The following three exercises make up what I call the "Fire Set," which are "Walk and Kick Back," "Walk Like a Warrior," and "Up Like Smoke, Down Like a Feather."
I designed this exercise after many years of experience working with martial artists as well as elders, and stumbling into many issues of leg strength, as well as osteoporosis and sarcopenia.

Osteoporosis is a condition in which the bones lose their strength and density. Sarcopenia is a lesser-known ailment. Another term for sarcopenia is vanishing flesh, meaning that you lose muscle mass, and that existing muscle mass becomes filled with deposits of fat. When you get on a scale, you are not aware that you have gained or lost any weight because your weight does not change. However, you notice the effects of sarcopenia in your daily performance. You suddenly cannot lift bags, you cannot open jars, your legs feel weak, and you do not understand why. What has happened is that your muscles have disappeared.

If you do not use it, you lose it. Muscle is replaced with deposits of fat, which does not give you the strength you need to perform your daily activities. When you lose independence because of issues such as osteoporosis or sarcopenia, you also have a tendency to become depressed, which inhibits your natural immune response to illness. It is a negative chain reaction.

In order to prevent these conditions, we can use physical resistance, using our own body weight. Studies done with sarcopenia and osteoporosis patients were done with common weight machines, free weights, cans, and resistance through lifting objects. I modified these exercises and created resistance by using our own body weight. It is just as if you are going to the gym and lifting your own body weight, again and again.

Remember to focus the mind and evoke your spirit throughout each exercise, and especially during these next three, which are the Fire Set of the preparation exercises. Your eyes should shine with a fiery spirit, as if you were in battle. In modern times, this warrior spirit can more often be utilized for raising the body's natural defense systems in the immune system, rather than engaging in combat.

Walk and Kick Back—First Piece of the Fire Set

Tai Chi Fire Set Exercises for Leg Strength Leg

This exercise will increase the heart rate, increase lung capacity, and strengthen circulation to get the blood to flow to the extremities, out to the surface of the skin, deep into the bone, and circulating smoothly through the organs.
In this exercise, you march in place, kicking the feet backwards as far you can. If you are very flexible, the foot can reach the buttocks during this movement. Put your hands on your waist and march!

Some days you can practice this slowly, some days you can speed it up. You may even do this at full speed, running in place. Be sure that you have loosened up first with other movements. Remember, keep breathing through the nose, and coordinate the abdominal muscles with the breath. Maintain the visualization of the baton. Keep the face relaxed and the spine straight. Deep, soft, quiet, inhalation. For a more advanced version of this exercise, which will also stretch the muscles around the lungs, put your hands over your head. This posture will be a little bit more challenging.

You will feel heat building up throughout the body. Remember, stop and close your eyes and then lead all of it into your energy centers when you are done.
Start with 3 minutes, and increase your training time gradually. Do not overdo it.

Walk Like a Warrior—Second Piece of the Fire Set

This second exercise of the Fire Set, or cardiovascular set, for increasing your heart rate and oxygen intake is "Walk Like a Warrior." It is similar to the previous exercise, except this time you bring your knees up as high as you can, emphasizing opposite arms with opposite legs. This can also get more advanced—the higher you lift the legs, and the faster you move.

Train the first two minutes just walking gently or lightly, and lifting the knees high. The higher you lift your knees, the more you demand from your heart, leg muscles, and lungs. Then, for one minute, run.

The reason I call this "Walk Like a Warrior" is so that students remember this is not only physical. I want you to literally sense as if you were running through a battlefield to save your family, or whatever it takes to evoke your spirit. The whole sensation of your body is like a warrior, with fiery eyes. You are in a fierce battle with yourself, against sickness, against feeling tired, or weak, or out of control. Take command of your health. When you finish, surrender, and take the time to return all of this energy back to your center.
Start with 3 minutes, and increase your training time gradually. Do not overdo it.

Third Piece of the Fire Set—Up Like Smoke, Down Like a Feather

Tai Chi Fire Set Exercises for Leg Strength Squat

The reason this next exercise or mind/body prescription is called "Up Like Smoke, Down Like a Feather" is because, eventually, you want to be able to have the spiritual sensation of a feather falling from the sky when you sink, and the sensation of smoke rising up from incense when you rise up. "Be" the feather, and "be" the smoke.

Up like Smoke, Down like a Feather will build muscle mass, stimulate bone growth, generate energy, and give you strong legs for a better quality of life, and for better Tai Chi.
In order to perform this mind/body prescription, you need to find a special place in your house, like a smooth wall, or the sides of a door jam, or even the door itself. Just make sure that you lean against the door in the direction it closes, and not the opposite.

For the First Set, lean against the wall, and open your legs as wide as is comfortable; wider than your shoulders, if possible. For the Second Set, close the legs so they are touching. Once you have advanced with this training, you can go up and down on the balls of the feet, without the wall.

Throughout each stage of this exercise, your goal is to glide down slowly like a feather, and then float up like smoke. When you slide up and down, you do not want your knees to extended further than your toes. If the knees are further then your toes, you can easily hurt your knees. Place your toes out far enough that when you bend to where the thighs are at a 90 degree angle, the toes are still in front of the knees.

The next thing that can happen is when you come up; if your inner muscles are stronger than your quadriceps in the outer thigh, the knee will move in. The inner muscle will say, "Let me do it. Let me lift your weight up." What I want you to do is to think from your skeleton, from the bones. Do not let the soft tissue determine the action or the alignment. You decide to let the bones determine it and maintain alignment.

In the exercise, you will quickly find out which one of your muscles is stronger. Is it the inner thigh muscles or the outer thigh muscles? If your legs are together, and on the way up you want to open your legs then your quads are stronger. When the legs are apart, some people want to bring the knees in to come up. When the legs are together, some people want to open the knees and come up. The bottom line is that your body is going to rely on whichever muscle is stronger. We want to re-educate the soft tissues and make them balanced and have confidence that each one has its own individual strength and independence, rather than relying on groups of muscles that are not needed for that specific action.

Some people cannot even use the inner or outer leg muscles to rise up, and they use their hands to come up. This is okay just to get started, but what we want to do is bring back the strength and flexibility of our legs.

Remember, once you are comfortable with this movement, to maintain all the other internal skills while you are sliding up and down on the wall.

First Part of the Third Piece of the Fire Set

Place the feet about shoulder's width apart, and if you can go a little further, it is even better. The wider you open the legs, the more you affect the quadriceps, the outer muscles of the thighs. Slowly slide down and try to imitate a feather dropping from the sky. Then pause. Do not go further than 90 degrees.

On the way up, make sure the knees stay over the middle toes. If you need to, you may open the knees a little bit, and push gently up to rise like smoke.

Then slide down again slowly. Maintain your sacrum, or tailbone, touching the wall. Maintain the knees over the toes and push up. Try to coordinate your Center of Gravity breathing, maintaining the sensation of the baton internally, between the Upper Pituitary Gland and Lower Center of Gravity. Repeat as many times as is comfortable, and gradually increase the repetitions as you condition the legs.

Second Part of the Third Piece of the Fire Set

Put your feet together. Maintain the same distance of your heels from the wall. Usually it is the distance between the knee and the sacrum. Another way of judging it is when you go down, the toes are still extended slightly farther than the knees.

Then on the way up, make sure that you do not open the knees. Sometimes the knees will want to open up to rely on the quads. Do not let them. You are in control of which muscles to use. Try to train so that the inner and outer leg muscles are balanced.

Once you are more advanced, you do not need the wall any more. The problem is that when training this without the wall, people tend to come up with their sacrum sticking out. This causes you to divert your weight from being directly aligned above the legs, and you lose all the weight resistance in the legs.

Once you have trained on the wall until your legs are strong and the ankles are flexible, then you can practice away from the wall as long as you can keep the sacrum dropped. Again, train both sets, with the legs open, and then closed, slowly adding repetitions.

Using the wall is a pure way to isolate the resistance into the desired soft tissue. To transition from using the wall to free standing, you can hold the doorknobs of a door and gently support yourself, without hanging on to it, while maintaining the tailbone in, and then float down like a feather and rise up like smoke.

Start with trying to do 50 repetitions with the legs open, and 50 with the legs closed, and increase your training time gradually. If this is too challenging at first, do fewer reps, but be sure that the inner and outer leg muscles get an equal workout. Again, be careful not to overdo it.

This excerpt is from Sunrise Tai Chi—Simplified Tai Chi for Health & Longevity by Ramel Rones with David Silver.