The aerobic threshold is the level of physical effort at which a person’s cells are just beginning to carry out fermentation to meet some of their energy needs. Marathon runners try to stay at or below the aerobic threshold so that they can go longer distances without experiencing difficulty. While running up and down the hills of San Francisco, the runner was focused on trying to maintain the pace he had when he was running on flat land. What impact did this focus have on his ability to maintain his aerobic threshold?