One of the main muscles that works directly on the knees is the quadriceps.
Opposing the muscles at the back of the knee joint is the hamstrings.
Another set of muscles that works on the knees are the calf muscles, particularly the gastrocnemius (though the soleus can affect the knee indirectly by helping to anchor the fibula).
When you activate your knees, making them feel strong or "active", you are generally activating some combination of these muscles.
If you are activating your knee while keeping it in the same position, i.e. not bending or straightening it, then you are more than likely using your quadriceps against your hamstrings or against your calf muscles (or some combination of both).
As part of your knee strengthening regime, try activating your calves in standing poses to see if calf activation makes your knees feel stronger or less subject to pain or both. Activate your calves to create a downward pull on your fibula (the smaller of the two lower leg bones), and to create even tension in the two hamstring tendons at the back of the knees.
Other muscles that work on the knee include the IT band muscles (tensor fascia latae and the superficial fibers of the gluteus maximus) and the pes anserinus muscles (sartorius, gracilis and semitendinosus.)
These latter muscles can work to stabilize the shin against rotation. For these muscles to activate effectively you'll need either to stabilize your hip bone or make your foot, ankle and lower leg bones rotationally stable.
A simple knee strengthening exercise that I sometimes use as a warm up in my yoga classes is to lift the heels while standing and then focus on making the knees feel strong. You could also think of this as "squeezing" the knees.
Try to make them feel strong at both the back and the front. Do this with the knees straight.
This same activation can be done while doing a standing forward bend.
With feet flat on the floor straighten your knees and make them feel strong at the front and the back. You can place your hands on the floor (or if you can't reach the floor, on chair or yoga blocks) or you can let your arms hand or reach them up behind you.
For a slightly different feel, try it with the heels lifted. Have your hands on the floor for balance or try pressing evenly through toes and forefeet for balancing without using your hands.
Another knee strengthening exercise while bending forwards is to lift the fronts of your feet so that you are on your heels. Here again make your knees straight (or straighter) after lifting your forefeet, and then make your knees feel strong.
The Difference Between Active and Straight
Straightening the knees and then making them feel “strong” or “active” can be two different and distinct steps. Straightening the knees doesn’t necessarily make them feel strong. Likewise, you don’t have to have the knees straight in order to activate them or make them feel strong.
The Calf Muscle that Affects the Knee
Lifting the fronts of the feet while in a forward bend can be used as a calf stretch but it can also be helpful for strengthening the knee since one of the calf muscles, the gastrocnemius, crosses the back of the knee.
Lifting the front of the foot adds tension to this muscle by lengthening it and that can add tension to the back of the knee.
Likewise, lifting the heels adds tension to the back of the knee but it does so by causing the calf muscle to activate.
Chair pose can also be used as a knee strengthening exercise. You can work towards doing it with your thighs level.
Start with your thighs at about 45 degrees. Your knees will automatically feel strong or active in this position but experiment with making them feel stronger.
Alternatively, work at making the fronts of your knees feel strong. Then work at making the back of your knees feel strong (or in the case of this yoga pose, stronger.)
Again, because the calf affects the back of the knee you can also try squeezing the calf while doing chair pose. I’d suggest do it at the same time as you are activating your hamstrings.
Squatting (The Deep Squat) and Kneeling
Two poses that are related because in them the back of the knee is closed is the deep squat and kneeling. Both can be used as knee strengthening exercises.
If you have trouble with kneeling or squatting I’d suggest working on kneeling first since it’s easier to control how much pressure you put on your knees. In either pose practice activating your knees and relaxing them.
The video below also includes a helpful tip for strengthening the knee while doing an exercise called Seated get up. At about the 5 minute mark is a muscle activation that I've found very helpful in making my knee stable, even in a pose like a one legged squat (which is basically what the seated get up is.)
Once you can kneel comfortably, work towards a deep squat. If you have trouble do it near a wall or door or pole so that you can hold on or use the support to help balance.
To make it easier to work towards squat, first find a foot position and pelvis position which allows your hips and knees and feet to be comfortable. Once you find that foot position (wider, closer together, turned out to varying degrees or parallel) practice activating and relaxing your knees. Also try activating the front of your knees and then your back (individually.)
As you practice the deep squat, work at expanding your foot position possibilities. You may have to explore hip muscle activation at the same time. But in either case work towards the ability to squat with feet closer and perhaps even with feet parallel.
Other variations of the deep squat include shifting your weight to one foot.
Here again you can practice strengthening the knee by focusing on the leg that is bearing your weight.
I’d suggest shift weight first. Do it slowly and gradually so that you can stop if you feel any discomfort or pain.
With weight on your target foot, then squeeze the knee. Try activating the front of the knee, then try activating the back. You can also try activating both together. Relax and then return to center or repeat a few times and then work on the other leg.
Balancing On One Leg to Strengthen the Knees
Standing on one leg balance poses can also be used as knee strengthening exercises.
Another knee strengthening exercise that exercises the knees in a slightly different configuration is standing side bend.
With feet hip width push your hips to the left and bend your spine to the right. Optionally reach your hands past your ears to the right (or up and to the right.)
Shift your body so that weight is even on both feet. Then squeeze your knees. Because the hips are pushed to the side and the legs are angled, you may find it helpful to activate one knee at a time, the inside knee, then the outside knee.
Practice the knees individually, then practice activating them together.
Another yoga pose that you can use as a standing knee strengthening exercise is Standing spinal twist (above right.)
With feet hip width and parallel turn your pelvis to the right, then turn your ribcage relative to your pelvis. Hold this position and and practice activating (and then relaxing) your knees. Here again you can try one leg at a time initially. Then try both knees simultaneously.
Two more of my favorite yoga poses that can be used as the framework for knee strengthening exercises are front triangle and triangle twist.
Both use the same foot position, one foot forwards the other back, with the back foot turned slightly out.
With hips square to the front foot bent forwards. Activate the front knee. Make it feel strong. For balance make your front foot feel strong. Then if you like lift your hands.
For triangle twist keep the foot position from front triangle.
With your right foot forwards place your left hand on the floor, lift your right hand and twist to the right. For this one you can keep your hips level, or push the hips back, or push them forwards. Activate and then release your front knee or if you like both knees. Do it with your breath. This can apply to all the exercises. Inhale as you engage the knee (or knees) and exhale as you relax the knees.
More knee strengthening exercises
For more on knee strengthening, check out these pages:
Strengthen your knees intelligently by learning to feel and control the muscles that work on them
To learn more about knee control, in particular by learning to feel and control the muscles that act on them, check out the following courses.
"Lessons in Muscle Control for Your Hip Flexors" focuses on the superficial hip flexors, the hip flexors that attach to your ASIC and that attach to the lower leg bones. It teaches you how to activate and relax these muscles and also how to anchor them for better control in a variety of positions.
The superficial hip flexors include the sartorius and the tensor fascia latae. These muscles both help to control knee rotation. Another superficial hip flexor is the rectus femoris.
"Lessons in Muscle Control for Your Hamstrings" focuses on the hamstrings and glutes. It teaches you how to activate and relax these muscles and also how to anchor them for better control in a variety of positions. As with the superficial hip flexors, some of the hamstrings help to control knee rotation.
"Lessons in Muscle Control for Your Thigh muscles" focuses on the muscles at the front, back, outside and inside of the thighs. You'll learn how to activate the vastus muscles as well as the adductors. Control of these muscles may make it easier in some positions to control the hip flexors and the hamstrings.
Mental models are created or modified whenever we learn. They drive habits, intuition and muscle memory. This understanding can be the basis for reducing frustration and making learning, problem solving and doing easier.