After focusing on single joint muscles of the hip, another way to experience your anatomy is to focus on contracting single joint muscles of the knee joint. However, there's a slight problem in that the single joint muscles at the front of the knee are quite a lot larger than those at the back.
The single joint muscles at the front of the knee are the three vastus muscles:
The single joint muscles at the back of the knee are:
Because the posterior single joint knee muscles are so small, I practice activating the multi-joint hamstring muscles at the back of the thigh along with the three vastus muscles at the front of the knee to help stabilize the knee joint.
The multi-joint hamstring muscles are
The feeling of this activation is a squeezing sensation at the front of the legs and an upward pulling sensation at the back of the legs.
In both cases, the sensation is just above the knee joints.
When standing on both feet with both knees bent the quadriceps work against the weight of the body to stabilize the knee joints. This may be one reason why the single joint muscles at the back of the knee are relatively small.
However, when standing with knees straight, the single joint muscles at the back of the knee may not be able to generate enough force to stabilize the knee against the quadriceps. For this reason it seems reasonable to use the hamstrings. The interesting thing is, when activating the hamstrings against the quadriceps in a straight knee standing position, the tension feels like it is isolated around the knee joint.
Ironically enough I rarely used to focus on activating my quadriceps in forward bends because they didn't flex the hips. I thought it more important to focus on the single joint hip flexors. However, activating both quads and hamstrings at the knee together seems to make hamstring stretches easier.
Learning to squeeze the front and back of the knees simultaneously offers another way to get around tight hamstrings. I found that when I squeezed the knees and then released, in time with my breath, or just slowly and rhythmically, my forward bends were a little more comfortable.
Another interesting aspect of activating the knee joint muscles, particularly the quadriceps, is that they can be used to rotate the femur at the knee joint.
Usually knee rotation is talked about as the tibia and fibular moving with respect to the femur. I found that while standing on one or both legs with knees bent, I could use the quadriceps to rotate the femur on top of the tibia. The action was slight but perceptable.
I used it to make my knee joint feel more comfortable. At the same time I was also controlling my ankle joint and my hip joint of the knee in question.
This can be a lot to juggle around with and so what I normally like to do is to activate my foot and ankle, then adjust my knee via my quads, and then adjust the hip. However, sometimes I get a better feel if I do my foot and ankle first, then my hip and then my knee. And then I repeat the adjustments so that I gradually move towards making my whole leg, ankle, knee and hip feel comfortable.
(The hamstrings can also be used to rotate the knee joint.)
If I'm standing with feet about hip width apart, parallel, and knees straight and then activate knee muscles, both front and back, I find that my legs want to rotate, particularly when I focus on even tension at either side of the back of the knee.
I'm guessing that the pull of the two sets of hamstrings (semitendinosus and semimembranosus on the inside and biceps femoris on the outside) is the cause of this feeling. I find that it's easier to equalize tension on inside and outside of the back of the knees if I stand with feet slightly outwardly rotated.
In any standing forward bend, I find that if I "squeeze the knees", lengthen neck and open chest, and then relax, I can get deeper into the forward bend. Squeezing the front and the back of the knee seems to give the hamstrings a better foundation to support the pelvis when lengthening the spine. Then when relaxing the spine the hamstrings also relax, allowing them to lengthen.
To standing forward bends that you can practice this is are wide leg standing forward bend and triangle forward bend
As an example, in triangle forward bend, you can
Then repeat. To add weight and strengthen the hamstrings, try reaching the arms to the sides after lenghtening the neck and opening the chest. Then try reaching them to the front.
For myself, each time I relax I find I can bend further forwards. It's as if after the work of supporting the upper body, my hamstrings want to relax.
Another yoga pose in which you can experiment with squeezing the knee joint muscles is chair pose. It's an alternative to focusing on the single joint muscles of the hip when doing this yoga pose.
One yoga pose where I found that squeezing the knees wasn't effective was wheel pose. For this backbending yoga pose, I found I got a better feeling when I focused on using my inner thighs to help press my pelvis upwards.
In terms of myofascial meridians, the three single joint anterior knee muscles are part of the superficial front line which connect to the abdominals. Meanwhile the hamstrings are part of the superficial back line which connect the spinal erectors.