Much has been written about strength training for runners—with articles describing every kind of exercise from lunges while holding dumbbells and calf raises to endless abdominal crunches and core training on a big, lime stability ball.
A stronger core aside, have you ever wondered if these training suggestions will really lead to a faster 5K or marathon?
Unlike most sports, which require strength, speed and power, distance running is primarily limited by the delivery and use of oxygen. There are no studies proving that strength training increases the supply of oxygen to your muscles and heart. Oxygen wouldn't recognize a dumbbell if it were hit on the head with one.
The responsibility of oxygen delivery rests on the shoulders of your cardiovascular system. The greater your stroke volume (the volume of blood pumped by your heart per beat) and cardiac output (the volume of blood pumped by your heart per minute), the more oxygen will be delivered to your muscles. And the more muscle capillaries and mitochondria you have, the greater your muscles' capacity to use the available oxygen.
Although a 25- or 30-minute 5K runner will likely improve his or her performance by strength training because the overall stimulus is greater when doing both running and strength training (and thus improving overall fitness), this runner is better served by spending more time running and improving the cardiovascular and metabolic traits associated with endurance than by strength training.