It’s all about balance. Those guy wires imitate how the muscles work to keep your bones and joints in good placement. Most muscles are attached to different bones on each end (like the ropes are attached to the ground and the tree) and when they contract, that action pulls one bone closer to the other. That’s how joints move. When one muscle shortens, its partner muscle is stretched and a joint moves.
Take your elbow for example. The muscles that control the elbow in flexion/extension (bending/straightening) are the biceps and triceps. Most of you are familiar with these muscles. The biceps is the muscle in the front part of your upper arm, and the triceps is in the back of the arm. When the biceps contracts and shortens, the arm bends at the elbow and the triceps muscle