This is an excerpt from Art of Running Faster, The by Julian Goater & Don Melvin.
2. Quadriceps and Hip Flexors
Runners cannot avoid working their quadriceps and hip flexors (the muscles in front of the hips and thighs which you use every time you lift your knees), and so it is important to keep these muscles stretched and loose.
First, stretch the hip flexors by holding a lunge position (figure 5.2), allowing the heel of your rear foot to come off the ground and keeping your body upright. After that, it's good to stretch both the quadriceps and hip flexors at once. Start with your right quad by standing on your left foot (figure 5.3). Hold your right foot with your right hand and pull it straight back and up behind your backside. Here's the key: Pull on that foot so your thigh moves back behind you, too, and gets a good stretch. The hip flexor in the front of your hip will feel a stretch, as well­—unless you allow yourself to lean forward. Don't. Keep tall, with your torso upright, reaching towards the sky with your left hand.
You are not trying to crush your knee joint. Stretch the front of your hips and quadriceps.
Often you see people pulling on their foot while leaving the knee pointing straight down, right next to the other one. That's not a complete or sufficient stretch; that's just holding onto your foot. Be sure to pull your leg back so your knee is pointing somewhat behind you and you feel the stretch along the front and top of your thigh. You can gain the PNF effect by pushing your foot against your hand and slightly straightening you knee. Then, of course, switch legs.
Quad strength and elasticity is vitally important in running downhill. When people who haven't run it think about the Boston Marathon, they tend to think of Heartbreak Hill. It does have an ominous-sounding name and, granted, it does come after the twenty-mile mark. But it's only 600 metres long. The challenge of the Boston Marathon isn't that half-mile—it's that the course is primarily downhill. The first 13 miles are downhill, there's a sharp downhill in mile 16, and it's all downhill between miles 21 and 24. And those who finish often complain about the punishment all that downhill running inflicted on their quads.
In a typical example, Lucas Meyer, Connecticut's top finisher in 2010, thought he could run a 2:16. But he ended up jogging across the line and posting a 2:21.
‘My quads are a mess,' he told the local paper. ‘At Mile 20, my quads just shut down.' And he had done a lot of downhill training, trying to get ready.
Runners often avoid stretching their adductors because they don't seem to work very hard when running. But ignore them at your peril! They perform an absolutely vital role when you're running, and if you strain them you will not be able to run. The simplest, and best, ways to stretch are to use the sideways lunge and a hurdler's stretch.
To perform the sideways lunge (figure 5.4), keep the rear foot sideways and flat on the ground, and bend the front leg gently until you feel a gentle stretch along the inside of your leg. Keep your body upright—there is no need to lean forward.
Read more from The Art of Running Faster by Julian Goater and Don Melvin.