Vertical punchers say the vertical punch uses the correct muscles. Vertical punchers claim the vertical punch uses the triceps muscle, which is mostly used for pushing, or punching, while the horizontal punch uses the bicep muscle, which is mostly used for lifting, not punching. This claim is absurd.
Horizontal punchers say their forearm is more protected against blocks. When the fist is horizontal, the any horizontal blocks against the sides of the forearm will strike bone. Downward blocks will strike the double bone top of the forearm. Upward blocks will strike the more vulnerable bottom of the forearm, but upward blocks the more weaker of the blocks.
When the fist is vertical, the underside of the forearm is exposed to powerful outward blocks that can damage pressure point nerves and blood vessels that are near the surface of the skin and are unprotected. Animals instinctively know not to expose their vulnerable undersides.