Great question posed in Competitor magazine.  Matt Fitzgerald responds;

Weightlifting and other forms of strength training help runners in three ways. 

First, they enhance performance, mainly by boosting running economy.  The legs function like springs during running.  Research has shown that weightlifting and plyometrics (jumping exercises) make the legs springier, so runners move faster with less energy.

Second, strength training also reduces injury risk.  Instability at key joints-especially the hips and knees-contributes to many common overuse running injuries.  Strength training increases joint stability and thereby reduces injury risk.

A third benefit of strength training is that it slows the loss of muscle mass and strength that occurs with aging, helping runners perform better later in life.  Adding more running to your routine will not increase the stability of your joints or help you preserve muscle mass and strength.  Those benefits are specific to the stress that strength training imposes on the muscles.

But what about performance?  It is reasonable to suppose that your running would improve more if you added an hour of running to your schedule instead of an hour of strength training.  But research proves otherwise.  In one study, elite runners improved their race times when they replaced one-third of their normal weekly running time with plyometrics.  It doesn't require extra time; just reallocate some of your existing running time to strength training and you will run faster and healthier.

I have trained a couple runners with my Stabilize Your Core in 12 Weeks program and here is a success story of one of my clients.   Strength training really works.

Raymond W, Manhattan I had just turned 57 when I cheered for my daughter at the Banco Popular Chicago Half Marathon. I remembered just how much fun I had running when I was a teenager. I even remembered when the first Boston Marathon was held and thought that would someday be my goal. I also realized that most runners are retiring at my age and not just starting. And I remember that just 10 years ago my doctor told me that I was too old to run. Well, that Christmas a gift certificate for running shoes and a beginners running book by Hal Higdon was under the Christmas tree. And I soon learned that he celebrated his 60th birthday by running 6 marathons in 6 weeks! And now he is in his 70’s and still running. So I thought-----I got time. I read the book and looked at his training program. Step one was to run for 30 seconds then walk for 30 seconds 15 times. I did that then took a 2 hour nap. I eventually worked myself up to run the Banco Popular half Marathon later that year at age 58 and did ok. I was told that if I wanted to improve I had to cross train to strengthen my core. I had no clue just what that was all about. So I joined the local Gym, as the weather was inclement, and my next big race was the Shamrock Shuffle. I figured that at the gym you just piled on the weights and did 75-100 repetitions of whatever. That is when my daughter came to my rescue and introduced me to Wanda. I arrived at the Gym and waited for her. I was pleasantly surprised as she drove up in a jeep, she jumped out with a clipboard and marched into the room with the enthusiasm of a drill sergeant. I liked that as it told me that she is serious about what she does. From the beginning she set me straight. She taught me the correct way to use the machines, the sequence of their use so that all the muscle groups are exercised, body posture and alignment, and most importantly how to breath, and to do the sequences deliberately so as isometrics are added into the equation. Well needless to say I give a lot of credit to Wanda as I finished my first Chicago Marathon. My time was 5 hours and 57 minutes. This sounds like a long time as by the time I was completing mile 10 the Kenyans were finishing the race. But when I looked back after I finished there were 7,000 runners in back of me. Thanks to my daughter and Wanda as I couldn’t have done this without your help.

Ray W
I wanted to let you know that I have been using your core training program throughout the winter. I took 6 minutes off the half marathon, 12 minutes off the 5 miler, and 3 minutes off the 5k. I am still not as fast as a Kenyan, but hopefully I will get there in the future. I ran the Chicago 1/2 Marathon and took 20 minutes off last years time.