Why horses have skinny legs? What’s the use of a skinny leg? The answer is Running. The skinniest lower legs belong to the fastest runners. Think of how a deer can move, as opposed to a tortoise. Why is that? It’s because when you run, or indeed walk, you are constantly swinging the bottom halves of your legs. For humans, one foot is stopped and essentially resting on the ground while the other is moving twice as fast as your overall speed.