The relationship between the two legs in running (Part 2 of 2)

So what is the relationship between the legs in running?

To keep the body torso balanced - as when standing - the center of mass of one leg has to be an equal distance in front of the body's center of mass as the other is behind.

Take-Off phase

So the point at which the leg on the ground is at its farthest behind the body's center of mass (just before take-off), the other leg's center of mas has to be equally far foward to keep the torso balanced.

After the back leg leaves the ground, its center of mass continues back just a few more inches and the heel begins to rise. So the center of mass of the front leg should continue moving forward as well if our goal is to keep the torso vertical. This is easy to accomplish. While the back leg is getting read to leave the ground, the front leg should have its heel tucked. The knee is up and forward, but the heel is under the butt. So to get the center of mass of the front leg to move forward more, all that needs to happen is for the heel to drop and move under the knee. Notice that the lines under the yellow dots in the figure to the left are farther apart than the figura above, yet they are still equidistant from the line under the blue dot.

Flight and Foot Strike

In efficient running, the foot strikes the ground almost directly under the body's center of mass (actually slightly in front, but does not becoem fully weight bearing until it is directly underneath). This means that the front leg's center of mass will have to move backward during the flight phase towards the body's center of mass before making contact with the ground. Consequently, this means the trail leg will also have to move towards the body's center of mass while in flight.

I see recreational runners (heel-strikers mostly) with the trail leg way behind the body through the whole flight phase. For them, bringing the trail leg under the body's center of mass before landing will reduce breaking forces (including shin-splint inducing forces) and cause them to run faster and more fluently.

Foot Strike and Stance

In efficient running, again, the foot strikes the ground almost directly under the body's center of mass. In truth, it becomes fully weight bearing directly under the body's center of mass. So, when the front foot is about to touch the ground a portion of that leg will actually be in behind the imaginary plane (vertical from the body's center of mass) and a portion will still be in front of it. Therefore, the trail leg should have a portion of it in front of that imaginary plane before the foot strike occurs with only part of it remaining behind the center of mass. So the knee, the lower femur, and upper shin should be in front of the plane and the lower calf, foot, and upper femur and glutes should be behind.

This is the relationship between the legs in running at a CONSTANT speed: the center of mass of one leg should be exactly as far in front of the body's center of mass as the other leg is behind it at any given time during all phases of the running cycle.

During GRADUALLY ACCELERATING speed: the center of mass of one leg should be slightly MORE in front of the the body's center of mass as the other leg is behind it. OR, the center of mass of the trail leg should be slightly LESS behind the body's center of mass as the front leg is in front of it at any given time.

During GRADUALLY DECELERATING speed: the center of mass of one leg should be slightly LESS in front of the center of mass as the other leg is behind it. OR the center of mass of the trail leg should be slightly MORE behind the body's center of mass as the front eg is in front of it at any given time. This is why fatigued hip flexors and a tired core - which causes a delay in the initiation of the recovery of the trail leg - leads to a shorter stride and fatigued hamstrings.