Flexibility, part of the trinity of fitness alongside endurance and strength, might be a bit overrated. It seems you're either born with the ability to touch your toes or you're not. There is some wiggle room to increase your flexibility, and many experts agree that daily stretching can lengthen tight muscles, but they also agree that you can be healthy and fit without being able to do the splits.
It was once believed that having stretchy hamstrings, the muscles on the back of the thigh, helped runners increase their speed — as in, looser hamstrings equal a longer stride. Recent research found that the tighter the hamstring, the more economical the runner. Economy in running is a measurement of how much oxygen is used while running, and using less energy with each step generally means you can farther and faster. The theory is that tighter hamstrings have a more efficient use of "elastic energy" during each stride. Biomechanics expert Dr. Duane Knudson explained to the New York Times that "flexibility is a functional thing. You only need enough range of motion in your joints to avoid injury. More is not necessarily better."
But don't give up on stretching altogether. Exceptionally tight hamstrings have been found to lead to overuse injuries in runners. Maybe you can't lay your chest on your thighs, but here are five simple hamstring stretches to limber up the backs of your legs.