Think about a stream where the water is constantly flowing. The water is clear and pure. Conversely, a body of water that sits motionless soon becomes stagnant and filthy.
Our bodies are filled with fluids. When we sit — sedentary — for extended periods of time, the fluids in our body become unhealthy, just like a stagnant body of motionless water. Conversely, when we are constantly moving, then the fluids in our body are clear and pure, just like a stream with constantly flowing water.
Tomorrow I am going to do an experiment. I am going to move all day long. I will not sit for even one minute.