To make sure you are up to speed (no pun intended), make sure you have had a look at Part 1 & Part 2 of The Most Important Muscles To Strengthen For Running and also read this article here.  Now the quadriceps group (as the latin translation suggests) is made up of four muscle heads: vastus lateralis, vastus medialis, vastus intermedius and rectus femoris.  These muscles primarily extend the knee while the rectus femoris also flexes the hip, which makes it a doubly important muscle for running (as we will see in the next part of this series).    Read This Article

So everyone understands that getting stronger by lifting weights will actually make you a faster runner and will not get big and bulky, please read this blog post here and this article here.  Also let's quickly revise the dynamics of running.  There are basically three parts to running: the stance phase (which is the amount of time you spend on the ground every time your foot touches the ground), the stride length and the stride rate (or frequency).  To run faster, you have to either a) decrease the stance phase, b) increase the stride length or c) increase the stride frequency.  As I have mentioned here, this applies to every distance from a 100m sprint to a ultramarathon.  The second group of muscles we are going to talk about in regards to their importance to running and why they need to be as strong as possible are the hamstrings and in this case when they extend the hip.  Read This Article