Male Speaker: Any individual muscle can only pull. The bicep contracts pulling the arm up that muscles can't push the exert force only when they contract. So to move the arm in the opposite direction you need another muscle on the opposite side of the joint. When the triceps contracts, it lowers the arm or the bicep relaxes and vice versa. Contraction is the key to spot the muscle that's doing work look for the one that's getting shorter during a movement.