Final answer:
Walking has the downside of having a narrow optimal speed and being less suitable for long distances compared to anaerobic activities like sprinting, which can cause rapid muscle fatigue.
Step-by-step explanation:
The downside of walking as a way of transportation could include that it has a narrow range of optimal speed and is only practical over relatively short distances compared to other modes of transport. While walking benefits from aerobic metabolism, which is efficient in energy production over long periods and sustainable for extended activity, it is not as rapid as the anaerobic metabolism used during high-intensity exercises like sprinting. A sprinter utilizes anaerobic metabolism, allowing for a burst of high speed but causing muscle fatigue more quickly than aerobic activities such as marathon running, which is characterized by longer duration and consistent pace.