What is the best way to strengthen the kidneys?

My master said that the body's kidney energy is limited, with a fixed amount, and using some will make it slightly less. Therefore, after middle age, one must learn to be moderate; moderation is the best way to supplement the kidneys. About five or six years ago, during the New Year, I went to Zhongnan Mountain to visit my master.

My master is an old child, over 90 years old, almost the same age as a child. In winter, he insisted I accompany him to climb the north wind. I didn’t dare not to go, so I followed my master out to hike. At the end of the year, Zhongnan Mountain was covered with snow everywhere, the mountain paths were rugged, steep, and slippery, making it very difficult to walk. I am an outdoor enthusiast, have been to many places, climbed many mountains, and consider myself physically capable.

No matter how skilled my master is, he is over 90 years old—he shouldn’t have better physical strength than me, right? It turned out that my master’s physical strength was much stronger than mine. After he reached the North Peak, it was more than half an hour before I finally reached the summit. My master teased me, saying I was a bit kidney-deficient. Master, can you not embarrass your disciple like this? After coming down from the north wind, we returned to my master’s small courtyard.

My master taught me a very simple and effective exercise to strengthen the kidneys. It’s just one move: stand upright, keep your feet close together, let your arms hang naturally at your sides, and lift your heels off the ground. Keep your heels off the ground, and according to your actual situation, hold for ten or twenty seconds. Then, forcefully press your heels down to the ground. It’s a pulling motion; just apply appropriate force. Repeat this movement 20 times. Doing this daily can help with kidney health. After middle age, no matter how good the method, it’s more important to root and stabilize oneself. That’s what my master also told me.