Seiza:
Head upright
Neck elongated
Shoulders down
Spine straight
Chest open and relaxed
Hip flexed
Knees flexed
Ankles straight.
Seiza, and it's Yoga counterpart the Diamond Pose or Vijrasana can be part of a healthy body and power development. This exercise is the foundation of power from structure - balance, stability, relaxation, and support.
That doesn't mean that it is automatically a healthy exercise for everyone. Wikipedia has two articles from the New England Journal of Medicine on the potentially injurious aspects of Yoga, and the Diamond Pose is one of them.
I am bow legged and active, and I like to eat, so I have a number of knee problems. I have been told to lose weight, but it is difficult for me to lose when my favorite cardiovascular exercise (running) is painful. Swimming is not a substitute, as the Whip Kick can aggravate knee issues. This little bit of Yoga helped me, but get up with someone who is a Yoga teacher. There are many great resources out there on Yoga. I am advocating the method more than the Art.
In Aikido and Japanese martial arts, we have a relatively macho approach - "do it until it feels good." Yoga has an approach that I find much more healthy - each of the Asanas has easier and more advanced variations, preliminary exercises that can lead to the development of the Asana, and increasingly the use of props for support. The quality of Yoga teachers varies widely (as of course does the quality of martial arts teachers) but efforts are made to define the Asana in concrete, healthful, physical terms. Martial artists do this for striking and throwing, but seldom for meditation postures.
For a stiff back:
Laying on the floor is a great place to start. A blanket or Yoga mat underneath you, and maybe a blanket on top. Bend the knees comfortably so that the pelvis tucks under a little bit. You may need to rest them on a box. You may need a pillow at your head - try something like a rolled up blanket that you can Very Gradually unroll. Find where you are comfortable, and take months to unroll the blanket. Relax and do some deep abdominal breathing (put your hand on your abdomen and feel it rise and fall with each breath). My wife (a long time student of the Alexander Method) does this exercise every night, for about ten deep slow breaths. Most people I know with back injuries do this as well. Also check out Feldenkrais Pelvic Clock exercises to loosen up the back and hips from this position. A Taiji teacher early on had me lay like this with a finger touching around the point on my abdomen that would represent my "one point" or dantien or hara. It is easy to take the time to find your body's center of gravity when your mind isn't distracted by the pain, and later you can use this sense of your one point to help you to sit longer and more effectively.
From here, spinal twists can help to elongate and loosen up the back, neck and shoulders - lay the knees down together to one side, and the opposite arm goes above your head.
A more advanced variation is to leave (using left and right for illustrative purposes) the left leg straight, the right leg extended from the hip and rotated to the left, and the right arm goes towards the right. Do both sides.
A favorite of mine for loosening the groin muscles is to put the soles of the feet together and gently rock the pelvis. Avoid this with any hip injuries. The lower back muscles loosen up for me with this one too.
Another favorite of mine is the Happy Baby Pose, which I learned as the Wind Relieving Posture (Ananda Balasana) which opens the hips and groin muscles a little more while.
Feel silly doing it or want to work the hips more? Do it upside down, try to keep your back flat. This opens the hips and the low back, and you can put weight on your hands if you need to. Or, crouch with your feet shoulder width apart if your knees can tolerate a more aggressive variation.
Gradual pressure on the knee:
The Cat Stretch (Marjariasana) introduces some pressure on the knee, but the knee is at a 90 degree angle. Your shoulders are also getting a good stretch. If you have injuries or difficulty with your weight, you can get the benefits to your spine by flexing your spine in a chair, but your knee is left out of the equation. Listen to your body and take the time.
The Child Pose (Balasana) has a few variations. You can leave your buttocks higher in the air which takes less flexion of the knee joint, and you can have your arms at your sides or above your head taking some weight. One teacher showed me a variation where the knees come together, bring your torso up to Cat Stretch and gently lower your pelvis to the left and the right. This should be done gently, and it has many of the same benefits as the first spinal twist but works the outside of the hip more so.
The classic Child's Pose is the buttocks on the heels (difficult to do with a bigger belly), so your knees come apart. Your neck hangs loosely, and your torso is laying forward and relaxed. Your torso coming forward puts much less weight directly on the hip and knee. Alternatively, I really like one variation I found online which uses a rolled yoga mat, but the support could be anything, like a firm pillow on it's side. You can adjust the pressure to your knees very effectively.
Remember that Yoga ends with the Corpse Pose (Shavasana) - let the legs out straight, go on your back, support your arms and head with pillows if need be and let your spine relax. Get the blood flow back to your legs - this is how the body heals!
Working on the lower limbs a little more:
Lunges: there are several variations. I can put weight on the hands. I can stretch the groin and hip muscles. I can leave my foot flat, or stretch the balls of the feet. Keep the knee directly above the toes, and change position for any knee pressure.
Pigeon Pose (Eka Pada Rajakapotasana) lets you focus on one knee at a time. Having had a meniscus surgery and being born bow-legged, I do not do the variations that put the knee sideways. The outside of my own knee stretches and opens before my hip muscles relax much, and that leaves me with leg swelling for days. I keep the one knee folded underneath me, and the other back.
There are several variations of the Pigeon, all the way from gentle to excessive. Take it slow.
If you keep weight on your hands and your torso leaning forward, you can take weight off the knee. You can rock gently left to right to open the hip, but I find the can aggravate old knee injuries. Another favorite variation of mine that also appeared in Runner's World is to lay forward on your knee - this looks just like the Child's Pose but with one leg straight. Use your arms to take weight, and then relax very slowly.
Do the opposite:
One of theories of Yoga practice that Kawahara Sensei embraced was the idea of Forward Bend, Followed by Back Bend. He would structure classes to have back falls alternating with forward falls, Nikkyo alternating with Kotegaeshi, Shihonage alternating with Kaitenage or with Ikkyo. So, when I practice the Pidgeon Pose, I also take time to fully straighten the leg too.
Camel Pose (Ultrasana) while the knee never goes much more than 90 degrees, the lower leg does support the weight of the body. The front of the hip opens and stretches, the chest opens and stretches, and the spine is kept straight. The easier version has you putting your hands on your lower back, advanced versions have you grabbing your heels, and you can support your weight on one hand and stretch your arms over your head to further open your chest. Rocking very gently forward and backward can loosen the hip further. I don't always do this one, and it would be awkward to do this with tight ankles.
The actual shape of Seiza, with the benefits to the spine, hips, and legs can be supported by a meditation bench. I have one made by a friend of mine for me - three pieces of 2x4 screwed together. There are many versions of the bench. Try to find one that has the seat resting flat against your butt - if your weight sits on the edge it will cut in after a while. The bench will take weight off your knees and ankles, and allow for blood flow.
Finally the Diamond Pose, which is not identical to Seiza but has all the painful features in common. The Diamond Pose keeps the knees together, Seiza will open the knees apart. You can brace better to the front with a wider base, but this changes the stresses on the knees. Again, using a rolled towel means you can slowly, over a long time, unroll the towel until the ankles sit flat. Don't rush it. The towel idea works with the meditation bench too.