Once a muscle action potential is initiated by the SA node where does impulse spread?

Your question appears to be based on a misunderstanding. The SA node (sinoatrial node) is located in the heart, and it generates electrical impulses that cause the heart to contract. These impulses spread through the heart muscle, causing the heart to pump blood. Muscle action potentials, on the other hand, occur in skeletal muscles, and they are initiated by signals from the central nervous system, not by the SA node.