The muscles are connected with bones through rope-like tendons and can only be contracted. Most of the muscles work in pairs so that different types of movements are possible. Such a muscle pair is needed, for instance, to bend the elbow. While one muscle, the biceps, contracts and bends the arm, the other muscle, the triceps, relaxes. Exactly the reverse happens when the arm is stretched. The signal for muscle contraction or relaxation comes from the brain, which is controlled by us. The musculature of the internal organs works by itself since we cannot control these muscles.