Olympic athletes train for years to increase the strength and efficiency of their body systems. All the body systems must work at their peak performance to compete at such a high level. Training just one system is not enough.
The human body is made up of multiple interacting systems.
Each system in the human body has a special function. Your skeletal system gives your body support and structure, so you can stand up. Your immune system helps you stay healthy by fighting off diseases. Each of these systems also interact with each other. For example, the bones in your skeletal system would not be very useful without the muscular system to help them move around. And how would your muscles know where to move your bones if your nervous system didn’t tell them?