your knee instead of strengthening it. Repeat the exercise for three sets of 10 to 20 repetitions, then do the same with the opposite leg. Pillow squats (exactly what they sound like — squats with a pillow between your knees) help to strengthen the MCL, along with a number of other muscles and ligaments in the inner knee. Strengthening the muscles and ligaments surrounding the MCL is important since they are basically the MCL support system. Squats target the highest number of muscles and ligaments in a single exercise, so don't forget to include them in your workout.  Stand up straight and place a thick pillow between your knees. Thicker pillows work better since using a thin pillow will significantly increase the difficulty level of the exercise. You could even use a medicine ball instead, if you prefer. Stand with your feet about hip-distance apart, keeping your back straight, your tummy tucked in and your eyes looking straight ahead. Bend your knees as if you are about to sit in an invisible chair, sticking your butt out behind you. You don't need to do a full squat (with your thighs are parallel to the floor). You can just do a half-squat to reap MCL-strengthening benefits. Slowly return to the starting position, then repeat the exercise for three sets of 10 reps. Allow one minute of rest time between each set — rest time is necessary for optimum muscle strengthening and growth. When performing leg strengthening exercises, it's important to exercise the entire leg throughout your routine (not just specific parts).  If you only focus on specific areas, you could be left with a weak area. Calf exercises help to strengthen the MCL while also building the calf muscles, which support the knee and help to relieve pressure on the joints and ligaments.  Stand with your feet shoulder-width apart. Make sure that you have something to grab onto for balance like an exercise bar or the back of a chair. Lift both heels off the ground, rising up onto the balls of your feet as high as you can go. Then slowly lower your heels, flattening your feet back onto the ground. Repeat this exercise for three sets of 15 to 20 repetitions. Sumo deadlifts train the muscles and ligaments in the inner knee, including the MCL and the VMO (the vastus medialis obliquus, a teardrop-shaped muscle that helps to stabilize the knee). The VMO helps to disperse stress within the kneecap, which is essential for preventing injury to the MCL.  Place your feet wider than your hips. Turn your toes out at a 45 degree angle — this is very important in order to make the exercise as effective as possible. Keep your back straight, your shoulders back and your stomach muscles engaged. This will help you to maintain good posture as you complete the exercise. Lower yourself by bending your knees and pushing your butt backwards, as if you were using it to close a door behind you. Slide your hands down along your thighs as you descend. When your hands reach your kneecaps, stop and slowly return to the starting position. Repeat for three sets of 10 repetitions, resting for one full minute between each set. Contracting the muscles in your quadriceps (the muscles at the front of your thighs) engages the muscles and ligaments in the knee. This helps to strengthen the MCL. Building up the quads themselves is also a good idea, since this will provide more support for the knee joints.  Lie down on an exercise mat on your back, extending your legs fully. If your knees need some extra support, place a rolled up towel or piece of foam underneath them. Contract (or squeeze) the muscles in your quads and hold for ten seconds before relaxing. Do 10 to 20 contractions, relaxing for thee seconds between each. Hip abductions are another great exercise for strengthening the MCL. Having strong, flexible hips will help absorb some of the impact while running. It also takes pressure off the knees.  Stand with your feet shoulder-width apart, holding onto the back of a chair for balance. Place an ankle weight or resistance band around your right ankle. Slowly lift your right foot off the floor and extend it out to the side, as high as your leg will go. Return to the starting position and repeat for three sets of 10 to 20 repetitions, before repeating on the other leg. Most gyms are equipped with a leg press machine. This machine is excellent for strengthening the muscles surrounding the knees, including the VMO. It also helps to provide a sturdy support system for the MCL.  Sit into the machine, lying back against the seat (which should recline at a 30 degree angle). Place your feet on the vertical plane in front of you. Adjust the weight according to your strength level, then use your feet to push the weight away from your body by extending your legs. Don't lock your knees when you extend your legs. Try to keep them slightly bent. Slowly return to the starting position, then repeat the exercise for two or three sets of 10 to 20 repetitions. Rest for a full minute between each set. Exercising with a loop band is a great way to engage the MCL. The band creates pressure against the outside of the knee, which forces the MCL to push the knee outward. This recreates the type of stress the MCL is exposed to during normal physical activity.  Place the loop band around your ankles and stand with your feet hip-width apart, toes pointing forwards. Keep your back straight and your knees slightly bent. Take five sideways steps towards the right, using your hips and maintaining your stance as much as possible. Now take five sideways steps towards the left. This is one full set. Do three sets of this exercise, resting for one full minute between each set.
Perform lunges. Do leg curls. Try pillow squats. Perform calf exercises. Give sumo deadlifts a try. Practice contracting your quads. Do hip abductions. Use the leg press machine. Exercise with a loop band.