Here’s a fun science project. Take the cardboard tube from a roll of paper towels. If you set the tube upright you can rest a brick on top of it even though the brick is many multiples of the tube’s weight.

Now place the tube at an angle. When the tube is angled force applies parallel to its cross section which creates shear stress. Under shear one of two things will happen. The weight of the brick will fold the tube in half, or the tube’s footing will wrench free.

Now imagine that the tube is your shin bone. How would you change the way you run?

First, keep the tube from getting too slanty. If you over-stride, or reach out with your foot, you will land with a severely angulated shin and your knee joint will absorb many multiples of your body weight in shear stress. The tube wouldn’t last for 400 meters much less 4 miles. Much better to let your foot drop straight down so that it lands under your center of mass, rather than ahead of it.

Second, center the brick over the tube. You want as much of your body mass as possible to be stacked perpendicular to the cross section of your shin. Remember that your head can be up to 20% of your body mass. At the moment you land your head, hips and feet should plot on the same line, near-perpendicular to the ground. Shear stress from forward head posture is why many runners report neck and shoulder pain, as well as knee and ankle pain.

The good news is we don’t have cardboard tubes for shin bones. The bad news is your body absorbs three times its mass with every stride. That’s a lot of bricks. If you run even casually, this adds up to many, many tons of force. With poor running mechanics eventually shear stress will wrench your shin bone away from your thigh bone. Your ACL is supposed to prevent this displacement but it can only take so much.  This is why so many runners trash their ACL.

Your body must obey the laws of physics, even if you try to defy them.