But we still have an unsatisfactory explanation because it does not take into account the fatigue factor. How could the front lines of two armies stand toe to toe and fight for hours?
Adrian Goldsworthy, author of the book The Complete Roman Army has some interesting ideas about this. He first argues that three quarters of the front line was fighting defensively to stay alive as opposed to attacking. This notion has been verified by examples from World War II where the majority of troops in a firefight did not fire their weapons. This is human nature. Secondly, Goldsworthy postulates that there were lulls in the fighting where the opponents drew back and rested. Makes sense to me. Drawing back also created an opportunity to launch pila at the enemy. We have evidence of front line soldiers killed by thrown missiles after the opponents made contact.