Your stride — or gait cycle — begins when one foot touches down on the ground and ends when that same foot hits the ground again. There are two phases: the stance phase, when your foot is in contact with the ground, and the swing phase, when that same foot swings back and then forward before landing again. Here is one complete gait cycle:

  1. Your foot makes initial contact with the ground.
  2. Your body rolls over your foot while it is planted on the ground (midstance).
  3. You toe off or push off the ground.
  4. Your leg swings behind you and then forward (swing phase).
  5. Your foot strikes the ground again to begin the next gait cycle.

Your foot has lots of moving parts — 26 bones, 33 joints, 107 ligaments, and 19 muscles and tendons — and for good reason: You need mobility in your foot to cushion impact and to help propel your body forward at toeoff.

Foot motion during running goes like this: You land on the outside of your foot (supinate); then, as your weight loads onto your foot, your arch flattens and your foot rolls inward (pronation); then your weight shifts onto the front of your foot toward your big toe, and you push off.

Rolling feet and flattening arches may sound like agents of injury, but trust me, pronation is a good thing; it's normal and necessary for injury prevention. As your arch flattens, ground force reaction (GRF) is better absorbed and spread out across your foot. If your arch doesn't flex, your foot remains stiff, and more GRF shoots straight up your shin, putting greater stress on your bone and increasing your chance of a stress injury or, worse, a stress fracture. This lack of motion is called underpronation or oversupination.

You can also overpronate, or have too much motion. This means your foot rolls too far inward before you push off, which puts more strain on the inner side of your shin, which can pull your knee a little out of alignment, increasing your vulnerability to shin and knee trouble.

Whether you overpronate or under-pronate or pronate just right indeed depends on your foot structure. If you have a high, rigid arch, you will likely underpronate; if you have a flat foot, you will have a tendency to overpronate.

But here's the rub: Foot motion is also influenced by what's happening farther up your body. Let's say there is a lack of stability around your left knee, and as you land, that knee, instead of staying straight ahead, collapses in toward your right knee. When your knee moves inward, it drags your lower leg along; when your lower leg leans in, it encourages your foot to roll further; when your foot rolls too far, you get overpronation; and when you overpronate, you are at greater risk for injuries — among them Achilles tendinitis, stress injuries to the shin, and knee trouble. That is why I'm such a stickler about the strength of your kinetic chain. The stronger the chain, the better you are able to control your body position from above and to dissipate GRF. Plus — I know what you all care about — you'll run faster too!

What I have just described is a series of events along your kinetic chain. Every part of your body has a job to do as you run.

  • Foot: your landing gear. It supports your entire body, but it also works as a lever and helps to propel you forward when you push off.
  • Ankle: next in line to your foot to absorb GRF, and if it doesn't have a decent amount of flexibility, those forces are headed straight up your leg. Your ankle has a huge motion role to play: It allows movement of your lower leg over your foot and facilitates foot motion as you push off. Finally, if the ligaments and muscles around your ankle aren't working for stability, you will be headed for a sprain or a spill. So let's give this joint the respect it deserves.
  • Lower leg: both upward and downward motion of your foot are controlled by the tibialis anterior in the front teaming up with calf muscles at the back. Lending power to propulsion, your calf muscles also help drive your body forward during toe-off.
  • Knee: Its movement is critical to shock absorption and propulsion. As your knee flexes during foot strike, it reduces GRF, and as you go to push off, extension of your knee produces propulsive force. Stability at this joint is crucial, which is why your knees love your quads.
  • Quadriceps: the muscles in the front of your thigh that keep your knee from collapsing as you hit the ground and prevent its rotation during the stance phase of running and as you get ready to push off.
  • Hamstrings: partner with your glutes to move your thigh forward during the second half of the swing phase before your foot strikes the ground. They also deliver powerful propulsive force when you push off.
  • Glutes: also hook up with other muscles in your hips and your core to control motion in your pelvis. Rock-solid stability is their signature performance.
  • Arms: don't have close contact with the ground (hopefully), they counterbalance what your legs are doing and can help make you a more efficient runner.

While every part of your body has a main job or two to do, all your muscles are working all the time as you run, and they are working together, not independently. To prevent injury and run your best, you must treat your body as a whole in which the parts are connected and work together in fluid running movement. This is the kinetic chain.

Excerpted from Dr. Jordan Metzl’s Running Strong by Jordan Metzl with Claire Kowalchik. Copyright (c) 2015 by Rodale Inc. By permission of Rodale Books. Available wherever books are sold.

Photo via Stocksy, illustration courtesy of the author


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